Kinetics, hydride transfer

Rhin(bpy)3]3+ and its derivatives are able to reduce selectively NAD+ to 1,4-NADH in aqueous buffer.48-50 It is likely that a rhodium-hydride intermediate, e.g., [Rhni(bpy)2(H20)(H)]2+, acts as a hydride transfer agent in this catalytic process. This system has been coupled internally to the enzymatic reduction of carbonyl compounds using an alcohol dehydrogenase (HLADH) as an NADH-dependent enzyme (Scheme 4). The [Rhin(bpy)3]3+ derivative containing 2,2 -bipyridine-5-sulfonic acid as ligand gave the best results in terms of turnover number (46 turnovers for the metal catalyst, 101 for the cofactor), but was handicapped by slow reaction kinetics, with a maximum of five turnovers per day.50... 

A number of mechanistic pathways have been identified for the oxidation, such as O-atom transfer to sulfides, electrophilic attack on phenols, hydride transfer from alcohols, and proton-coupled electron transfer from hydroquinone. Some kinetic studies indicate that the rate-determining step involves preassociation of the substrate with the catalyst.507,508 The electrocatalytic properties of polypyridyl oxo-ruthenium complexes have been also applied with success to DNA cleavage509,5 and sugar oxidation.511... 

The rate also varies with butadiene concentration. However, the order of the rate dependence on butadiene concentration is temperature-de-pendent, i.e., a fractional order (0.34) at 30°C and first-order at 50°C (Tables II and III). Cramer s (4, 7) explanation for this temperature effect on the kinetics is that, at 50°C, the insertion reaction to form 4 from 3, although still slow, is no longer rate-determining. Rather, the rate-determining step is the conversion of the hexyl species in 4 into 1,4-hexadiene or the release of hexadiene from the catalyst complex. This interaction involves a hydride transfer from the hexyl ligand to a coordinated butadiene. This transfer should be fast, as indicated by some earlier studies of Rh-catalyzed olefin isomerization reactions (8). The slow release of the hexadiene is therefore attributed to the low concentration of butadiene. Thus, Scheme 2 can be expanded to include complex 6, as shown in Scheme 3. The rate of release of hexadiene depends on the concentra-... 

The primary hydrogen-deuterium kinetic isotope effect is obtained from the percent cw-2-butene obtained from the deuterated and undeuterated stannanes. This is possible because a hydride and a deuteride are transferred to the carbocation when the undeuterated and deuterated stannane, respectively, forms c -2-butene. The secondary deuterium kinetic isotope effect for the hydride transfer reaction is obtained from the relative amounts of fraws-2-butene in each reaction. This is because a hydride is transferred from a deuterated and undeuterated stannane when trans-2-butene is formed. 

The exo and the endo ring closures (the kc reactions) are in competition with the aryl radical-tributyltin hydride transfer (the ks or ku reaction). These workers162 used this competition to determine the primary hydrogen-deuterium kinetic isotope effect in the hydride transfer reaction between the aryl radical and tributyltin hydride and deuteride. 

The pyridine-catalysed lead tetraacetate oxidation of benzyl alcohols shows a first-order dependence in Pb(OAc)4, pyridine and benzyl alcohol concentration. An even larger primary hydrogen kinetic isotope effect of 5.26 and a Hammett p value of —1.7 led Baneijee and Shanker187 to propose that benzaldehyde is formed by the two concurrent pathways shown in Schemes 40 and 41. Scheme 40 describes the hydride transfer mechanism consistent with the negative p value. In the slow step of the reaction, labilization of the Pb—O bond resulting from the coordination of pyridine occurs as the Ca—H bond is broken. The loss of Pb(OAc)2 completes the reaction with transfer of +OAc to an anion. 

Experiments have implied that quantum mechanical effects can be involved in the proton and hydride transfer of several biological processes. Specifically, large kinetic isotope effects (KIE) observed for... 

Hydride transfer from [(bipy)2(CO)RuH]+ occurs in the hydrogenation of acetone when the reaction is carried out in buffered aqueous solutions (Eq. (21)) [39]. The kinetics of the reaction showed that it was a first-order in [(bipy)2(CO)RuH]+ and also first-order in acetone. The reaction proceeds faster at lower pH. The proposed mechanism involved general acid catalysis, with a fast pre-equilibrium protonation of the ketone followed by hydride transfer from [(biPy)2(CO)RuH]+. 

The kinetics of the ionic hydrogenation of isobutyraldehyde were studied using [CpMo(CO)3H] as the hydride and CF3C02H as the acid [41]. The apparent rate decreases as the reaction proceeds, since the acid is consumed. However, when the acidity is held constant by a buffered solution in the presence of excess metal hydride, the reaction is first-order in acid. The reaction is also first-order in metal hydride concentration. A mechanism consistent with these kinetics results is shown in Scheme 7.8. Pre-equilibrium protonation of the aldehyde is followed by rate-determining hydride transfer. 

A competition between stoichiometric hydrogenation of acetone and acetophenone resulted in hydrogenation of the acetone [42]. Competitions of this type could be influenced by both the basicity of the ketone, as well as by the kinetics of hydride transfer to the protonated ketone. An intramolecular competition between an aliphatic and aromatic ketone resulted in preferential hydrogenation of the aliphatic ketone, with the product shown in Eq. (24) being isolated and fully characterized by spectroscopy and crystallography. Selective ionic hydrogenation of an aldehyde over a ketone was also found with HOTf and [Cp(CO)3WH],... 

Experimental results characterizing hydride transfer in zeolites are scarce, as it is a secondary reaction, which cannot be observed directly. Data from kinetics... 

In another article by Corma et al. (178), ITQ-7, a three-dimensional large-pore zeolite, was tested as an alkylation catalyst and compared with a BEA sample of comparable Si/Al ratio and crystal size. The ratio of the selectivities to 2,2,4-TMP and 2,2,3-TMP, which have the largest kinetic diameter of the TMPs, and 2,3,3-TMP and 2,3,4-TMP, which have the lowest kinetic diameter, was used as a measure of the influence of the pore structure. Lower (2,2,4-TMP + 2,2,3-TMP)/ (2,3,3-TMP + 2,3,4-TMP) ratios in ITQ-7 were attributed to its smaller pore diameter. The bulky isomers have more spacious transition states, so that their formation in narrow pores is hindered moreover, their diffusion is slower. The hydride transfer activity, estimated by the dimethylhexane/dimethylhexene ratio,... 

Andres, J., Moliner, V. and Safont, V. S. Theoretical kinetic isotope effects for the hydride-transfer step in Lactate Dehydrogenase, J. Chem. Soc. Faraday Trans., 90 (1994),... 

Isotope effects have been used to determine whether the hydride transfer from the enzyme cofactor nicotinamide-adenine dinucleotide (NADH) (reaction (43)) takes place as a hydride ion transfer in a single kinetic step or in a multistep reaction via an uncoupled electron and hydrogen transfer. 

Tetraphenylcyclopent-3-enone and dimethyl phosphonate are the major products from the base-catalysed reaction of methyl phosphonate with tetra-cyclone.75 A mechanism involving initial hydride transfer from dimethyl phosphinate anion to the ketone followed by kinetically controlled protonation to give (98) is suggested. 

An understanding of kinetic acidity is necessary in order to distinguish such mechanisms from other ways in which hydrogen may become attached to a substrate, e.g., hydrogen atom transfer, reaction 8, and hydride transfer, reaction 9. 

A study of the stereoselectivity of reduction of 3,3,5-trimethylcyclohexanone (5) with TIBA in benzene showed that under kinetically controlled conditions (excess reagent and short reaction time) 96% of trans-3,3,5-trimethylcyclo-hexanol (trans-6) was formed (148). This high degree of stereoselectivity was explained by proposing a cyclic 6-center transition state with hydride transfer occurring preferentially from the less hindered side (Scheme 17). 

Ashby and Yu have studied the kinetics of reduction of benzophenone with TIBA in ether and showed that the overall kinetic rate expression is second order, first order in TIBA and first order in ketone (151). The observed activation parameters were AG - 18.8 kcal/mol AH = 15.8 kcal/mol and AS = - 10.1 e.u. The negative entropy of activation is consistent with a cyclic transition state for the rate-determining hydride-transfer step. A Hammett study gave a value of p = 0.362, supporting nucleophilic attack by the aluminum alkyl on the carbonyl group in the rate-determining step. 

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