Hydrogenation mechanistic studies

Tri-rc-butylstannane is able to reductively replace halogen by hydrogen. Mechanistic studies indicate a free radical chain mechanism.199 The order of reactivity for the halides is RI > RBr > RC1 > RF, which reflects the relative ease of the halogen atom abstraction.200... 

Thus, using a catalytic amount of A adduct of Scheme 15.1 and heating to 80-120°C under 1-5 atm H2 resulted in the hydrogenation of a variety of imines in high isolated yields. Similarly the A-aryl aziridines were catalytically hydrogenated. Mechanistic studies point to the initial protonation of the imine, followed by hydride transfer to the carbon of the iminium salt. 

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). 

Hydrogenation of epoxides lends itself well to both synthetic applications and mechanistic studies. The reaction is complex, for either carbon-oxygen bond may break with or without inversion of configuration, and the product may contain deoxygenated products (92,93) as well as ketones derived by isomerization (26). The reaction is especially sensitive to both catalyst and environment (74). 

Complex 5 was more active than the well-known precious-metal catalysts (palladium on activated carbon Pd/C, the Wilkinson catalyst RhCl(PPh3)3, and Crabtree s catalyst [lr(cod)(PCy3)py]PFg) and the analogous Ai-coordinated Fe complexes 6-8 [29] for the hydrogenation of 1-hexene (Table 2). In mechanistic studies, the NMR data revealed that 5 was converted into the dihydrogen complex 9 via the monodinitrogen complex under hydrogen atmosphere (Scheme 4). 

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]. 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

Another example of selective C=C bond hydrogenation has arisen from mechanistic studies on an iron m-hydride dihydrogen complex, [Fe(PP3)(FI)(H2)](BF4) [PP3 = P(CH2CH2PPh2)3], a catalyst inactive with alkene substrates. Scheme 6 shows that no decoordination of dihydrogen is required in any step of the cycle and that the vacant site is created by unfastening of one of the P-donor atoms (species (16)).50 Extensive studies on catalytic alkene hydrogenation by analogous tripodal (polyphosphine) Rh, Os, and Ir complexes have been carried by Bianchini and co-workers.51,52... 

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). 

Agnelli, M., Swaan, H. M., Marquez-Alvarez, C., Martin, G. A., and Mirodatos, C. 1998. CO hydrogenation on a nickel catalyst. II. A mechanistic study by transient kinetics and infrared spectroscopy. J. Catal. 175 117-28. 

In aqueous solution, a metal complex is enclosed by several solvation spheres involving many solvent molecules. As recently shown, for mechanistic studies a reduced coordination sphere is sufficient (157,159), and therefore here inclusion of a third solvent molecule is sufficient. Addition of a third water molecule results in the exothermic formation of [Be(H20)2(L)--H20] and [Be(H20)2(LH)--H20]+, where the third hydrogen-bonded... 

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 study revealed that the reaction proceeds through intramolecular 1,3-hydrogen migration, and the chiral rhodium catalyst differentiates the enantiotopic C-l hydrogens of allylic alcohols (Scheme 29).53... 

Catalytic cycles involving both alkyne hydrorhodation and silylrhodation are proposed.613 However, mechanistic studies performed on related hydrogen-mediated enyne reductive cyclizations (vide supra) suggest oxidative cyclization of the enyne followed by hydrosilylytic cleavage of the resulting metallacycle via cr-bond metathesis is also plausible (Scheme 27). 

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