Comparative Mechanistic Study on the [Au(NHC)]+‑Catalyzed Hydration of Alkynes, Alkenes, and Allenes

The addition of water molecules to unsaturated substrates is a highly desirable process. Additions to alkynes are very common, whereas additions to allenes and specially alkenes are rather scarce. One of the main aims here is to perform a comparative analysis of their reaction mechanisms for the process catalyzed by Au­(I); another objective is to analyze why alkenes are much less reactive than their alkyne or allene counterparts. With this purpose the reaction mechanism for the addition of water to terminal and internal alkynes, alkenes, and allenes catalyzed by an [Au­(NHC)]+ complex (NHC = N-heterocyclic carbene) is analyzed by means of DFT calculations. The general catalytic cycle for the three kinds of substrates can be described by three main steps: (i) reactant π coordination to the Au­(I) complex, (ii) water nucleophilic addition, and (iii) protodeauration, with subtle differences among the reactants. A comparative analysis, from the evolution of the centroids of localized molecular orbitals (CLMO), of the electronic rearrangements taking place in the protodeauration step reveals different mechanisms for these three substrates, both regarding the electron pair that accepts the proton and the fate of the Au–C bond pair. For alkenes calculations show that nucleophilic addition is highly demanding but affordable, whereas protodeauration is in any case energetically prohibitive. The main reason is not the intrinsic barrier of the protodeauration step, just a few kcal mol–1 higher than that of alkynes, but the high energy of the water-added intermediate. This issue is not related to the strength of the Au­(I)–CC bond but to that of the C–O bond.