Catalysis of .alpha.-hydrogen exchange. VII. Isobutyraldehyde-2-d exchange in the presence of primary amine salts and pyridine buffers

General acid-specific base catalyzed isomerization of tert-butyl thiolbut-3-enoate to tert-butyl thiolcrotonate

“…Diminished relative reactivity of triethylamine as a catalyst may reflect steric hindrance to proton transfer. 30 (2) Values of A:b and kc appear to approach the diffusion-controlled limit31 as acidity of ammonium ions increases and rate constants of these magnitudes are expected for proton transfer to a highly basic carbanion. 27 The Bronsted a values for A:b (=Arc) are 0.48 (0.38 without triethylamine constants), which could be interpreted to mean that proton transfer occurs about midway along the reaction coordinate.32 This interpretation finds support in the result that experimental deuterium solvent kinetic isotope effects are greater than six, a result we shall address below.…”

“…Diminished relative reactivity of triethylamine as a catalyst may reflect steric hindrance to proton transfer. 30 (2) Values of kb and k, appear to approach the diffusion-controlled limit3I as acidity of ammonium ions increases and rate constants of these magnitudes are expected for proton transfer to a highly basic ~a r b a n i o n .~' T h e Bronsted cy values for kb ( = k c ) are 0.48 (0.38 without triethylamine constants), which could be interpreted to mean that proton transfer occurs about midway along the reaction c~o r d i n a t e .~~ This interpretation finds support in the result that experimental deuterium solvent kinetic isotope effects are greater than six, a result we shall address below. Although the "data" of Table V are fictitious, they are useful reminders that, for isomerization of 1 to 2, a slow proton transfer (k,) coupled by an equilibrium with a quick proton transfer (k,) gives rise to a situation wherein the rate-determining step is the quick proton transfer.…”

Catalysis of .alpha.-hydrogen exchange. IX. Isobutyraldehyde-2-d exchange in the presence of amino acids

“…Catalysis by the small amount of free RNH2 in equilibrium with other RNH3+ species (of acidity comparable to that of HZ) is negligible at the pH's at which the present runs were made. 4,6 Significant catalysis by the carboxylate anion group of Zis even less plausible. According to earlier work, an acetate ion is more than 2800 times as reactive as a water molecule in removing deuterium from the N-methyliminium ion derived from isobutyraldehyde-2-d (and pyridine is even more reactive).4,5 We assumed that the relative reactivities toward Me2CDCH=NH- (CH2)"C02H's are similar and therefore neglected the kw" term.…”

“…The fact that the best straight line through these points passes within the experimental error of the origin shows that the first term in eq 6 is negligible, and the fact that the four points at pH's all runs except where stated otherwise. 6 " pH 5.02. * pH 5.40. pH 5.48.…”

“…Previous work has provided evidence that the dedeuteration of isobutyraldehyde-2-d in the presence of buffered solutions of primary amine salts involves the rate-controlling attack of the buffer base (B~) on the N-isobutylidenealkylammonium ion to give an enamine. [3][4][5][6] We have now studied the catalytic activity MeiCDCHO + RNHa+ ^±1 Me2CDCH=NHR+ Me2CDCH=NHR+ + B~-> BD + Me2C=CHNHR of several -amino-n-alkanoic acids, species in which a primary amine salt and a basic group are both present in the same molecule, in order to learn whether bifunctional catalysis would occur.…”