Mechanistic Insights on the Hydration of Terminal and Internal Allenes Catalyzed by [(NHC)Au]+

The reaction mechanism for the hydration of internal and terminal allenes catalyzed by [Au­(NHC)]+ is analyzed by means of DFT calculations. Several reaction pathways for generating the two possible regioisomers were evaluated. Direct addition on coordinated allenes or to an intermediate with a σ-allylic cation structure as suggested for the Au­(I)-catalyzed hydroamination of allenes were considered. The isomerization between both regioisomeric products catalyzed by the same Au­(I) catalyst was also investigated as suggested for hydroalkoxylation of allenes. The regioselectivity of the reaction predicted by computation agrees with experiment for both terminal and internal allenes. The presence of alkyl or aryl substituents introduces differences in the reaction mechanism for the hydration process.