Unusual Rate Enhancement in the RhCl(PPh₃)₃-Catalyzed Hydrosilylation by Organosilanes Having Two Si-H Groups at Appropriate Distances: Mechanistic Aspects

Unusual rate enhancement observed in the RhCl(PPh₃)₃-catalyzed hydrosilylation of carbonyl compounds with certain α,ω-bifunctional organosilanes was studied in two series of experiments. First, the reactions with Me₂HSi(CH₂)_nSiHMe₂[1 (n = 1), 2 (n = 2), 3 (n = 3), and 4 (n = 4)], R₂HSi(CH₂)₂SiHPh₂[8 (R = Me) and 9 (R = Ph)], and 1,2-[Me₂HSi-(CH₂)_n][Me₂HSi(CH₂)_n']C₆H₄[5 (n = n' = 0), 6 (n = 0, n' = 1), and 7 (n = n' = 1)] were investigated in order to understand the rate acceleration by these two closely spaced Si-H groups. The reactions of five of these bifunctional organosilanes, 2, 3, and 5–7, with acetone were unusually rapid and resulted in selective conversion of only one of their Si-H bonds to a Si-OiPr group within several hours at room temperature. The reaction of their remaining Si-H bonds was as slow as the hydrosilylation with monofunctional organosilanes such as EtMe₂SiH and PhMe₂SiH; the conversion was below 25% after 1 day at room temperature. The rate of the reaction of acetone with 1 or 4 was similar to that of EtMe₂-SiH or PhMe₂SiH. These results suggest that the large enhancement in rate occurred in those bifunctional organosilanes in which two closely spaced Si-H groups were connected by 2–4 carbon units. Similar rate enhancement, compared with monofunctional organosilanes, was observed at 50 °C in the hydrosilylation of 8 or 9 and led to the selective conversion of one Si-H group to a Si-OiPr moiety. Analysis of the products revealed involvement of redistribution of methyl groups in the reaction with 5. Hydrosilylation of unsymmetrical bifunctional organosilanes, 6 or 8, gave a 1:1 mixture of two isomers. In the second approach, the stoichiometric reaction of 8 or 9 with RhCl(PPh₃)₃ was studied by ¹H and ³¹P NMR spectroscopy. The product obtained was dependent on the solvent used; in CDCl₃, Rh(III)-oxidative adducts, R¹ ₂HSi(CH₂CH₂)R² ₂Si-RhHCl(PPh₃)₂(R¹, R² = Me or Ph), having a trigonal bipyramidal structure with two PPh₃ ligands at the apical positions were obtained, whereas the spectra of such reaction mixtures obtained in toluene-d₈ suggested the formation of Rh(V)-double oxidative adducts, R¹ ₂Si(CH₂CH₂)R² ₂Si-RhH₃(PPh₃)₂. Since the catalytic hydrosilylation of acetone with 2 or 8 proceeded in toluene-d₈, but did not in CDCl₃, it is likely that the Rh(V)-double oxidative adducts play an important role in the rapid hydrosilylation of ketones with one end of the bifunctional organosilanes. These experimental results allow us to discuss two probable mechanisms involving disilametallacyclic intermediates for the rate enhancement by the two closely spaced Si-H groups.