Homogeneous catalysis: IX. Hydrosilylation using tris(Pentanedionato)rhodium(III)-trialkylaluminium as catalyst

Abstract

The two component (Ziegler) catalyst [Rh(acac) 3]-AlEt 3 (or an analogue with an alternative cocatalyst) has been investigated for the hydrosilylation by SiHX 3 of alkynes, dienes, alkenes, styrene, or allylbenzene at 60° C. Terminal alkynes did not yield adducts, but internal alkynes RCCR′ gave products of cis-addition with SiHEt 3 or SiHEt 2Me (but not SiH(OEt) 3), without regiospecificity for the case of R ≠ R′. Acyclic dienes gave 1/1 adducts with SiHX 3 (X = Me, Et, OEt, or OSiMe 3; but not X = Ph), predominantly (or, for penta-1,3-diene, exclusively) the products of 1,4-addition. Among cyclic dienes, only cyclohexa-1,3- (or -1,4)-diene was hydrosilylated with SiHEt 3 to yield cyclohex-2-enyltriethylsilane; cycloocta-1,3-diene was merely rearranged to the 1,5-isomer, norbornadiene was polymerised, and no reaction was observed with 2,5-dimethylhexa-2,4-diene. Internal straight-chain alkenes RR′CCHR″ , RR′CCR″Rṫ, or cyclohexene proved unreactive; however disubstituted olefins RCHCHR′ gave the terminal (isomerised) 1/1-adducts, e.g., n-C 5H 11SiEt 3 from MeCHCHEt and SiHEt 3. Likewise terminal alkenes RCHCH 2 gave RCH 2CH 2SiX 3 (X=Ph or OEt) or (X = Et) a mixture of isomeric 1/1 adducts. With styrene and SiHEt 3, or to a lesser extent SiH(OR) 3 (R = Me or Et), the dehydrogenative hydrosilylated material, the vinylsilane PhCHCHSiX 3, was the principal product with isomeric 1/1 adduct byproducts; with allylbenzene, likewise, PhCH 2CHCHSiX 3 was a significant, but less important, component of the reaction mixture. Mechanistic pathways are proposed; for the dehydrogenative