Reactions of O‐aryl S‐aryl dithiocarbonates with pyridines in aqueous ethanol: kinetics and mechanism

Abstract

The reactions of O-(4-methylphenyl) S-(4-nitrophenyl) dithiocarbonate (1), O-(4-chlorophenyl) S-(4-nitrophenyl) dithiocarbonate (2), and O-(4-chlorophenyl) S-phenyl) dithiocarbonate (3) with a series of pyridines were subjected to a kinetic investigation in 44 wt% ethanol-water, at 25.0 -C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically. Under amine excess, pseudo-first-order rate coefficients (k obs ) were determined. For the studied reactions, plots of k obs versus free pyridine concentration at constant pH were linear, with the slope (k N ) independent of pH. The Brønsted-type plots for (1) and (2) were biphasic, suggesting a stepwise mechanism with a change in the rate-determining step, from breakdown to the formation of a tetrahedral intermediate (T W ), as the basicity of the pyridines increases. For the reactions of (3), at the pK a range of the pyridines studied, only the breakdown to products of T W was observed.a Both the pK a and k N values were determined in 44 wt% ethanol-water, at 25.0 8C, and ionic strength 0.2 M (KCl).

The nucleofuge in the pyridinolysis of O‐(4‐nitrophenyl) S‐aryl thio and dithiocarbonates

“…Several works have appeared in the literature on the kinetics of the nucleophilic reactions of O ‐aryl S ‐aryl thio and dithiocarbonates . Among these studies are the reactions of secondary alicyclic amines and pyridines with O ‐aryl S ‐(4‐nitrophenyl) thiocarbonates, and O ‐aryl S ‐(4‐nitrophenyl) dithiocarbonates, where there are two possible nucleofuges of different nature (benzenethiolates versus phenoxides). The question is which of the groups is the nucleofuge.…”

Reactions of O‐aryl S‐aryl dithiocarbonates with secondary alicyclic amines in aqueous ethanol. Kinetics and mechanism

“…The reactions of dithiocarbonates 1 and 2 with pyridines,6 anilines,7 and SA amines (this work) under the same experimental conditions were found to be stepwise, albeit with an important difference: the reactions with pyridines proceed through only one tetrahedral intermediate (T ± ), while in the reactions with SA amines and anilines two tetrahedral intermediates, the zwitterionic (T ± ) and the anionic (T − ), are formed. The absence of the intermediate T − in the reactions with pyridines is obvious since these tertiary amines lack a proton and, therefore, the proton transfer from T ± to the amine is impossible.…”

“…Dithiocarbonates 1 , 2 , and 3 were synthesized as described 6 . O ‐(4‐methylphenyl) S ‐phenyl dithiocarbonate ( 4 ) was synthesized by the reaction of phenyl chlorodithioformate with 4‐methylphenoxide, as previously described for the preparation of dithiocarbonate 3 6. The solid product 4 showed the following characteristics: m.p.…”

“…On the other hand, the aminolysis of diaryl dithiocarbonates has received less attention 5–8. The reactions of SA amines with O ‐phenyl S ‐(4‐nitrophenyl) dithiocarbonate in aqueous ethanol have been shown to proceed through T ± and T − intermediates 8.…”

“…The reactions of SA amines with O ‐phenyl S ‐(4‐nitrophenyl) dithiocarbonate in aqueous ethanol have been shown to proceed through T ± and T − intermediates 8. We have recently described the pyridinolysis of O ‐(4‐methylphenyl) S ‐(4‐nitrophenyl) dithiocarbonate ( 1 ), O ‐(4‐chlorophenyl) S ‐(4‐nitrophenyl) dithiocarbonate ( 2 ), and O ‐(4‐chlorophenyl) S ‐phenyl dithiocarbonate ( 3 ) in aqueous ethanol, showing that these reactions are stepwise and occur through a T ± intermediate 6. Also, a kinetic and mechanistic study of the reactions of dithiocarbonates 1 and 2 with anilines has demonstrated the presence of the above two (T ± and T − ) intermediates 7…”