Carbonyl reduction, mechanism

Kejrwords Dynamic kinetic asymmetric transformation (DYKAT) Dynamic kinetic resolution (DKR) Hydrogenation Imine reduction Ketone reduction Mechanism of carbonyl reduction Mechanism of imine reduction Mechanism of dUiydrogen activation Ruthenium catalysis Shvo s catalyst Transfer hydrogenation... 

Asymmetric transfer hydrogenation of imines catalyzed by chiral arene-Ru complexes achieves high enantioselectivity (Figure 1.34). Formic acid in aprotic dipolar solvent should be used as a hydride source. The reaction proceeds through the metal-ligand bifunctional mechanism as shown in the carbonyl reduction (Figure 1.24). 

Tetracyanoethylene oxide (TCNEO) not only oxidizes sulfides to sulfoxides but also reduces sulfoxides to sulfides with generation of two molecules of carbonyl cyanide (17). The reduction mechanism involves a zwitterion intermediate (15) that produces sulfide and two molecules of (17) by simultaneous cleavage of the C—C and O—S bonds. A mechanism (Scheme 14) that involves a zwitterion (16) as a common intermediate is proposed for the formation of ylide and sulfoxide.311... 

The reduction mechanism of carbonyl compounds and its dependence on pH has been outlined in section 8.2. Pinacol formation occurs either by dimerization of the hydroxymethyl radical 101 (Eq. (185) ) or by mixed coupling of 101 with the ketyl 102. Dimerization of 102 seems less probable due to electrostatic repulsion of the two negative charges. Besides coupling, 101 or 102 may be further reduced to the alcohol 103. With active cathodes (e.g., Hg, Sn) 101 forms organo-... 

The reduction mechanism was elucidated by calculating the electronic structures (ab initio STO 3G method) of the possible complexes and by comparing them to the a-enone structures. Thus it was demonstrated that these tropones are reduced under frontier orbital control and that carbonyl complexation by the alkali cations is essential. 

We have studied the electrolysis of y-butyrolactone (BL) and methyl formate (MF) in TBAP solutions. A typical voltammogram of y-BL/TBAP with a gold electrode is also shown in Figure 1. Butyrate (CH3CH2CH2COO ) and a cyclic (3-keto ester were identified as the major electrolysis products. The latter is a product of a nucleophilic attack of y-BL anion (in the a position) on the carbonyl center of another molecule [3], The FTIR spectra of this product, as well as its lithiated derivative, are shown in Figure 2. The basic reduction mechanisms of y-BL, based on the above product analysis, as well as on other arguments [3], are presented in Scheme 2. 

Procter has suggested that a study of the dependence of reaction outcome on the stereochemistry of the alkene in the substrate can be used to gain information on the mechanistic direction of reductive couplings.42,43 In cases where the alkene stereochemistry has a marked effect on the reaction outcome, a traditional carbonyl first mechanism may be in operation, whereas in reactions where alkene stereochemistry has little effect, an alternative mechanism in which the alkene is reduced first and a common reactive intermediate is formed, regardless of the geometry of the starting alkene, may operate (Scheme 5.20).42,43... 

The nicotinamide ring of nicotinamide adenine dinucleotide can exist in both oxidized (NAD+) and reduced (NADH) forms, where the reduced form can be viewed as a double vinylogous amine, i.e. a double enamine. The hydrogen transfer from the C4 atom is widely believed to proceed by a hydride transfer mechanism, reminiscent of the mechanism of carbonyl reduction by metal hydrides. 

On the basis of experimental as well as theoretical studies on similar systems, the mechanism of carbonyl reduction is expected to involve ... 

MECHANISM OF CARBONYL REDUCTION I.4.2.J Early Mechanistic Studies... 

Scheme 7.4. Limiting mechanisms for carbonyl reduction of carbonyls by a trialkylborane (a) pericyclic mechanism, (b) Two step mechanism involving dehydroboration of a trialkylborane followed by carbonyl reduction by the resultant dialkylborane.

In contrast to other reactions implemented so far with artificial metalloenzymes there is, to the best of our knowledge, no enzyme known that catalyzes such C-C bond-forming allylic alkylations. The final section of this chapter focuses on carbonyl reduction via a transfer hydrogenation mechanism. 

The catalysts used to make hydrogen react with double bonds are transition metals usually palladium (as in this example) or platinum, but sometimes nickel, rhodium, or ruthenium. We will talk about several different reductions in this section, but the mechanisms of all of them are similar, and very different from those involved in carbonyl reductions. 

Since the presence of these highly electronegative species at positions where they may, by induction, withdraw electrons from carbonyl does have the anticipated effect, we infer support for the interpretation that hydride ion transfer is rate-controlling in this oxidation-reduction mechanism. 

Other Zinc Enzymes. Studies aimed at elucidating the mechanism of dimeric alcohol dehydrogenase enzymes have also been reported. Cobalt(n) can replace zinc(ii) at the two catalytic and non-catalytic sites in the liver enzyme, LADH. Reduction of pyridinealdehyde derivatives by the yeast enzyme has been examined. In acetonitrile solution, reduction by NADH analogues is catalysed by metal ions and mechanistic studies of the reaction have been carried out. Biomimetic studies of zinc-catalysed carbonyl reduction have also been reported. ... 

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