[en] Kinetic studies on the pyridinolysis of aryl dithiocyclopentanecarboxyaltes 2 were carried out at 60.0 .deg. C in acetonitrile. In the aminolysis of 2, the β_X values were 0.5 - 0.8 with anilines, and there was no breakpoint. However, in the pyridinolysis of 2, biphasic Bronsted plots were obtained, with a change in slope from a large value (β_X ≅ 0.7) to a small value (β_X ≅ 0.4) at pK_a".deg. = 5.2. This was attributed to a change in the rate-limiting step from breakdown to the formation of a zwitterionic tetrahedral intermediate, T"±, in the reaction path, with an increase in the basicity of the pyridine nucleophile. An obvious change in the cross-interaction constant ρ_X_Z from a large positive (ρ_X_Z = +1.02) value to a small negative value (ρ_X_Z = -0.17) supports the proposed mechanistic change

[en] We propose a concerted mechanism with a hydrogen bonded cyclic transition state for the aminolysis of aryl N-isopropyl thiocarbamates with benzylamines in acetonitrile based on the negative cross-interaction constant, failure of RSP and the kinetic isotope effects greater than unity. The four conducive factors for the concerted aminolysis mechanism for the present reaction series are: strong (strongest) push provided to expel ArS^- by the nonleaving group, (CH₃)₂CHNH, destabilization of the intermediate, T^±, by a powerful expulsion rate of the benzylamine from T^±, greater leaving ability of -SAr than -OAr due to the greater electron acceptor ability of σ^*_C-S than σ^*_C-O bond orbital, and instability of the intermediate, T^±, in a less polar solvent, MeCN than in water due to the ionic nature of T^±. The stepwise aminolysis mechanism of 2a through a tetrahedral intermediate, however, shifts to a concerted process where the leaving group is changed to a thiophenoxide (EtOC(=O)-SAr) instead of a phenoxide (^-OAr). The push provided by an EtO group to expel ^-SAr in T^± is now strong enough to make the intermediate so unstable that the intermediate can not exist. In view of the similar strong push expected from a (CH₃)₂CNH group to expel the ^-SAr group in T^±, it is of interest to see whether the aminolysis mechanism of the thiol analog of aryl N-isopropyl thiocarbamates, (CH₃)₂CHNHC(=O)SC₆H₄Z, also shifts to a concerted mechanism or not

[en] The aminolysis reactions of phenyl N-benzyl thiocarbamate with benzylamines in acetonitrile at 50.0 .deg. C are investigated. The reactions are first order in both the amine and the substrate. Under amine excess, pseudo-first coefficient (k_obs) are obtained, plot of k_obs vs free amine concentration are linear. The signs of ρXZ (< 0) are consistent with concerted mechanism. Moreover, the variations of ρX and ρZ with respect to the sustituent in the substrate and large ρXZ value indicate that the reactions proceed concerted mechanism. The normal kinetic isotope effects (k_H/k_D = 1.3 ∼ 1.5) involving deuterated benzylamine nucleophiles suggest a hydrogen-bonded, four-centered-type transition state. The activation parameters, ΔH^† and ΔS^†, are consistent with this transition state structure

[en] The aminolysis of phenyl dithiocyclohexane-carboxylates with anilines in acetonitrile proceeds by rate-limiting breakdown of a tetrahedral intermediate, T^±. The large β_X (β_nuc) values can be accounted for by a strong localized cationic charge on the nitrogen atom of anilines in T^±, which is lost in the aniline expulsion from T^± (k_-a). The breakdown rate ratio of k_-a/k_b is large due to large k_-a and relatively small k_b. The proposed mechanism is also supported by a large positive cross-interaction constant, ρ_XZ (= 1.68), adherence to the RSP, and low activation parameters. The greater than unity k_H/k_D values involving deuterated anilines suggests a four-center type hydrogen-bonded TS

[en] The kinetics of the addition of X-substituted benzylamines(BA) to Y-substituted methyl α-cyanocinnamate(MCC) in acetonitrile at 25.0 .deg. C have been studied. The reaction is studied under pseudo-first-order conditions by keeping a large excess of BA over MCC, respectively. The addition of BA to MCC occurs in a single step in which the additon of BA to C_α of MCC and proton transfer from BA to C_β of MCC take place concurrently with a four-membered cyclic transition state structure. The sign and magnitude of the cross-interaction constant, ρ_XY (= 0.49), are comparable to those found in the normal bond formation processes in the S_N2 and addition reactions. The normal kinetic isotope effect (k_H/k_D > 1.0) and relatively low ΔH^≠ and large negative ΔS^≠ values are also consistent with the mechanism proposed. the amine additions to olefins in acetonitrile were found to occur by a concerted formation of the C_α-N and C_β-H bonds in a single step process leading to a neutral product. The rates of amine additions in acetonitrile are in general extremely slower than in aqueous solution (k_a(aq) >> 10⁴·k_a(MeCN)), but the relative order remains the same, k_a (CN, CN) >> k_a (H, NO₂). An unexpected trend was observed in acetonitrile, however, that the α-carbon becomes more positive (ρ_Y < 0) in the TS than in the reactant for Z, Z'=CN, CN (benzylidenemalononitrile, BMN) in contrast to the negatively charged C_α (ρ_Y > 0) for Z, Z'= H, NO₂ (β-nitrostyrene, NS)

[en] The aminolysis reactions of aryl thionocarbamates (ATNC) with benzylamines in acetonitrile are investigated at 10.0 .deg. C. The rate of ATNC is slower than the corresponding aminolysis of aryl thiocarbamate (ATC), which has been interpreted in terms of cooperative effects of atom pairs O and S on the reactivity and mechanism. For concerted processes, these effects predict a rate sequence, -C(=S)-S- < -C(=S)-O- <-C-(=O)-S- < -C-(=O)-O-, and our results are consistent with this order. The negative cross-interaction constant, ρ_XZ = -0.68, the size of β_Z (= 0.69∼0.88) and failure of the RSP are in accord with the concerted mechanism. The normal kinetic isotope effects, k_H/k_D = 1.55 ∼ 1.74, involving deuterated benzylamines suggest a hydrogen bonded cyclic transition state. In spite of the large amount of research that has been carried out, there are still many facets of the aminolysis of aryl esters, carbonates, carbamates, and their thio analogs, that are not well understood. These include the effects of the nonleaving group R, RO or RNH, and the combined or cooperative effects of the atom pairs, X and X', on the aminolysis reactivity and mechanism of

[en] The aminolysis of thiophenyl cyclopentanecar-boxylates with benzylamines in acetonitrile proceeds by rate-limiting breakdown of a tetrahedral intermediate, T^±. The unusually large β_X (β_nuc) values can be accounted for by a strong localized cationic charge on the nitrogen atom of benzyl-amines in T^±, which is lost in the benzylamine expulsion from T^± (k_-a). The breakdown rate ratio of k_-a/k_b is large due to large k_-a and relatively small k_b. The proposed mechanism is also supported by a large positive cross-interaction constant, ρ_XZ (=1.72), adherence to the RSP, and low activation parameters. The greater than unity k_H/k_D values involving deuterated benzylamines suggests a four-center type hydrogen-bonded TS. The mechanisms of the aminolysis of aryl esters and carbonates, and their thiol, thiono and dithio derivatives have been extensively studied. Curved Bronsted plots in the aminolysis reactions have been interpreted in terms of a zwitterionic tetrahedral intermediate, T^±, in the reaction path and a change in the rate-limiting step from leaving group expulsion to attack by the nucleophile as the nucleophile becomes more basic. In some of the cases, however, the aminolysis has been found to proceed concertedly in a single step through a tetrahedral intermediate transition state (TS). The mechanistic change from a stepwise through an intermediate, T^±, to a concerted via a single TS has been reported to be caused by destabilization of the tetrahedral intermediate, T^±, due to several factors, e. g., an enhanced leaving ability of the leaving group (LG), strong electronic push provided by the substrate (nonleaving group) and destabilization rendered by the amines and by substitution of S^- by O^- in the tetrahedral intermediate, T^±

[en] We propose a concerted mechanism with a hydrogen bonded cyclic transition state for the aminolysis of aryl N-allyl thiocarbamates with benzylamines in acetonitrile based on the negative cross-interaction constant, failure of RSP, the kinetic isotope effects greater than unity and relatively low ΔH^≠. with large negative ΔS^≠. values. The aminolysis mechanism of aryl carbamates, 1, is quite similar to that of aryl carbonates, 2, and aryl esters, 3. A change in the mechanism of the aminolysis with benzylamines in acetonitrile has been observed from stepwise through a tetrahedral intermediate, T^±, to concerted for the carbamates (1 with R = Ph) as well as for the carbonates (2 with R = Et) when leaving group is changed from phenoxides (a : ^-OAr) to thiophenoxides (b : ^-SAr). This suggests that the strength of push provided to expel the leaving group from T^± by PhNH is similar to that by EtO, and the destabilization of T^± due to this push is strong enough for ^-SAr but is too weak for ^-OAr to lead the aminolysis to a concerted process

[en] In this work, we invoke the mechanistic criteria based on the sign of cross-interaction constants, ρ_XY where X and Y are the substituents in the nucleophile and substrate, respectively; for a stepwise mechanism the sign of ρ_XY was invariably positive and the reactivity-selectivity principle (RSP) was found to hold. Although there is abundant literature on the kinetics and mechanism of the aminolysis of aryl carbonates and esters, the aminolysis reactions of aryl carbamates have been less studied in terms of kinetics. The mechanism of the aminolysis of substituted diphenyl carbonates has been studied, and structure-reactivity relationships for those reactions have also been examined in detail by Gresser and Jencks. Castro and co-workers have reported a number of mechanistic studies on the aminolysis of aryl carbonates and esters. These and other studies showed that most of the aminolysis of aryl carbonates and esters proceed by either a stepwise mechanism through a zwitterionic tetrahedral intermediate, T^±, or a concert mechanism depending on the amine, substrate, and solvent involved

[en] We propose a concerted mechanism with a hydrogen bonded cyclic transition state for the aminolysis of aryl N,N-dimethyl thiocarbamates with benzylamines in acetonitrile based on the negative cross-interaction constant, failure of RSP, a strong push provided to expel ArS^- by the nonleaving group, Me₂N, the kinetic isotope effects greater than unity and relatively low ΔH^≠ with large negative ΔS^≠ values. Kinetic studies on the aminolysis mechanisms of aryl carbamates, are however relatively scarce, albeit they are structurally similar to the corresponding esters and carbonates. Recent works on the aminolysis of aryl thiocarbamates, with R = Et^4e and Ph^4d have indicated that the aminolysis rates with benzylamines in acetonitrile are more than 3 times faster with R = Et than with R = Ph in concerted processes. This rate enhancement with R = Et relative to R = Ph has been attributed mainly to a stronger push to expel the thiophenoxide leaving group by EtNH than by PhNH in the tetrahedral transition state. It is, however, not well understood that exactly what type of electronic effect is responsible for this push, e. g. is it a polar or a charge transfer effect? and whether there is a steric inhibition effect operative with a bulkier phenyl group relative to an ethyl group or not. In order to shed more light on the aminolysis mechanism of aryl N,N-dimethyl thiocarbamates by elucidating effects of the nonleaving (RNH) group in, we carried out kinetic studies on the aminolysis of aryl N,N-dimethyl thiocarbamates with benzylamines in acetonitrile, eq