Structure-reactivity correlations in the aminolysis and pyridinolysis of bis(phenyl) and bis(4-nitrophenyl) thionocarbonates

The reactions of a series of secondary alicyclic amines with bis(phenyl) and bis(4-nitrophenyl) thionocarbonates (BPTOC and BNPTOC, respectively), and a series of pyridines with the latter substrate, are subjected to a kinetic investigation in water, 25.0 degrees C, ionic strength 0.2 M (KCl). All the reactions obey pseudo-first-order kinetics under amine excess over the substrate. The reactions of piperidine with BPTOC are first order in amine, but those of the same substrate with the other secondary alicyclic amines exhibit a complex order, consistent with the existence of both a zwitterionic (T+/-) and an anionic (T-) tetrahedral intermediates on the reaction pathway. Deprotonation of T+/- by a secondary amine to give T- (rate coefficient k(3)) competes with expulsion of the amine moiety from T+/- (k(-1)), except in the reactions of 1-formylpiperazine whereby k(-1) much greater than k(3) [1-formylpiperazine], and a kinetics second order in amine is observed. The reactions of secondary alicyclic amines with BNPTOC are all first order in amine and show a nonlinear Bronsted-type plot with limiting slopes beta = 0.1 (high amine pK(a)) and beta = 0.5 (low amine pK(a)). This slight curvature is consistent with a concerted mechanism (one step). The intermediate T+/- is not formed because of its high kinetic instability due to a large k(-1) value. The pyridinolysis of BNPTOC exhibits a first order in amine kinetics and a linear Bronsted-type plot of slope beta = 1.0, which is consistent with the existence of T+/- whereby the expulsion of the leaving group from T+/- is the rate-determining step. This intermediate is less unstable than that with a secondary amine due to the fact that k(-1) is smaller for a pyridine compared to an isobasic secondary alicyclic amine.