The effect of oxygen on the regioselectivity in the rhodium catalysed hydrosilylation of 1,3-dienes

The regioselectivity of the hydrosilylation of substituted 1,3-dienes catalysed by several rhodium complexes in the presence and absence of oxygen was studied. In addition to the already known accelerating effect, the presence of oxygen strongly affected the product distribution. For 2-substituted 1,3-dienes in the presence of oxygen the regioselectivity was in the range of 1 ∶ 6 to 1 ∶ 10 in favour of the head-product, while the absence of oxygen changed the ratios to 1 ∶ 1 to 3 ∶ 1 in favour of the tail-product. When HSiPh3 was used in the presence of oxygen a single isomer was isolated in 87% yield, while in the absence of oxygen a mixture of products was produced. Control experiments indicated that a heterogeneous/colloidal catalytic system may be responsible for the preferred head-product formation.