An Ultimate Stereocontrol in Asymmetric Synthesis of Optically Pure Fully Aromatic Helicenes

Šámal, Michal; Chercheja, Serghei; Rybáček, Jiří; Chocholoušová, Jana Vacek; Vacek, Jaroslav; Bednářová, Lucie; Šaman, David; Stará̈, Irena G.; Starý, Ivo

doi:10.1021/jacs.5b02794

Cited by 104 publications

(32 citation statements)

References 42 publications

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“…This nicely illustrates a point-to-helical chirality transfer utilizing a traceless chiral auxiliary. 128 Using similar strategies, the same group prepared pseudohelicenic 2,2′-bipyridines (91-95), 129 together with pseudohelicenic N-containing structures 96-98, 130 in almost diastereomerially pure forms ( Figure 14 and Table 7).…”

Section: [2+2+2] Alkyne Cyclotrimerizationmentioning

confidence: 99%

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Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P)

2019

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In this review, we discuss the rich chemistry of helicenes and helicenoids containing maingroup elements. Enantioenriched helicenic derivatives containing main-group elements B, Si, N and P, either incorporated within the helical backbone or grafted to it, will be thoroughly presented. We will describe their synthesis, resolution, and asymmetric synthesis, their structural features, electronic and chiroptical properties, emission, together with other photochemical properties and applications. OUTLINE 14 The absorption spectrum of 19c recorded in CH 2 Cl 2 solutions exhibited a maximum at 376 nm which is significantly red shifted compared with that of double [5]helicene 19a (310 nm) due to the more extended π-conjugation in 19c. According to TD-DFT calculations, the low-energy absorption band from 400 to 500 nm is assignable to the HOMO→LUMO transition corresponding to a π−π* transition of the fully conjugated bis-helical system. DFT calculations revealed a very high theoretical isomerization barrier of 45.1 kcal/mol for the (P,P)/(M,M)-19c stereoisomer, i.e. 4.4 kcal/mol higher than the (P,M) stereoisomer (not observed experimentally). The high configurational stability of 19c enabled to obtain the pure (P,P)and (M,M)-19c by chiral HPLC (Daicel Chiralcel IE column; EtOAc/MeOH = 9:1 as eluent) and to study their chiroptical properties. The ECD spectrum of (P,P)-19c in CH 2 Cl 2 displayed two strong positive bands at 275 and 320 nm, a strong negative one at 375 nm and a broad and less intense one between 425-475 nm. 52 Note that the isomerization barrier of the OBO-fused double [7]helicene is much higher than for oxabora[6]helicene 9 (vide supra) and higher than carbo[7]helicene (42.0 kcal/ mol), indicating the advantage of double helicenes over monohelicenes in terms of conformational stability. Indeed no racemization was observed upon heating enantiomer (P,P) and (M,M)-19c for 24 h up to 200 °C. 52 Note that in 2017 a longer double OBO-helicene analogue was deposited on a Au(111) under UHV conditions and its surface-assisted cyclodehydrogenation into a planar OBO-perihexacene was observed by STM. 54 Similarly to 6 and 9, the strong bond alternation found in the BOC4 rings of 19a 51 may account for the small NICS value of 1.3 while the surrounding C6 rings, including the distorted central benzene ring, show large negative NICS values (Scheme 3c). For comparison, in the all-carbon analogue tetrabenzo-[a,f,j,o]perylene shown in Scheme 3c, the central C6 ring shows a positive NICS value, indicating substantial antiaromatic character. Notably, molecular orbital calculations of 19a indicate that the HOMO and LUMO are spread over the C6 rings rather than on the boron and oxygen atoms, accounting for the substantial stability of 19a (Scheme 3c). Borahelicenes are efficient blue fluorophores. Indeed, azabora[6]helicene 6 displays blue fluorescence at 447 nm (Table 2), while smaller achiral [4]helicenic systems were successfully used as host materials in phosphorescent OLEDs with efficiencies better than the classical 4,4'...

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Section: [2+2+2] Alkyne Cyclotrimerizationmentioning

confidence: 99%

“…Stereoselective synthesis of tolyl-substituted azahelicenes. 126,128 Table 7. Specific rotation values of enantioenriched tolyl-substituted azahelicenes.…”

Section: Scheme 20mentioning

confidence: 99%

Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P)

2019

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“…This results in the one-step enantioselective synthesis of oxa [9]helicenes in good yields with up to 94% ee [111] (Scheme 36). In 2015, a new general asymmetric synthetic methodology was successfully developed for [5]-, [6]-, and [7]helicenes with ultimate enantioselectivity (ee >99) based on the controlled transfer of reactant point chirality to the product with unidirectional helicity [112]. At first, triyne207, having inbuilt point chirality, was converted into tetrahydrohelicene diastereomers 208 and 209, being in the thermodynamic equilibrium, by cobalt-catalyzed tandem [2 + 2 + 2] cycloisomerization.…”

Section: Enantiopure Helicenes: Enantioselective Synthesis Chiral Sementioning

confidence: 99%

Helicene-Based Chiral Auxiliaries and Chirogenesis

Hasan

Borovkov

2017

Symmetry

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Helicenes are unique helical chromophores possessing advanced and well-controlled spectral and chemical properties owing to their diverse functionalization and defined structures. Specific modification of these molecules by introducing aromatic rings of differing nature and different functional groups results in special chiroptical properties, making them effective chiral auxiliaries and supramolecular chirogenic hosts. This review aims to highlight these distinct structural features of helicenes; the different synthetic and supramolecular approaches responsible for their efficient chirality control; and their employment in the chirogenic systems, which are still not fully explored. It further covers the limitation, scope, and future prospects of helicene chromophores in chiral chemistry.

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“…To explore the emerging potential of helical cations and to address the issues that depend on their helical chirality, robust preparative entry to nonracemic series is essential. [39][40][41][42][43][44][45] For resolutions in general, the most popular method is the formation and separation of crystalline diastereomeric salts of enantiomers and optically active resolving agents, so called Pasteurian resolution. 43,[46][47][48][49][50] In the context of cationic helicenes, an elegant resolution of a configurationally stable [4]heterohelicenium cation has been published by Lacour et al 20 They took advantage of diastereomeric ion pairs of the helicenium with their original BINPHAT anion.…”

Section: Introductionmentioning

confidence: 99%

Dutch Resolution of a configurationally stable [5]helquat

et al. 2018

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Synthesis and nontrivial optical resolution of a helicene-like dication, helquat 1, has been accomplished. Starting with gram scale of the racemic helquat 1 sample, Dutch Resolution using family of 3 tartrate anions was key to achieve successful separation of M and P helical enantiomers of 1. Hundreds of milligrams of each enantiomer of this configurationally stable C -symmetric helquat have been obtained. Racemization barrier of 1 has been determined. To our knowledge this is the first report on Dutch Resolution performed with a helicene-like compound. Moreover, there are no literature precedents for Dutch Resolution of chiral quaternary ammonium cations.

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An Ultimate Stereocontrol in Asymmetric Synthesis of Optically Pure Fully Aromatic Helicenes

Cited by 104 publications

References 42 publications

Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P)

Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P)

Helicene-Based Chiral Auxiliaries and Chirogenesis

Dutch Resolution of a configurationally stable [5]helquat

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