In this tutorial review recent mechanistic studies on transition metal-catalyzed hydrogen transfer reactions are discussed. A common feature of these reactions is that they involve metal hydrides, which may be monohydrides or dihydrides. An important question is whether the substrate coordinates to the metal (inner-sphere hydrogen transfer) or if there is a direct concerted transfer of hydrogen from the metal to substrate (outer-sphere hydrogen transfer). Both experimental and theoretical studies are reviewed.

No abstract

No abstract

The mechanism of the Ru(arene)(amino alcohol)-catalyzed transfer hydrogenation of ketones using isopropyl alcohol as the hydrogen source has been studied by means of hybrid density functional methods (B3PW91). Three mechanistic alternatives were evaluated, and it was shown that the reaction takes place via a six-membered transition state, where a metal-bound hydride and a proton of a coordinated amine are transferred simultaneously to the ketone. Further calculations provided a general rationale for the rate of the reaction by comparison of steric effects in the ground and transition states of the ruthenium hydride complex. It was found that the TS has a strong preference for planarity, and this in turn is dependent on the conformational behavior of the O,N-linkage of the amino alcohol ligand. Finally, a general model, rationalizing the enantioselectivity of the reaction, was developed. Experimental studies of both rate and enantioselectivity were used in order to support the computational results.

No abstract

The reaction mechanism of the iridium-phosphanooxazoline-catalyzed hydrogenation of unfunctionalized olefins has been studied by means of density functional theory calculations (B3LYP) and kinetic experiments. The calculations suggest that the reaction involves an unexpected Ir(III)-Ir(V) catalytic cycle facilitated by coordination of a second equivalent of dihydrogen. Thus, in the rate-determining migratory insertion of the substrate alkene into an iridium-hydride bond, simultaneous oxidative addition of the bound dihydrogen occurs. The kinetic data shows that the reaction is first order with respect to hydrogen pressure. This is interpreted in terms of an endergonic coordination of this second equivalent of dihydrogen, although a rate-determining step, in which coordinated solvent is replaced by dihydrogen, could not be ruled out. Furthermore, the reaction was found to be zeroth order with respect to the alkene concentration. This correlates well with the calculated exothermicity of substrate coordination, and the catalyst is thus believed to coordinate an alkene in the resting state. On the basis of the proposed catalytic cycle, calculations were performed on a full-sized system with 88 atoms to assess the appropriateness of the model calculations. These calculations were also used to explain the enantioselectivity exerted by the catalyst.

The mechanism of the copper-catalyzed aziridination of alkenes using [N-(p-toluenesulfonyl)imino]phenyliodinane (PhINTs) as the nitrene source has been elucidated by a combination of hybrid density functional theory calculations (B3LYP) and kinetic experiments. The calculations could assign a Cu(I)/Cu(III)-cycle to the reaction and demonstrate why a higher oxidation state of copper cannot catalyze the reaction. A mechanism whereby Cu(II)-catalyst precursors can enter the Cu(I)/Cu(III)-cycle is suggested. Three low-energy pathways were found for the formation of aziridines, where the two new N-C bonds are formed either in a nonradical concerted or consecutive fashion, by involvement of singlet or triplet biradicals. A close correspondence was found between the title reaction and the Jacobsen epoxidation reaction in terms of spin-crossings and the mechanism for formation of cis/trans isomerized products. The kinetic part of the study showed that the reaction is zero order in alkene and that the rate-determining step is the formation of a metallanitrene species.

The iridium-catalyzed asymmetric hydrogenation of largely unfunctionalized olefins has been studied by DFT calculations using a full, experimentally tested combination of ligand and substrate. All possible diastereomeric pathways were considered within four different hydrogenation mechanisms. The effect of a solvent continuum was also considered, and both the gas-phase and solventcontinuum calculations favored the same mechanism. This mechanism passed through Ir III and Ir V intermediates and was consistent with the sense of stereoselection observed experimentally. Comparing the calculations to those performed on a model system permitted an evaluation of the model system's utility in representing the full one. A simple, general method for predicting the sense of stereoselection in iridium-catalyzed olefin hydrogenation was developed and tested against published data.