Unsaturated substrates, computational studies

A combined experimental and computational study of substrate-induced decomposition pathways for alkene metathesis catalysts reveals the importance of / -hydride transfer from ruthenacyclobutane intermediates. Subsequent steps afford allyl hydride species, which then eliminate H2 to give catalytically inactive unsaturated complexes of the form [RuL(77 -alkene)Gl2]. ... 

Three pathways for the ruthenium-catalyzed hydrogenations of ketones, aldehydes, and a,(3-unsaturated aldehydes with several catalyst systems were studied the direct insertion, the migratory insertion, and the concerted hydrogen transfer highlighting the catalyst/substrate configurations that lead to stereoselective differentiation. The contribution of computational chemistry were essential to differentiate these mechanisms of ruthenium-catalyzed reactions and to rationalize the experimentally observed stereochemical outcome. 

Thus, the delicate process of enantioselection in the asymmetric hydrogenation of p-dehydroamino acids can be roughly described as being determined by the mode of coordination of the double bond of the substrate in octahedral Rh(III) intermediates. The enantioselectivity could be correctly predicted for all studied cases if only the dihyride pathway is considered. The order of enantioselection in each particular case can be affected either by the entropic effect on the activation barrier for the double bond coordination specific for each substrate or by the interference of other catalytic pathways, for example, the unsaturated route, which has been computed to be more energetically demanding and significantly less stereoselective in the case of (Z)-P-dehydroamino acids. 

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