Catalytic reduction of benzoate esters and lactones in the presence of PhMeSiH2 and a titanocene-based catalyst
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Abstract
PhMeSiH 2 reduces ethyl benzoate in the presence of a Cp 2 TiMe 2 (Cp = η 5 -cyclopentadienyl) catalyst to give PhMeSi(OEt)(OCH 2 Ph) (1). Small to moderate amounts of toluene are also produced, depending on the reaction conditions. Under 40 psi of H 2 the same reaction gives quantitative reduction of the ester to toluene. PhCHO is reduced, under the same conditions as the ester, to a mixture of PhMeSi(OCH 2 Ph) 2 and toluene in proportions similar to that observed with the ester. A stoichiometric reaction of PhMeSiH 2 , PhCOOEt, and Cp 2 TiMe 2 resulted in the formation of known [Cp 2 Ti(OEt)] 2 (2) in 75% isolated yield. A mechanism is proposed in which the C=O of the ester inserts into a TiH bond. A Ti-mediated transfer of the ethoxy group from the resulting (α-ethoxy)benzyloxytitanium intermediate to silicon produces PhMeSi(OEt)H and PhCHO. 1 is generated via hydrosilation of PhCHO by PhMeSi(OEt)H. Both β- and γ-butyrolactones react with PhMeSiH 2 in the presence of a catalytic amount of Cp 2 TiMe 2 to produce copolymers -[O-PhMeSi-O(CH 2 ) 3 CHR] n - (R = H, 3; R = Me, 4) (M w [Formula: see text] 1 × 10 3 ; M w /M n [Formula: see text] 1.3). A mechanism analogous to that proposed for the ethyl benzoate reduction is proposed. A reaction of PhMeSiH 2 and γ-butyrolactone with a stoichiometric amount of Cp 2 TiMe 2 in the presence of tetrahydofuran (THF) produces Cp 2 Ti(µ-H)(µ-η 1 ,η 5 -C 5 H 4 )Ti(OC 4 H 8 )Cp(5) (65% isolated yield) whose structure is determined by X-ray crystallography.Key words: hydrosilation, esters, lactones, titanocene, catalysis.
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