The origins of the enantioselection in asymmetric catalytic hydrogenation of amino-acid precursors

A kinetic study of the hydrogenation of hexene-1 catalyzed by a ferrocenyl-phosphine derivative of ruthenium(II)

“…The lack of reactivity may be related to solubility factors since the usual precursor and product hydrides have very low solubilities in the hexane medium (21,26). Alternatively a hydridoruthenium(ll) conlplex containing PPFA just may not form readily; formation of a detectable hydride via reaction of I1 with H2 is not observed (see below), in spite of the possibility of a reaction such as [3] with the released proton being associated with a dissociated chloride or arsine donor (cf. eq.…”

“…Presumably, reduction to the ruthenium(1) state involves hydride formation (eq. [3]), followed by a reaction such as [5]. Such dihydrogen reduction of Ru(ll1) and Ru(l1) species to Ru(l1) [5] RuHH (or RU' + H ' ) + Ru" + 2~u ' + Hi and Ru(1) species, respectively, has been studied previously (36,37) and found to be promoted by base (cf.…”

“…[l], [3]) is the lack of accompanying isomerization of hexene-1 during the catalytic hydrogenation. Such monohydride catalysts are usually quite active for isomerization via the usual metal hydride addition -elimination sequence involving a metal alkyl intermediate (1,35).…”

“…[I], [3], [5]). Enhanced reactivity here in butanol compared with benzene suggests solvent polarity is important, perhaps in stabilizing transition states akin to that shown in eq.…”

Stereoselective Hydrogenation ofCCBonds: Application to Drug and Natural Product Synthesis

“…The most often encountered mechanism in asymmetric Rh-catalyzed hydrogenation was proposed by Halpern and Landis,140,143 and by Brown and Maddox 144 on the basis of nuclear magnetic resonance (NMR) and X-ray crystallographic studies of the reaction intermediates, as well as of detailed kinetic analyses. 140,145,146 …”