δ-Cyano substituted para-quinone methides enable access to unsymmetric tri- and tetraarylmethanes containing all-carbon quaternary stereocenters†
para-Quinone methides bearing an electron-withdrawing cyano group at the exocyclic methylene δ-position were identified as valuable 1,6-conjugate addition building blocks for acyclic all-carbon quaternary stereocenter construction. A wide variety of electron-rich arenes as nucleophiles were tolerated, effectively furnishing diverse unsymmetrical triarylmethanes bearing all-carbon quaternary stereocenters. The robust transformable abilities of the cyano group provide a platform to access other valuable functional group-containing unsymmetrical tri- and tetraarylmethanes that are otherwise difficult to be prepared. Computational studies supported the hypothesis that the cyano group at the δ-position tunes the molecular electron-density distribution, and the stability of para-quinone methides is enhanced by lowering their polymerizability.