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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry