Solvent effects hydride transfer

Additionally, it has been noted that Tetralin operates via hydride transfer, at least in its reduction of quinones. Thus it has been shown that Tetralin readily donates hydrogen to electron-poor systems, such as quinones at 50°-160°C. The reaction is accelerated by electron-withdrawing substituents on the H-acceptor and polar solvents, and is unaffected by free radical initiators (6). These observations are consistent with hydride transfer, as is the more recent finding of a tritium isotope effect for the reaction (7). 

The solvent isotope effect produces an A-ratio (HOH/DOD) of three with isotope-independent A// of 17-18 kJ/mol. This result is more difficult to interpret, because it is unknown how many isotopic sites in the enzyme or water structure contribute to the isotope effect of 2-3. If a single site should be the origin of the effect, then the site could reasonably be a solvent-derived protonic site of the enzyme involved in general-acid catalysis of the hydride transfer, most simply by protonic interaction with the carbonyl oxygen of cyclohexenone or possibly by proton transfer to an olefinic carbon of cyclohexenone. 

The pyridinium chlorochromate (PCC) oxidations of pentaamine cobalt(III)-bound and unbound mandelic and lactic acids have been studied and found to proceed at similar rates.Free-energy relationships in the oxidation of aromatic anils by PCC have been studied. Solvent effects in the oxidation of methionine by PCC and pyridinium bromochromate (PBC) have been investigated the reaction leads to the formation of the corresponding sulfoxide and mechanisms have been proposed. The major product of the acid-catalysed oxidation of a range of diols by PBC is the hydroxyaldehyde. The reaction is first order with respect to the diol and exhibits a substantial primary kinetic isotope effect. Proposed acid-dependent and acid-independent mechanisms involve the rapid formation of a chromate ester in a pre-equilibrium step, followed by rate-determining hydride ion transfer via a cyclic intermediate. PBC oxidation of thio acids has been studied. ... 

The kinetics and mechanism of hydride transfer between Michler s hydride and 2,3,5,6-tetrabromo-/3-benzoquinone have been investigated spectrophotometrically, examining both solvent and pressure effects.330... 

Tapia, O., Andres, J., Moliner, V. and Stamato, F. L. M. G. (1997) Theory of solvent effects and the description of chemical reactions. Proton and hydride transfer processes, in Hadzi, D. (edr), Theoretical treatments of hydrogen bonding, John Wiley and Sons, New York, pp. 143-164,... 

Substituent or solvent effects may be similar for concerted and stepwise processes. It has been shown that provided the rates of reverse reactions are almost independent of changes in oxidation potential, plots of E°, the standard reduction potential for the half cell (8) against log kf for a series of acceptors, Ox +, reacting with a hydride donor must have a slope of 30 mV/ log unit whether the rate-limiting step is hydride transfer, or hydrogen-atom transfer, or electron transfer (Kurz and Kurz, 1978). 

Substituent effects on rates and equilibria in hydride transfers between a range of NAD+ analogues have been examined. Anhydrous and aqueous acetonitrile and aqueous isopropyl alcohol have been used as reaction media, and earlier caveats as to possible complicating kinetic effects of nonproductive adduct formation apply, particularly where hydroxylic solvents are used. Data from hydride transfers have been compared with equilibria, kR +, and rates for pseudo-base formation, or for formation of cyanide adducts. The aqueous alcoholic solvent has an added disadvantage that p/fR+-values for the cations are necessarily composite. 

Quantum dynamics effects for hydride transfer in enzyme catalysis have been analyzed by Alhambra et. al., 2000. This process is simulated using canonically variational transition-states for overbarrier dynamics and optimized multidimensional paths for tunneling. A system is divided into a primary zone (substrate-enzyme-coenzyme), which is embedded in a secondary zone (substrate-enzyme-coenzyme-solvent). The potential energy surface of the first zone is treated by quantum mechanical electronic structure methods, and protein, coenzyme, and solvent atoms by molecular mechanical force fields. The theory allows the calculation of Schaad-Swain exponents for primary (aprim) and secondary (asec) KIE... 

The reduction of camphor enamines by the same method has been found to be highly selective, leading to the endo isomer as the predominant product (85-87%)204 (Scheme 136). The lack of solvent effect and the incorporation of up to 3 deuterium atoms when using DCOOD, are in favour of a two-step mechanism. This would involve reversible protonation of the / -carbon atom with formation of the iminium ion, followed by irreversible transfer of hydride from the formate ion to this ion. 

Summary BusSnH is an effective reagent for partial conversion of Si-Cl into Si-H groups. The hydrogenation mechanism postulates the coordination of the catalyst or the solvent to silicon giving a hypervalent intermediate in the first step, followed by the attack of tributyltin hydride by a single electron transfer or a synchronous hydride transfer. This mechanism implies that the intermediate containing a hypervalent silicon atom reacts faster than the starting tetracoordinate silane. 

Some of the new theoretical relations, the cross-relation between the rates of a cross-reaction of two difierent redox species with those of the two relevant selfexchange reactions, were later adapted to non-electron transfer reactions involving simultaneous bond rupture and formation of a new bond (atom, ion, or group transfer reactions). The theory had to be modified, but relations such as the crossrelation or the effect of driving force (—AG°) on the reaction rate constant were again obtained in the theory, in a somewhat modified form. For example, apart from some proton or hydride transfers under special circumstances, there is no predicted inverted effect. Experimental confirmation of the cross-relation followed, and an inverted effect has only been reported for an H+ transfer in some nonpolar solvents. The various results provide an interesting example of how ideas obtained for a simple, but analyzable, process can prompt related, yet different, ideas for a formalism for more complicated processes. 

An important step in the definition of combined solute and solvent coordinates is presented in some papers by van der Zwan and Hynes (1984) and particularly by Lee and Hynes (1988), who have shown how Marcus treatment of nonequilibrium polarization effects may be accomplished for model charge-transfer reactions (e.g. proton transfer, hydride transfer, and Sjv2 reactions) in a generalized continuum description of nondissipative... 

---