Catalytic reduction of benzoate esters and lactones in the presence of PhMeSiH 2 and a titanocene-based catalyst

• Published in: Canadian journal of chemistry Vol. 80; no. 5; pp. 489 - 495
• Main Authors: Shu, Ronghua, Harrod, John F, Lebuis, Anne-Marie
• Format: Journal Article
• Language: English
• Published: 01-05-2002
• ISSN: 0008-4042; 1480-3291
• DOI: 10.1139/v02-047

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 Ti–H 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.