A Novel Approach to Functionalized Heterohelicenes via Chromium-Templated Benzannulation Reactions

Abstract

A series of functionalized oxa- and thiaheterohelicenes has been prepared by a new progressive annulation technique based on the chromium-templated [3+2+1]benzannulation reaction of hetero[5]helicene carbene complexes with various alkynes partly in competition of angular and linear annulation. Oxidative work-up with CAN yields the respective quinones along with small amounts of nitrated products or a nitrate ester byproduct. Thermal recomplexation of an acylated monobenzannulation product by Cr(CO)6 affords the corresponding, thermodynamically favored η6-oxa[6]helicene-Cr(CO)3 complex. Bidirectional benzannulation reveals a more delicate regioselectivity and yields three different isomers including a bisquinone with an unprecedented bislinear annulation pattern. Molecular structures determined by X-ray analysis indicate a decreasing helical distortion as a result of progressive annulation, steric demand of the quinone substitution pattern (Ph>Pr>Et), or the nature of the embedded heterocycle (SO2>S>O).