Highly Stereoselective <i>Gem</i> ‐Dimethylcyclopropanation of Allylic Alcohols Promoted by Alkylidene Titanocenes

Abstract A novel methodology for the direct gem ‐dimethylcyclopropanation of a series of acyclic and cyclic primary and secondary allylic alcohols is herein described. The reaction proceeds in a regio‐, chemo‐, and stereoselective manner, affording the corresponding gem ‐dimethylcyclopropane derivatives in moderate yields. We postulate that an alkylidene titanocene, generated in situ from sub‐stoichiometric amounts of low‐valent titanium species, Mg and 2,2‐dichloropropane under mild reaction conditions, serves as the active species. The scope, limitations and a plausible mechanism of this metal‐promoted transformation are also presented.