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-1-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 π-allene complex (IPr)Au[η2-H2C═C═CMe2][OTf] (5·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·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.