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

Abstract

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'...

Phosphahelicenes with (Thio)Phosphinic Acid and Ester Functions by the Oxidative Photocyclisation Approach

“…Although helicenes have attracted attention for more than one hundred years now, their chemistry still represents a hot topic, due to their peculiar physicochemical and optoelectronic properties and applications, as well as to the chirality related phenomena displayed by these inherently chiral polyaromatic compounds . A common strategy to broaden and modulate the structural features and properties of helicenes is the incorporation of heterocyclic units in their backbones . Thus, for instance, nitrogen and phosphorus heterocycles have been incorporated, which opened the way, notably, to new applications in coordination chemistry, organometallic catalysis, and organocatalysis .…”

Synthesis, Structure, and Complexation of an S‐Shaped Double Azahelicene with Inner‐Edge Nitrogen Atoms

“…To obtain furtheri nformation on the behavior of 16 in solution, 1 HNMR measurements were carried out. However, 16 is insoluble in most conventional organic solvents (e.g.,c hloroform, tetrahydrofuran, ethyl acetate, and methanol), and sparingly soluble only strongly coordinating solvents such as DMSO andD MF.A las, such strongly coordination solvents lead to ag radual dissociation of the ligand to give palladium-free dl-1 within af ew hours.…”

“…Helicenes exhibitascrew-shaped p-conjugated structure that endows these compounds with unique opticala nd electronic properties. [1] Beyond the realm of carbohelicenes, interest has recently intensified on heteroatom-containing helicenes or analogousd erivatives as chiral building blocks for functional molecules. Amongt hese compounds,a zahelicenes, whichc ontain nitrogen atoms in the carbonaceoush elicene framework, have been recognized as promising prospective ligands and organocatalysts due to their efficient chiral environment.…”

Chiral Diketopyrrolopyrrole-Helicene Polymer With Efficient Red Circularly Polarized Luminescence

“…Fully characterized by NMR spectroscopy and mass spectrometry (see Supplementary Information), the structures of the two latters were further ascertained by X-ray crystallographic analysis (see Scheme 1 and Supplementary Information). Both rac-4 and rac-H6(Alkoxy) 2 crystallized in a P-1 space group and displayed helicity (dihedral angle between the two terminal rings) of 41.1 • and 54.4 • , respectively, which is in the range of classical carbo[6]helicenes (Gingras, 2013;Chen and Shen, 2016;Dhbaibi et al, 2019). Moreover, the lateral octyloxy chains point toward the outside of the molecules, which may disfavor the formation of polymer aggregates in solution.…”