Abstract
The chemical reduction of Pt(acac)~2~ by DIBALH in the presence of phosphanes, which is used to generate active Pt^0^ complexes in the Pt‐catalyzed silaboration of cyclohexadiene by 2‐(dimethylphenylsilyl)‐4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane (1) leading to the 1,4‐silaborated product, was mimicked by the electrochemical reduction of Pt(acac)~2~ in the presence of 2 equiv. of PR~3~ (R = Ph, __n__Bu). The electrochemical reduction generates free acac anions and neutral Pt^0^(PR~3~)~2~ complexes. The kinetics of the oxidative addition of bromobenzene (used first as a model molecule) and silylborane 1 to the Pt^0^ complexes was investigated and the rate constants determined. Pt^0^(P__n__Bu~3~)~2~ is much more reactive than Pt^0^(PPh~3~)~2~ towards 1. From the electrochemical study, it emerges that the acac anions released in the reduction of Pt(acac)~2~ do not coordinate to the Pt^0^(PR~3~)~2~ complexes. Consequently, the rate of the oxidative addition of 1 to Pt^0^(PR~3~)~2~, generated either by the electrochemical reduction or by the chemical reduction by DIBALH, is not affected by the acac anions and a posteriori not by aluminum cations. The oxidative addition and the further step of the catalytic cycle [insertion of the diene into the Pt–B bond of the Si–Pt–B complex generated in the oxidative addition, with formation of the (η^3^‐allyl)Pt–Si complex] were monitored by NMR spectroscopy. Pt^0^ and Pt^II^ complexes involved in the catalytic cycle were characterized. The oxidative addition is faster when the ligand is PMe~2~Ph relative to that obtained with PPh~3~, in agreement with the electrochemical data. No reductive elimination within the (η^3^‐allyl)Pt–Si complex is observed when the ligand is PMe~2~Ph, whereas reactions in the presence of PPh~3~ proceeded to give the final product. As a consequence, PPh~3~ is a better ligand than PMe~2~Ph for the catalytic reaction, as observed experimentally. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)