The quest for alternative routes to racemic and nonracemic azahelicene derivatives

Andronova, Angelina; Szydlo, Florence; Teplý, Filip; Tobrmanová, Miroslava; Volot, Amandine; Stará̈, Irena G.; Starý, Ivo; Rulı́s̆ek, Lubomı́r; Šaman, David; Cvačka, Josef; Fiedler, Pavel; Vojtı́šek, Pavel

doi:10.1135/cccc2008180

Cited by 16 publications

(7 citation statements)

References 50 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%

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%

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

2019

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“…As there is a close structural resemblance between oxahelicenes and parent carbohelicenes, oxahelicenes represent attractive helical heteroaromatics that can also be obtained in an optically pure form by asymmetric synthesis (vide infra). Analogously, the incorporation of the nitrogen or silicon atom(s) into the triyne tether(s) affords, after the intramolecular Co I -, Rh I -, or Ir I -catalyzed [2 + 2 + 2] cycloisomerization, partially saturated azahelicenes (Stará, Starý et al, Carbery and co-workers , ) or silahelicenes (Shibata et al, 66 + 67 → 69 → 70 , Scheme C), respectively.…”

Section: The Synthesis Of Helicenes By [2 + 2 + 2] Cycloisomerization...mentioning

confidence: 99%

Helically Chiral Aromatics: The Synthesis of Helicenes by [2 + 2 + 2] Cycloisomerization of π-Electron Systems

2019

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CONSPECTUS: Advanced molecular nanocarbons are now in the spotlight reflecting the basic discoveries of fullerenes, carbon nanotubes, and graphene. This research area includes also the chemistry, physics, and nanoscience of nonplanar polycyclic hydrocarbons, many of which exhibit helical chirality, such as iconic helicenes and their congeners. The combination of unique π-electron systems with the chirality phenomenon makes them highly attractive in various fields of science. Helicenes are polyaromatic compounds that are composed of all-angularly annulated benzene units, but other (hetero)cycles can also be embedded into their backbone. Even though they do not contain any stereogenic center, they are inherently chiral owing to the helical shape they adopt. Hexahelicene and higher homologues are conformationally stable within a reasonable range of temperatures and, therefore, can be obtained in an enantiopure form through a racemate resolution or asymmetric synthesis. An amazing array of synthetic methods for their preparation has been developed, but only a few of them have passed the tough scrutiny to be general, robust and practical methods such as traditional photocyclodehydrogenation of diaryl olefins and recently developed transition-metal-catalyzed [2 + 2 + 2] cycloisomerization of π-electron systems, which is discussed in this Account. Alkyne [2 + 2 + 2] cycloisomerization is a highly exergonic process and is therefore suitable for forming the strained helicene backbone, three (or more) cycles of which are closed in a single operation. The typical starting materials are aromatic triynes (optionally cyanodiynes or ynedinitriles) or tetraynes with diynes that undergo intramolecular or intermolecular cyclization, respectively, catalyzed by various complexes mainly of Ni 0 , Co I , or Rh I . Utilizing this synthetic methodology, various [5]-, [6]-, [7]-, [9]-, [11]-, [13]-, [16]-, [17]- , and [19]helicenes or their congeners, including functionalized derivatives, can be effectively prepared. Moreover, asymmetric synthesis (both catalytic and stoichiometric) of nonracemic helicenes has already been demonstrated. It relies on [2 + 2 + 2] cycloisomerization of centrally chiral triynes followed by an asymmetric transformation of the first order (controlled by the 1,3-allylic-type strain) or on enantioselective [2 + 2 + 2] cycloisomerization of alkynes catalyzed by chiral complexes mainly of Ni 0 or Rh I . Intriguingly, advanced helical architectures were formed such as the longest helicenes (up to oxa [19]helicene by closing 12 rings in a single synthetic operation) or laterally extended helicenes (e.g., pyreno[7]helicenes). Utilizing the aforementioned synthetic methodology, the tailor-made helical molecular nanocarbons are now better accessible to be applied in enantioselective catalysis, chirality sensing, spintronics (based on chirality induced spin selectivity), chiroptics (to produce circularly polarized light emission), organic/molecular electronics, or chiral single molecule devices.

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“…The most widely used method for the enantioselective synthesis of carbohelicenes is the transition‐metal‐catalyzed [2+2+2] cycloaddition [4,6] . However, this reaction has not been applied to the enantioselective synthesis of azahelicenes and azahelicene‐like molecules, although the diastereoselective synthesis of aza[5] and [6]helicene‐like molecules, as represented by C , [3d] was reported by Starý and Stará (Figure 1a) [3d–g] …”

Section: Introductionmentioning

confidence: 99%

Rhodium‐Catalyzed Enantioselective Synthesis, Structures, and Properties of Single and Double Azahelicene‐Like Molecules

Hanada

Nogami

Miyamoto

et al. 2021

Chemistry A European J

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The enantioselective synthesis of aza [6] and [7] helicene-like molecules have been achieved by the cationic rhodium(I)/axially chiral biaryl bisphosphine complex-catalyzed intramolecular [2 + 2 + 2] cycloaddition of cyanodiynes. This protocol was successfully applied to the diastereo-and enantioselective synthesis of an S-shaped double aza [6] helicene-like molecule with a high ee value of 89 %. Although no epimerization and racemization were observed in the double carbo [6]helicene-like molecule at 80°C, epimerization and racemization of the double aza[6]helicene-like molecule proceeded at 80°C. This double aza [6]helicene-like molecule showed good fluorescent quantum yields and chiroptical responses under both neutral and acidic conditions.

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The quest for alternative routes to racemic and nonracemic azahelicene derivatives

Cited by 16 publications

References 50 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)

Helically Chiral Aromatics: The Synthesis of Helicenes by [2 + 2 + 2] Cycloisomerization of π-Electron Systems

Rhodium‐Catalyzed Enantioselective Synthesis, Structures, and Properties of Single and Double Azahelicene‐Like Molecules

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