Mechanisms of syn -insertion of alkynes and allenes into gold-silicon bonds: A comprehensive experimental/theoretical study
Résumé
A detailed mechanistic study is reported for the syn-insertion of alkynes and allenes in the Au-Si bonds of complexes (R3P)Au-SiR′Ph 2 (R = Ph, Me and R′ = t-Bu, Ph). Kinetic experiments indicate that (i) the reaction is first-order in alkyne and gold silyl complex and (ii) it requires a rather low enthalpy of activation and a relatively large negative entropy of activation [ΔH‡ = 13.7 (±1.6) kcal·mol-1 and ΔS‡ = -32.0 (±5.0) cal·mol-1·K-1 for the reaction of (Ph3P)Au-Si(t-Bu)Ph2 with methyl propiolate], in line with a bimolecular associative transformation. The different mechanistic pathways have been explored by DFT calculations. Accordingly, the reaction is found to proceed via a two-step inner-sphere mechanism: (i) first, the alkyne coordinates to the gold silyl complex to form a π-complex; (ii) the subsequent migratory insertion step is rate determining and occurs in a concerted manner. Provided dispersion effects are taken into account (B97D functional), the enthalpy of activation estimated theoretically [ΔH ‡ = 11.5 kcal·mol-1] is in good agreement with that measured experimentally. The influence of the π substrate (methyl propiolate, dimethyl acetylene dicarboxylate, phenyl acetylene, ethyl 2,3-butadienoate) has been analyzed theoretically, and the regioselectivity of the insertion has been rationalized. In particular, the unexpected selectivity observed experimentally with the allene is shown to result from the insertion of the terminal nonactivated C=C double bond into the Au-Si bond of (Ph 3P)Au-SiPh3, followed by an original isomerization (Au/Si exchange process). This study provides unambiguous evidence for coordination-insertion at gold and thereby strongly supports the possible occurrence of inner-sphere mechanisms during the functionalization of alkynes and allenes. © 2014 American Chemical Society.