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[RuCl(SiMe 3 )(CO)(PPh 3 ) 2 ] ( 1 ) was found to catalyze dimerization of aldehydes RC(O)H yielding esters RC(O) OCH 2 R ( 3 ) in a Tishchenko type reaction. Aldehydes with α-hydrogen atoms (R = Me, Et, i Pr, i Bu) reacted quantitatively within 2 days (5 mol% catalyst, 70 °C in C 6 D 6 ) whereas those without α-hydrogen atoms (R = t Bu, Ph) did not react at all. In the case of benzaldehyde (R = Ph), well shaped crystals of [RuCl(O 2 CPh)(CO)(PPh 3 ) 2 ] ( 4 ) precipitated from the reaction mixture. Molecular structures of catalyst 1 and benzoato complex 4 were established by single-crystal X-ray diffraction analyses exhibiting square pyramidal ( 1 ) and octahedral ( 4 ) complex, respectively. Using 1:1 mixture of acetaldehyde and propanal a crossed esterification reaction took place forming esters RC(O) OCH 2 R′ ( 3 , R/R′ = Me/Me, Et/Et; 5 , R/R′ = Me/Et, Et/Me). In the case of t BuC(O)H a crossed esterification reaction with RC(O)H (R = Me, Et) took place yielding only two products, namely the symmetrical esters RC(O) OCH 2 R ( 3 , R = Me, Et) and the crossed products of the type RC(O) OCH 2 t Bu ( 6 , R = Me, Et). As shown with propanal, ester formation is completely suppressed by water. Because ruthenium catalyst 1 was prepared by reaction of [RuCl(H)(CO)(PPh 3 ) 3 ] ( 2 ) with H 2 C CHSiMe 3 , complex 2 was also used as catalyst in the reaction with aldehydes. It proved to be non-active, but in the presence of H 2 C CHSiMe 3 (H 2 C CHSiMe 3 :RC(O)H = 1:1) catalytic formation of cyclotrimers of aldehydes (RCHO) 3 ( 7 ) and aldol condensation products RCH CR′ C(O)H ( 8 , R = Et, i Bu; R′ = R−CH 2 ) was observed. |
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