Mechanistic study on the substitution reactions of O-ethyl S-aryl dithiocarbonates with quinuclidines

摘要

The quinuclidinolysis of 4-nitrophenyl, 2,4-dinitrophenyl and 2,4,6-trinitrophenyl O-ethyl dithiocarbonates (1, 2, and 3, respectively) are studied kinetically at 25.0 degrees C, ionic strength 0.2 M; the two former in water and the latter in 44 wt% ethanol-water. The Bronsted plots (log k(N) vs. pK(a) of quinuclidinium ions) for the reactions of 1 and 2 are linear with slopes beta=0.94 and 0.76, respectively, and that of 3 shows a smooth curvature. From these results and other arguments it can be concluded that the quinuclidinolysis of dithiocarbonate 1 in water is stepwise, in contrast to those of dithiocarbonates 2 in water and 3 in aqueous ethanol, which are concerted. The kinetics and mechanistic effects of the leaving and electrophilic groups and the amine nature are discussed.