Surprisingly Difficult Resolution of <i>N</i>‐Methylated Cationic [4]Helicenes

Mehanna, Nathalie; Grass, Stéphane; Lacour, Jérôme

doi:10.1002/chir.22032

Cited by 7 publications

(5 citation statements)

References 55 publications

(86 reference statements)

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“…258 However, the efficiency of this strategy appeared to be dependent on the nitrogen substituents, especially when Nmethyl side chain(s) are used, resulting in poor diastereomeric separation of the corresponding sulfoxide derivatives on silica gel. 259 While the two chemical diastereomeric resolution approaches mentioned above afforded very high level of enantiopurity for [4]helicenium derivatives with high quantities of carbocationic helicenes (thus enabling reactivity and property studies), a direct and precise determination of enantiomeric purity by chiral chromatography was still problematic due to their ionic nature. 260 To extend the structural diversity of [4]helicenium derivatives, the same research group started to investigate other heteroatoms than nitrogen ones for the S N Ar substitutions reactions.…”

Section: Chemical Reviewsmentioning

confidence: 99%

“…This nonclassical Pummerer bond fragmentation mechanism was further investigated to rationalize the proposed rearrangement pathway (Scheme ), which was found to be directly related to the high stability of the helical carbocation . However, the efficiency of this strategy appeared to be dependent on the nitrogen substituents, especially when N -methyl side chain(s) are used, resulting in poor diastereomeric separation of the corresponding sulfoxide derivatives on silica gel . While the two chemical diastereomeric resolution approaches mentioned above afforded very high level of enantiopurity for [4]helicenium derivatives with high quantities of carbocationic helicenes (thus enabling reactivity and property studies), a direct and precise determination of enantiomeric purity by chiral chromatography was still problematic due to their ionic nature.…”

Section: Helicenes Substituted With Nitrogenmentioning

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: Chemical Reviewsmentioning

confidence: 99%

Section: Helicenes Substituted With Nitrogenmentioning

confidence: 99%

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

2019

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“…The resulting adducts (e.g., 135.10 and 135.11 ) retained the sense of helicity present in the cation. The addition of enantiopure sulfoxide anions yielded diastereomeric products 135.11 , which could be resolved and converted back to the diazahelicenium cations in an unusual type of Pummerer rearrangement. − Diazahelicenium cations unsymmetrically substituted with two different R′ groups reacted with hydride and organolithium nucleophiles in a highly diastereoselective manner (dr > 49) . Reduction of 135.3b with sodium borohydride yielded the neutral helicene 135.14 .…”

Section: Phenalenoidsmentioning

confidence: 99%

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

et al. 2016

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Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

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“…The dye properties have been investigated, and a high degree of solvent-sensitive behaviour was observed. [7] The [4]helicenium ions can be resolved into enantiomers, [8] owing to the methoxy groups in positions 1 and 13. Whereas non-substituted [4]helicene racemises almost instantaneously, the insertion of two methoxy groups locks the structure and raises the barrier of inversion to 130-175 kJ mol À1 .…”

Section: Introductionmentioning

confidence: 99%

Synthesis and Stability of N,N′‐Dialkyl‐1,13‐dimethoxyquinacridinium (DMQA⁺): A [4]Helicene with Multiple Redox States

2014

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N,N′‐Dialkyl‐1,13‐dimethoxyquinacridinium (DMQA+) is a configurationally locked [4]helicene, a stable carbenium ion and a potent near‐infrared emitter. The versatile synthesis and resolution into M and P enantiomers has made DMQA+ and its congener helicenium derivatives attractive chiral dyes and building blocks. In the present study, electrochemical and spectroscopic studies showed that several redox states of DMQA+ are accessible and stable. The cation stability towards nucleophiles is determined and it is shown that the addition of OH− proceeds with a surprisingly low rate of reaction. The synthetic methodologies leading to DMQA+ are described, and the chemistry occurring from the different redox states of DMQA is identified. This [4]helicene is shown to be a unique molecular framework that is stable as a neutral radical and very stable as a carbenium ion.

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Surprisingly Difficult Resolution of N‐Methylated Cationic [4]Helicenes

Cited by 7 publications

References 55 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)

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Synthesis and Stability of N,N′‐Dialkyl‐1,13‐dimethoxyquinacridinium (DMQA⁺): A [4]Helicene with Multiple Redox States

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Surprisingly Difficult Resolution of N‐Methylated Cationic [4]Helicenes

Cited by 7 publications

References 55 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)

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Synthesis and Stability of N,N′‐Dialkyl‐1,13‐dimethoxyquinacridinium (DMQA+): A [4]Helicene with Multiple Redox States

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Synthesis and Stability of N,N′‐Dialkyl‐1,13‐dimethoxyquinacridinium (DMQA⁺): A [4]Helicene with Multiple Redox States