Ti(III)-Mediated Selective α-α' Homocoupling of Allylic Epoxides: A Direct Route to Symmetrical Homoallylic 1,4-Diols.

Titanocene(III)-mediated opening of allylic epoxides is commonly associated with γ-coupling processes rationalized through allyltitanium intermediates. Herein, we show that the regioselectivity of this process is strongly dependent on the substitution pattern of the allylic epoxide. Minimally substituted allylic epoxides undergo highly selective α-α' homocoupling under Cp2TiCl2/Mn conditions to afford symmetrical homoallylic 1,4-diol derivatives, with no detectable formation of γ-γ' or γ-α' coupling products. The absence of cyclization products in substrates bearing pendant alkenes capable of intramolecular radical trapping, together with the unchanged product distribution observed in the presence of 1-methyl-1,4-cyclohexadiene, is consistent with the involvement of organotitanium intermediates.