An Isolable Vinylidene ortho-Quinone Methide: Synthesis, Structure and Reactivity

Commonly, an elusive intermediate is generated from a precursor and then trapped and consumed in a reaction. Vinylidene ortho-quinone methides (VQMs) have been demonstrated as transient axially chiral intermediates in asymmetric catalysis due to their orthogonal π-bonds forming an allene motif. The current understanding of VQMs is primarily based on time-resolved absorption, trapping experiments and computational studies. Herein, we report the first isolation and comprehensive characterization of a VQM, including crystallographic analysis. The disturbed aromaticity of the VQM led to its high reactivity as an electrophile or a 4π-component capable of asymmetric dearomatization of an electron-deficient phenyl group. Notably, the VQM could be isolated in enantiomerically enriched form, and the subsequent transformation was stereospecific, indicating that the generation of the VQM was involved in the enantiodetermining step. This study paves the way for the direct application of VQMs as starting materials.