Synthetic Methods for Conjugated Polymer and Carbon Materials E-Book
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- Herausgeber: Wiley-VCH Verlag GmbH & Co. KGaA
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
A concise and practical overview of the most important modern synthetic aspects of conjugated polymers and carbon materials, including their properties and applications.
Well structured, this book summarizes recent achievements, outlines the current state and reviews research trends. As such, a wide variety of polymerization techniques are included on both a strategic as well as a practical level, including Stille, Suzuki , and direct (hetero)arylation polymerizations. Furthermore, it covers various carbon-rich materials, such as graphene and carbon nanotubes, followed by a look at how the different synthetic pathways and strategies influence their final properties, for example, for use in organic electronic devices. The whole is rounded off with a discussion of future technology advances.
An essential reference for newcomers as well as experienced researchers in the field.
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Seitenzahl: 522
Veröffentlichungsjahr: 2016
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Table of Contents
Title Page
Copyright
List of Contributors
Chapter 1: Stille Polycondensation: A Versatile Synthetic Approach to Functional Polymers
1.1 Introduction
1.2 Reaction Mechanism
1.3 Reaction Conditions
1.4 Examples of Functional Materials Synthesized by Stille Polycondensation
1.5 Challenge and Outlook
1.6 Summary
References
Chapter 2: Suzuki Polycondensation
2.1 Introduction
2.2 Mechanism of Suzuki Coupling and Suzuki Polycondensation
2.3 Catalysts
2.4 Reaction Conditions for Suzuki Coupling
2.5 Side Reactions
2.6 AB versus AA/BB Suzuki Polycondensation
2.7 Monomer Purity, Stoichiometry, and Solvents
2.8 Monomers for SPC
2.9 Chain Growth SPC
2.10 Scope and Applications of SPC
2.11 Conclusion
References
Chapter 3: Controlled Synthesis of Conjugated Polymers and Block Copolymers
3.1 Introduction
3.2 Approaches to Controlled Polymerizations
3.3 End-Functionalized Polymers
3.4 Block Copolymers
3.5 Other Copolymers
References
Chapter 4: Direct (Hetero)arylation Polymerization
4.1 Introduction
4.2 First Examples of Direct (Hetero)arylation Polymerization
4.3 Selectivity and Reactivity Problems
4.4 En Route to Defect-Free Conjugated Polymers
4.5 Outlook
References
Chapter 5: Continuous Flow Synthesis of Conjugated Polymers and Carbon Materials
5.1 Introduction to Flow Chemistry
5.2 Conjugated Polymers
5.3 Carbon Materials
5.4 Material Processing
5.5 Summary
References
Chapter 6: Synthesis of Structurally Defined Nanographene Materials through Oxidative Cyclodehydrogenation
6.1 Introduction
6.2 Synthesis of Nanographene Molecules through Oxidative Cyclodehydrogenation
6.3 Bottom-Up Synthesis of Graphene Nanoribbons
6.4 Conclusions
References
Chapter 7: Photochemical and Direct C–H Arylation Routes toward Carbon Nanomaterials
7.1 Introduction
7.2 Photochemical Routes toward PAHs and Carbon Nanomaterials
7.3 Intramolecular Direct Arylation C–H
References
Chapter 8: Carbon-Rich Materials from sp-Carbon Precursors
8.1 Introduction
8.2 Carbyne
8.3 Solid-State Reactions of Polyynes: Topochemical Polymerizations
8.4 Diyne Polymerization
8.5 Tubular Structures
8.6 Beyond Diynes – Topochemical Polymerization of Polyynes
8.7 Toward “Nanographene”
8.8 Pentalenes
8.9 Modification of sp-Precursors with Tetracyanoethylene (TCNE)
8.10 Thermal Dimerization of Cumulenes
8.11 Outlook: From Solution to Surface?
8.12 Summarizing Comments
Acknowledgments
References
Index
End User License Agreement
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Guide
Table of Contents
Begin Reading
List of Illustrations
Chapter 1: Stille Polycondensation: A Versatile Synthetic Approach to Functional Polymers
Figure Scheme 1.1 The Stille coupling reaction scheme.
Figure Scheme 1.2 Synthesis of aryltin compounds by Eaborn
et al
. [9].
Figure Scheme 1.3 Coupling of halides and organostannanes by Kosugi
et al
. [10–12].
Figure Schemes 1.4 Synthesis of PPT by Stille polycondensation [16, 17].
Figure 1.5 Synthesis of PPTs with metalloporphyrin or pendent carbazole units [18].
Figure Scheme 1.6 A simplified mechanism for Stille coupling [7, 19].
Figure Scheme 1.7 Formation of
cis
complex and
cis–trans
isomerization [19]. L = ligand.
Figure Scheme 1.8 Cyclic and open transition states [26, 27].
Figure Scheme 1.9 A ligand dissociation to form a T-shaped complex [29].
Figure Scheme 1.10 A more complex mechanism by Espinet
et al
. [20].
Figure Scheme 1.11 The structure of some bulky phosphine ligands [34–37].
Figure Scheme 1.12 Selectivity of ArCl over ArOTf in Stille coupling [37].
Figure Scheme 1.13 Bulky ligands assist Stille coupling [20].
Figure Scheme 1.14 Microwave conditions to shorten reaction time [54].
Figure Scheme 1.15 The structure of some polyimide polymers for second-order NLO [59–61].
Figure Scheme 1.16 Synthesis of polymers with NLO chromophores on side chains [62].
Figure Scheme 1.17 Synthesis of fluoro- and alkoxy-substituted PPVs for third-order NLO [63].
Figure Scheme 1.18 Synthesis of polythiophene derivatives for third-order NLO [64].
Figure Scheme 1.19 Synthesis of PPTs with tricyanodihydrofuran subunit for PR material [65]
Figure 1.20 Synthesis of a polyalkylthiophene polymer [71].
Figure Scheme 1.21 Synthesis of PTB polymers.
Figure Scheme 1.22 The structures of PBDTTPD [73, 84–86], PBnDT-DTBT [87], PBnDT-FTAZ [88], and PBDTT-DPP [89].
Figure Scheme 1.23 Synthesis of PDTP-DFBT [90], PffBT4T, PBTff4T, and PNT4T [42].
Figure Scheme 1.24 Synthesis of CPDT-BT polymer [91, 92].
Figure Scheme 1.25 Synthesis of Si-bridged CPDT-BT polymer [56].
Figure Scheme 1.26 Synthesis of Si- and Ge-bridged CPDT-TPD polymers [93–95].
Figure Scheme 1.27 Synthesis of IID-based polymers [96, 97].
Figure Scheme 1.28 Synthesis of lactam-based donor polymers [98–102].
Figure Scheme 1.29 Conjugated polymers based on thiophene, benzothiadiazole, and benzobis(thiadiazole) [103].
Figure Scheme 1.30 Synthesis of DTBT-IDT and DTABT-IDT [104].
Figure Scheme 1.31 The synthesis of PDI containing polymer and small molecule [105, 106].
Figure Scheme 1.32 Synthesis of NDI-based polymer P(NDI2OD-T2) [107–109].
Figure Scheme 1.33 Synthesis of PNDIT and PNDIS polymers [110–112].
Figure Scheme 1.34 Synthesis of BFI-based acceptor materials [113–116].
Figure Scheme 1.35 Some polythiophene derivatives for OFETs [119, 120].
Figure Scheme 1.36 Some IID-based polymers for OFETs [121–123].
Figure Scheme 1.37 DPP-based polymers for OFETs [117, 125–127].
Figure Scheme 1.38 Synthesis of soluble PPV derivatives [131, 132].
Figure Scheme 1.39 Synthesis of PPV-based random copolymers for P-OLED [132, 133].
Figure Scheme 1.40 Synthesis of PPyV polymers and their pyridinium forms [134].
Figure Scheme 1.41 Synthesis of random and regioregular PPyV polymers [136].
Figure Scheme 1.42 Synthesis of regioregular polythiophenes using Stille method [137].
Figure Scheme 1.43 Synthesis of V-shaped polythiophene V-PT [138].
Figure Scheme 1.44 Synthesis of some OFET polymers based on CPDT with different bridging atoms [139, 140].
Figure Scheme 1.45 Examples of polythiophenes that can detect purine and pyrimidine via H-bonds [141–143].
Figure Scheme 1.46 Synthesis of polythiophene sensor polymer and its interaction with metal ions [144].
Figure Scheme 1.47 Synthesis of phenylene–thiophene-based liquid crystal polymer.
Figure Scheme 1.48 Stille coupling in the total synthesis of rapamycin and dynemicin A [147, 148].
Figure Scheme 1.49 Reaction scheme of Suzuki coupling for conjugated polymers.
Figure Scheme 1.50 Reaction scheme for Kumada coupling.
Figure Scheme 1.51 Catalytic use of tin in Stille coupling [150, 151].
Figure Scheme 1.52 Ionic liquid supported Stille coupling [152].
Chapter 2: Suzuki Polycondensation
Figure Scheme 2.1 Suzuki reaction catalytic cycle.
Figure Scheme 2.2 Transmetallation process involving a quaternary boronate anion.
Figure Scheme 2.3 Relationship between pH and an organoboron compound's nature.
Figure Scheme 2.4 Transmetallation involving Pd-OR intermediates.
Figure Scheme 2.5 Reactions in the absence of base or catalyzed by weak bases.
Figure Scheme 2.6 Ligands for palladium-catalyzed Suzuki reactions.
Figure Scheme 2.7 Reactions facilitated by ligandless catalysts.
Figure Scheme 2.8 Water-soluble ligands.
Figure Scheme 2.9 The first reported microwave-assisted Suzuki polycondensation.
Figure Scheme 2.10 Side reactions of Suzuki coupling: (a) formation of homocoupling products; (b) B–C bond cleavage; and (c) ipso-coupling.
Figure Scheme 2.11 Phosphine-mediated aryl–aryl exchange.
Figure Scheme 2.12 Possible routes to phosphorus incorporation during Suzuki polycondensation.
Figure Scheme 2.13 AA/BB versus AB Suzuki polycondensation.
Figure Scheme 2.14 Some commercially available monomers for AA/BB SPC.
Figure Scheme 2.15 Synthesis of AB- and BB-type monomers via aryllithiums.
Figure Scheme 2.16 Halide–boronate exchange for producing monomers and polymers.
Figure Scheme 2.17 Direct borylation via transition metal-catalyzed C–H activation of arenes.
Figure Scheme 2.18 Boron-based monomers for producing PAVs or PAEs.
Figure Scheme 2.19 Synthesis of a cyclododecaphenylene by iterative Suzuki coupling.
Figure Scheme 2.20 Use of masking groups to protect boronic acids during Suzuki coupling.
Figure Scheme 2.21 A cascade Heck–Suzuki route to a PAV.
Figure Scheme 2.22 Modified Suzuki coupling using a triolborate.
Figure Scheme 2.23 The first example of SPC using dichloroarene monomers.
Figure Scheme 2.24 SPC by a chain growth mechanism.
Figure Scheme 2.25 Synthesis of ladder-type polyphenylenes using SPC.
Figure Scheme 2.26 Regioregular polythiophene- and thiophene-containing copolymers made by SPC.
Figure Scheme 2.27 Units which have been integrated into polymers by SPC.
Figure Scheme 2.28 Carbazole copolymers made by SPC for OPV applications.
Chapter 3: Controlled Synthesis of Conjugated Polymers and Block Copolymers
Figure Scheme 3.1 Mechanism of the catalyst transfer polycondensation, illustrated via the polymerization of thiophenes.
Figure Scheme 3.2 Poly(thiophene)s obtained in the case of unidirectional growth (a) or bidirectional growth (b).
Figure Scheme 3.3 Grignard metathesis and the effect of the catalyst on the regioregularity of the polymer (without LiCl).
Figure Scheme 3.4 The selective formation of only the desired isomer of the monomer can be achieved in two ways. This results in regioregular poly(thiophene)s.
Figure Scheme 3.5 Mechanism of the Pd(RuPhos) protocol.
Figure Scheme 3.6 Chain growth polymerization of 2-halothiophenes, exemplified for chlorodibutylpropylenedioxythiophene and SnCl
4
as the initiator.
Figure Scheme 3.7 Overview of the different methods for the synthesis of functionalized Ni initiators for KCTP.
Figure Scheme 3.8 Visualization of different isolated Ni complexes equipped with (protected) functionalized initiators.
Figure Scheme 3.9 Visualization of multifunctional external initiators.
Figure Scheme 3.10 Suzuki–Miyaura polymerization of fluorene with Pd(P
t
Bu
3
)
2
as external initiator.
Figure Scheme 3.11 Overview of end groups resulting in mono-capped or di-capped polymer chains.
Figure Scheme 3.12 Different methods to obtain block copolymers.
Figure Scheme 3.13 All-conjugated block copolymers consisting of different conjugated moieties.
Chapter 4: Direct (Hetero)arylation Polymerization
Figure Schemes 4.1 Synthesis of thiophene-based molecules.
Figure 4.2 Thiophene-based organic materials prepared by direct (hetero)arylation [14].
Figure Scheme 4.3 First example of direct (hetero)arylation polymerization.
Figure Scheme 4.4 Synthesis of poly(3,4-alkylenedioxythiophene)s.
Figure Scheme 4.5 First efficient synthesis of regioregular poly(3-hexylthiophene) (P3HT) by DHAP.
Figure Scheme 4.6 Synthesis of a
push–pull
arene-based copolymer.
Figure Scheme 4.7 Synthesis of a
push–pull
hetero(arene) copolymer.
Figure Scheme 4.8 First conjugated polymers synthesized by DHAP.
Figure Scheme 4.9 Selectivity and reactivity issues.
Figure Scheme 4.10 Synthesis of a
push–pull
copolymer by DHAP.
Figure Scheme 4.11 Monomers bearing β-blocking groups.
Figure Scheme 4.12 Selectivity from a directing group.
Figure Scheme 4.13 Branched and cross-linked poly(3-hexylthiophene).
Figure Scheme 4.14 Structural defects in conjugated alternating copolymers: (a) branching and (b) homocoupling.
Figure Scheme 4.15 (a) Regiosymmetric AB-type monomers give polymers insensitive to homocoupling defects. (b) P3HT is an example of non-regiosymmetric polymer.
Figure Scheme 4.16 Screening of DHAP conditions for P3HT synthesis.
Figure Scheme 4.17 Synthesis of P3HS and P3HT: effect on the regioregularity (rr).
Figure Scheme 4.18 Synthesis of P(Cbz-
alt
-TBT) via DHAP: homocoupling defects.
Figure Scheme 4.19 Defect-free copolymers from time-controlled DHAP.
Figure Scheme 4.20 Arenes and heteroarenes coupling investigated in DHAP.
Figure Scheme 4.21 Coupling of unprotected thiophene units.
Figure Scheme 4.22 Possible sources of defects in DHAP.
Figure Scheme 4.23 Polymers synthesized by DHAP and studied in: (a) plastic solar cells and (b) organic field effect transistors and light-emitting diodes.
Figure Scheme 4.24 Polymers synthesized by DHAP and studied in: (a) electrochromic windows; (b) chemical sensors; (c) memory devices; and (d) gas storage.
Chapter 5: Continuous Flow Synthesis of Conjugated Polymers and Carbon Materials
Figure 5.1 Pictures illustrating conventional batch processing and flow chemistry. The top graphic is reproduced with permission from Tekno Scienze Publisher [5].
Figure 5.2 Monomer components used in Suzuki polycondensation to produce
P1
and
P2
for application in OLED devices.
Figure 5.3 Polymer synthesis via Suzuki polycondensation for (a)
PFO
; (b)
PCDHTBT
; and (c) the associated continuous flow setup.
Figure 5.4 Flow synthesis of (a)
PTB
by Stille coupling and (b)
MEH-PPV
by the Gilch method.
Figure 5.5 Synthesis of
PBDT-BT
by Stille or Suzuki polycondensation.
Figure 5.6 Flow synthesis of
PiLEDOT
by direct arylation in a column reactor.
Figure 5.7 Synthesis of (a)
PBDTTPD
and (b)
PBDTTTz-4
by Stille polycondensation in flow.
Figure 5.8 (a) The distribution of the photovoltaic parameters (PCE,
V
oc
, FF,
I
sc
, and
P
max
) of 375 modules, processed with two different solvent combinations, represented as histograms. (b)
I–V
curves of a champion module from each of the two solvent combinations. (c) Roll-to-roll slot-die coating of the active layer. (d) Photograph of a finished module.
Figure 5.9 (a) Preparation of thiophene Grignard monomer and synthesis of P3HT via GRIM. (b) Schematic representation for the flow setup using Ni(dppp)Cl
2
and (c) nickel complex
24
.
Figure 5.10 (a) Schematic of droplet reactor, comprising a droplet generator and coiled PTFE tubing in a temperature-stabilized oil bath; (b) close-up of the droplet generator; note, the droplet phase has been dyed with colored ink for clarity; and (c) photograph showing droplet flow through coiled PTFE tubing as the polymerization proceeds. The stated flow conditions correspond to a 2-min residence time in the oil bath.
Figure 5.11 The three-step reaction sequence to
PCBtB
in microfluidic reactors.
Figure 5.12 (a) General scheme showing the flow synthesis of
PC
61
BM
and
PC
71
BM
and (b) large-scale flow synthesis of
PC
61
BM
.
Figure 5.13 Synthesis of indene-C
60
bisadduct (
IC
60
BA
) and indene-C
70
bisadduct (
IC
70
BA
) under (a) conventional batch reaction and (b) continuous flow conditions.
Figure 5.14 Illustration of the oxidation reaction of graphite flakes in the Couette–Taylor flow reactor. (a) Schematic diagram of Couette–Taylor flow reactor system. (b) Conceptual diagram of Vortex structure in the Couette–Taylor reactor.
Figure 5.15 Flow processing of
P3HT
solution to align polymer chains and produce ordered polymer aggregates.
Figure 5.16 (a) The UV–Vis spectrum of
P3HT
film with CU and MCU indicating various solution treatment conditions; films deposited from pristine (b,d) and flow processed (c,e) P3HT solution examined using the atomic force microscopy (b,c) and grazing incidence angle X-ray scattering (d, e).
Chapter 6: Synthesis of Structurally Defined Nanographene Materials through Oxidative Cyclodehydrogenation
Figure Scheme 6.1 Synthesis of
meso
-naphthodianthrone (
2
) through the oxidative cyclodehydrogenation of helianthrone (
1
) with AlCl
3
reported by Scholl and Mansfeld.
Figure Scheme 6.2 Synthesis of HBC
4
through the intramolecular oxidative cyclodehydrogenation of hexaphenylbenzene
3
under various conditions.
Figure Scheme 6.3 Examples of nanographene molecules synthesized through intramolecular oxidative cyclodehydrogenation.
Figure Scheme 6.4 Two-step synthesis of
C
3
symmetrical nanographene molecule
15
.
Figure Scheme 6.5 Synthesis of nanographene molecule
17
(C78) through the intramolecular oxidative cyclodehydrogenation of oligophenylene precursor
16
under different conditions.
Figure Scheme 6.6 Oxidative cyclodehydrogenation of hexaphenylbenzene derivatives with DDQ.
Figure Scheme 6.7 Synthesis of teranthenes
19
and quarteranthene
22
through the oxidative cyclodehydrogenation with DDQ/Sc(OTf)
3
.
Figure Scheme 6.8 Oxidative cyclodehydrogenation of indolyl-pentapyrrolylbenzene
24
with BAHA.
Figure Scheme 6.9 Base-induced cyclodehydrogenation for the synthesis of rylene diimides.
Figure Scheme 6.10 Anionic cyclodehydrogenation of precursors
37
and
39
toward 1-azaperylene (
38
) and imidazo(naphtho)quinolizine
40
, respectively.
Figure Scheme 6.11 Synthesis of “defective” nanographene molecules
42
and
45
with seven-membered rings.
Figure Scheme 6.12 Synthesis of warped nanographene molecules
49
with one five-membered ring and five seven-membered rings.
Figure 6.15 Synthesis of pyrrole-fused azacoronenes
61
,
64
, and
67
.
Figure Scheme 6.13 Synthesis of tetrabenzo[8]circulenes
51
and PAH
53
with an eight-membered ring through the oxidative cyclodehydrogenation.
Figure Scheme 6.14 Synthesis of N-substituted HBCs
56
and
59
with pyrimidine rings.
Figure Scheme 6.16 Synthesis of S-containing nanographene molecules
69
and
71
.
Figure Scheme 6.17 Synthesis of B-containing nanographene molecule
74
and B- and S-containing PAH
76
.
Figure 6.5 (a) Synthesis of N-doped chevron-type GNR
119
, GNR heterojunction
121
, and B-doped
N
= 7 armchair GNR
124
through the surface-assisted polymerization and cyclodehydrogenation. (b,c) High-resolution STM images on Au(111) surfaces of (b) GNR
119
and (c) GNR heterojunction
121
with (b) partly overlaid DFT-based STM simulation model and a formula chemical structure. (d) Differential conductance d
I
/d
V
map observed at the bias voltage of −0.35 V. The heterostructure profiles seen in (c) are drawn as white dashed lines as a guide to the eye. Scale bars in (c) and (d) indicate 2 nm. (e,f) Atomic resolution AFM images of (e) GNR
124
and (f) B-doped
N
= 14 armchair GNR formed via the fusion of GNR
124
[114]a.
Figure 6.1 Surface-assisted cyclodehydrogenation of (a) CHP
77
to TBC
78
and (c) 6,6′-bipentacene precursor
79
to peripentacene
80
. (b) High-resolution STM image of TBC
78
. (
Figure Scheme 6.18 Structures of
N =
9 armchair and
N =
5 zigzag GNRs with instruction for counting the number “
N.
”
Figure Scheme 6.19 Synthesis of GNRs
84
,
87
, and
90
through A
2
B
2
-type Suzuki, AA-type Yamamoto, and AB-type Diels–Alder polymerization, respectively, followed by oxidative cyclodehydrogenation.
Figure Scheme 6.20 Synthesis of laterally extended GNRs
94
,
97
, and
100
through A
2
B
2
-type Suzuki, AA-type Yamamoto, and AB-type Diels–Alder polymerization, respectively, followed by oxidative cyclodehydrogenation.
Figure 6.2 Schematic illustration for the surface-assisted synthesis of
N
= 7 armchair GNR
103
, starting from 10,10′-dibromo-9,9′-bianthryl (
101
).
Figure 6.3 (a) Synthesis of
N
= 7 and 13 armchair GNRs
103
and
106
, respectively, as well as their heterojunction such as
107
through the surface-assisted polymerization and cyclodehydrogenation. (b–d) High-resolution STM images on Au(111) surfaces of (b) GNR
103
, (c) GNR
106
, and (d) GNR heterojunction
107
with (b,c) partly overlaid molecular models (light blue) and (b) partially overlaid DFT-based STM simulation (gray scale). Inset of (d) displays an STM image of a larger area with a variety of
N
= 7–13 GNR heterojunctions.
Figure 6.4 (a) Synthesis of
N
= 5 armchair GNR
110
, cove-edge GNR
113
, and
N
= 6 armchair GNR
116
through the surface-assisted polymerization and cyclodehydrogenation. (b–e) High-resolution STM images on Au(111) surfaces of (b) GNR
110
, (c) GNR
113
[154], (d) PPP
115
, and (e)
N
= 6 armchair GNR
116
with partly overlaid (b,c) chemical structures and (d,e) molecular models. (b) Inset: DFT-simulated STM image of GNR
110
. GNR
116
displayed in (e) was prepared with annealing at ∼300 °C to avoid the chemisorption of Br radicals on the Au(111) surface.
Chapter 7: Photochemical and Direct C–H Arylation Routes toward Carbon Nanomaterials
Figure Scheme 7.1 Formation of phenanthrene from
cis
-stilbene using the photochemical dehydrogenation method.
Figure Scheme 7.2 Synthesis of contorted hexabenzocoronene using the Katz-modified Mallory reaction as the final synthetic step.
Figure Scheme 7.3 Synthesis of hexabenzocoronene from substituted pentacene quinone core.
Figure Scheme 7.4 Synthesis of thiophene-fused hexabenzocoronenes with different peripheral substituents.
Figure Scheme 7.5 Regioselective synthesis of fused perylenediimide molecules.
Figure Scheme 7.6 Synthesis of fused perylenediimide molecules.
Figure Scheme 7.7 Synthesis of fused perylenediimide molecules.
Figure Scheme 7.8 Synthesis of PDI-based fused dimers.
Figure Scheme 7.9 Synthesis of pyridine-fused PDIs.
Figure Scheme 7.10 Synthesis of PAHs prepared using the CDH reaction.
Figure Scheme 7.11 Photochemical synthesis of dibenzo[
fg,op
]naphtacene derivatives with liquid crystalline properties.
Figure Scheme 7.12 Synthesis of PAHs through the photochemical cyclodehydrohalogenation (CDH) reaction. The yields per cyclization reaction are given in parentheses.
Figure Scheme 7.13 Photochemical synthesis of heterocycle-fused molecules.
Figure 7.1 Intramolecular palladium-catalyzed arylation and key intermediates for the proposed mechanism by Rice
et al.
Figure Scheme 7.14 Synthesis of dibenzo[
a
,
g
]corannulene by palladium-catalyzed intramolecular direct C–H arylation.
Figure Scheme 7.15 Synthesis of picene derivatives by palladium-catalyzed intramolecular direct C–H arylation.
Figure Scheme 7.16 Synthesis of indeno[1,2,3]-annelated PAHs by palladium-catalyzed intramolecular direct C–H arylation.
Figure Scheme 7.17 Synthesis of indenopyrenes by palladium-catalyzed intramolecular direct C–H arylation.
Figure Scheme 7.18 Synthesis of pentaindenocorannulene and tetraindenocorannulene from multiple palladium-catalyzed intramolecular direct C–H arylation.
Figure Scheme 7.19 Synthesis of corannulene and sumanene from multiple intramolecular palladium-catalyzed direct C–H arylation.
Figure Scheme 7.20 Sequential ICl-induced alkyne cyclization followed by an intramolecular direct C–H arylation.
Figure Scheme 7.21 Synthesis of low π-sextet PAHs by palladium-catalyzed intramolecular direct C–H arylation.
Chapter 8: Carbon-Rich Materials from sp-Carbon Precursors
Figure 8.1 TEM images of carbon structures formed by pyrolysis of acetylenic scaffolds, (a) carbon onions, (b) carbon nanotubes containing cobalt, (c) carbon nanotubes containing iron, and (d) carbon “ropes.”
Figure 8.2 Schematic chemical structures of α-carbyne, β-carbyne, polyynes, and cumulenes.
Figure 8.3 Schematic formation of a polyyne with sterically demanding end groups from a trialkylsilyl-protected precursor.
Figure 8.4 Schematic structure of polyynes encased in a (a) single-walled and (b) double-walled carbon nanotube.
Figure 8.5 (a) Schematic formation of a polyyne rotaxane via active metal templation. Polyyne rotaxanes from the groups of (b) Saito, see [39], (c) Gladysz, see [40–42], (d) Anderson and Tykwinski, see [43, 44], and (e) Anderson, see [45].
Figure 8.6 Synthesis of a hexayne [3]rotaxane by Frauenrath and coworkers, using α-cyclodextrins.
Figure 8.7 Schematic depiction of the synthesis of “odd” [
n
]cumulenes and the formation of a [9]cumulene rotaxane.
Figure 8.8 Schematic crystal packing for several modes of polyyne polymerization, and optimal parameters θ,
R
, and
d
for each addition pattern. (a) Polydiacetylene formation via 1,4-addition of a diyne moiety, leading to a ladder polymer formation via a second 1,4-addition. (b) Polydiacetylene formation via 3,6-addition of a diyne moiety. (c) Polytriacetylene formation via 1,6-addition of a triyne moiety. (d) Polytetraacetylene formation via 1,8-addition of a tetrayne.
Figure 8.9 Goroff's supramolecular approach to PIDA/PBDA formation from diiodo- or dibromo-1,3-butadiynes.
Figure 8.10 Schematic description of Campos' polydiphenyldiacetylene (PDPDA) formation templated by a functionalized block copolymer.
Figure 8.11 Morin's PDA formation via (a) a
meta
-linked dimeric phenylene butadiynylene derivative and (b) a
para
-linked oligo(phenylene butadiynylene).
Figure 8.12 (a) Chemical structure of
1a–c
(b–d) observed molecular stacking of macrocycles 1a–c, respectively.
Figure 8.13 (a) Chemical structure of
2
, (b) molecular stacking of monomer
2
, and (c) structure of the polymer obtained by slow annealing of
2
at 40 °C, (d) chemical structure of
3
, (e) solid-state stacking of monomer
3
, and (f) solid-state structure of the polymer obtained by slow annealing of
3
at 190 °C.
Figure 8.14 (a) Chemical structures of Morin's substituted phenylacetylenyl and phenylbutadiynyl macrocycles (PAM/PBM) for organic nanotube/nanorod formation. (b) Schematic depiction of topochemical polymerization of macrocycles.
Figure 8.15 (a) Fowler's supramolecular host–guest approach toward a polytriacetylene and (b) crystallographic investigation before and after irradiation.
Figure 8.16 X-ray analysis of Frauenrath's substituted octa-2,4,6-triyne-1,8-diol derivatives for either 1,4- or 1,6-polymerization.
Figure 8.17 Tetraynes with packing parameters suitable for 1,6-addition polymerization. (a) θ = 30°,
R
1,6
= 3.7 Å,
R
3,8
= 3.7 Å, and
d
= 7.4 Å; (b) θ = 28°,
R
1,6
= 3.7 Å,
R
3,8
= 3.6 Å, and
d
= 7.7 Å; and (c) θ = 29°,
R
1,6
= 3.5 Å,
R
3,8
= 3.5 Å, and
d
= 7.5 Å.
Figure 8.18 (a) Chemical structure of nonamphiphilic hexayne
7
and amphiphilic hexayne
8
. (b) Structural depiction of the self-assembly of amphiphile
8
into colloids in aqueous solution and polymerization into carbonized nanocapsules under UV irradiation.
Figure 8.19 (a) Chemical structure of hexayne amphiphile 9, (b) predicted structural model for self-assembled monolayers of
9
at the air–water interface, and (c) close packing of hexayne outlining parameters significant for topochemical polymerization: tilt angle θ = 62.5° relative to the normal layer, short contact between the acetylene carbons of neighboring molecules 3.42–3.53 Å along the
a
-axis, and packing distance 5.20 Å.
Figure 8.20 Dichtel's Asao–Yamamoto benzannulation reaction toward 2,3-diarylnaphthalenes.
Figure 8.21 Alabugin's (a) Wolff–Kishner-type reaction toward tetracenediones and (b) examples of radical intramolecular cascade reactions.
Figure 8.22 Selected reactions toward pentalenes (a) Itami's C–H activation protocol mediated by Pd/Ag and (b,c) Diederich's protocol mediated by Pd/Zn.
Figure 8.23 (a) Chemical structure of the dendritic “molecular battery”
11
from 12-fold addition of TCNE. (b) Cascade procedure to form [AB]-type oligomer
13
with a dendralene backbone.
Figure 8.24 Cyano-functionalized diaryltetracenes through [2+2] cycloaddition of TCNE with tetraaryl[3]cumulenes, and ORTEP drawing of parent derivative (R=H).
Figure 8.25 Products from the reaction of a [5]cumulene with TCNE.
Figure 8.26 Thermal dimerization of [5]-, [7]-
,
and [9]cumulenes.
Figure 8.27 (a) Synthetic route toward cove-edged GNRs, where blue highlights Clar sextets consistent with the two most likely canonical structures,
25[1]
and
25[1]′
; ORTEP drawing of
25[1]
. (b) Chemical structure of cove-edged GNR
26
grown on a Au(111) surface under UHV conditions. (c) Long-range STM image of the oligomers after cyclodehydrogenation. (d) High-resolution STM image of isolated GNR with structural model superimposed.
Figure 8.28 (a) Chemical structures of the glycosylated and ester-terminated hexaynes
27
and
28
, respectively, while (b,c) show STM images of
28
on a Au(111) at low temperatures, with submolecular resolution. The islands show two different contrasts within the ribbons, which are 2.3-nm wide and appear in pairs with a width of 4.6 nm. Molecules are ordered in a head-to-head and tail-to-tail motif with head (blue) and tail (red).
Figure 8.29 (a) Schematic synthesis of oligo-(
E
)-1,1′-bi(indenylidene) through thermally induced C1–C5 radical cyclizations of enediyne precursors followed by step growth polymerization on Au(111). (b) Nc-AFM image of an individual oligomer chain. (c) Experimental STM d
I
/d
V
map (constant height) at
V
s
= 0.125 V reveals an extended electronic state along the conjugated backbone of oligomer shown in (b).
Figure 8.30 (a) Chemical structure of 1,3,5-tris-(4-ethynylphenyl)benzene (exTEB), (b) trimer after cyclotrimerization of exTEB on Au(111), (c) STM image of trimer on Au(111), (d) hexamer after cyclotrimerization of exTEB on Au(111), (e) STM image of hexamer on Au(111), and (f) honeycomb-like polyphenylene nanostructures after annealing at 433 K on Au(111).
Figure 8.31 (a) Chemical structure of 1,3,5-triethynylbenzene (TEB). STM topographic images of (b) TEB molecules and reaction products on Ag(111) showing both TEB molecules (green) and dimeric products (red), and (c) carbon network after annealing a dimer-dominated sample to 370 K. (d) Chemical structure of exTEB. STM topographic images of (e) exTEB molecules and reaction products on Ag(111) showing covalently bonded exTEB dimers after annealing at 300 K (lower inset shows a high-resolution image of a dimer superimposed with a calculated model and the upper inset magnifies an area with dimers in red and monomers in green), and (f) a magnified area of the network after annealing to 400 K. (Inset shows a single honeycomb segment superimposed with a calculated model.) Scale bars in (b) and (c) denote 10 Å while those in (e) and (f) denote 50 nm.
Figure 8.32 (a) Schematic depiction of the elaboration of corannulene into sp
2
carbon allotropes and the analogous carbomerization into sp
2
/sp carbon allotropes (graphynes) and (b) example of a carbomer synthesized by Chauvin and coworkers (with X-ray crystal structure).
List of Tables
Chapter 2: Suzuki Polycondensation
Table 2.1 Aryl–aryl interchange (10 to 12) measured at 50 °C after 3 h in CDCl
3
Chapter 5: Continuous Flow Synthesis of Conjugated Polymers and Carbon Materials
Table 5.1 Reaction conditions of flow and batch reactions and the molecular weight data of the resulting polymers
P1
and
P2
Table 5.2 Reaction conditions
a
and molecular mass data
b
for Suzuki polycondensations in batch and flow
Table 5.3 Reaction conditions and molecular mass data
a
for PTB synthesized using Stille polycondensation
b
(entries 1–4) and MEH-PPV synthesized using the Gilch method
c
(entries 5 and 6) in batch and flow
Synthetic Methods for Conjugated Polymers and Carbon Materials
Edited by Mario Leclerc and Jean-Francois Morin
The Editors
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Université Laval
Department of Chemistry
1045, Avenue de la médecine
G1V 0A6 NK
Canada
Prof. Jean-Francois Morin
Université Laval
Department of Chemistry
1045, Avenue de la médecine
G1V 0A6 NK
Canada
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List of Contributors
Serge Beaupré
Université Laval
Department of Chemistry
Pavillon Alexandre-Vachon
Quebec City, QC G1V 0A6
Canada
Maxime Daigle
Université Laval
Département de chimie et Centre de recherche sur les matériaux avancés (CERMA)
1045 Ave de la Médecine
Québec G1V 0A6
Canada
Maude Desroches
Université Laval
Département de chimie et Centre de recherche sur les matériaux avancés (CERMA)
1045 Ave de la Médecine
Québec G1V 0A6
Canada
Andrew C. Grimsdale
Nanyang Technological University
School of Materials Science and Engineering
Block N4.1, 50 Nanyang Avenue
Singapore 639798
Singapore
Tine Hardeman
KU Leuven
Department of Chemistry
Celestijnenlaan 200F, bus 2404
3001 Heverlee
Belgium
Guy Koeckelberghs
KU Leuven
Department of Chemistry
Celestijnenlaan 200F, bus 2404
3001 Heverlee
Belgium
Anurag Krishna
Nanyang Technological University
School of Materials Science and Engineering
Block N4.1, 50 Nanyang Avenue
Singapore 639798
Singapore
Mario Leclerc
Université Laval
Department of Chemistry
Pavillon Alexandre-Vachon
Quebec City, QC G1V 0A6
Canada
Andrey V. Lunchev
Nanyang Technological University
School of Materials Science and Engineering
Block N4.1, 50 Nanyang Avenue
Singapore 639798
Singapore
Valerie D. Mitchell
University of Melbourne
School of Chemistry, Bio21 Institute
Bldg. 102, Room 438, 30 Flemington Road
Parkville, VIC 3010
Australia
Jean-Francois Morin
Université Laval
Département de chimie et Centre de recherche sur les matériaux avancés (CERMA)
1045 Ave de la Médecine
Québec G1V 0A6
Canada
Akimitsu Narita
Max-Planck-Institut für Polymerforschung
Ackermannweg 10
55128 Mainz
Germany
Dominik Prenzel
Friedrich-Alexander-University Erlangen-Nürnberg (FAU)
Department of Chemistry and Pharmacy Interdisciplinary Center for Molecular Materials (ICMM)
Henkestrasse 42
91054 Erlangen
Germany
Alexander M. Schneider
University of Chicago
Department of Chemistry
929 E. 57th St. GCIS E 419 A
Chicago, IL 60637
USA
Rik R. Tykwinski
Friedrich-Alexander-University Erlangen-Nürnberg (FAU)
Department of Chemistry and Pharmacy Interdisciplinary Center for Molecular Materials (ICMM)
Henkestrasse 42
91054 Erlangen
Germany
Marie-Paule Van Den Eede
KU Leuven
Department of Chemistry
Celestijnenlaan 200F, bus 2404
3001 Heverlee
Belgium
Lize Verheyen
KU Leuven
Department of Chemistry
Celestijnenlaan 200F, bus 2404
3001 Heverlee
Belgium
Wallace W.H. Wong
University of Melbourne
School of Chemistry, Bio21 Institute
Bldg. 102, Room 438, 30 Flemington Road
Parkville, VIC 3010
Australia
Luping Yu
University of Chicago
Department of Chemistry
929 E. 57th St. GCIS E 419 A
Chicago, IL 60637
USA
Tianyue Zheng
University of Chicago
Department of Chemistry
929 E. 57th St. GCIS E 419 A
Chicago, IL 60637
USA
Chapter 1Stille Polycondensation: A Versatile Synthetic Approach to Functional Polymers
Tianyue Zheng, Alexander M. Schneider and Luping Yu
1.1 Introduction
The development of functional polymers is a very active research field that covers every aspects of our lives and has had huge impact on human society due to their applications in many cutting-edge technologies, such as energy conversion and storage, electronic devices, biotechnology, and health care, to name a few [1]. Scientists from different disciplines have invented numerous new materials for those purposes. Integral to these efforts is the development of efficient, versatile, and scalable synthesis techniques, which in turn enable the development of new functional materials. Thus, new synthetic methodologies are always a critical research topic that is actively pursued. A large number of recent advances can be cited to support this view, such as ring-opening metathesis polymerization (ROMP), atom transfer radical polymerization (ATRP, a type of “living” radical polymerization), and controlled Ziegler–Natta polymerization [2, 3]. Most recently, polycondensations based on transition metal-catalyzed CC bond formation reactions have emerged as important methodologies for synthesis of electro-optic materials containing large systems. These reactions include Stille, Suzuki, Negishi, Heck, and so on [4–7]. The Stille reaction is one of the best methods for the synthesis of organic functional materials due to its excellent compatibility with various functional groups and high reaction yield.
The most attractive application of the Stille coupling reaction is in the synthesis of conjugated, polyaromatic semiconducting materials, which are an important class of materials for organic electronics. These materials exhibit good solubility in various solvents, which allows them to be fabricated into devices using inexpensive solution-phase printing techniques [8]. Over the past several decades, the development of semiconducting polymers has led to the advent of new technologies for numerous applications, ranging from organic light-emitting diodes (OLEDs), field effect transistor (FET), and organic photovoltaic (OPV) solar cells [7]. Among these semiconducting polymers, the majority of them, especially those containing thiophene moieties, can be synthesized via Stille polycondensation from-related monomers. These polymers bear a wide variety of functional groups and their emergence is enabled by the power and broad scope of the Stille polycondensation. This chapter summarizes recent progress in investigating the Stille polycondensation and its application to the development of functional materials.
1.1.1 History of the Stille Reaction (and Polycondensation)
The Stille coupling reaction refers to the reaction between an organostannane (also called organotin) and an organic electrophile in the presence of palladium catalyst to generate new C–C single bond (Scheme 1.1) .
Scheme 1.1 The Stille coupling reaction scheme.
In 1976, Eaborn reported a Pd-catalyzed reaction using bis(tributyltin) to prepare aryltin compounds, where C–Sn bonds were formed (Scheme 1.2) [9]. Later, in 1977, Kosugi used a similar method to report the first C–C bond formation from cross-coupling between acyl chlorides or aryl halides and organostannanes (Scheme 1.3) [10–12]. These disclosures were considered the first examples of cross-coupling reactions between organostannanes and electrophilic partners.
Scheme 1.2 Synthesis of aryltin compounds by Eaborn et al. [9].
Scheme 1.3 Coupling of halides and organostannanes by Kosugi et al. [10–12].
Following these examples, John K. Stille carried out extensive studies on the reaction scope and mechanism beginning in 1978. The original report from Stille and coworkers involved the synthesis of ketones from acyl chlorides and organostannanes [13]. Following that, the general features of this reaction were revealed and it quickly became a standard method in organic synthesis and one of the most useful procedures for carbon–carbon bond formation, especially of sp2–sp2 C–C bonds. His major body of work was summarized in a very influential review in 1986 [13–15]. Together with the Suzuki reaction, a palladium-catalyzed cross-coupling of organoboranes and electrophiles, the Stille reaction is one of the most efficient methods for preparing functional materials, especially those containing extended conjugation systems that are linked by sp2–sp2 C–C bonds.
By incorporating a ditin compound and a difunctional electrophile, the Stille reaction was used to synthesize polymers as early as the 1980s and the early 1990s, when polycondensation between organo-ditin monomers and dihalide monomers was developed [7]. Yu and coworkers further developed this methodology, including reaction scope and conditions, for making high molecular weight heteroaromatic diblock copolymers in the early 1990s (Schemes 1.4 and 1.5) [16–18].
Scheme 1.4 Synthesis of PPT by Stille polycondensation [16, 17].
Scheme 1.5 Synthesis of PPTs with metalloporphyrin or pendent carbazole units [18].
1.2 Reaction Mechanism
1.2.1 Simplified Mechanism
The Stille reaction is a Pd(0)-catalyzed cross-coupling reaction. The active Pd(0) species may be generated from Pd(II) precursor that is reduced by the organostannane before entering the catalytic cycle. In his review article in 1986 [15], Stille proposed the reaction mechanism based on the study of coupling between benzoyl chloride and tributyl(phenyl)stannane with Pd(Bn)Cl(PPh3)2 (PPh3 = triphenylphosphine) as the catalyst. The proposed mechanism is similar to other Pd(0)-mediated cross-coupling reactions, in which the PdL2 (L = PPh3) complex was assumed to be the active catalytic species. The PdL2 undergoes oxidative addition with organic electrophile R1–X to form R1PdL2X, which then undergoes a slow transmetallation with organostannane R2SnMe3 to form R1PdL2R2, followed by a reductive elimination step to give the product R1–R2 and regenerate the PdL2 active species. A general feature of this mechanism is that a trans–cis isomerization step is needed for the ensued reductive elimination. Though this mechanism (Scheme 1.6) was generally accepted by the research community, more extensive investigation revealed more complexity of the mechanism. Espinet and coworkers have written in-depth reviews of the mechanistic study of the Stille reaction in 2004 [19] and most recently in 2015 [20]. It was shown that the actual mechanism may vary according to different reaction conditions, including catalyst, ligands, solvents, and additives. There is no simple answer to the actual mechanism and thus is referred as the mechanistic black box [19]. In the three major steps of the mechanism, the oxidative addition and reductive elimination steps are extensively studied and relatively well understood, but the transmetallation step is more complicated and not well understood.
Scheme 1.6 A simplified mechanism for Stille coupling [7, 19].
For halides with a C(sp3)–X bond, the oxidative addition of R–X to Pd(0) is usually a bimolecular reaction (SN2) and the configuration of product will be affected by the choice of different solvents. For C(sp2)–X, this step is considered to go through a three-center transition state between the electrophile R–X and the active Pd(0)L2 (L = ligand) to give a kinetic product of cis-[Pd(II)RXL2] complex, which can isomerize to the more thermodynamically stable trans-[PdRXL2] complex [19]. This cis- to trans-isomerization is usually fast; very often only the trans-complex is found. However, with bidentate ligands to stabilize the intermediate, the cis complex may be observed (Scheme 1.7). For example, in the reaction of ArOTf (Ar = C6F5, C6Cl2F3) and RSnBu3 (R = vinyl), Espinet and coworkers were able to observe the cis complexes [(dppe)Pd(Ar)(OTf)] (dppe = 1,2-bis(diphenylphosphino)ethane), which were stable in the solid state and fully characterized by nuclear magnetic resonance (NMR) spectroscopies [21]. Milstein and coworkers studied the mechanism of the oxidative addition of chlorobenzene to Pd(dippp)2 (dippp = 1,3-bis(diisopropylphosphanyl)propane) in dioxane [22]. They monitored the intermediates by 31P NMR and found that the cis-(dippp)Pd(Ph)Cl and trans-(η1-dippp)2Pd(Ph)Cl are formed in parallel pathways. While in equilibrium with each other, the cis-complex is favored both kinetically and thermodynamically.
Scheme 1.7 Formation of cis complex and cis–trans isomerization [19]. L = ligand.
(Adapted with permission from [19]. Copyright 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)
The major difference between the Stille reaction and other similar Pd-catalyzed cross-coupling reactions is in the transmetallation step. During the transmetallation step, the organostannanes interact with the Pd center, resulting in Sn–C bond cleavage and Pd–C bond formation. Unlike in other modern Pd-catalyzed coupling reactions, the nature of the Sn–C bond is neither as strong nor as polar as other metal–carbon bonds, such as B–C, Zn–C, and Mg–C bonds. Espinet and Echavarren point out that the transmetallation step in the Stille reaction involves the electrophilic cleavage of Sn–C bond (SE2) by the Pd(II) complex (from oxidative addition), which could also be viewed as a ligand substitution (SN2) on the Pd(II) complex [19]. These complexes are usually 16-electron, square planar, tetracoordinated, and can experience ligand substitution via two possible pathways. One pathway is dissociative, which would involve a 14-electron, T-shaped intermediate with substitution being determined by the ligand with the highest trans influence that weakens the bond trans to it. The other is associative, which would involve an 18-electron, trigonal bipyramidal intermediate with substitution being determined by the ligand with the highest trans effect that leads the lowest energy transition state [23]. The solvent could play a role in this step, by assisting the ligand substitution or serving as ligand itself, such as THF or DMF [24, 25]. For the intermediate in the electrophilic cleavage process, both an open and cyclic (Scheme 1.8) transition states are possibilities, which have been proposed to explain this (SE2) step. Stille considered this step to involve an open transition state from his studies on the [Pd(Bn)Cl(PPh3)2]-catalyzed coupling of benzoyl chloride with (S)-PhCHDSnBu3 [15, 28], which explains the fact that the transmetallation step can be very fast and that the inversion configuration of the alpha carbon sometimes occurs. Espinet and coworkers also reported an open transition state in the coupling of organotriflates, again using triphenylarsine (AsPh3) as ligand [26]. However, with the same ligand but organohalide substrate, Espinet and coworkers reported that the trans complex reacts with organostannane through a cyclic transition state with release of ligand [27], which explains the inverse dependence on the concentration of ligand on the reaction rate [15, 23]. All of these findings demonstrate the complexity of the transmetallation step, which may go through different pathways according to different reaction conditions.
Scheme 1.8 Cyclic and open transition states [26, 27].
Reductive elimination leads to formation of the final product and regenerates the active Pd(0) species into the catalytic cycle. Before the formation of the coupled product from the transmetallation intermediate, a trans- to cis-isomerization places the coupling partners in cis-position to each other [15]. A three-coordinate, T-shaped 14-electron complex resulting from ligand dissociation has also been proposed to be the intermediate [15, 19, 23]. For example, Hartwig and coworkers reported the formation of 14-electron ArPdXL (L = PPh3) complexes by dissociation of one ligand L from 16-electron trans-ArPdXL2 complexes (Scheme 1.9) [29]. The ArPdXL complexes then react with organostannane to generate the ArPdRL complexes, which then undergo a fast reductive elimination to produce the Ar-R product. The rate of the reductive elimination step is usually fast, but might be slow when allyl groups or chelating ligands are involved [30].
Scheme 1.9 A ligand dissociation to form a T-shaped complex [29].
Taking this knowledge together, a more complicated mechanism has been proposed by Espinet and coworkers (Scheme 1.10) [20]. In this mechanism, in addition to the regular three major steps, more details about the configuration of intermediate species have been added, taking into account the effect of ligands, solvents, and so on. This more detailed mechanism may give clues to the nature of side reactions, which could affect the structure of resulting polymers. A more detailed understanding of reaction mechanism under proper conditions is crucial to synthesize high-quality polymers. This point will be further illustrated in the later section of this chapter.
Scheme 1.10 A more complex mechanism by Espinet et al. [20].
(Reprinted with permission from [20]. Copyright 2015 American Chemical Society.)
1.3 Reaction Conditions
The Stille polycondensation reaction involves two types of monomers, an organodihalide (or organoditriflate) and an organodistannane. Typically, diiodo monomers are more reactive than dibromo compounds, and dichlorides are the least reactive primarily due to their low reactivity in the oxidative addition step. However, while organochlorides can be activated in the synthesis of small molecules by using special catalyst systems [31, 32], the examples of using chlorides to synthesize polymers are rare. In general, it has been found that the combination of electron-rich organotin compounds and electron-deficient halide or triflate is beneficial for the synthesis of polymers exhibiting high molecular weight, as the electron-withdrawing groups may facilitate the oxidative addition step and the electron-rich organostannane favors the transmetallation step [7, 18].
1.3.1 Catalyst and Ligand
There are many palladium compounds that provide catalytic centers for the Stille reaction, including Pd(II) sources such as dichlorobis(triphenylphosphine)-palladium(II) [PdCl2(PPh3)2] and palladium(II) acetate [Pd(OAc)2] and Pd(0) sources such as tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4] and tris(dibenzylideneacetonyl) dipalladium(0) [Pd2(dba)3]. However, since the active species in the catalytic cycle is Pd(0) complex, a reducing agent will be needed if Pd(II) is added as the Pd source. For example, adding PPh3 to Pd(OAc)2 rapidly leads to the formation of [Pd(OAc)2(PPh3)2] complex, which undergoes slow intramolecular reduction to form a Pd(0) complex [33]. Pd(PPh3)4 and Pd2(dba)3 are the most frequently used, commercially available Pd catalysts for the Stille coupling, particularly for polymerization. Pd(PPh3)4 is reactive but is not stable against air or moisture, since the free PPh3 can be easily oxidized by air to form triphenylphosphine oxide (OPPh3), accordingly, Pd2(dba)3 is a more air-stable compound [7].
In addition to catalyst, the selection of ligand is also important in developing a robust catalytic system. Size and bulkiness, as well as electron-donating ability are some key parameters for ligands applied in the Stille reaction, in which phosphine ligands are the most commonly used ligands. For example, Yu and coworkers examined the reaction scope and conditions of Stille coupling for making high molecular weight conjugated copolymers [18]. It was found that catalyst concentration, different solvents and ligands, and structures of monomers could largely affect the polymerization. If a Pd(II) catalyst is used, a stoichiometric excess of the distannyl monomer is necessary to generate the Pd(0) complex and enhance the molecular weight of the resulting polymer. When different ligands are used, they also found that the molecular weight and dispersity of resulting polymers showed a trend as AsPh3 > P(2-furyl)3 > PPh3, indicating the reactivity of ligands.
Though they suffer from sensitivity to trace amounts of oxygen and moisture, the bulky electron-rich phosphine ligands are widely used in the Stille coupling reaction to extend the reaction scope to weakly active organotin and electrophile (4–7, Scheme 1.11) [34–37]. For example, proazaphosphatrane ligands (6) are very effective for enabling the coupling of aryl chlorides with a variety of organostannanes, including sterically hindered ones [36]. Fu and coworkers studied the bulky ligand P(t-Bu)3 (7), leading to the first effective Stille couplings of unactivated aryl chlorides with organostannanes [37]. This catalyst system was found to be highly effective; highly hindered tetrasubstituted biaryls may be produced, and the reaction can take place at room temperature in some cases. They also found the unexpected selectivity of Ar-Cl over Ar-OTf in the reaction involving ArCl/ArOTf or ClC6H4OTf when P(t-Bu)3 was used (Scheme 1.12).
Scheme 1.11 The structure of some bulky phosphine ligands [34–37].
Scheme 1.12 Selectivity of ArCl over ArOTf in Stille coupling [37].
In fact, bulky phosphine ligands can assist every step in the catalytic cycle of the Stille reaction, as concluded by Espinet and coworkers in 2015 (Scheme 1.13) [20]. This is shown to be due to the stabilization of monoligated Pd intermediates due to the bulk of the phosphine ligand. At the oxidative addition step, the monoligated Pd(0) species facilitates nucleophilic attack at the Ar–X bond from the ligand free side; while the electron richness of the phosphine provides efficient electron back-donation to the σ* Ar–X orbital, making the oxidative addition possible even for Ar–X bond with very low nucleophilicity. At the transmetallation step, a three-centered, 14-electron Pd(II) complex is stabilized by the ligand, beneficial for the nucleophilic attack by the organostannane, the Sn–C bond of which is of low polarity. Moreover, there is no need for trans- to cis-isomerization as in the case of a tetracoordinated Pd(II) complex, further inducing reductive elimination.
Scheme 1.13 Bulky ligands assist Stille coupling [20].
(Reprinted with permission from [20]. Copyright 2015 American Chemical Society.)
Another factor that is of no less importance is the electronic effect of the ligands. For example, the reduction of [Pd(OAc)2(PPh3)2] to Pd(0) complex will be enhanced by electron-withdrawing groups at the para position of the aryl groups on the phosphine [33]. [Pd(dba)(AsPh3)2] was found to be more stable than analogous phosphine complexes [Pd(dba)L2] (L = PPh3 or tri(2-furyl)phosphine (TFP)) due to its better electron-donating ability [38]. Farina and coworkers carried out a kinetic study of ligands with different donicities [39]. The coupling was between a model reaction system of iodobenzene and vinyltributyltin with Pd2(dba)3 as the palladium source, and the ligands studied were PPh3, tri(p-anisyl)phosphine (TAP), TFP, and AsPh3. It was shown that the coupling rate when using TFP and AsPh3 is three and four orders of magnitude faster, respectively, than that of PPh3. They rationalized this by the observed inhibitory effect on the cross-coupling of “strong” ligands, such as both PPh3 and TAP. Stronger electron-donating ligands are also more easily oxidized, leading to formation of palladium black, deactivating the catalyst [7].
Since the pathways in the Stille coupling reaction may be influenced by the reaction conditions, it is crucial to carefully optimize the conditions in order to obtain desired products. For the Stille polycondensation, the reaction conditions are even more critical because in addition to high yield (thus high degree of polymerization), molecular weight distribution (characterized by dispersity) is important in controlling the quality of the resulting polymers. Selection of the correct set of conditions for Stille polycondensation is often a trial-and-error process when different monomer combinations are used [7]. The optimized catalytic systems for Stille polycondensation to achieve high-quality polymers may vary according to different target polymers.
1.3.2 Solvent
The solvent lays the foundation for the complex system of reaction conditions, which involves the interplay of many factors including catalyst, ligand, and additives, in addition to the solvent. The commonly used solvents for the Stille reaction include benzene, toluene, xylene, tetrahydrofuran (THF), dimethylfluoride (DMF), N-methylpyrrolidone (NMP), dioxane, and chloroform. They show a wide range of polarity, as well as solubility toward organic molecules. Choosing the appropriate solvent is critical for the reaction to be efficient, since the solvent not only solubilizes the organic reagents and intermediates, but also takes part in the catalytic cycle by serving as ligand to Pd catalyst or assisting in ligand dissociation. For example, Amatore and coworkers [25] studied the coupling of PhI with tributyl(vinyl)tin in DMF with [Pd(dba)(AsPh3)2] as the catalyst, and found that the transmetallation takes place in the solvent-coordinated trans-[PdPhI(AsPh3)(DMF)] complex. Moreover, the solvent is found to affect the configuration (retention or inversion) of final product based on its polarity and coordinating ability [40, 41].
For polymerization, the demand of the solvent is even greater than those in small-molecule Stille coupling. Like in the small-molecule reaction, the solvent should dissolve the starting monomers, stabilize the catalyst, and maintain catalytic ability; for Stille polymerization, the solvent must also keep the growing polymer chain in solution as long as possible in order to obtain polymers with high molecular weight and narrow dispersity. For example, DMF is highly polar and can coordinate to the catalyst center as ligand; however, many polymers, especially conjugated polymers used in functional materials, show low solubility in DMF. On the contrary, polymers show good solubility in toluene, which is less polar and coordinating. Yu and coworkers found that mixed solvents such as toluene/DMF (typically in a 4 : 1 ratio) can provide benefits of each individual solvent while avoiding the disadvantages, enabling a good yield of high molecular weight polymers [18]. In addition, high-boiling solvents are always used for polymerization procedures, which often require high temperature to facilitate the polymerization reaction and increase the solubility of final polymers. As a result, toluene (b.p. = 110 °C) and chlorobenzene (b.p. = 131 °C) are often used in polymerization reactions. For example, Yan and coworkers used chlorobenzene as solvent to carry out the Stille polycondensation either in a conventional or a microwave-assisted conditions, with the reaction temperature reaching over 150 °C in the latter case [42].
1.3.3 Additive
The additives used in the Stille coupling are usually inorganic salts, such as LiCl, CsF, and CuI. The active species may be either anion or cation; the role additives play in the catalytic cycle can be varied according to different combinations of other reaction conditions, such as ligand and solvent.
LiCl is a common additive in the Stille coupling since the very early stages of this methodology. Prof. Stille found that LiCl could accelerate the coupling of organostannanes with vinyl and aryl triflates [43]. LiCl was proposed to transform the triflato complex into the more reactive chloro complex, which then enters the catalytic cycle as with other organic halides. Similar effects have been found with iodide and bromide salts as well [44, 45]. However, Farina and coworkers reported that the effect of LiCl additive was largely dependent on the reaction conditions, leading to both accelerating and retarding effects [39, 46]. For example, in the coupling of vinyl triflate and aryl tributylstanne with Pd2(dba)3 as the catalyst in NMP, LiCl was found to retard the reaction when TFP or PPh3 was used as ligand, but accelerate the reaction for AsPh3. Interestingly, they found that the accelerating effect was extremely significant when no additional ligand was added. Espinet and coworkers have also reported both positive and negative effects of LiCl [26]. LiCl favors the coupling of C6F5I with organostannanes when catalyzed by [Pd(AsPh3)4] in THF, by promoting the oxidative addition step. By contrast, with the more nucleophilic [Pd(PPh3)4], LiCl retards the reaction since the oxidative addition has already taken place without LiCl.
Some Lewis bases have been reported to facilitate Stille coupling by activating the organostannane. The most widely studied are fluoride salts, such as CsF, KF, and Bu4NF, which can activate the tin compounds due to its fluorophilicity. It is suggested that a pentavalent tin complex with enhanced reactivity toward transmetallation be formed by the coordination of F− anion to tin compounds [7]. Fu and coworkers used CsF to activate the organostannane, enabling its coupling with aryl chlorides, and particularly with aryl bromides at room temperature, in their Pd/P(t-Bu)3 catalytic system [37]. Examples of applying other Lewis bases have also been reported. Besides fluoride salts, Fu and coworkers have studied the activating effect of bases such as Cs2CO3, NaOH, NaOMe, N(i-Pr)2Et, and others in assisting the Stille coupling reaction [47]. In addition, amines can also be beneficial since that they can stabilize the tin compounds by coordination [48]. Finally, reagents such as (n-Bu)4N+Ph2P(O)O− can act as a “tributyltin scavenger” to improve cross-coupling efficiency, as reported by Liebeskind and coworkers [49].
Another important category of additive is CuIor other Cu(I) salts, which could enhance coupling of Stille reaction, referred as the copper effect. Liebeskind and coworkers studied the effect of addition of CuI on the kinetics of Pd-catalyzed coupling between iodobenzene and vinyltributyltin in dioxane [50]. They observed a >100-fold rate increase when a strong ligand, such as PPh3, was used, but little effect when a soft ligand, such as AsPh3, was used. They concluded that CuI is a scavenger for the free ligand, especially for strong ligands such as PPh3, which is known to inhibit the transmetallation. In addition, they proposed that in very polar solvents such as NMP and in the absence of strong ligand, a Sn/Cu transmetallation takes place to yield an organocopper species, which more easily transmetallates to the Pd(II) complex. They also observed that a stoichiometric ratio of Pd:L:Cu = 1 : 4 : 2 (L = ligands) gave the best result with both enhanced reaction rate and yield. Further increase of CuI did not increase the rates significantly, but did decrease the yield, because too much CuI removes ligand from the active catalytic species and thus reduces the catalyst stability. Many other Cu(I) salts (CuX, X = Cl, Br, CN, thiophene-2-carboxylate (TC)) have been reported to have similar effect [51–53].
