Hydrogenation, catalyzed mechanistic studies

Kinetic Considerations. Extensive kinetic and mechanistic studies have been made on the esterification of carboxyHc acids since Berthelot and Saint-GiHes first studied the esterification of acetic acid (18). Although ester hydrolysis is catalyzed by both hydrogen and hydroxide ions (19,20), a base-catalyzed esterification is not known. A number of mechanisms for acid- and base-catalyzed esterification have been proposed (4). One possible mechanism for the bimolecular acid-catalyzed ester hydrolysis and esterification is shown in equation 2 (6). 

In mechanistic studies, monodeuterated alcohols were obtained by using PrOD (Scheme 14). These results indicate that the intermediate for this transfer hydrogenation was not a dihydride complex but rather a monohydride complex, which was generally accepted by analogous transition-metal-catalyzed reactions [55-57]. 

Doi and co-workers379 carried out kinetic and mechanistic studies for the hydrogenation of ethylene with [H4Ru(CO)i2] as the precatalyst. The hydrogenation rate is first-order with respect to cluster concentration, and increased to constant values with increasing ethylene and hydrogen pressure. An inverse dependence of the reaction rate on CO pressure was also observed. The mechanism proposed is in accordance with a cluster-catalyzed reaction (Scheme 74). 

Further mechanistic insights into hydrogenations catalyzed by HRuCl(PPh3)3 (7, p. 83) have been obtained indirectly, from studies on hydrogenation of some ruthenium(III) phosphine complexes (83). A frequently considered mechanism for hydrogen reduction of metal salts involves slow formation of an intermediate monohydride, followed by a faster reaction between the hydride and starting complex (/, p. 72), Eqs. (2) and (3) ... 

Hydrogenation of 1,3-dienes to terminal olefins is catalyzed by HRh(PPh3)4 and [Rh(CO)2(PPh3)2]2 in the presence of excess phosphine diene insertion into a metal- hydride bond to give a-alkenyl rather than 7r-allyl intermediates was postulated for the initial step (141). Mechanistic studies of the HRh(PPh3)4 catalyst (142) and a more reactive phosphole analog (143) HRh(DBP)4 [5-phenyl-5//-dibenzophosphole (DBP), 7] for... 

The hydrogenations become analogous to those involving HMn(CO)5 (see Section II,D), and to some catalyzed by HCo(CN)53 (see below). Use of bis(dimethylglyoximato)cobalt(II)-base complexes or cobaloximes(II) as catalysts (7, p. 193) has been more thoroughly studied (189, 190). Alkyl intermediates have been isolated with some activated olefinic substrates using the pyridine system, and electronic and steric effects on the catalytic hydrogenation rates have been reported (189). Mechanistic studies have appeared on the use of (pyridine)cobaloxime(II) with H2, and of (pyridine)chlorocobaloxime(III) and vitamin B12 with borohydride, for reduction of a,/3-unsaturated esters (190). Protonation of a carbanion... 

Mechanistic Studies of Rhodium-Catalyzed Asymmetric Homogeneous Hydrogenation... 

Chloroperoxidase derived from the microorganism Caldariomyces fumago combined with sodium chloride and hydrogen peroxide in buffered aqueous solution converts olefins to chlorohydrines374"376. Chloroperoxidase immobilized on a polymeric support catalyzes the bromination of barbituric acid in a continuous flow system377. Detailed mechanistic study on the mono- and dibromination of barbituric acid catalyzed by bromoperoxidase from Ascophyllum nodosum has been reported by Franssen and coworkers378. Pyrimidine bases are chlorinated and brominated by chloroperoxidase379. 

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