Kinetic isotope effects, Group 14 hydrides

An interesting catalytic ruthenium system, Ru(7/5-C5Ar4OH)(CO)2H based on substituted cyclopentadienyl ligands was discovered by Shvo and coworkers [95— 98]. This operates in a similar fashion to the Noyori system of Scheme 3.12, but transfers hydride from the ruthenium and proton from the hydroxyl group on the ring in an outer-sphere hydrogenation mechanism. The source of hydrogen can be H2 or formic acid. Casey and coworkers have recently shown, on the basis of kinetic isotope effects, that the transfer of H+ and TT equivalents to the ketone for the Shvo system and the Noyori system (Scheme 3.12) is a concerted process [99, 100]. 

Besides obvious participation of protons, hydrids and hydrogen atoms in a chemical reaction in enzymes active sites, two main criteria are used for discrimination of particle involvement in the reaction limiting stage site-directed substitution of chosen enzyme groups and kinetic isotope effects (KIE). 

That alkenes are formed from those alkyl groups containing a jS-hydrogen atom strongly implies that the mechanism of alkene formation involves a /3-hydride abstraction step. There is a very pronounced kinetic isotope effect when C6D5CD2CH2COCI is decarbonylated, which indicates that not only does a jS-deuteride abstraction take place but that it is also rate determining. Further evidence for the participation of a /3-hydride abstraction comes from the decarbonylation of erythro- or t/ reo-2,3-diphenylbutanoyl chloride, where the former yields the (ii)-alkene and the latter the (Z)-isomer. 

Considering this result and proposals of alkane complexes in matrix isolation studies, both before and after Kubas s discovery, Bergman explained results of H-D exchange between the hydride and cyclohexyl group of the complex in Equation 2.20 and kinetic isotope effects for reductive elimination by invoking an intermediate in which the alkane is bound to the transition metal through a C-H bond, This proposal of an alkane complex is now fully accepted, although the precise structure of such a complex in solution is unknown. 

Specifically, the acylic dialkyl complex in Equation 10.7a decomposes in the presence of modest concentrations of added ligand with rates that are inhibited by added triph-enylphosphine. The inhibition by phosphine shows that dissociation of phosphine occurs prior to p-hydrogen elimination. A small kinetic isotope effect implies that the p-hydrogen elimination step is fully or partially reversible. In tliis case, the final irreversible step of the overall reaction would be the reductive elimination of the hydride and alkyl groups to form n-butane. 

These results indicate that DMD directs its action on the negatively charged nitro group of the a -adducts to form intermediate cyclohexadienone and subsequent aromatization. Sensitivity of KMnO oxidation of the a -adducts to the steric effects at the addition site and high value of kinetic isotope effect (KIE) k lk 10 at -70°C [12b] suggests that the oxidation proceeds via direct interaction of the oxidant with the oxidized site, hence probably via abstraction of the hydride anion. 

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