Alkyne-Assisted Stereospecific Skeletal Rearrangement via Nonclassical Carbocations.

We report stereospecific nucleophilic substitutions of homopropargylic alcohols enabled by anchimeric assistance from an alkyne. Upon activation, neighboring-group participation of the alkyne generates a nonclassical cyclopropylidenemethyl cation. The transformation provides access to densely-substituted tertiary homopropargyl fluorides with broad functional-group tolerance and efficient chirality transfer from enantioenriched alcohols. Computational studies support the involvement of the nonclassical cationic intermediate. The calculations further indicate that alkyne-derived nonclassical cations are less stabilized than their alkene analogues.