Asymmetric Catalysis on the Nanoscale: The Organocatalytic Approach to Helicenes

Kötzner, Lisa; Webber, Matthew J.; Martínez, Alberto; Fusco, Claudia De; List, Benjamin

doi:10.1002/anie.201400474

Cited by 115 publications

(41 citation statements)

References 49 publications

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“…In 2014, List and coworkers reported the first asymmetric organocatalytic synthesis of helicenes, using a SPINOL-derived phosphoric acid bearing extended π-substituents as the chiral Brönsted acid asymmetric catalyst. 175 Upon condensation of a phenylhydrazine with an appropriate polyaromatic ketone, enantiopure Brönsted acid 157 might promoted asymmetric [3,3] sigmatropic rearrangement and furnished enantioenriched pseudo-helical (tetrahydro-helicene) 155a and its corresponding azahelicenes 156a after oxidation. The approach appeared modular and a variety of azahelicenes (155a-e) could be obtained with good to excellent yields and enantioselectivities (see Scheme 39 and Table 12).…”

Section: Enantioselective Fischer Indolizationmentioning

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: Enantioselective Fischer Indolizationmentioning

confidence: 99%

“…Extended π-substituents in the 3,3'position were needed for π−π stacking interaction with the polyaromatic system present in the formed enehydrazine, holding the intermediate in a chiral nanometer-sized pocket, and the catalyst could induce the screw sense of the helicene. 175 175 .…”

Section: Enantioselective Fischer Indolizationmentioning

confidence: 99%

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

2019

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“…The paper highlights that the amination site is mainly determined by electronic effects, and the reaction is supposed to occur at the carbon adjacent to the electron-rich aryl group owing to a carbocation intermediate involved in the process. A fascinating application of chiral BA catalysis is the synthesis of helicenes applying the Fischer indolization ( Figure 16c) [87]. The helical chirality strongly attracts attention in very diverse fields, such as material sciences, catalysis and biology.…”

Section: Organocatalysis and Non-covalent Interactions Activation Modmentioning

confidence: 99%

Organocatalysts for enantioselective synthesis of fine chemicals: definitions, trends and developments

Palumbo

2015

ScienceOpen Research

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“…The paper highlights that the amination site is mainly determined by electronic effects and the reaction is supposed to occur at the carbon adjacent to the electron-rich aryl group owing to a carbocation intermediate involved in the process. A fascinating application of chiral BA catalysis is the synthesis of helicenes applying the Fischer indolisation (Figure 16c) [82]. The helical chirality strongly attracts attention in very diverse fields, such as material sciences, catalysis and biology.…”

Section: Brønsted Acid Catalysismentioning

confidence: 99%

Organocatalysts for enantioselective synthesis of fine chemicals: definitions, trends and developments

Federsel¹

2014

ScienceOpen Research

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Organocatalysis, that is the use of small organic molecules to catalyse organic transformations, has been included among the most successful concepts in asymmetric catalysis and it has been used for the enantioselective construction of C-C, C-N, C-O, C-S, C-P, and C-halide bonds. Since the seminal works in early 2000, the scientific community has been paying an ever-growing attention to the use of organocatalysts for the synthesis, with high yields and remarkable stereoselectivities, of optically active fine chemicals of interest for the pharmaceutical industry. A brief overview is here presented about the two main classes of substrate activation by the catalyst: covalent organocatalysis and non-covalent organocatalysis, with a more stringent focus on some recent outcomes in the field of the latter and of hydrogen-bond-based catalysis. Finally, some successful examples of heterogenisation of organocatalysts are also discussed, in the view of a potential industrial exploitation.

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Asymmetric Catalysis on the Nanoscale: The Organocatalytic Approach to Helicenes

Cited by 115 publications

References 49 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)

Organocatalysts for enantioselective synthesis of fine chemicals: definitions, trends and developments

Organocatalysts for enantioselective synthesis of fine chemicals: definitions, trends and developments

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