Kinetics Brpnsted acid-catalysis

An NMR kinetic study of a phosphine-catalysed aza-Baylis-Hillman reaction of but-3-enone with arylidene-tosylamides showed rate-limiting proton transfer in the absence of added protic species, but no autocatalysis.175 Brpnsted acids accelerate the elimination step. Study of the effects of BINOL-phosphinoyl catalysts sheds light not only on the potential for enantioselection with such bifunctional catalysis, but also on their scope for catalysing racemization. 

A detailed study of the Biginelli has varied acid catalysis (Brpnsted vs Lewis), and also covered a wide range of solvents. " " The yield is found to depend on the keto-enol equilibrium of the / -ketoester, a factor which in turn depends on the solvent. This control via an equilibrium is in turn facilitated by the catalyst, which can eliminate kinetic control. 

A number of studies of fhe acid-cafalyzed mechanism of enolizafion have been done, and the case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their values. When plotted according to the Brpnsted catalysis law (Section 3.6.1) the value of fhe slope a is 0.74. When deuterium or tritium is introduced in the a-position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C-H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone. 

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