RH4(CO)12, CO2RH2(CO)12, AND CO3RH(CO)12 AS EFFECTIVE CATALYSTS FOR HYDROSILYLATION OF ISOPRENE, CYCLOHEXANONE, AND CYCLOHEXENONE

Hydrosilylation of isoprene, cyclohexanone, and cyclohexenone catalyzed by Rh4(CO)12, Co2Rh2(CO)12, and Co3Rh(CO)12 is studied. Their catalytic activities are compared with those of RhCl(PPh3)3 and Co2(CO)8, and it is found that these Rh and Co-Rh carbonyl clusters are excellent new catalysts for the reactions. The reaction of isoprene catalyzed by those carbonyl clusters is highly regioselective and extremely stereoselective to give (Z)-(2-methylbut-2-en-1-yl)dimethylphenylsilane as the predominant product (88-94%). The reactions of cyclohexanone with HSiEt3 catalyzed by Rh4(CO)12 and Co2Rh2(CO)12 give (cyclohexyloxy)triethylsilane exclusively, while the reactions with HSiMe2Et and H2SiPh2 afford a mixture of (cyclohexyloxy)silane (major) and (cyclohex-1-en-1-yloxy)silane (minor). The reaction of cyclohexenone with HSiMe2Ph catalyzed by these Rh and Co-Rh carbonyl clusters proceeds smoothly at ambient temperature to give the 1,4-addition product (cyclohex-1-en-1-yloxy)dimethylphenylsilane exclusively. On the other hand, the reaction with H2SiPh2 proceeds at -35-degrees-C to give the 1,2-addition product (cyclohex-2-en-1-yloxy)diphenylsilane exclusively, which upon desilylation affords cyclohex-2-en-1-ol in quantitative yield. Kinetic study shows that (i) the reaction is first order to cyclohexenone and HSiMe2Ph and (ii) Rh4(CO)12 and Co2Rh2(CO)12 possess more than 1 order of magnitude higher catalytic activity than that of RhCl(PPh3)3, while Co3Rh(CO)12 is a 1 order of magnitude less active catalyst than RhCl(PPh3)3.