Helicity-modulated remote C-H functionalization

Abstract

Remote C-H functionalization is highly important for the conversion and utilization of arenes, but the conventional routes are comprehensively developed with the assistance of transition metal catalysts or templates. We report a facile metal/template-free electrochemical strategy for remote C-H functionalization in a helical system, where aromatic or aliphatic hydrogen act as a directing group to promote the alkoxylation at the opposite site of the helical skeleton by generating a unique helical “back-biting” environment. Such helicity-modulated C-H functionalization is prevalent for carbo[ n ]helicenes ( n = 6 to 9, primitive or substituted) and hetero[6]helicenes and also occurs when the aryl hydrogen on the first position is replaced by a methyl group or a phenyl group. Thus, the relatively inert helicene skeleton can be precisely furnished with a rich array of alkoxy pendants with tunable functional moieties. Notably, the selective decoration of a methoxy group on N -methylated aza[6]helicene close or distant to the nitrogen atom leads to distinct luminescence variation upon changing the solvents.

Oxidation of a triple carbo[5]helicene with hypervalent iodine

“…19 Lately, the electrochemical methoxylation of carbo[n]helicenes (n = 6–9) was realized in 61–85% yield. 20 The critical factor for the functionalization of helicenes is the regioselectivity. Because of the lack of site-specific methods to functionalize carbohelicenes, the community has instead relied on the synthesis of pre-functionalized analogues, incorporating alkoxyl groups, bromine atoms, and alkynes as substituents at various positions.…”