Synthesis, Structures, and Optical Properties of Azahelicene Derivatives and Unexpected Formation of Azahepta[8]circulenes

Abstract

Polycyclic heteroaromatic compounds including pyrrole units are promising functional scaffolds owing to their electron-rich nature, bright fluorescence, and applicability to anion recognition at the pyrrolic hydrogen atom. We report herein the effective synthesis of pseudo-aza[5]helicene and aza[7]helicene derivatives, and unexpected formation of azahepta[8]circulenes by oxidative fusion reactions. By choosing reaction conditions and peripheral substituents attached at the terminal indole moieties, we obtained aza[7]helicenes 10 a-c and azahepta[8]circulenes 11 a,c selectively in moderate to good yields. Their solid-state structures have been revealed by X-ray diffraction analysis. UV/Vis absorption, emission, and cyclic voltammetry of these compounds were studied in comparison with those of previously reported tetraaza[8]circulene (TA8C), a symmetric and planar molecule. Furthermore, the enantiomeric separation of dimethyl-substituted aza[7]helicene 10 b was achieved, and the racemization kinetics have been elucidated both theoretically and experimentally. This work illustrates a wealth of advantages of pyrrole incorporation into the polycyclic aromatic scaffolds in terms of synthetic aspects, structural variation, and optical tuning.

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

“…Scheme 41. Synthesis of trisaza [7]helicenes 163a-c from 164a-c. 177 In 2012, Hiroto, Shinokubo, et al reported the selective oxidative fusion of 2 aminoanthracenes mediated by DDQ to provide pyrazine-fused bisanthracenes together with pyrrole-fused dimer 167a,b obtained as by-products and displaying an aza [5]helicenic structure with a stable helical conformation thanks to the presence of bulky ethynylsilylated groups (Scheme 42). 178 ECD and CPL activity were indeed measured for enantiopure compounds prepared by chiral HPLC resolution over a CHIRALPAK-IA column using ethyl acetate/hexane (1:10) as the eluent.…”

Synthetic Applications of Oxidative Aromatic Coupling—From Biphenols to Nanographenes

“…Here, a macrocycle consisting of four pyrrole rings connected by ortho ‐phenylene moieties was oxidatively contracted to 182 in excellent yield by the action of DDQ and scandium(III) triflate in boiling toluene. Products 183 – 185 were prepared in a similar way . Depending on the conditions and oxidant used, the bis(indolyl)helicene 183 or the closed, nonplanar circulene analogue 184 can be obtained in good yields from the same precursor.…”

“…Products 183-185 were prepared in asimilar way. [191] Depending on the conditions and oxidant used, the bis-(indolyl)helicene 183 or the closed, nonplanar circulene analogue 184 can be obtained in good yields from the same precursor.The eight-membered ring in the latter case readily forms under the same conditions as those used for the synthesis of circulene 182,whereas the preparation of its open form 183 requires the use of much milder conditions (PIFAin CH 2 Cl 2 at À78 8 8C). Thee fficiencyo ft he DDQ/Sc(OTf) 3 system for pyrrole-pyrrole intramolecular oxidative coupling has also been demonstrated by the synthesis of paracyclophane 186,which contains asegregated donor-acceptor-donor system with stacked tetrafluorobenzene rings as acceptors.…”

Facile synthesis of fluorescent hetero[8]circulene analogues with tunable solubilities and optical properties

“…Recently, we reported the synthesis of triaza-quasi-[8]circulene 12a, 11 which lacks one heterole moiety from the hetero[8] circulene structure. 12 Due to the structural constraint, 12a takes on a non-planar geometry. Interestingly, under milder conditions, triaza[7]helicene 13a was exclusively obtained from the identical precursor 11a.…”