Formation of Cyclopropanes via Activation of (γ-Methoxy)alkyl Gold(I) Complexes with Lewis Acids

Treatment of the gold 3-methoxy-3-phenylpropyl complex (P)­AuCH2CH2CH­(OMe)­Ph [P = P­(t-Bu)2o-biphenyl] with AlCl3 at −78°C led to the immediate (≤5 min) formation of a 4:1 mixture of phenylcyclopropane and (1-methoxypropyl)­benzene in 86 ± 5% combined yield. Lewis acid activation of the stereochemically pure isotopomer erythro-(P)­AuCH2CHDCH­(OMe)­Ph led to the formation of cis-2-deuterio-1-phenylcyclopropane in 84 ± 5% yield as a single stereoisomer, which established that cyclopropanation occurred with inversion of the γ-stereocenter. Similarly, ionization of the stereochemically pure cyclohexyl gold complex cis-(P)­AūCHCH2CH­(OMe)­CH2CH2̄CH2 at −78°C formed bicyclo[3.1.0]­hexane in 82% ± 5% yield, which validated a low energy pathway for cyclopropanation involving inversion of the α-stereocenter. Taken together, these observations are consistent with a mechanism for cyclopropane formation involving backside displacement of both the Cγ leaving group and the Cα (L)­Au+ fragment via a W-shaped transition state.