Rate and Mechanism of the Oxidative Addition of a Silylborane to Pt 0 Complexes – Mechanism for the Pt‐Catalyzed Silaboration of 1,3‐Cyclohexadiene

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)