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

Abstract

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.

Multiwavelets applied to metal–ligand interactions: Energies free from basis set errors

“…We have build our test set to include ligands of various size, many of which are common incoming ligands in metal-catalyzed reactions (such as H2, CO, alkenes, methanol). 3,4,5,6,7,8,9,10 Several features are observed from our computed results (Figure 2, see also SI, Figure S1 to S4). Firstly, the magnitude of the BSSE is largest for DZ basis sets, with an average value of 9.92 kcal/mol for the 26 kcal/mol).…”

“…1,2 Many of the studied reactions involve metal complexes that throughout the course of the reaction bind or lose a ligand, for example, there may be incoming substrates such as alkenes or hydrogen (H2), or leaving ligands such as solvent or product molecules (Scheme 1). 3,4,5,6,7,8,9,10 The difficulty of contemporary DFT functionals to accurately compute metal-ligand interactions has been highlighted in the literature. 11,12,13,14,15,16,17,18,19,20,21,22 Many of the reported studies focus on the performance of different DFT functionals and the importance of including dispersion corrections in the computed energies.…”

Gold-Catalyzed Intermolecular Alkyne Hydrofunctionalizations—Mechanistic Insights

“…An internal nucleophilic attack of a gold-precoordinated HNu has been sometimes proposed as the main alternative to this mechanism (see also below). Very recent computational analyses support this alternative mechanism in the case of the hydration of internal alkynes [12]; however, to the best of our knowledge this proposal still awaits experimental verification.…”

“…First of all, as already outlined above, π-coordinated terminal alkynes can transform into coordinated alkynyls, thereupon contributing to catalyst deactivation. However, π-coordinated terminal alkynes also undergo nucleophilic attack more quickly and form more stable vinyl intermediates [12]. Sterically more encumbered alkynes can experience difficulties in coordinating to gold, with consequent slower, less effective reaction.…”

Multiwavelets Applied to Metal-Ligand Interactions: Energies Free from Basis Set Errors

“…1,2 Many of the studied reactions involve metal complexes that throughout the course of the reaction bind or lose a ligand, for example, there may be incoming substrates such as alkenes or hydrogen (H2), or leaving ligands such as solvent or product molecules (Scheme 1). 3,4,5,6,7,8,9,10 The difficulty of contemporary DFT functionals to accurately compute metal-ligand interactions has been highlighted in the literature. 11,12,13,14,15,16,17,18,19,20,21,22 Many of the reported studies focus on the performance of different DFT functionals and the importance of including dispersion corrections in the computed energies.…”