Helicenes E-Book

• Herausgeber: Wiley-VCH GmbH
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• Sprache: Englisch

Seitenzahl: 919

Veröffentlichungsjahr: 2022

Beschreibung

Helicenes

A thorough introduction to everything there is to know about this fascinating compound class

The intriguing nature of this highly interesting compound class has inspired much research over the last decade, and much of what has been produced is highly interdisciplinary, with applications found in catalysis, material science, and spectroscopy. Indeed, the field has reached maturity such that elegant synthetic methods are now available and novel applications in arenas such as enantioselective catalysis and optoelectronics are appearing.

Helicenes provides not only an introduction to the synthesis of the fascinating compound class of helicenes, but also describes the properties and, most importantly, their applications. The book thoroughly explains several synthetic routes from classical to state-of-the-art methods. In addition, the different classes of helicene-based molecules – organic, organometallic, oligo-meric, and multidimensional – are covered.

Helicenes readers will also find:

• Edited by world-leading experts in the field of chirality
• A comprehensive and well-structured approach that deals with every aspect of this compound class
• Discussions of the applications of helicenes in organic synthesis, photophysics, material science, optoelectronic devices, on-surface chemistry, and theoretical calculations
• A special focus on the applications of helicenes in catalysis, optoelectronics, non-linear optics, and chiroptical spectroscopy
• Chapters focusing on helicenes as theoretical helically chiral models

Helicenes is an essential reference for organic chemists, materials scientists, spectroscopists, polymer chemists, inorganic chemists, and catalytic chemists, as well as students in these fields and libraries supplying them.

Leseprobe

Helicenes

Synthesis, Properties, and Applications

Edited by Jeanne Crassous, Irena G. Stará, and Ivo Starý

Editors

Dr. Jeanne CrassousInstitut des Sciences Chimiques de Rennes (UMR CNRS 6226)Université de Rennes 1Campus de Beaulieu35042 Rennes CedexFrance

Dr. Irena G. StaráInstitute of Organic Chemistry and Biochemistry (IOCB Prague)Czech Academy of SciencesFlemingovo náměstí 542/2166 10 Prague 6Czech Republic

Dr. Ivo StarýInstitute of Organic Chemistry and Biochemistry (IOCB Prague)Czech Academy of SciencesFlemingovo náměstí 542/2166 10 Prague 6Czech Republic

Cover Image: Tomáš Bellon

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Preface

Helicenes are the archetypal helical molecules simply composed of ortho‐condensed aromatic rings whose helical shape derives from the steric repulsion of terminal cycles or substituents. It leads to configurationally stable helices with minus (M) or plus (P) handedness. These chiral polycyclic hydrocarbons have attracted the attention of chemists for decades, not only because of their beautiful 3D helical topology artificially mimicking natural helical structures such as double‐stranded DNA or snail shells but also because of their many appealing physicochemical properties and consequent applications in various fields of research. Over time, the structural diversity of helicenes has grown considerably and many synthetic approaches have been developed to access a variety of helical scaffolds decorated with a decent range of heteroatoms, functional groups, or metal ions. With advances in chiral high‐performance liquid chromatography (HPLC) methodologies, nonracemic helicenes have become accessible via racemate resolution. Alternatively, new stereoselective synthetic procedures have been developed to receive helicenes in a nonracemic form.

These helically chiral extended π‐conjugated molecules strongly interact with circularly polarized (CP) light and as a result exhibit remarkable chiroptical properties such as unusually high values of specific optical rotation, very intense electronic and vibrational circular dichroism spectra, CP emission, and nonlinear optical responses. From a technology perspective, helicenes can behave as semiconductor materials and as such can constitute attractive optoelectronic devices, in particular chiral photovoltaic cells, transistors, and CP organic light emitting diodes (OLEDs). Moreover, their helical topology combined with extended π‐conjugation makes them unique models for studying the basis of optical activity, the role of chirality in 2D self‐assembly of molecules on surfaces, and principles of chirality transfer in enantioselective catalysis.

Undoubtedly, helicene research is undergoing a dynamic development, and the number of groups focusing on this topic is steadily increasing. This monograph collects chapters written by internationally recognized experts in their field and covers all of the aforementioned aspects.

Chapters 1–9 describe manifold facets of the synthesis of helicene architectures and discuss selected properties derived from their molecular structure. In Chapter 1, one of the most widely used and historically important synthetic methods for accessing helicene derivatives, “the photochemical approach to helicenes” is reviewed by Jan Storch, Jaroslav Žádný, Vladimír Církva, and Martin Jakubec from the Institute of Chemical Process Fundamentals of the CAS, Prague; Jan Hrbáč from Masaryk University, Brno; and Jan Vacek from Palacký University Olomouc, Czech Republic. In Chapter 2, another popular and efficient method for obtaining racemic or enantioenriched helicenes, “synthesis of helicenes by [2 + 2 + 2] cycloisomerization,” is presented by Irena G. Stará and Ivo Starý, coeditors, from the Institute of Organic Chemistry and Biochemistry of the CAS, Prague, Czech Republic. In Chapter 3, Ken Tanaka from the Tokyo Institute of Technology, Japan, describes the most efficient methods for “enantioselective synthesis of helicenes.” Chapter 4 reports on a class of charged helicene scaffolds, “cationic triarylcarbenium helicenes,” written by Johann Bosson and Jérôme Lacour from University of Geneva, Switzerland, and Niels Bisballe and Bo W. Laursen from University of Copenhagen, Denmark. “Organometallic and coordination chemistry of helicenes” is the subject of Chapter 5, authored by Rafael Rodriguez, Natalia Del Rio, and Jeanne Crassous, coeditor, from University of Rennes, France, while Chapter 6 focuses on “tetrathiahelicenes” by Silvia Cauteruccio, Emanuela Licandro, and Patrizia Romana Mussini from University of Milan, Italy, and Andreas Dreuw from Heidelberg University, Germany. In Chapter 7, Masahiko Yamaguchi from Tohoku University, Japan, describes the “synthesis and properties of helicenes with small numbers of benzene rings” to introduce the smallest configurationally stable [4]helicene derivatives. The next two chapters, 8 and 9, focus on multihelicene architectures, specifically on “preparation of multihelicenic platforms from halogenated helicenes” by Myriam Roy from Sorbonne Université, Paris, and Marc Gingras from Aix Marseille University, France, and on “helical nanographenes and their synthetic and chiroptical achievements” by Sandra Míguez‐Lago, Juan P. Mora‐Fuentes, Carlos M. Cruz, and Araceli G. Campaña from Granada University, Spain.

After the first part of the monograph discusses mainly the chemical aspects of helicene research (Chapters 1–9), the second part presents primarily the properties and applications of helicenes (Chapters 10–16). Accordingly, in Chapter 10, Xiao Xiao and Colin Nuckolls from Columbia University, New York, United States, describe the properties of “helicene‐based electron acceptors.” Chapters 11 and 12 focus on the “chiroptical properties of helicenes” through a “historical perspective and structure‐property relationships” by Sergio Abbate and Giovanna Longhi from Università degli Studi di Brescia, Italy, and Tadashi Mori from Osaka University, Japan, and on “photophysical and chiroptical properties of helicene‐based systems: experiment vs. theory” by Monika Srebro‐Hooper, Jagiellonian University, Krakow, Poland; Jochen Autschbach, University of Buffalo, United States; and Jeanne Crassous, coeditor. Chapter 13 discusses “helicene derivatives with circularly polarized luminescence” written by Meng Li from Beijing National Laboratory for Molecular Science and Wen‐Long Zhao, Hai‐Yan Lu, and Chuan‐Feng Chen from University of Chinese Academy of Sciences, Beijing, China. In Chapter 14, Benoît Champagne from University of Namur, Belgium, describes the “nonlinear optical properties” of helicenes, and in Chapter 15, the “helicenes for optoelectronic devices” research area is reported by Seán T.J. Ryan and Matthew J. Fuchter from Imperial College London, United Kingdom. Finally, this book could not be completed without addressing the important application fields of helicenes, which are surface science and asymmetric catalysis, as very briefly reviewed in the last Chapter 16 by coeditor Jeanne Crassous.

Jeanne Crassous would like to warmly thank the Centre National de la Recherche Scientifique (CNRS), which employs her, and the University of Rennes and the Institut des Sciences Chimiques de Rennes for giving her a pleasant and enriching environment to work in, with so many friendly and professional colleagues from both the scientific and administrative parts. She also thanks all the students and coworkers with whom she has shared her passion for chirality and helicenes during the past 25 years. She expresses special thanks to her former and current close collaborators, Régis Réau and Ludovic Favereau. She dedicates this book to the memory of her mentors André Collet and François Diederich, who passed away too early.

Irena G. Stará and Ivo Starý would like to express their sincere thanks to the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences for creating an inspiring environment and for all financial and technical support. They still owe enough thanks to their great teachers Pavel Kočovský, Jiří Závada, Wolfgang Oppolzer, Peter E. Kündig, and Josef Michl, who fundamentally influenced their scientific career. Last but not least, they are grateful to all their former students whom they have had the privilege of teaching, so that they can now learn from them themselves.

This book has been written under the spiritual influence of many chirality‐related communities, especially those from international symposia (Chirality and Chiroptical Spectroscopy conference series) and national and international consortia, including the HEL4CHIROLED ITN project Grant Agreement (H2020‐MSCA‐ITN‐2019 n° 859752).

1The Photochemical Approach to Helicenes

Jan Storch1, Jaroslav Žádný1, Vladimír Církva1, Martin Jakubec1, Jan Hrbáč2, and Jan Vacek3

1Department of Advanced Materials and Organic Synthesis, Czech Academy of Sciences, Institute of Chemical Process Fundamentals, Prague, Czech Republic

2Department of Chemistry, Faculty of Science, Masaryk University, Brno, Czech Republic

3Department of Medical Chemistry and Biochemistry, Faculty of Medicine and Dentistry, Palacky University, Olomouc, Czech Republic

1.1 Introduction

In this chapter, the authors have decided to follow the nomenclature recommendations of IUPAC [1] for class names of organic compounds, which classifies helicenes as “ortho‐fused polycyclic aromatic or heteroaromatic compounds in which all rings (minimum five) are angularly arranged so as to give helically shaped molecules, which are thus chiral.” Therefore, the following text includes [n]helicenes where n ≥ 5 and the photoreaction is the very last step of their preparation, unless stated otherwise.

Two basic photo‐approaches (oxidative photocyclodehydrogenation and photoinduced elimination; for details, see Section 1.2) were used for the preparation of all helicenes listed in this chapter. The photochemically created bonds are highlighted in red in all figures. Non‐oxidative photochemical approaches are discussed in specific cases.

At the beginning of the chapter, some general features are mentioned, including the mechanism of the photocyclization, reaction conditions, attempts at asymmetric photosynthesis, and the synthetic approach to starting materials. The appropriate helical structures and their preparations are described in sections on carbo‐, aza‐, thia‐, and phosphahelicenes and other helicenes. Helicene‐like molecules, including dihydrohelicenes, are discussed separately, as well as photochemical transformations of helicenes.

1.2 General Features

Historically, the photocyclization of stilbenes [2, 3] was discovered during the investigation of their cis/trans photoisomerization [4], but the reaction was not used synthetically until Mallory found that iodine catalyzed this reaction in 1964 [5, 6].

Figure 1.1 Photochemical reaction pathways for stilbene derivatives.

From the mechanistic point of view, the cis/trans photoisomerization of stilbenes is very fast with a high quantum yield, allowing stilbenes to be used as an isomeric mixture (Figure 1.1), although only the cis‐isomer is capable of cyclization. The symmetry‐allowed photoreaction typically takes place from the singlet S1 state by a conrotatory process according to the Woodward–Hoffmann rules. Thus triplet sensitizers do not sensitize the photocyclization, and triplet quenchers (such as oxygen) do not quench it. The unstable 4a,4b‐dihydrophenanthrene (DHP) intermediate possesses trans‐configuration [7] and can, unless trapped, relax back to the stilbene. In the presence of an oxidant, DHP forms a phenanthrene derivative. This type of photocyclization is called the Mallory reaction[8]. If the stilbene contains suitable leaving groups (R = OMe, Cl, Br, etc.) in ortho‐position, the elimination reaction producing cyclization product can take place (in the absence of an oxidant) [9–11]. Photocyclizations are typically carried out at concentrations of 10−3 M and lower to avoid the competing photodimerization [12]. The proposed photocyclization mechanism is also applicable to aza‐, thia‐, and other stilbene derivatives.

Originally, air was used as an oxidant until Mallory discovered that oxidative trapping occurs much faster when iodine (5 mol%) is used together with air [5]. It was proposed that iodine is photochemically cleaved into radicals reacting with hydrogen to form hydrogen iodide, which is then reoxidized to iodine by oxygen [13]. Other oxidants (e.g. selenium radicals, TCNE, TCNQ, chloranil, etc.) were investigated by Laarhoven without any practical significance for photocyclizandation of helicenes [14].

Higher amounts of hydrogen iodide may contribute to side reactions including double bond saturation of stilbenes [13]. In 1986, Katz developed new photocyclization conditions using propylene oxide as a hydrogen iodide scavenger under inert conditions [15, 16]. As a consequence, the iodide could not be reoxidized by air, and its stoichiometric amount is needed. Accordingly, it allows for a higher concentration of starting material in the reaction mixture without the undesired formation of dimers. When speaking about photocyclodehydrogenation leading to [n]helicenes, these conditions are sometimes familiarly referred to as Katz's conditions. Other cyclic ethers (such as THF) are often used as HI scavengers too.

The previously mentioned conditions provide [n]helicenes in strictly racemic mixtures (1 : 1 ratio of (P)‐ and (M)‐enantiomer). Although attempts to lead photocyclizations asymmetrically using circularly polarized light sources [17–23], chiral solvents [24–26], or cholesteric liquid crystals [27, 28] were made, the obtained results (% ee) were more or less at the level of experimental error and did not have any practical importance. Thus, the photochemical approach has to be followed by an optical resolution to obtain helicenes in their optically pure forms. The nonracemic helicenes are often photochemically accessible as corresponding diastereomers with (photo‐stable) chiral auxiliaries (providing up to >99% de), which can be synthetically cleaved or transformed after cyclization [29–31]. Eventually, such diastereomers might be separated using standard chromatographic methods. When an enantiomerically pure [6]helicene moiety was a part of the precursor, nonracemic [n]helicenes (n = 8–11, 13) were obtained [32]. Other asymmetric photosyntheses to enantioenriched metallocene helicenes were developed by Katz [33–35]. Some studies suggested that only one chiral auxiliary is not sufficient, and a better result might be obtained with the chiral substitution at the most sterically hindered position [36]. The same phenomenon was observed by Carbery and Pearson [37]. For an overview see Ref. [26] and the references therein.

Conventional (Figure 1.2a) and transition metal‐catalyzed (Figure 1.2b) methods of preparation of stilbene‐like molecules as a common starting material for the photochemical synthesis of helicenes are described in the literature [38, 39], including several other synthetic procedures. In practice, the Wittig reaction (and its variations) and Pd‐catalyzed cross‐coupling reactions belong to commonly used methods.

Figure 1.2 Commonly used preparations of stilbene‐like molecules by conventional (a) and metal‐catalyzed (b) procedures.

The most widely used sources of UV–vis light for continuous irradiation in laboratory experiments are commercially available mercury lamps (low, medium, and high pressure) [40]. Their spectral irradiance is strongly dependent on the mercury vapor pressure. The lamp also produces a considerable amount of infrared radiation and heat. Therefore, cooling‐water circulation must be utilized to protect the reaction solution from heating. Recently, new energy‐efficient light sources like light‐emitting diodes (LEDs) [41–43] (Figure 1.3d) became available, thus avoiding the use of optical filters and reducing consumption costs.

Figure 1.3 Common photoreactor types: (a) immersion well, (b) external chamber, (c) continuous‐flow, (d) LED‐type, and (e) electrodeless discharge lamps.

Photoreactors with an immersion well (Figure 1.3a) and external chamber (merry‐go‐round, Figure 1.3b) are the most common types of photochemical equipment on a preparative laboratory scale. Both reactor types are well established and in widespread utilization. The use of quartz allows light of all wavelengths above about 200 nm to enter the reaction mixture. For some photoreactions, higher yields can be obtained by employing Pyrex glass. This excludes from the reaction mixture light of wavelengths below about 300 nm and thereby protects the forming products from further photochemical degradation.

Following the experiments by Mallory [6], Scholz [44], and Martin [45], the batch setup of photocyclization of stilbene derivatives under UV–vis irradiation has become one of the most popular methods for the synthesis of helicenes [13]. This is mainly due to the synthetic accessibility of the stilbene precursors and functional group tolerance under the reaction conditions (pH, temperature, etc.). However, the development of this setup is limited by the necessary dilution of the reaction mixture (∼10−3 mol·l–1) to prevent the undesired [2 + 2] cycloaddition, requiring large volumes of pure solvents and prolonged reaction times (>15 hours).

Therefore, finding an efficient protocol for the photocyclization of stilbene derivatives in a flow mode (Figure 1.3c) would be useful for the synthesis of helicenes on a gram scale. The photochemical transformations conducted under continuous‐flow conditions are commonly characterized by shorter reaction times, higher yields, increased selectivities, easier purification, improved productivities, and high photonic and energy efficiencies compared with the conventional batch methods [46]. The equipment requirements are a special tubing (fluorinated ethylene propylene [FEP], which is highly transmittable, flexible, and anticorrosive) wound around the light source and a diaphragm pump.

Collins et al. reported the first example of the [5]helicene preparation by the photochemical continuous‐flow strategy under visible light in the presence of Cu complex formed in situ [47]. In comparison to the traditional setup in a batch reactor, the flow method under photocatalytic conditions prevented the product from overannulation and enabled its gram‐scale preparation. The same group used it also for the synthesis of pyrene–helicene dyads [48]. Rueping et al. [49] used a photo‐flow methodology for the synthesis of [5]‐ and [6]helicenes with different substitution patterns. Murase et al. [50] accomplished the synthesis of amide‐type aza[6]helicene in high yield by photocyclization of boron hydroxamate complex using a continuous‐flow reactor in order to minimize its decomposition. Sýkora et al. [51] prepared 2‐bromo[6]helicene as a key intermediate for helicene functionalization utilizing a continuous‐flow photoreactor. Finally, Církva et al. [52] synthesized [6]helicenes fluorinated at terminal rings in the same setup.

A special photochemical reactor utilizing electrodeless discharge lamps (EDLs; Figure 1.3e) was also designed [53, 54]. It generates UV irradiation when placed in a microwave (MW) field. This methodology was used for the preparation of [6]helicene derivatives. Next to low costs, the arrangement in which the EDLs are placed inside the reaction vessel has several other advantages, such as simultaneous UV and MW exposure of the sample, enabling the performance of photochemistry at high temperatures.

1.3 Photochemical Preparation of Carbohelicenes

This part is further divided into oxidative and eliminative photocyclization according to the mechanism of the reactions. More details on photocyclizations toward carbohelicenes can be found in reviews [8, 13, 14, 26,55–60] and a book chapter [61].

1.3.1 Oxidative Photocyclizations of Unsubstituted Carbo[n]helicenes

The first preparation of helicene molecule by photocyclization reaction was reported by Martin in 1967 on the preparation of [7]helicene (3) [45]. Since then, photocyclization has become one of the most important methods for the synthesis of helicene homologs, from [5]‐ to [16]helicenes (Figure 1.4) and, of course, their derivatives.

Figure 1.4 Unsubstituted carbo[n]helicenes.

[n]Helicenes can be photochemically formed from olefinic precursors of [a] + [b] + [c] structure, where “[a–c]” denotes the number of ortho‐fused benzene subunits and “+” stands for the vinylene linker(s).

In the case of [5]helicene (1), great attention must be paid to the design of precursors due to possible benzo[ghi]perylene formation (see Section 1.6), which was observed by Dietz [44, 62], who attempted the synthesis of 1 using the [2] + [2] or [3] + [1] strategy. Using the [1] + [1] + [1] photocyclization strategy fails as well, as Liu and Katz prepared [5]helicene only in small amounts (6% yield) [63]. A possible solution to this problem was recently shown by Collins [47], who used copper(I) complexes as photocatalyst and [2] + [2] under visible light irradiation to prepare [5]helicene in 57% yield.

In the case of [6]helicene (2), several strategies have been investigated. Martin reported on [1] + [2] + [1], [2] + [1] + [1], and [4] + [1] cyclization leading to 2 with yields up to 80% [64, 65