40662-08-6Relevant academic research and scientific papers
Tetrahedral Intermediate in Acyl Transfer Reactions. A Revaluation of the Significance of Rate Data Used in Deriving Fundamental Linear Free Energy Relationships
DeTar, DeLos F.
, p. 7205 - 7212 (2007/10/02)
A theoretical investigation of model mechanisms applicable to acyl transfer reactions in solution has shown that the interpretations of experimental rate constants in terms of mechanistic constants are all subject to an ambiguity that is well-known in principle but usually ignored or incorrectly evaluated in practice.For all models involving reversibly formed tetrahedral intermediates, the experimental constants are products of the form kffp in which kf is equal to ki+>n or to ki and fp is a product distribution fraction.Each assesible pH range can give a maximum of one constant that depends on the pH; there is no way to dissect out the desired mechanistic constants or the equilibrium constants for several tetrahedral intermediates unless some independent means can be developed to measure the fp.These conclusions are of major concern to all studies that attempt to relate reactivity to structure.Representative acyl transfer reactions have been reinterpreted.One example of the so-called trialkyl lock acceleration is now shown to amount to a factor of about 4000 for the mechanistic rates in contrast to the factor of 5*1010 originally proposed.Most of the decrease in estimate arises from recent reevaluations on observed rates, but there is a further decrease by a factor of 100 in the mechanistic rates due to considerations treated in present study.Evidence is also presented that certain acyl transfer reactions in solution may proceed by direct displacement rather than though a reversibly formed tetrahedral intermediate.
Formation and Hydrolysis of Lactones of Phenolic Acids
Caswell, Michael,Schmir, Gaston L.
, p. 4815 - 4821 (2007/10/02)
The lactonization of 2'-hydroxybiphenyl-2-carboxylic acid to 3,4-benzocoumarin has been studied in aqueous solution, 30 deg C, in the range of pH 0-6.5.As with simpler coumarinic acids, the rate-determining step changes from the breakdown of tetrahedral intermediates in the neutral or weakly acidic pH range to formation of intermediates below pH 2.In contrast, the lactonization of methyl-substituted dihydrocoumarinic acids shows no eviednce for a change in rate-limiting step over a wide pH range.The hydrolysis of 8-hydroxy-1-naphthoic acid lactone in the range of pH 0-10 also does not undergo a transition in rate-limiting step.These results suggest that conjugation of the phenolic oxygen with the carbonyl carbon atom through an extended double-bond system may be necessary for the expulsion of water from a cationic tetrahedral intermediate and hence for the change in the rate-determining step.Revision of the rate constants for the acid-catalyzed lactonization of dihydrocoumarinic acid and of 4,4,5,7-tetramethyldihydrocoumarinic acid indicates that the rate-enhancing effect of the "trimethyl lock" is significantly smaller than previously believed.
