Lewis Acid Catalyzed Transfer Hydromethallylation for the Construction of Quaternary Carbon Centers

The design and gram‐scale synthesis of a cyclohexa‐1,4‐diene‐based surrogate of isobutene gas is reported. Using the highly electron‐deficient Lewis acid B(C6F5)3, application of this surrogate in the hydromethallylation of electron‐rich styrene derivatives provided sterically congested quaternary carbon centers. The reaction proceeds by C(sp3)−C(sp3) bond formation at a tertiary carbenium ion that is generated by alkene protonation. The possibility of two concurrent mechanisms is proposed on the basis of mechanistic experiments using a deuterated surrogate. Quat a wonderful world! A surrogate of isobutene gas based on cyclohexa‐1,4‐diene has been designed that enables the transfer hydromethallylation of alkenes under Lewis acidic conditions. The method allows for the construction of a congested quaternary carbon center and a C(sp3)−C(sp3) bond by way of a tertiary carbenium ion generated by alkene protonation.