Aspects on the Regio and Stereo Selectivity in the Hydrolsilylation of 1,3-Dienes

Abstract: This thesis is a summary of four papers concerning various aspects on the transition metal catalysed hydrosilylation of 1,3-dienes. The hydrosilylation reaction is formally an addition of a hydrosilane to an unsaturated system and depending on the reaction conditions, control of stereo-, regio-, and chemo-selectivity can be achieved. Optically active heterobidentate P,N ligands were synthesised from phenylglycine and their coordination to PdCl2(CH3CN)2 was studied by 1H, 31P and 15N-1H-HMQC NMR spectroscopy. Further, the catalytic efficiency of a PdL complex was studied in the asymmetric hydrosilylation of cyclic 1,3-dienes. Enatiomeric excess up to 84% for cyclohexenol was achieved after a hydrosilylation/oxidation sequence of 1,3-cyclohexadiene (paper I). The regioselectivity in the hydrosilylation of substituted 1,3-dienes catalysed by several rhodium complexes in the presence and absence of oxygen was studied. Beside the already known accelerating effect, the presence of oxygen strongly affected the product distribution in the hydrosilylation of 2-substituted 1,3-dienes. The head-product (II) was dominating under the O2-conditions, while under Ar-conditions; the tail-product (I) was produced in excess. At best, a I:II ratio of approximately 1:10 was achieved under the O2-conditions (paper II). Other reaction parameters such as temperature, solvent, transition-metal cocatalysts and ligands also have an effect on the regioselectivity. The ligands around the catalyst-metal was found to have the largest impact on I:II ratio. A surprising catalytic difference was observed between RhCl(CS)(PPh3)2 and RhCl(CO)(PPh3)2. The former gave a I:II ratio of 70:30, while the latter gave a I:II ratio of 15:85 (paper III). Allylsilanes can be used as effective terminator groups in electrophilic ring closure. A suitable precursor for the ring closure is an epoxy-allylsilane. The silyl unit can be introduced by a chemo-selective hydrosilylation on a functionalised diene (60%) and the following ring closure is initiated by a proper Lewis acid such as EtAlCl2 or BF3.OEt2 (50-70%) (paper IV).