Protolytic catalysis of anilide methanolysis. Spectator catalysis of leaving-group departure

Abstract

Substituted phenols serve as general-acid catalysts of leaving-group departure from the adduct of methoxide ion with 7n-N02C6H4N(CH3)COCF3 in methanol at 25 °C. Sufficiently high concentrations of general acid convert methoxide addition to the rate-limiting step, allowing determination of rate constants for methoxide addition to substrate carbonyl (fea = 300 M-1 s-1), for overall solvent-assisted leaving-group departure (fee = kake'/k^= 5.9 M-1 s-1) and for overall general-acid-catalyzed leaving-group departure (fcBH = kJtBB fk^= 2400 ± 1200 M"2 s-1 for five substituted phenols with pKa's from 12.7 to 14.6). Thus the Bronsted a ~0. It is suggested that the general acid is a spectator at spontaneous expulsion of the leaving group, producing catalysis by fast subsequent trapping of CH3NAr-. The Jencks clock shows the tetrahedral intermediate to have a minimum characteristic lifetime of 1-10 ns.

Proton inventory of phosphate monoanion catalyzed hydrolysis of S-ethyl trifluorothioacetate

“…This technique can be used to obtain information on transition state structures in both enzyme model and actual enzymic reactions. 4 Several examples of the application of this technique to elucidate transition state structures are reported in literature.5 An abridged version of the theory is given below. The measured rate constant kn, in a solution of atom fraction of deuterium n, is related to the rate constant k0 in pure H20 by eq 2.…”

“…The synthetic sequence for 2 is as shown in Scheme I. 4-Phenanthrol (10). Compound 10 was prepared by the sequence in Scheme I according to the method of Haworth.17 Yields: 7, 49%; 8, 72%; 9, 60%; 10, 50%.…”

“…3-N02 group. It can be seen that 4 is far less stable than 5 and 3, which is attributed largely to incomplete localization of the negative charge donated by ~OCH3.…”

Catalyzing separation of carbon dioxide in thiamin diphosphate‐promoted decarboxylation

“…Jencks [37] and Venkatasubban and Schowen [38] observed catalysis in reactions where separation of products from one another by diffusion is rate‐determining. They reasoned that in such a case, the catalyst must form an initial, stable complex with the reactant.…”

“…Applying that concept to the present case, the pyridinium catalyst must be associated with MTh prior to the decarboxylation process. In the reported instances [37,38], the pre‐associated catalyst is held in place by hydrogen bonding, and is in a position to transfer a proton to the acceptor in competition with a reversible step. However, in the decarboxylation of MTh, there are no groups with which a proton donor could form a hydrogen bond to promote the reaction.…”