Hydride transfer process

M is an unsaturated hydrocarbon or an organic compound such as CH3OH, CH3I, CH3N02, (CH3)2CO, CH3NH2, etc. When M is an olefin, Reaction 27 or 28 will compete with a hydride transfer process (see earlier discussion) and a condensation process. For instance, in the radiolysis of C3D8-CH3CHCH2 mixtures (9), the relative rates of Reactions 29, 30, 31, and 32... 

Figure 7. Transition state TS(IVa-Va) and product Va process involving hydride transfer process from original ethylene complex IVa.

Triazasilatranes 179 and 180 react with various nucleophiles such as organometallic reagents (equation 176), metal alkoxides (equation 177) and amides (equations 178 and 179) to give the substitution products 172, 181-184 as well as hydride transfer products 169, 170. The relative ratios of these products depend on stereoelectronic factors, the nature of the nucleophilic reagents and the reaction conditions312. Thus, the reaction of triazasilatrane 180 with /i-butyllithiurn affords 181a, which is the product of substitution, while only 1-hydrotriazasilatrane (170) is formed from 180 and /e/t-butyllithiurn in a hydride transfer process. 

Although the remaining steps required for the conversion of 463 to haemanthidine (382) were conceptionally straightforward, it was an experimentally difficult task owing to the poor stereoselectivity encountered in the reduction of the neopentyl ketone function at C-l 1. Moreover, the more forcing conditions required in some experiments to effect the reduction of the sterically hindered carbonyl function at C-ll resulted in an internal Cannizzaro hydride transfer process that produced nortazettine (467). The best results were eventually ob-... 

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,... 

The first step in the catalytic cycle of flavocytochrome i>2 is the oxidation of L-lactate to pyruvate and the reduction of the flavin. Our understanding of how this occurs has been dominated by what can only be described as the dogma of the carbanion mechanism. Although this mechanism for flavoprotein catalysed substrate oxidations is accepted by many, doubts remain, and the alternative hydride transfer process cannot be ruled out. The carbanion mechanism has been extensively surveyed in the past, reviews by Lederer (1997 and 1991) and Ghisla and Massey (1989) are recommended, and for this reason there is little point in covering the same ground in the present article in any great detail. 

Several enzymes such as reductases and dehydrogenases utilize nicotinamide derivatives as reversible carriers of redox equivalents. The reduced dihydronicotinamide moiety NAD(P)H acts by donating a hydride equivalent to other molecules. In the corresponding two-electron oxidized NAD(P) form, the cofactor formally accepts a hydride ion from the substrate. Functional models of such reversible hydride transfer processes are of considerable interest for biomimetic chemistry, and the strategies to regenerate nicotinamide-type cofactors are crucial for the performance of many organic transformations involving biocatalytic key steps 139,140). 

Any of these hydride transfer processes yields the observed configuration of the methyl-bearing methyne. Reduction of the ketone through the more favorable equatorial approach finishes the sequence that leads to IV... 

Similar considerations may be applied to j8-hydride transfer processes. For example, °ox(ReCH2CHMe2) = +0.06 V relative to Ag/Ag+ (reversible) [52a] and red(Re(CH2=CMe2)+) = —1.34 V (irreversible) which translates to a 135 kJ mol bond weakening of the f C-H bond upon oxidation. In this case, H loss from the Re-alkyl cation radical generates an alkene complex in which a new Re C bond has been formed and this contributes substantially to the thermodynamic weakening of the C H bond. 

As emphasized earlier, the redistribution or transfer of hydrogen is one of the most dominant and recurrent reactions of olefins in the presence of acidic zeolite catalysts. The formation of hydrogen-rich paraffins and hydrogen-deficient aromatics is superimposed constantly on any reaction where a low molecular weight olefin—or olefin precursor — is either a reactant, a product, or an intermediate. Even more specifically, numerous examples of hydride-transfer processes during reactions over zeolite catalysts have been observed, and we will discuss some of these in detail. 

Of particular interest are the results obtained from the conversion of methanol on the ammonium salt of HPW (refs. 14-15). With this solid the yield of hydrocarbons is enhanced as compared with that obtained on the parent acid, but in addition the products with the former are largely aliphatic as compared with the olefinic species from the latter catalyst (Fig. 10). This observation implies that the ammonium salt is active in catalyzing the hydride transfer process, somewhat reminiscent of that previously observed with many zeolites in which micropores are present. 

This chapter is concerned with mechanistic observations on hydride-transfer processes in non-enzymic systems, but under conditions and with structures such that the observations are considered relevant to enzyme-catalyzed reactions. Even... 

A simple interpretation is that powerful, obligate one-electron oxidants may elicit single-electron donation from NADH, but its reaction with two-electron acceptors is normally a single-step hydride transfer process. 

In a remarkable article published in 1991 [39], Bunting reviewed structure-reactivity studies relevant to the nature of the hydride-transfer process between materials that can be regarded as related to nicotinamide cofactors. Much of the article concerned the large quantity of work published from Bunting s own laboratory. 

The data from ht-ADH raise provocative questions regarding hydride transfer processes. In particular, it would appear that a model that goes beyond a simple tunnel correction is needed to explain the composite data for ht-ADH. One possible explanation is that, at elevated temperatures, hydride transfer is a full tunnel-... 

DHFR catalyzes the reduction of 7,8-dihydrofolate (H2F) to 5,6,7,8-tetrahydrofolate (H4F) using nicotinamide adenine dinucleotide phosphate (NADPH) as a cofactor (Fig. 17.1). Specifically, the pro-R hydride of NADPH is transferred stereospecifi-cally to the C6 of the pterin nucleus with concurrent protonation at the N5 position [1]. Structural studies of DHFR bound with substrates or substrate analogs have revealed the location and orientation of H2F, NADPH and the mechanistically important side chains [2]. Proper alignment of H2F and NADPH is crucial in enhancing the rate of the chemical step (hydride transfer). Ab initio, mixed quantum mechanical/molecular mechanical (QM/MM), and molecular dynamics computational studies have modeled the hydride transfer process and have deduced optimal geometries for the reaction [3-6]. The optimal C-C distance between the C4 of NADPH and C6 of H2F was calculated to be 2.7A [5, 6], which is significantly smaller than the initial distance of 3.34 A inferred from X-ray crystallography [2]. One proposed chemical mechanism involves a keto-enol tautomerization (Fig. 

Scheme 5. The catalytic cycle proposed for the hydroformylation mechanism with rac-1 as catalyst precursor. The two intramolecular hydride transfer processes reflect the cooperativity of the two rhodium centers.

Usually, the reaction leads predominantly to the more stable alcohol of a stereoisomeric pair because the reaction conditions promote equilibration by the hydride-transfer process discussed in Section 4.1. 

The general process of a C(sp )—H bond functionalization through [l,5]-hydride transfer is illustrated by a model reaction shown in Scheme 4.1. The typical substrate 1 in this type of reaction usually has both a hydride donor moiety and a hydride acceptor moiety. A [l,5]-hydride transfer process generates a zwitterionic intermediate which undergoes subsequent cycliza-tion readily, yielding the desired C—C or C—X bond-forming product. The key features of the reaction include (1) the reaction is redox neutral, thus no external oxidant is required (2) the reaction is waste minimized in that all atoms in the starting material are present in the final product. In this context,... 

Tejero J, Perez-Dorado 1, Maya C, Martinez-Julvez M, Sanz-Aparicio J, Gomez-Moreno C, Hermoso JA, Medina M (2005) C-terminal tyrosine of ferredoxin-NADP+ reductase in hydride transfer processes with NAD(P)+/H. Biochemistry 44 13477-13490... 

The final step is the actual hydride transfer process to produce the products. The multi-step mechanism for the hydride transfer fits Hao s pseudo-first-order rate constant analysis and the KIEapp observations. ... 

The other route is the dihydride route, involving sequential abstraction of both the 0-H and p-C-H of the alcohol onto the TM center (Scheme 9) [26, 49-51]. It is commonly held that zero-valent TM first inserts into the O-H bond of the alcohol, followed by P-H elimination to give [MH2] species. [MH2] then reduces C=Y compounds via a similar but reverse route. As a result, this route is not selective regarding the hydride transfer processes. If a deuterium-labeled alcohol is used, the deuterium contents of Y-H and C-H in the product should both be around 50 % (Scheme 10) [50, 51]. 

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