Discrete acyl anion

It should be pointed out that every method mentioned above involves the in situ trapping of the generated acyllithium. Otherwise the reactions proceed in very complex manners. Thus far, essentially no examples of an acyllithium as a discrete intermediate are known. Some, in the form of cuprate or nickelate complexes, behave like a discrete acyl anion intermediate, as will be described later. 

These enzymes do not catalyze any proton exchange at C-3 of pyruvate or at C-2 of an acyl-CoA unless the biotin is first carboxylated. This suggested that removal of the proton to the biotin oxygen and carboxylation might be synchronous. However, 13C and 2H kinetic isotope effects and studies of 3H exchange66 support the existence of a discrete enolate anion intermediate as shown in Eq. 14-11.165/67 This mechanism is also consistent with the observation that propionyl-CoA... 

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