Kinetics and Mechanism of the Gold-Catalyzed Intermolecular Hydroalkoxylation of Allenes with Alcohols

The mechanism of the hydroalkoxylation of 3-methyl 1,2-butadiene (1) with 1-phenylpropan-l-ol (2) catalyzed by (IPr)AuOTf in toluene has been evaluated through a combination of kinetic analysis, deuterium labeling studies, and in situ spectral analysis of catalytically active mixtures. These data are consistent with a mechanism involving endergonic conversion of (IPr)AuOTf and 1 to the cationic gold pi-allene complex (IPr)Au[eta(2)-H2C=C=CMe2]-[OTf] (5 center dot OTf), which undergoes outer-sphere addition of 2 followed by rapid protodemetalation to form 1-(3-methyl-2-butenyloxy)-propyl)benzene (3a) as the kinetic product. The microscopic rate constants associated with the formation and consumption of 5 center dot OTf are of similar magnitude, such that the kinetic behavior of catalytic hydroalkoxylation changes as a function of the relative and absolute concentrations of allene and alcohol.