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Chemical Synthesis Using Highly Reactive Metals E-Book

Reuben D. Rieke

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Herausgeber: John Wiley & Sons
Kategorie: Wissenschaft und neue Technologien
Sprache: Englisch

Beschreibung

Written by the creator of Rieke metals, valuable for chemical reaction methods and efficiency, this groundbreaking book addresses a significant aspect of organic and inorganic chemistry. The author discusses synthetic methods, preparation procedures, chemical reactions, and applications for highly reactive metals and organometallic reagents. * Addresses a new generation of chemistry that goes beyond the standard use of metals and activation * Provides step-by-step guidelines, chemical equations, and experimental descriptions for handling metals including zinc, magnesium, copper, indium, nickel, manganese, calcium, barium, iron, palladium, platinum, uranium, thorium, aluminum, cobalt, and chromium * Uses a unique approach to highlight methods and techniques that make chemical synthesis and activation of Rieke metals more safe and efficient * Discusses novel applications and special topics, such as highly reactive metals for novel organometallic reagents, semiconducting polymers, plastics electronics, photovoltaics, and the Reformatsky reagent

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Leseprobe

Cover

Title Page

Preface

1 Genesis of Highly Reactive Metals

2 General Methods of Preparation and Properties

2.1 General Methods for Preparation of Highly Reactive Metals

2.2 Physical Characteristics of Highly Reactive Metal Powders

2.3 Origin of the Metals’ High Reactivity

References

3 Zinc

3.1 General Methods for Preparation of Rieke Zinc

3.2 Direct Oxidative Addition of Reactive Zinc to Functionalized Alkyl, Aryl, and Vinyl Halides

3.3 Reactions of Organozinc Reagents with Acid Chlorides

3.4 Reactions of Organozinc Reagents with α,β‐Unsaturated Ketones

3.5 Reactions of Organozinc Reagents with Allylic and Alkynyl Halides

3.6 Negishi Cross‐Coupling of Vinyl and Aryl Organozinc Halides

3.7 Intramolecular Cyclizations and Conjugate Additions Mediated by Rieke Zinc

3.8 The Formation and Chemistry of Secondary and Tertiary Alkylzinc Halides

3.9 Electrophilic Amination of Organozinc Halides

3.10 Reformatsky and Reformatsky‐Like Reagents and Their Chemistry

3.11 Configurationally Stable Organozinc Reagents and Intramolecular Insertion Reactions

3.12 Preparation of Tertiary Amides via Aryl, Heteroaryl, and Benzyl Organozinc Reagents

3.13 Preparation of 5‐Substituted‐2‐Furaldehydes

3.14 Preparation and Chemistry of 4‐Coumarylzinc Bromide

3.15 Preparation and Cross‐Coupling of 2‐Pyridyl and 3‐Pyridylzinc Bromides

3.16 Preparation of Functionalized α‐Chloromethyl Ketones

3.17 Rieke Zinc as a Reducing Agent for Common Organic Functional Groups

3.18 Detailed Studies on the Mechanism of Organic Halide Oxidative Addition at a Zinc Metal Surface

3.19 Regiocontrolled Synthesis of Poly(3‐Alkylthiophenes) Mediated by Rieke Zinc: A New Class of Plastic Semiconductors

References

4 Magnesium

4.1 General Background and Mechanistic Details of Grignard Reaction

4.2 General Methods for Preparation of Rieke Magnesium

4.3 Grignard Reagent Formation and Range of Reactivity of Magnesium

4.4 1,3‐Diene‐Magnesium Complexes and Their Chemistry

4.5 Regioselectivity of Reaction of Complexes with Electrophiles

4.6 Carbocyclization of (1,4‐Diphenyl‐2‐butene‐1,4‐diyl) magnesium with Organic Dihalides

4.7 1,2‐Dimethylenecycloalkane‐Magnesium Reagents

4.8 Synthesis of Fused Carbocycles, β‐γ‐Unsaturated Ketones, and 3‐Cyclopentenols from Conjugated Diene‐Magnesium Reagents

4.9 Synthesis of Spiro‐γ‐Lactones and Spiro‐δ‐Lactones from 1,3‐Diene‐Magnesium Reagents

4.10 Synthesis of γ‐Lactams from Conjugated Diene‐Magnesium Reagents

4.11 Low‐Temperature Grignard Chemistry

4.12 Typical Procedures for Preparation of Active Magnesium and Typical Grignard Reactions as Well as 1,3‐Diene Chemistry

References

5 Copper

5.1 Background of Copper and Organocopper Chemistry

5.2 Development of Rieke Copper

5.3 Phosphine‐Based Copper

5.4 Lithium 2‐Thienylcyanocuprate‐Based Copper

5.5 Copper Cyanide‐Based Active Copper

5.6 Formal Copper Anion Preparation and Resulting Chemistry

5.7 Typical Experimental Details of Copper Chemistry

References

6 Indium

6.1 Background and Synthesis of Rieke Indium

6.2 Preparation of Organoindium Compounds

6.3 Preparation and Reactions of Indium Reformatsky Reagents

6.4 Experimental Details for Preparation and Reactions of Activated Indium

References

7 Nickel

7.1 Preparation of Rieke Nickel, Characterization of Active Nickel Powder, and Some Chemistry

7.2 Preparation of 3‐Aryl‐2‐hydroxy‐1‐propane by Nickel‐Mediated Addition of Benzylic Halides to 1,2‐Diketones

7.3 Preparation of 3‐Arylpropanenitriles by Nickel‐Mediated Reaction of Benzylic Halides with Haloacetonitriles

7.4 Reformatsky‐Type Additions of Haloacetonitriles to Aldehydes Mediated by Metallic Nickel

7.5 Preparation of Symmetrical 1,3‐Diarylpropan‐2‐ones from Benzylic Halides and Alkyl Oxalyl Chlorides

7.6 Nickel‐Mediated Coupling of Benzylic Halides and Acyl Halides to Yield Benzyl Ketones

7.7 Nickel‐Assisted Room Temperature Generation and Diels–Alder Chemistry of

‐Xylylene Intermediates

7.8 Active Nickel‐Mediated Dehalogenative Coupling of Aryl and Benzylic Halides

References

8 Manganese

8.1 Preparation of Rieke Manganese

8.2 Direct Formation of Aryl‐, Alkyl‐, and Vinylmanganese Halides via Oxidative Addition of the Active Metal to the Corresponding Halide

8.3 Direct Formation of Organomanganese Tosylates and Mesylates and Some Cross‐Coupling Reactions

8.4 Benzylic Manganese Halides, Sulfonates, and Phosphates: Preparation, Coupling Reactions, and Applications in New Reactions

8.5 Preparation and Coupling Reactions of Thienylmanganese Halides

8.6 Synthesis of β‐Hydroxy Esters Using Active Manganese

8.7 Reductive Coupling of Carbonyl‐Containing Compounds and Imines Using Reactive Manganese

8.8 Preparation of Heteroarylmanganese Reagents and Their Cross‐Coupling Chemistry

References

9 Calcium

9.1 Preparation of Rieke Calcium

9.2 Oxidative Addition Reactions of Rieke Calcium with Organic Halides and Some Subsequent Reactions

9.3 Preparation and Reaction of Calcium Cuprate Reagents

9.4 Preparation and Reactions of Calcium Metallocycles

9.5 Synthesis of Polyphenylcarbynes Using Highly Reactive Calcium, Barium, and Strontium: A Precursor for Diamond‐like Carbon

9.6 Chemical Modification of Halogenated Polystyrenes Using Rieke Calcium or Rieke Copper

References

10 Barium

10.1 Preparation of Rieke Barium

10.2 Oxidative Addition of Rieke Barium to Allylic Halides: Preparation of Stereochemically Homogeneous Allylic Barium Reagents

References

11 Iron

11.1 Preparation of Highly Reactive Iron and Some Oxidative Addition Chemistry

References

12 Palladium and Platinum

12.1 Preparation of Highly Reactive Palladium and Platinum and Some Oxidative Addition Chemistry

References

13 Highly Reactive Uranium and Thorium

13.1 Two Methods for Preparation of Highly Reactive Uranium and Thorium: Use of a Novel Reducing Agent Naphthalene Dianion

References

14 Aluminum

14.1 Preparation of Highly Reactive Aluminum and Reaction with Aryl Halides

References

15 Cobalt

15.1 Two Methods for Preparing Rieke Cobalt: Reaction with CO and Also Fischer–Tropsch Chemistry

References

16 Chromium

16.1 Preparation of Highly Reactive Chromium Metal and Its Reaction with CO to Yield Cr(CO)

References

Index

End User License Agreement

List of Tables

Chapter 03

Table 3.1 Preparation of organozinc compounds.

Table 3.2 Reactions of organozinc halides mediated by copper with acid chlorides.

Table 3.3 Reactions of organozinc halides mediated by Pd(PPh

)

and with acid chlorides.

Table 3.4 Reactions of organozinc halides mediated by copper iodide and with acid chlorides.

Table 3.5 Reactions of alkylzinc chloride reagents with benzoyl chloride.

Table 3.6 Copper‐mediated conjugate additions of organozinc halides with α,β‐unsaturated ketones.

Table 3.7 Reactions of RZnX with allylic halides mediated by CuCN∙2LiBr.

Table 3.8 Reaction of organozinc halides with Y─CH

─C≡C─CH

─Y.

Table 3.9 Preparation of 2‐bromo‐1‐alkene.

Table 3.10 Coupling reactions of RZnX with aryl and vinyl halides catalyzed by Pd(PPh

)

Table 3.11 Catalyst screening.

Table 3.12 Coupling reactions with benzoyl chlorides.

Table 3.13 Coupling reactions with acid chlorides.

Table 3.14 Preparation of functionalized ketones.

Table 3.15 Formation and coupling reactions of

sec

‐ and

‐alkylzinc bromides.

Table 3.16 Conjugate addition of secondary and tertiary alkylzinc bromides to enones.

Table 3.17 Preparation of compounds

Table 3.18 Coupling reactions of arylzinc halides.

Table 3.19 Coupling reactions of heteroarylzinc halides.

Table 3.20 Coupling reaction of benzylzinc halides.

Table 3.21 Coupling reactions with arylzinc halides.

Table 3.22 Coupling reactions with heteroarylzinc halides.

Table 3.23 Pd‐catalyzed coupling reactions with pyridylzincs.

Table 3.24 Pd‐catalyzed synthesis of 2‐(5‐arylfuran‐2‐yl)1,3‐dioxolanes.

Table 3.25 Coupling with haloamines and alcohols.

Table 3.26 Cu‐catalyzed coupling reaction.

Table 3.27 Coupling reaction with benzoyl chloride.

Table 3.28 Coupling reaction with heteroaryl acid chloride.

Table 3.29 Coupling reaction with arylhalide.

Table 3.30 Coupling reaction of 2‐pyridylzinc bromide (

) with benzoyl chlorides.

Table 3.31 Coupling reaction of

with acid chlorides.

Table 3.32 Study of substituent effect.

Table 3.33 Steric effect on cross‐coupling reaction.

Table 3.34 Pd‐catalyzed coupling of

with arylhalide.

Table 3.35 Coupling reactions of

with heteroaryl halides.

Table 3.36 Preparation of 2,2′‐bipyridines.

Table 3.37 Coupling reaction with haloaromatic amines.

Table 3.38 Coupling reaction with haloaromatic alcohols.

Table 3.39 Pd‐catalyzed coupling reactions of 3‐pyridylzinc bromide (

Table 3.40 Copper‐catalyzed coupling reaction of

Table 3.41 Preparation of quinoline and isoquinoline derivatives via heteroarylzinc reagent.

Table 3.42 Preliminary test for the coupling reaction of

Table 3.43 Preparation of aminophenylpyridines.

Table 3.44 Preparation of hydroxyphenylpyridines.

Table 3.45 Preparation of quinoline and isoquinoline derivatives.

Table 3.46 Reaction of organozinc reagent mediated by copper with chloroacetyl chloride.

Table 3.47 Reduction of compounds with Rieke zinc.

Table 3.48 Relative rates (

Table 3.49 Relative rates of reaction of aryl, vinyl, benzyl, and allyl halides (

Table 3.50 Relative rates of reaction of organic halides in various electron transfer processes.

Table 3.51 Relative rates of reaction of aryl bromides (

Table 3.52 Relative rates of reaction of aryl iodines (

Table 3.53 Hammett constants (

) for some ET reactions (1–14) and one S

(15) involving aryl halides.

Table 3.54 Synthetic applications.

Table 3.55 Regioselectivities of the reactions of Zn* with 2,5‐dibromo‐3‐alkylthiophenes.

Table 3.56 Regioregularity of P3ATs controlled by different catalysts.

Table 3.57 NMR chemistry shift (ppm) of

H and

C in different regiostructures.

Chapter 04

Table 4.1 Reactions of activated magnesium with various halides.

Table 4.2 Reaction of

‐fluorotoluene and 1‐fluorohexane with activated magnesium.

Table 4.3 Cyclization of (1,4‐diphenyl‐2‐butene‐1,4‐diyl)magnesium with α,ω‐alkylene dihalides.

Table 4.4 Stepwise reactions of (2,3‐dimetyl‐2‐butene‐1,4‐diyl)magnesium with electrophiles.

Table 4.5 Reactions of (2,3‐dimethyl‐2‐butene‐1,4‐diyl)magnesium with organodihalides.

Table 4.6 Reactions of magnesium complexes of 1,2‐dimethylenecycloalkanes with bis‐electrophiles.

Table 4.7 Reactions of diene‐magnesium reagents with carboxylic esters: formation of cyclopentenols.

Table 4.8 Reactions of diene‐magnesium reagents with carboxylic esters: formation of β,γ‐unsaturated ketones.

Table 4.9 Synthesis of spiro‐γ‐lactones from conjugated diene, ketones, and CO

Table 4.10 Reactions of conjugated diene‐magnesium reagents with epoxides followed by carbon dioxide.

Table 4.11 Reactions of conjugated diene‐magnesium reagents with epoxides followed by acidic hydrolysis.

Table 4.12 Lactamization of conjugated diene‐magnesium reagents with imines and CO

Table 4.13 Reactions of conjugated diene‐magnesium reagents with imines followed by acidic hydrolysis.

Table 4.14 Formation of the functionalized Grignard reagents and their reactions with electrophiles.

Chapter 05

Table 5.1 Reactions of activated copper with alkyl halides.

Table 5.2 Intramolecular epoxide‐opening reactions of epoxy aryl halides using phosphine‐based active copper.

Table 5.3 Intramolecular cyclizations of bromoepoxides with activated copper.

Table 5.4 Cross‐coupling reactions of thienyl‐based organocopper reagents with acid chlorides.

Table 5.5 Reactions of thienyl‐based allylic organocopper reagents with electrophiles.

Table 5.6 1,4‐Conjugate addition reactions with thienyl‐based organocopper reagents.

Table 5.7 Reactions of thienyl‐based organocopper reagents with 1,2‐epoxybutane.

Table 5.8 Cross‐coupling of benzoyl chloride with organocopper reagents derived from CuCN∙2 LiBr‐based active copper.

Table 5.9 Conjugate additions with organocopper reagents derived from CuCN∙2 LiBr‐based active copper.

Table 5.10 Reaction of CuCN∙2 LiBr‐derived copper with 2,3‐dichloropropene.

Table 5.11 Reaction of copper anion with organohalides and subsequent cross‐coupling reactions with benzoyl chloride.

Table 5.12 1,4‐Conjugate addition reactions of anion‐based organocopper reagents with 2‐cyclohexen‐1‐one.

Table 5.13 Formation and reactions of copper anion‐based higher‐order allyl cyanocuprates.

Table 5.14 1,4‐Conjugate additions with copper anion‐based allyl organocopper reagents.

Chapter 06

Table 6.1 Reactions of activated indium and diorganomercury.

Table 6.2 Reactions of aryl halides with activated indium.

Table 6.3 Solvent effect on the yield of the reactions with activated indium.

Table 6.4 Effect of stoichiometric ratio of ethyl bromoacetate to carbonyl compounds on the yield of the reaction with activated indium.

Table 6.5 Summary of Reformatsky reaction of carbonyl compounds with activated indium and ethyl bromoacetate.

Chapter 07

Table 7.1 Molar ratio of lithium to metal halide for reduction in glyme.

Table 7.2 Bulk elemental analysis of active nickel powder.

Table 7.3 Relative atomic concentrations in percent as a function of sputtering depth—activated Ni.

Table 7.4 3‐Aryl‐2‐hydroxy‐1‐propanones (4) prepared.

Table 7.5 3‐Arylpropanenitriles (3) prepared by the reaction of benzyl halides (1) with bromoacetonitrile (2; X

= nickel).

Table 7.6 The preparation of β‐hydroxynitriles by the reaction of bromoacetonitriles with aldehydes mediated by the metallic nickel.

Table 7.7 Preparation of symmetrical 1,3‐diarylpropan‐2‐ones by the reaction of benzylic halides with alkyl oxalyl chlorides in the presence of metallic nickel.

Table 7.8 Coupling reaction of benzyl halides with acyl halides mediated by metallic nickel.

Table 7.9 Nickel‐mediated cycloadditions of

‐xylylene with dienophiles.

Table 7.10 Nickel‐mediated cycloadditions of substituted dibromides with dienophiles.

Table 7.11 Reaction of iodobenzenes with activated nickel powder.

Table 7.12 Reaction of bromobenzenes with activated nickel powder.

Table 7.13 Reaction of benzyl halides with metallic nickel powders.

Table 7.14 Reaction of benzylic mono‐ and polyhalides with metallic nickel.

Chapter 08

Table 8.1 Formation and coupling reactions of organomanganese bromides.

Table 8.2 Coupling reaction with acid chlorides.

Table 8.3 Cross‐coupling reaction of benzylic manganese halides.

Table 8.4 Reactivity of Mn*.

Table 8.5 Coupling reaction of arylmanganese bromides with benzoyl chloride.

Table 8.6 Coupling reaction of arylmanganese bromides with phenyl isocyanate.

Table 8.7 Cross‐coupling reaction of benzylic and functionalized benzylic manganese sulfonates.

Table 8.8 Preparation of organomanganese tosylates and their coupling reaction.

Table 8.9 Coupling reaction with acid chlorides.

Table 8.10 Cross‐coupling reaction of benzylic manganese bromide.

Table 8.11 Cross‐coupling reaction of benzylic and functionalize benzylic manganese sulfonates.

Table 8.12 Preparation of organomanganese tosylates and their coupling reaction.

Table 8.13 Coupling reactions of benzylic manganese phosphates with acid chlorides.

Table 8.14 Addition reactions of benzylic manganese phosphates.

Table 8.15 Homocoupling reactions of benzyl bromides at different reaction conditions.

Table 8.16 Homocoupling reactions of benzyl halides.

Table 8.17 Palladium‐catalyzed cross‐coupling reactions of benzyl manganese halides and phosphate with aryl iodides.

Table 8.18 Coupling reaction of 3‐bromo‐4‐thienylmanganese bromide.

Table 8.19 Coupling reaction of 4‐substituted 3‐bromothiophene.

Table 8.20 Coupling reactions of 3‐thienylmanganese bromide.

Table 8.21 Reactions of α‐bromoester with carbonyl compound using Mn*.

Table 8.22 Reactivity study of Mn* on halides.

Table 8.23 Study of reactivity depending on halides.

Table 8.24 Reductive coupling of arylaldehydes.

Table 8.25 Reductive coupling of arylketones.

Table 8.26 Reactions of heteroaryl manganese bromides with benzoyl chloride.

Table 8.27 Cross‐coupling reactions of heteroaryl manganese reagents.

Chapter 09

Table 9.1 Grignard‐type reactions of organocalcium reagents with cyclohexanone.

Table 9.2 Cross‐coupling reactions of calcium organocuprate reagents with benzoyl chloride.

Table 9.3 Conjugate 1,4‐addition reactions of calcium organocuprate reagents with enones.

Table 9.4 Reactions of 1,3‐diene/calcium complex with organic dihalides.

Table 9.5 Reactions of organocalcium reagents prepared from ρ‐halopolystyrene.

Table 9.6 Reactions of calcium cuprate reagents from chloromethylated polystyrene and Rieke calcium with various electrophiles.

Chapter 13

Table 13.1 Stoichiometry effect on reaction of benzophenone with U*.

Table 13.2 Temperature effect on reaction of benzophenone with U*.

Chapter 14

Table 14.1 Reactions of aryl halides with activated aluminum.

Chapter 15

Table 15.1 Production from the reaction of synthesis gas over cobalt powders.

Table 15.2 Weight percent composition of the hydrocarbons formed in the hydrolysis of cobalt powders.

Table 15.3 Calculated lattice spacings found for active cobalt powders.

List of Illustrations

Chapter 01

Figure 1.1 Graduate research proposal.

Figure 1.2 First research proposal.

Chapter 02

Figure 2.1 Active magnesium.

Figure 2.2 Active indium.

Chapter 03

Scheme 3.1 1,4‐Addition of 2°, 3° alkylzinc bromides.

Scheme 3.2 (i) Zn*(1.5–3 equiv), THF. (ii) DTBAD (1 equiv), 0°C. (iii) Sat. NaHCO

. X═Br(

1a–1i

), I(

Scheme 3.3 Olefinic π‐bond insertion into the carbon─zinc bond.

Scheme 3.4 Representative synthetic routes for 5‐substituted 2‐furaldehydes.

Scheme 3.5 Preparation of 5(1,3‐dioxolan‐2‐yl)‐2‐furanylzinc bromide (

Scheme 3.6 Preparation of 5‐aryl‐substituted furans.

Scheme 3.7 Synthetic routes for 4‐substituted coumarin.

Scheme 3.8 Preparation of 4‐coumarinylzinc bromide (

Scheme 3.9 Expansion of coupling reaction.

Scheme 3.10 Preparation of intermediates.

Scheme 3.11 Preparation of amino and hydroxyl bipyridines.

Scheme 3.12 Coupling of alcohols and amines.

Scheme 3.13 Preparation of α‐chloroketones.

Figure 3.1 R

Br versus 1‐bromopentane plots.

Scheme 3.14 Reaction of Zn* with optically active bromides: intermediate radicals promote racemization.

Scheme 3.15 Indirect radical detection in the reaction of Zn* with radical probes.

Scheme 3.16 Oxidative addition via S

2 transition state.

Scheme 3.17 Oxidative addition via an ate complex transition state/intermediate.

Scheme 3.18 Oxidative addition via S

1 transition state.

Scheme 3.19 Oxidative addition via an outer‐sphere electron transfer.

Scheme 3.20 Oxidative addition via an inner‐sphere electron transfer.

Figure 3.2 Structure‐reactivity profiles for the reactions of a set of ET reagents with primary, secondary, and tertiary alkyl halides. Inner‐sphere processes dominate the left side of the plot, whereas outer‐sphere processes are dominant on the right side.

Scheme 3.21 Oxidative addition via a radical‐anion intermediate.

Figure 3.3 Hammett plots for the reactions ArI/Mg (Et

O), ArBr/Mg (THF, Et

O), ArI/Zn* (THF), and ArBr/Zn* (THF). Reaction constants,

; regression coefficients are in parentheses.

Figure 3.4 Relative rates of reaction.

Figure 3.5 Orientation of 3‐alkylthiophene units in polymers.

Scheme 3.22 Regiocontrolled synthesis of poly(3‐alkylthiophene) mediated by Zn*.

Scheme 3.23 Regiocontrolled synthesis of poly(3‐alkylthiophenes) starting with 3‐alkyl‐2‐bromo‐5‐iodothiophenes.

Figure 3.6

H NMR spectra of (a) regiorandom P3HT

(1 : 1 : 1 : 1 HT–HT/HT–HH/TT–HT/TT–HH) and (b) regioregular P3HT

(HT linkage >98.5%).

Figure 3.7 Terminal polymer groups.

Figure 3.8 Expanded

H NMR spectra of (a) regiorandom P3HT

(1 : 1 HT/HH) and (b) regioregular

(HT linkage >98.5%).

Figure 3.9

C NMR spectra of (a) regiorandom P3HT

(1 : 1 : 1 : 1 HT–HT/HT–HH/TT–HT/TT–HH) and (b) regioregular P3HT

(HT linkage only).

Chapter 04

Scheme 4.1 Carbocyclization of (1,4‐diphenyl‐2‐butene‐1,4‐diyl)magnesium with organic dihalides.

Scheme 4.2 Synthesis of spiro‐olefins.

Scheme 4.3 Reaction of the 1,2‐bismethylenecyclohexane‐magnesium complex with an ester.

Scheme 4.4 Synthesis of spiro‐γ‐lactones.

Scheme 4.5 Synthesis of spiro‐δ‐lactones.

Scheme 4.6 Preparation of alcohols containing a quaternary carbon center.

Scheme 4.7 Formation of spiro‐γ‐lactams.

Scheme 4.8 Low‐temperature magnesium chemistry.

Chapter 05

Scheme 5.1 Intramolecular epoxide opening to afford a bicyclic product.

Chapter 07

Scheme 7.1 Postulated mechanisms.

Figure 7.1 Proposed intermediates.

Scheme 7.2 Proposed reaction scheme.

Figure 7.2 Precursors.

Scheme 7.3 Reaction with acetyl chloride.

Figure 7.3 Proposed organometallic intermediates.

Figure 7.4 Intermediates and product.

Figure 7.5 Reaction intermediates.

Scheme 7.4 Semmelhack et al. [126] proposed scheme for aryl halide coupling.

Scheme 7.5 Proposed coupling scheme of Tsou and Kochi [127].

Scheme 7.6 Aryl coupling scheme proposed by Rieke and Kavaliunas [123].

Scheme 7.7 Proposed reaction mechanism.

Chapter 08

Scheme 8.1 Preparation and coupling reaction of benzylic manganese halides.

Scheme 8.2 Palladium‐catalyzed coupling reaction.

Scheme 8.3 Reaction of manganese with bromothiophene.

Scheme 8.4 Proposed reaction scheme.

Scheme 8.5 Preparation and coupling reaction of benzylic manganese halides.

Scheme 8.6 Proposed reaction scheme.

Scheme 8.7 Coupling of benzylic halides.

Scheme 8.8 Cross‐coupling of benzylic halides.

Scheme 8.9 Preparation of 3,4‐disubstituted thiophenes.

Scheme 8.10 Manganese Reformatsky reactions.

Scheme 8.11 Reductive coupling of aldimines.

Scheme 8.12 Reaction of heterocyclic compounds with electrophiles.

Chapter 09

Figure 9.1 (a) UV‐vis electronic spectrum (cyclohexane) and (b) fluorescence spectrum (cyclohexane, excitation wavelength = 300 nm) of poly(phenylcarbyne) obtained by this methodology.

Scheme 9.1 Proposed reaction scheme. M = Ca, Sr, or Ba; Ar = biphenyl.

Scheme 9.2 Modifications of halogenated polystryene resin using Rieke calcium or copper.

Scheme 9.3 Reactions of organocalcium and calcium cuprate reagents prepared from chloromethylated polystyrene and Rieke calcium.

Chapter 10

Scheme 10.1 Various allylation reactions using allylic barium reagents.

Scheme 10.2 Synthesis of (3

)‐2,3‐oxidosqualene.

Scheme 10.3 Preparation of cembrol A.

Scheme 10.4 Coupling of allylic barium reagent.

Chapter 13

Scheme 13.1 Proposed reaction scheme.

Figure 13.1 Surface metallopinacols on active titanium and uranium.

Figure 13.2 Product of mixed pinacol reaction with U*.

Figure 13.3 Mixed pinacol reaction at 50°C.

Figure 13.4 Pinacol exchange on the surface of U.

Guide

Cover

Table of Contents

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Chemical Synthesis Using Highly Reactive Metals

Reuben D. Rieke

Published by John Wiley & Sons, Inc., Hoboken, New JerseyPublished simultaneously in Canada

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Library of Congress Cataloging‐in‐Publication Data:

Names: Rieke, Reuben D., 1939– author.Title: Chemical synthesis using highly reactive metals / Reuben D. Rieke.Description: Hoboken, New Jersey : John Wiley & Sons, Inc., [2017] | Includes bibliographical references and index.Identifiers: LCCN 2016034805 | ISBN 9781118929117 (cloth) | ISBN 9781118929131 (epub) | ISBN 9781118929148 (Adobe PDF)Subjects: LCSH: Organometallic compounds–Synthesis. | Reactivity (Chemistry)Classification: LCC QD411.7.S94 R54 2017 | DDC 547/.050453–dc23LC record available at https://lccn.loc.gov/2016034805

Cover image courtesy: JacobH/Gettyimages

Preface

It is obvious that such a large body of work as the summary of our active metal research for over 50 years requires the acknowledgement of many people. There is also no doubt that there is one key person without whose lifelong help this book would not be possible. That person is my wife Loretta. From the day we met at the entrance examinations for the chemistry graduate program at the University of Wisconsin–Madison in September 1961 until today, she has been of incredible help. From our early days at the University of North Carolina at Chapel Hill, where she carried out research with my research group, to 1991 when the two of us founded Rieke Metals, Inc., in Lincoln, Nebraska, where she served as Vice President and Business Manager, she has been a cornerstone of my travels through life. Another major force in these efforts is our daughter Elizabeth, who started working part time in Rieke Metals, Inc., and rose to the position of CEO before we sold the company in July 2014. Finally, our son Dennis was a constant supporter of our efforts and an excellent sounding board for our ideas.

Of course, this work would not be possible if I did not have an excellent group of graduate students, postdoctoral students, and undergraduate students. From the initial two students who worked on the active metals, Dr. Phillip Hudnall and Dr. Steven Bales, to my final student, Dr. S. H. Kim, I had an outstanding group of people to work with. This book only covers my research on active metals so my students that worked on radical anion chemistry, electrochemistry, electron paramagnetic chemistry, and quantum mechanical calculations are not mentioned in the book. The active metal students are all referenced in this book in the metal sections that they were involved with. Of special note is my postdoctoral student from Spain, Professor Alberto Guijarro of the University of Alicante, Alicante, Spain, who carried out the beautiful mechanistic studies on the oxidative addition of Rieke zinc with organic halides as well as several synthetic studies. The three years he spent with us were particularly productive. The history of active metals is discussed in the early part of the book. However, special thanks must go to Professor Saul Winstein of UCLA who allowed me to follow my idea of studying through‐space interactions by preparing radical anions and determining their EPR spectra. My other mentors, my undergraduate research director at the University of Minnesota–Minneapolis, Professor Wayland E. Noland, and my PhD mentor, Professor Howard E. Zimmerman of the University of Wisconsin–Madison, were also of major help in my early training.

Of final note is the assistance of our cat, Buddy. He always felt that it was his duty to come and sit in the middle of my papers as I was writing this book. When he was banished to the side of the papers, he insisted on placing his head and two front paws on my arm.

Chemical research is a long, hard road but the rewards of discovery are hard to describe. As the old saying goes, the train ride has been long and many times bumpy, but we have not reached the station yet.

1Genesis of Highly Reactive Metals

Modern life without metals is inconceivable. We find them at every turn in our existence: transportation, buildings and homes, transporting our water, carrying our electricity, modern electronics, cooking utensils, and drinking vessels. Perhaps this is not to be unexpected as 91 of the 118 elements in the periodic table are metals. Accordingly, we can surely expect to find them in all aspects of our lives. The early chemistry of metals or processing of metals is one of the oldest sciences of mankind. Its history can be traced back to 6000 BC. Gold was probably the first metal used by man as it can be found as a relatively pure metal in nature. It is bright and attractive and is easily formed into a variety of objects but has little strength and accordingly was used mainly for jewelry, coins, and adornment of statues and palaces. Copper articles can also be traced to ~6000 BC. The world’s oldest crown made of copper was discovered in a remote cave near the Dead Sea in 1961 and dates to around 6000 BC. The smelting of copper ores is more difficult and requires more sophisticated techniques and probably involved a clay firing furnace which could reach temperatures of 1100–1200°C. Silver (~4000 BC), lead (~3500 BC), tin (~1750 BC), smelted iron (~1500 BC), and mercury (~750 BC) constituted the metals known to man in the ancient world. It would not be until the thirteenth century that arsenic would be discovered. The 1700s, 1800s, and 1900s would see the rapid discovery of over 60 new metals. The bulk of these metals were prepared by reducing the corresponding metal salt with some form of carbon or, in a few cases, with hydrogen. A small number of difficult to free metals were eventually prepared by electrochemical methods such as the metals sodium, potassium, and aluminum. Eventually the concept of a metal alloy was understood. It became readily apparent that the presence of one or more different metals dispersed throughout a metal could dramatically change the chemical and physical properties of any metal. The extensive and broad field of metal alloys will not be discussed in this text. The main point to be made is that the presence of a foreign material, whether it be another metal or a nonmetal, can have a significant effect on a metal’s chemical and physical properties. Pure metals prepared by different methods have essentially all the same chemical and physical properties. The one caveat in this statement is particle size or surface area. Whitesides clearly demonstrated the effect of surface area on the rate of Grignard formation at a magnesium surface. Taking this to the extreme, Skell and Klabunde have demonstrated the high chemical reactivity of free metal atoms produced by metal vaporization. These two topics will be discussed in greater depth later in the text. Thus it is clear that preparation of metals which leads to the presence of foreign atoms throughout the metal lattice can have a profound effect on the metal’s chemical and physical properties. This will be discussed in greater detail later in the text.

The genesis of highly reactive metals from our laboratories can be traced back to my time spent in a small two‐room schoolhouse in a small town of 180 people in southern Minnesota (1947–1949) and then to graduate school at the University of Wisconsin–Madison where I was working on my PhD degree under the direction of Professor Howard E. Zimmerman. My research proposal, which was part of the degree requirements, was the synthesis of the naphthalene‐like molecule shown in Figure 1.1. The ultimate goal of the project was to determine if there was through‐space interaction between the two 1,3‐butadiene units via the bridging ethylene unit (4N + 2 electrons). To verify the through‐space interaction, I proposed preparing the radical anion and measuring the electron paramagnetic resonance (EPR) spectrum. EPR became an available experimental technique, thanks to the explosion of solid‐state electronics in the 1960s. Simulating the spectrum in conjunction with quantum mechanical calculations should provide a reasonable estimate of the influence of through‐space interaction. My postdoctoral mentor, Professor Saul Winstein, at UCLA allowed me to pursue this general idea and we went on to produce the monohomocyclooctatetraene radical anion. The experience gained in this project working with solvated electrons in THF allowed me to write my first proposal as an assistant professor of chemistry at the University of North Carolina at Chapel Hill. The project was the reduction of 1,2‐dibromobenzocyclobutene with solvated electrons to generate the radical anion of benzocyclobutadiene as shown in Figure 1.2. The reduction was to be carried out in the mixing chamber of a flow mixing reactor in the sensing region of an EPR spectrometer. However, even at −78°C, the only spectrum we could see was the radical anion of benzocyclobutene. It became clear that the radical anion (II) and/or the dianion was so basic that even at −78°C in extremely dry THF, the anions were protonated to yield benzocyclobutene which was then reduced to the radical anion. Quenching with D2O verified the presence of II and its dianion. In order to trap or stabilize the dianion, we attempted to carry out this chemistry in the presence of MgCl2 and generate the di‐Grignard. However, we mistakenly mixed the solvated electrons (we were using potassium naphthalenide) with MgCl2, generating a black slurry of finely divided black metal. Upon reflection, it became clear that we had generated finely divided magnesium. We quickly determined that this magnesium was extremely reactive with aryl halides and generated the corresponding Grignard reagent. Thus, the field of generating highly reactive metals by reduction of the metal salts in ethereal or hydrocarbon solvents was born.

Figure 1.1 Graduate research proposal.

Figure 1.2 First research proposal.

2General Methods of Preparation and Properties

2.1 General Methods for Preparation of Highly Reactive Metals

In 1972 we reported a general approach for preparing highly reactive metal powders by reducing metal salts in ethereal or hydrocarbon solvents using alkali metals as reducing agents [1–5]. Several basic approaches are possible, and each has its own particular advantages. For some metals, all approaches lead to metal powders of identical reactivity. However, for other metals one method can lead to far superior reactivity. High reactivity, for the most part, refers to oxidative addition reactions. Since our initial report, several other reduction methods have been reported including metal‐graphite compounds, a magnesium‐anthracene complex, and dissolved alkalides [6].

Although our initial entry into this area of study involved the reduction of MgCl2 with potassium biphenylide, our early work concentrated on reductions without the use of electron carriers. In this approach, reductions are conveniently carried out with an alkali metal and a solvent whose boiling point exceeds the melting point of the alkali metal. The metal salt to be reduced must also be partially soluble in the solvent, and the reductions are carried out under an argon atmosphere. Equation 2.1 shows the reduction of metal salts using potassium as the reducing agent:

(2.1)

The reductions are exothermic and are generally completed within a few hours. In addition to the metal powder, one or more moles of alkali salt are generated. Convenient systems of reducing agents and solvents include potassium and THF, sodium and 1,2‐dimethoxyethane (DME), and sodium or potassium with benzene or toluene. For many metal salts, solubility considerations restrict reductions to ethereal solvents. Also, for some metal salts, reductive cleavage of the ethereal solvents requires reductions in hydrocarbon solvents such as benzene or toluene. This is the case for Al, In, and Cr. When reductions are carried out in hydrocarbon solvents, solubility of the metal salts may become a serious problem. In the case of Cr [7], this was solved by using CrCl3∙3 THF.

A second general approach is to use an alkali metal in conjunction with an electron carrier such as naphthalene. The electron carrier is normally used in less than stoichiometric proportions, generally 5–10% by mole based on the metal salt being reduced. This procedure allows reductions to be carried out at ambient temperature or at least at lower temperatures compared with the previous approach, which requires refluxing. A convenient reducing metal is lithium. Not only is the procedure much safer when lithium is used rather than sodium or potassium, but also in many cases the reactivity of the metal powders is greater.

A third approach is to use a stoichiometric amount of preformed lithium naphthalenide. This approach allows for very rapid generation of the metal powders in that the reductions are diffusion controlled. Very low to ambient temperatures can be use for the reduction. In some cases the reductions are slower at low temperatures because of the low solubility of the metal salts. This approach frequently generates the most active metals, as the relatively short reduction times at low temperatures restrict the sintering (or growth) of the metal particles. This approach has been particularly important for preparing active copper. Fujita et al. have shown that lithium naphthalenide in toluene can be prepared by sonicating lithium, naphthalene, and N,N,N′,N′‐tetramethylethylenediamine (TMEDA) in toluene [8]. This allows reductions of metal salts in hydrocarbon solvents. This proved to be especially beneficial with cadmium [9]. An extension of this approach is to use the solid dilithium salt of the dianion of naphthalene. Use of this reducing agent in a hydrocarbon solvent is essential in the preparation of highly reactive uranium [10].

For many of the metals generated by one of the three general methods in the preceding text, the finely divided black metals will settle after standing for a few hours, leaving a clear, and in most cases colorless, solution. This allows the solvent to be removed via a cannula. Thus the metal powder can be washed to remove the electron carrier as well as the alkali salt, especially if it is a lithium salt. Moreover, a different solvent may be added at this point, providing versatility in solvent choice for subsequent reactions.

Finally, a fourth approach using lithium and an electron carrier such as naphthalene along with Zn(CN)2 yields the most reactive zinc metal of all four approaches [11].

The wide range of reducing agents under a variety of conditions can result in dramatic differences in the reactivity of the metal. For some metals, essentially the same reactivity is found no matter what reducing agent or reduction conditions are used. In addition to the reducing conditions, the anion of the metal salt can have a profound effect on the resulting reactivity. These effects are discussed separately for each metal. However, for the majority of metals, lithium is by far the preferred reducing agent. First, it is much safer to carry out reductions with lithium. Second, for many metals (magnesium, zinc, nickel, etc.), the resulting metal powders are much more reactive if they have been generated by lithium reduction.

An important aspect of the highly reactive metal powders is their convenient preparation. The apparatus required is very inexpensive and simple. The reductions are usually carried out in a two‐necked flask equipped with a condenser (if necessary), septum, heating mantle (if necessary), magnetic stirrer, and argon atmosphere. A critical aspect of the procedure is that anhydrous metal salts must be used. Alternatively, anhydrous salts can sometimes be easily prepared as, for example, MgBr2 from Mg turnings and 1,2‐dibromoethane. In some cases, anhydrous salts can be prepared by drying the hydrated salts at high temperatures in vacuum. This approach must be used with caution as many hydrated salts are very difficult to dry completely by this method or lead to mixtures of metal oxides and hydroxides. This is the most common cause when metal powders of low reactivity are obtained. The introduction of the metal salt and reducing agent into the reaction vessel is best done in a dry box or glove bag; however, very nonhygroscopic salts can be weighed out in the air and then introduced into the reaction vessel. Solvents, freshly distilled from suitable drying agents under argon, are then added to the flask with a syringe. While it varies from metal to metal, the reactivity will diminish with time, and the metals are best reacted within a few days of preparation.

We have never had a fire or explosion caused by the activated metals; however, extreme caution should be exercised when working with these materials. Until one becomes familiar with the characteristics of the metal powder involved, careful consideration should be taken at every step. With the exception of some forms of magnesium, no metal powder we have generated will spontaneously ignite if removed from the reaction vessel while wet with solvent. They do, however, react rapidly with oxygen and with moisture in the air. Accordingly, they should be handled under an argon atmosphere. If the metal powders are dried before being exposed to the air, many will begin to smoke and/or ignite, especially magnesium. Perhaps the most dangerous step in the preparation of the active metals is the handling of sodium or potassium. This can be avoided for most metals by using lithium as the reducing agent. In rare cases, heat generated during the reduction process can cause the solvent to reflux excessively. For example, reductions of ZnCl2 or FeCl3 in THF with potassium are quite exothermic. This is generally only observed when the metal salts are very soluble and the molten alkali metal approach (method one) is used. Sodium–potassium alloy is very reactive and difficult to use as a reducing agent; it is used only as a last resort in special cases.

2.2 Physical Characteristics of Highly Reactive Metal Powders

The reduction generates a finely divided black powder. Particle size analyses indicate a range of sizes varying from 1 to 2 µm to submicron dimensions depending on the metal and, more importantly, on the method of preparation. In cases such as nickel and copper, black colloidal suspensions are obtained that do not settle and cannot be filtered. In some cases even centrifugation is not successful. It should be pointed out that the particle size analysis and surface area studies have been done on samples that have been collected, dried, and sent off for analysis and are thus likely to have experienced considerable sintering. Scanning electron microscopy (SEM) photographs reveal a range from spongelike material to polycrystalline material (Figures 2.1 and 2.2). Results from X‐ray powder diffraction studies range from those for metals such as Al and In, which show diffraction lines for both the metal and the alkali salt, to those for Mg and Co, which only show lines for the alkali salt. This result suggests that the metal in this latter case is either amorphous or has a particle size <0.1 µm. In the case of Co, a sample heated to 300°C under argon and then reexamined showed diffraction lines due to Co, suggesting that the small crystallites had sintered upon heating [12].

Figure 2.1 Active magnesium.

Figure 2.2 Active indium.

ESCA (XPS) studies have been carried out on several metals, and in all cases the metal has been shown to be in the zerovalent state. Bulk analysis also clearly shows that the metal powders are complex materials containing in many cases significant quantities of carbon, hydrogen, oxygen, halogens, and alkali metal. A BET [13] surface area measurement was carried out on the activated Ni powder showing it to have a specific surface area of 32.7 m2/g. Thus, it is clear that the highly reactive metals have very high surface areas which, when initially prepared, are probably relatively free of oxide coatings.

2.3 Origin of the Metals’ High Reactivity

There are several characteristics of the metal powders prepared by these methods which clearly explain their high reactivity. They all exhibit very high surface areas. Particle sizes of a few microns or in some cases <0.1 µm point to very high surface areas. The BET studies [13] on Ni powder indicated surface areas of over 30 m2/g. Moreover, the lack of diffraction lines for several metals suggests particle sizes of <0.1 µm. Also the possibility of some metals being amorphous would increase their internal energy and lead to higher reactivity compared to the corresponding highly crystalline counterpart. In addition, the metals are produced under nonequilibrium conditions and exhibit many dislocations and imperfections. This would also be expected to lead to increased chemical reactivity. The metals are also prepared under a pure argon atmosphere which would result in a relatively oxide‐free surface being produced. Bulk analysis of the metals is quite varied depending on the metal. However, in all cases, there is a significant amount of other elements generally including carbon, hydrogen, halogens, and alkali metal ions from the alkali metal reducing agent. As will be pointed out in detail later, finely divided metal powders prepared by methods which do not introduce these materials into the metal lattice are all significantly less reactive than Rieke metals. For example, metal powders prepared by metal vaporization methods are far less reactive in oxidative addition reactions compared to the corresponding Rieke metals even though they are of comparable or even smaller particle size [14]. There is also one extremely important difference between the Rieke metals and finely divided metals prepared by other methods, and that is the presence of alkali metal salts. Whitesides’ [15] work on magnesium and our studies [16] on zinc clearly show that the rate‐determining step in oxidative addition reactions is the electron transfer from the metal surface to the organic halide. As in an electrochemical reduction reaction, the alkali salt can act as an electrolyte and facilitate this electron transfer. In most of the reductions presented in this text, the alkali salt is LiCl or LiBr. We will see later in the text that these alkali salts can also increase the reactivity of the resulting organometallic reagents RMX toward many electrophiles. In summary, the Rieke method of producing metal powders yields metals which are far from pure metal powders. The presence of these foreign materials along with the features mentioned yields metal powders which undergo many new and novel reactions which cannot be achieved by standard metals or their chemically activated counterparts.

References

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6 (a) Csuk, R.; Glanzer, B.L.; Furstner, A.

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14 Klabunde, K.J.

Chemistry of Free Atoms and Particles

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3Zinc

3.1 General Methods for Preparation of Rieke Zinc

In 1973 we reported the formation of Rieke zinc. For the first time, this zinc was shown to add oxidatively to alkyl and aryl bromides. The Rieke zinc used in those reactions was prepared using anhydrous zinc chloride or zinc bromide and potassium or sodium metal in refluxing tetrahydrofuran (THF) or 1,2‐dimethoxyethane (DME) for 4 h. The reaction is very exothermic, and extreme care must be exercised while carrying out this reaction. The reaction should be heated very slowly at first and carried out in a hood. A large oversized flask should be used to allow expansion of a refluxing solution. A deep black zinc powder is generated during the reduction. Particle size analysis indicates that the average size is 17 µm. Powder patterns show both the characteristic lines of KCl and ordinary zinc metals. For preparation details see Method 1 presented later in this chapter.

A wide variety of zinc salts and various reducing agents have been tried, but the aforementioned conditions seem to lead to the most active zinc. The addition of other alkali salts such as KI, NaI, LiI, KBr, LiBr, or LiCl prior to the reduction step does affect the activity of the zinc.

A far superior method of preparing the highly reactive zinc is to use lithium metal as the reducing agent along with an electron carrier such as naphthalene (Method 2). This approach is considerably safer as there is no rapid burst of heat. The dark green lithium naphthalenide also serves as an indicator, signaling when the reduction is over.

Rieke zinc is prepared by placing lithium metal (10 mmol), a catalytic amount of naphthalene (1 mmol), and 12–15 ml of THF in one flask placed in an ice bath. Once this mixture has stirred for about 30–60 s, it will turn dark green, indicating the formation of lithium naphthalenide. Zinc chloride dissolved previously in 12–15 ml of THF is then cannulated dropwise (ca. 3 s per drop) into the lithium naphthalenide, and stirring is continued for 30 min after the transfer is complete. This method is not only safer due to the use of lithium metal rather than sodium or potassium but also yields a more reactive zinc. A third method sometimes employed to prepare Rieke zinc uses a stoichiometric amount of naphthalene with respect to lithium (Method 3). Both methods yield Rieke zinc with the same reactivity. It should also be noted that the reactivities are similar regardless of the choice of solvent (THF or DME) and that of the halide salt. Further, the electron carrier is not limited to naphthalene. Other carriers such as biphenyl and anthracene have also been used. The zinc settles very rapidly allowing the solvent and electron carrier to be removed if deemed desirable. Washing it a second or third time removes the majority of the electron carrier. Finally, the desired solvent for the following chemistry can be then added. It should be noted that Rieke zinc in THF or other solvents can be purchased commercially (Rieke Metals, LLC). The zinc metal can be transferred readily either by a cannula or by a syringe yielding the highly reactive zinc in a dry solvent ready for further chemistry. Details for the three procedures are given later in the text.

Method 1 Active Zinc Prepared from the Potassium Reduction of Zinc Chloride:

Using a dry box or a glove bag with an argon or nitrogen atmosphere, charge an oven‐dried, two‐necked, round‐bottomed flask (250 ml), containing a magnetic stirring bar, with anhydrous zinc chloride (9.54 g, 0.07 mol) and thinly cut potassium metal (5.47 g, 0.14 mol). Fit the flask with a condenser capped with a gas adapter (with stopcock). Close the stopcock and cap the side neck with a rubber septum.

Remove the apparatus from the dry box or glove bag, and connect it to a vacuum/argon or nitrogen manifold. Before opening the stopcock to the inert atmosphere from the manifold, evacuate the system (5 min) and refill with argon or nitrogen (1 min) in three cycles.

Open the stopcock, and add freshly distilled THF (40 ml) through the septum inlet using a glass syringe.

The mixture is heated without stirring until the zinc chloride visibly reduces at the surface of the potassium. The heating is then stopped, and the vigorous exothermic reduction of the zinc chloride proceeds. At this point cooling in a water or ice bath may be required to moderate the progress of the reaction. After the reduction subsides, the mixture is refluxed for 2.5 h with rapid stirring. The active zinc is then ready for use.

Method 2 Active Zinc Prepared from the Lithium Reduction of Zinc Chloride Using Catalytic Naphthalene

Using a dry box or glove bag with an argon atmosphere, charge an oven‐dried, two‐necked flask (50 ml), containing a magnetic stirring bar, with anhydrous zinc chloride (1.09 g, 8 mmol). Secure a gas adapter (with stopcock) to the flask, and cap the side neck with a septum. Charge a second dry, two‐necked round‐bottomed flask (50 ml) containing a magnetic stir bar, with thinly cut lithium metal (0.11 g, 1.6 mmol) and naphthalene (0.2 g, 1.6 mmol). Fit the flask with a gas adapter (with stopcock), and secure a septum to the side neck. Close the stopcock before removing the flasks from the dry box or glove bag.

Remove the flasks from the dry box or glove bag, and connect them to a vacuum/argon manifold. Before opening the stopcocks to the inert atmosphere from the manifold, evacuate the system (5 min) and refill with argon or nitrogen (1 min) in three cycles. Open the stopcocks to positive argon pressure.

Add dry, freshly distilled THF (15 ml) through the septum inlet using a glass syringe to the flask containing the lithium metal and naphthalene. The stirring mixture will turn green in <30 s. Add freshly distilled THF (20 ml) through the septum inlet using a dried glass syringe to the flask containing the zinc chloride. This addition should be performed with rapid stirring.

Transfer the stirring zinc chloride solution to the flask containing the stirring green mixture of lithium and naphthalene in THF, using a cannula, dropwise, over a period of 1.5 h. Perform the addition slowly enough so that the green color of lithium naphthalenide persists. If the mixture becomes clear during the addition of the zinc chloride, stop the addition, and allow the mixture to stir until the green color returns before resuming the addition of the zinc chloride solution. When addition of the zinc chloride is complete, stir the mixture until all the residual lithium is consumed. The resulting black slurry of active zinc is then ready for use. The rate of addition of the zinc chloride solution is crucial. If the addition of zinc chloride is performed over a period of four or more hours, the active zinc formed may not settle completely from the THF solution. The reduction of the zinc chloride in approximately 1.5–2 h produces a mossy form of active zinc which rapidly settles.

If the presence of naphthalene or lithium chloride (from the reduction) is not desired in the active zinc, they can be removed at this point by repeated washing with dry THF. After the reduction is complete, turn the stir plate off and allow the active zinc to settle (1–2 h). Monitor the progress of the settling by shining a strong light through the slurry by use of a flashlight. Remove the THF solution, by cannula, down to the surface of the settled zinc. Tip the flask slightly to facilitate the removal of the last portion of THF. Add freshly distilled THF (25 ml), and stir for several minutes. Turn off the stirring, allow the zinc to settle for a few minutes, and remove the supernatant by cannula. Repeat the washing cycle two additional times.

Method 3 Active Zinc Prepared from the Stoichiometric Lithium Naphthalenide Reduction of Zinc Chloride

Using a dry box or glove bag with an argon atmosphere, charge an oven‐dried, two‐necked, round‐bottomed flask (50 ml), containing a magnetic stirring bar, with anhydrous zinc chloride (2.09 g, 15.4 mmol). Secure a gas adapter (with stopcock) to the flask, and cap the side neck with a septum. Charge a second dry, two‐necked, round‐bottomed flask (50 ml) containing a magnetic stirring bar with thinly cut lithium metal (0.213 g, 30.6 mmol) and naphthalene (3.99 g, 31.2 mmol). Fit the flask with a gas adapter (with stopcock), and secure a septum to the side neck. Close the stopcocks before removing the flasks from the dry box or glove bag.

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