Regioselective Catalytic Transformations Involving β-Silyl-Substituted (η3-Allyl)palladium Complexes:  An Efficient Route to Functionalized Allylsilanes

Various alkyl derivatives of 1-(trimethylsilanyl)but-3-en-2-ol acetate (1a−e) undergo regioselective palladium-catalyzed nucleophilic substitution via β-silyl-substituted (η3-allyl)palladium intermediates. With external nucleophiles, such as malonates and enolates, the nucleophilic substitution occurs with complete allylic rearrangement, providing functionalized allylsilanes as building blocks of high synthetic potential. Internal nucleophiles, such as disilanes and NaBPh4, afford bisallylic disilanes and (allylsilyl)benzene derivatives with good regioselectivity. For both types of nucleophiles, the double bond geometry of the resulting allylsilane is selectively trans. The β-silyl-substituted (η3-allyl)palladium intermediates of the reaction were also isolated. The 1H NMR studies indicate selective formation of the syn-isomer of the key (η3-allyl)palladium intermediates, which explains the high trans-selectivity of the double bond formation in the allylsilane products. According to the 13C NMR studies, the β-silyl functionality exerts deshielding effects on the nearest allylic terminal carbon (C3), which can be ascribed to hyperconjugative interactions between the silyl functionality and the allylpalladium moiety. It was concluded that, together with the steric effects of the silyl group, these electronic interactions are responsible for the high regioselectivity of the nucleophilic attack in the catalytic process.