Photochemical Intramolecular C−H Addition of Dimesityl(hetero)arylboranes through a [1,6]‐Sigmatropic Rearrangement

A new reaction mode for triarylboranes under photochemical conditions was discovered. Photoirradiation of dimesitylboryl‐substituted (hetero)arenes produced spirocyclic boraindanes, where one of the C−H bonds in the ortho ‐methyl groups of the mesityl substituents was formally added in a syn fashion to a C−C double bond of the (hetero)aryl group. Quantum chemical calculations and laser flash photolysis measurements indicated that the reaction proceeds through a 1,6]‐sigmatropic rearrangement. This behavior is reminiscent of the photochemical reaction mode of arylalkenylketones, thus demonstrating the isosteric relation between tricoordinate organoboron compounds and the corresponding pseudo‐carbocationic species in terms of pericyclic reactions. Despite the disrupted π‐conjugation, the resulting spirocyclic boraindanes exhibited a characteristic absorption band at relatively long wavelengths (λ =370—400 nm).