Hydride transfer reducing agent

Bimolecular Decomposition of the Transient Complexes Methyl Transfer Reactions Rearrangement of the Carbon-Skeleton of R (H20)sCrCH02+ as a Hydride Transfer Reducing Agent Alkene Complexes... 

Table 4.1. Reactivity of Hydride-Transfer Reducing Agents...

As all four of the hydrides can eventually be transferred, there are actually several distinct reducing agents functioning during the course of the reaction.60 Although this somewhat complicates interpretation of rates and stereoselectivity, it does not detract from the synthetic utility of these reagents. Reduction with NaBH4 is usually done in aqueous or alcoholic solution and the alkoxyboranes formed as intermediates are rapidly solvolyzed. 

When the enzyme alcohol dehydrogenase converts acetaldehyde to ethanol, NADH acts as a reducing agent by transferring a hydride from C4 of the nicotinamide ring to the carbonyl group of acetaldehyde. 

The catalytic cycle with Ni catalysts is generally similar. The essential difference is the deactivation process, which in this case occurs not via the formation of a precipitate of Ni°, but rather due to interception of the highly reactive Ni° species by any fortuitous oxidant, such as oxygen. As Ni11 is not so easily reduced to Ni° as Pdn is to Pd°, Ni-catalyzed systems often require the addition of a stoichiometric reducing agent (Zn, DIBAL-H, other hydride transfer agents, BuLi, etc.). 

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... 

Sodium dithionite is well established [ 1 ] as a powerful reducing agent under alkaline conditions. Its redox potential is close to that of sodium borohydride [2] and, in several respects, there are advantages in the use of sodium dithionite as an alternative to the metal hydrides under phase-transfer catalytic conditions, particularly in the reduction of carbonyl compounds [3],... 

An Alternative Mechanism. Considering the facility of the electron transfer reactions to which a great deal of this symposium has been devoted, we have to worry whether our "proton transfer" reactions may not really be the result of electron transfer in the reverse direction followed by hydrogen transfer. As Bergman (26) has recently reported that another hydride anion may act as a one-electron reducing agent, and as we have evidence implicating 0s(C0) H as an intermediate in a number of... 

This complex can also transfer hydride to another molecule of the carbonyl compound in a similar manner, and the process continues until all four hydrides have been delivered. Since all four hydrogens in the complex metal hydride are capable of being used in the reduction process, 1 mol of reducing agent reduces 4 mol of aldehyde or ketone. Finally, the last complex is decomposed by the addition of water as a proton source. 

The initial reaction is effectively the same as with an aldehyde or ketone, in that hydride is transferred from the reducing agent, and that the tetrahedral anionic intermediate then complexes with the Lewis acid aluminium hydride. However, the typical reactivity of the carboxylic acid derivatives arises because of the presence of a leaving group. 

Fig. A shows the oxidation of an alcohol into an aldehyde (1) and the reduction of the aldehyde to alcohol (2). In the process, one hydride ion is transferred (two electrons and one proton see p.32), which moves to the oxidizing agent A in reaction 1. The superfluous proton is bound by the catalytic effect of a base B. In the reduction of the aldehyde (2), A-H serves as the reducing agent and the acid H-B is involved as the catalyst.

Apparently the preference of the formyl ligand for the aldehyde form over the hydrated form stems mainly from the large steric requirements of the (H20)sCr moiety. Surprisingly the transient formyl complex acts as a reducing agent towards the formaldehyde present in the solution, via hydride transfer to yield CO and methanol (52) ... 

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