Stereocontrolled Synthesis of Chiral Heteroaromatic Propellers with Small Optical Bandgaps

Abstract

Chiral heteroaromatic propellers based on radially p-extended hexapyrrolohexaazacoronenes were obtained in ac oncise synthesis from suitably functionalizedd onoracceptor monopyrroles.T oo vercome steric hindrance,anew cyclodehydrogenation method was developed, and it uses bromine electrophiles as oxidative coupling agents instead of the commonly employed high-potential oxidants.T he new reaction offers high yields of propeller-shaped targets,even for electron-deficient precursors,a nd shows electrophile-dependent stereoselectivity,w ith N-bromosuccinimide and dibromine yielding,r espectively D 6 -a nd C 2 -symmetric products. The propeller azacoronenes are chiral and can be separated into configurationally stable enantiomers.I na ddition to providing steric bulk, peripheral functionalization considerably affects the electronic properties of the propellers,w hich exhibit reduced optical and electrochemical band gaps,a nd amore clearly defined electroreduction behavior.

Aromatic Nanosandwich Obtained by σ-Dimerization of a Nanographenoid π-Radical

“…CD spectra of the enantiomers showed significantly lower intensities than those of the related propeller-shaped HPHACs. 37 Voltammetric analyses of 4 2 showed two oxidations at 0.60 and 0.85 V and at least eight reduction events in the −1.37 to −1.99 V potential range (vs Fc/Fc + , cf. Figures S7–S10 ).…”

“…This feature resembles the monkey-saddle distortion of the original NMI-HPHAC system, 36 which displayed alternating helicities of the bay regions around the HPHAC core, with a ( P , M , P , M , P , M ) relative stereochemistry. 37 In 4 2 , each subunit has only five regions with defined helicity, because the sixth bay is expanded by the inserted sp 3 bridge. Relative helicities for the front subunit of 4 2 (as oriented in Figure 1 , clockwise) are ( P , P , M , P , M ), whereas the back subunit has a different stereochemistry, namely, ( P , M , P , P , M ).…”

“…This radical is derived from hexapyrrolohexaazacoronene (HPHAC), 30 a versatile platform for structural modification that has so far been transformed by subunit replacement, 31 , 32 edge expansion, 32 − 34 core replacement, 35 peripheral fusion, 36 , 37 and helical distortion. 37 These alterations have produced π-conjugated systems exhibiting variable emission behavior, 31 susceptibility to multiple oxidation 33 or reduction, 36 , 37 and tunable aromaticity. 32 , 34 The present design combines radial fusion of electron-deficient subunits with edge expansion, to produce an odd-electron donor–acceptor π system, which undergoes spontaneous σ-dimerization.…”

A Nitrogen‐Doped Hexapole [7]Helicene versus Its All‐Carbon Analogue

“…Herein, we present ap air of propeller-shaped NGs: [8][9][10] the nitrogen-doped hexapole [7]helicene (H7H) N-H7H [11,12] and its all-carbon analogue C-H7H. N-H7H and C-H7H share acommon hexabenzocoronene (HBC) platform, to which six dibenzoullazine (DBU) [13] or dibenzopyrene (DBP) blades are attached at the periphery in ahelical structure ( Figure 1).…”

Synthesis and Properties of NitroHPHAC: The First Example of Substitution Reaction on HPHAC

“…Due to the pyrrole moieties, on the other hand, multiple oxidation levels of HPHAC are reversibly generated, and its dication shows global aromaticity owing to the cyclic conjugation of a 22π-electron system. So far, HPHAC-hexabenzocoronene-hybridized [18], ethylene-bridged [19], periphery-expanded [20,21], core-expanded [16], azulene-fused [22], β,β-thieno-fused [23], chiral [24], antiaromatic [25], and σ-dimerized [26] types of its analogues have been reported. However, a peripheral substitution reaction of HPHAC has never been reported yet.…”