Regioselective hydrosilylation of 1,3-dienes catalyzed by phosphine complexes of palladium and rhodium

The hydrosilylation of isoprene catalyzed by a palladium complex using chlorohydrosilanes proceeded via 1,4-addition to give Z-2-methylbuten-2-yl-silanes exclusively, the stereochemistry of which was determined on the basis of NMR spectroscopy observing the nuclear Overhauser effect (NOE). The result clearly demonstrates that the reaction is highly regioselective and stereo-selective. On the other hand, the reaction catalyzed by a rhodium complex afforded 3-methyl-buten-2-ylsilane as major product accompanied by Z-2-methylbuten-2-ylsilane, i.e., the regioselectivity of the reaction is opposite to that of other catalyst systems. Similar regioselectivities were observed in the hydrosilylation of myrcene and ocimene, although a considerable substituent effect was apparent, especially in the case of ocimene. Possible mechanisms of these reactions are proposed.