Kinetics and mechanism of the aminolysis of O‐phenyl 4‐nitrophenyl dithiocarbonate in aqueous ethanol

Abstract

The reactions of the title substrate (1) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt% ethanol-water, at 25.0ЊC, ionic strength 0.2 M (KCl). Under amine excess over the substrate, pseudo-first-order rate coefficients (k obs ) are obtained. Plots of k obs against [NH], where NH is the free amine, are nonlinear upwards, except the reactions of piperidine, which show linear plots. According to the kinetic results and the analysis of products, a reaction scheme is proposed with two tetrahedral intermediates, one zwitterionic and another anionic with a kinetically significant proton transfer from Ϯ Ϫ (T ) (T ), to an amine to yield (k 3 step). By nonlinear least-squares fitting of an equation derived Ϯ Ϫ T T from the scheme to the experimental points, the rate microcoefficients involved in the reactions are determined. Comparison of the kinetics of the title reactions with the linear k obs vs.[NH] plots found in the same aminolysis of O-ethyl 4-nitrophenyl dithiocarbonate (2) in the same solvent shows that the rate coefficient for leaving group expulsion from (k 2 ) is larger Ϯ T for 2 due to a stronger push by EtO than PhO. The k 3 value is the same for both reactions since both proton transfers are diffusion controlled. Comparison of the title reactions with the same aminolysis of phenyl 4-nitrophenyl thionocarbonate (3) in water indicates that (i) the k 2 value is larger for the aminolysis of 1 due to the less basic nucleofuge involved and the small solvent effect on k 2 , (ii) the k 3 value is smaller for the reactions of 1 due to the more viscous solvent, (iii) the rate coefficient for amine expulsion from is larger for theaminolysis of 1 than that of 3 due to a solvent effect, and (iv) the value of the rate coefficient for amine attack (k 1 ) is smaller for the aminolysis of 1 in aqueous ethanol, which can be explained by a predominant solvent effect relative to the electron-withdrawing effect from the nucleofuge.

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

“…The different behavior exhibited by all these reactions can be explained by the mechanism shown in Scheme . A similar reaction mechanism has been obtained for the aminolysis (SA amines) of some O ‐alkyl S ‐aryl dithiocarbonates,1 O ‐phenyl S ‐4‐nitrophenyl dithiocarbonate,8 alkyl aryl1 and diaryl thionocarbonates,5 and for the reactions of 1 and 2 with anilines 7…”

“…According to this estimation, the proton transfer from T ± to the amine ( k 3 ) must be thermodynamically favorable and, therefore, diffusion controlled 8. A value of ca .…”

“…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.…”

“…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. 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.…”

Kinetics and mechanism of the aminolysis of 4‐nitrophenyl and 2,4‐dinitrophenyl 4‐methylphenyl carbonates in aqueous ethanol

“…Therefore, the k 3 value for 1 and 2 should be ca. 10 10 s −1 M −1 in water 13a, 23, 24 and 2 × 10 9 s −1 M −1 for piperazinium ion and 4 × 10 9 s −1 M −1 for the other secondary alicyclic amines in 44 wt% ethanol–water 5, 25.…”