HOMOGENEOUS CATALYSIS. IX. HYDROSILYLATION USING TRIS(PENTANEDIONATO)RHODIUM(III)-TRIALKYLALUMINIUM AS CATALYST

Article abstract of DOI:10.1016/0022-328X(84)85171-2

The two component (Ziegler) catalyst -AlEt3 (or an analogue with an alternative cocatalyst) has been investigated for the hydrosilylation by SiHX3 of alkynes, dienes, alkenes, styrene, or allylbenzene at 60 deg C.Terminal alkynes did not yield adducts, but internal alkynes RC<*>CR' gave products of cis-addition with SiHEt3 or SiHEt2Me (but not SiH(OEt)3), without regiospecificity for the case of R <*> R'.Acyclic dienes gave 1/1 adducts with SiHX3 (X = Me, Et, OEt or OSiMe3; 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 SiHEt3 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''R''', or cyclohexene proved unreactive; however disubstituted olefins RCH=CHR' gave the terminal (isomerised) 1/1-adducts, e.g., n-C5H11SiEt3 from MeCH=CHEt and SiHEt3.Likewise terminal alkenes RCH=CH2 gave RCH2CH2SiX3 (X = Ph or OEt) or (X = Et) a mixture of isomeric 1/1 adducts.With styrene and SiHEt3, or to a lesser extent SiH(OR)3 (R = Me or Et), the dehydrogenative hydrosilylated material, the vinylsilane PhCH=CHSiX3, was the principial product with isomeric 1/1 adduct byproducts; with allylbenzene, likewise, PhCH2CH=CHSiX3 was a significant, but less important, component of the reaction mixture.Mechanistic pathways are proposed; for the dehydrogenative hydrosilylation of styrene, crucial steps are styrene insertion into a Rh^III-SiX3 bond and a subsequent intramolecular hydrogen transfer, which are consistent with both a labelling experiment using SiDEt3 and the lack of dehydrogenation (under the reaction conditions) of PhCH2CH2SiEt3.