The Mechanism Of Transition Metal-catalyzed Dehalogenative Si-C Coupling Reaction

Arylsilanes are not only common intermediates for carbon-carbon bonds formation in modern organic synthesis but also important building block of organic photoelectric materials. For transition-metal catalyzed dehalogenative Si-C coupling reaction of hydrosilane with aryl halide, the main difficulty lies in that the chemoselectivity of silylation to reduction was relative poor. Although important progress have obtained experimentally, the reaction mechanism is still not clear, which will prevent the furhter improvement in the catalyst design apsect.The reaction mechanism of the transition metals(Pd and Pt) catalyzed Si-C coupling reaction have been studied by means of density functional theory(DFT) method. Density functional B3 LYP in combination with basis set scheme with LanL2 DZ for Pt, Pd, Br and I atoms and 6-31g(d, p) for other element was used for all the calculations. The potential energy surface invovled by 10 possible reaction paths, including different initiation mode, different types of tranmetallation manner and intermediates PdIV/PtIV have been taken into consideration. The preferred reaction path has been determined. The effect of substituent on two substrates, the status of catalyst have also been discussed in relation to the experimental facts.For palladium catalytic system, the catalytic cycle initiated by oxidative addition of monoligated Pd0 species(Pd0PMe3) to aryl halide C-X was found be favorred over that to hydrosilane Si-H bond, which has been proposed as the first step in previous studies. The tranmetallation step starting from intermediates PdII(X)(Ar) compounds was found to be both the rate-determining and chemoselectivity-determining step. The reaction barrier for this step was found to be strongly affected by the substituent on aryl ring of aryl halide. The reaction path involving intermediate PdIV has been excluded for it is quite unstable.For platinum catalytic system, the prefered initiation manner was similar to that of palladium catalytic system. The main difference lies in that the intermediate PtIV was qutie stable, whose energy is even lower than the initial reactants about 8 kcal/mol, and the reaction path involved by intermediate PtIV seems to be the preferred reaction path. The reductive eliminating starting from intermediate PtIV is the rate-determining step. In addition to that, the effect of substituents on aryl halide to this rate-determing step accrod well with the experimental facts, which give further support for this reaction pathway.