Enoate catalytic mechanism

Another approach to preparing enantiomerically pure carboxylic acids and related compounds is via enanhoselective reduction of conjugated double bonds using NAD(P)H-dependent enoate reductases (EREDs EC 1.3.1.X), members of the so-called Old Yellow Enzyme family [44]. EREDs are ubiquitous in nature and their catalytic mechanism is well documented [45]. They contain a catalytic flavin cofactor and a stoichiometric nicotinamide cofactor which must be regenerated (Scheme 6.23). 

Scheme 2.2 General catalytic mechanism of enoate reductases [10,11],...

Another way to enhance ( )-enoate selectivity of a-alkoxy aldehyde reactions is available, based on the catalytic effect of benzoic acid in ether solvents (99a, 101b, 128, 131). Detailed stereochemical comparisons of catalyzed and uncatalyzed reactions are not available in the examples in Table 19, but entry 98 is reported to benefit significantly by comparison to the uncatalyed process (131). It has not been shown whether benzoic acid catalysis affects the mechanism of C—C bond formation or whether it equilibrates product stereochemistry in this case. The combination of the acid catalyst and the elevated reaction temperature in dimethoxyethane increases the risk of Z E equilibration. However, in an earlier example discussed in connection with Table 16, the ( )-enoate-enhancing effect of benzoic acid is due to kinetic control. It has also been shown that benzoic acid catalysis is more effective in CH2CI2 than in dimethylsulfoxide (99a). 

To explain the preceding results, the authors proposed the mechanism depicted in Scheme 4.15. First, both the enoate and alkyne substrates coordinate to the Ni(0) catalyst, which bears the chiral N-heterocyclic carbene ligand. Next, the enantioselectivity-determining step of the process is the oxidative cyclisation, giving metallocyclic intermediate E. Then, cyclisation and subsequent (3-alkoxide elimination releases the enone. Transmetallation with BEtj, followed by p-hydride elimination, gives a nickel hydride. Reductive elimination then closes the catalytic cycle. 

A possible mechanism for the catalytic [2+2] cycloaddition reaction catalyzed by Tf2NH is depicted in Scheme 4.9. The Mukaiyama-type Michael addition of silyl enol ether to enoate catalyzed by silyl triflic imide aHbrds the corresponding silyl ketene acetal, and then it proceeds successively to the intramolecular silyl oxonium carbon to... 

An efficient method for palladium(II) catalytic desulfitative conjugate addition of arylsulfinic acids with a, -unsaturated carbonyl compound has been developed. Experimental evidences for the key reacting intermediates including aryl Pd(II) sulfinic intermediate, aryl Pd(II), and C=0-Pd complexes were provided. The mechanism of the dehydrosulfenylation of 2-arylsulfinyl esters for furnishing enoates has been determined to be a homolytic process. The interception of the radical intermediate using a nitroxyl radical and mass spectrometry techniques were useful for identification of the intermediates involved in the dehydrosulfenylation of 2-arylsulfinyl esters. These data indicated that a radical-mediated process is operative. Tetrabutylammonium iodide has been found to promote deselenylation reaction of ) -chloro- and -oxyselenides to afford alkenes efficiently with formation of selenenyl iodides. A catalytic version of the transformation has also been developed. 

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