Hyuck Keun Oh

Kinetics and mechanism of the aminolysis of aryl thiocarbamates: effects of the non-leaving group

The kinetics of the aminolysis of aryl thiocarbamates [ATC: H2NC(=O)SC6H4Z] with benzylamines (XC6H4CH2NH2) in acetonitrile at 10.0 degrees C have been studied. The rate order with variation of the non-leaving amino group, RNH, in RNHC(=O)SC6H4Z is NH2 < PhNH < EtNH indicating that the polar (sigma*) and steric (E(s)) effects of the RNH group are insignificant, and the strength of push to expel the leaving group in the tetrahedral transition state is the sole, important effect. The strong push provided by the NH2 group, the negative rhoXZ(-0.38) value, the size of betaZ(-0.54), and failure of the reactivity-selectivity principle are all consistent with the concerted mechanism. The kinetic isotope effects involving deuterated amine nucleophiles (XC6H4CH2ND2) are normal (k(H)/k(D)approximately 1.40-1.73) suggesting a hydrogen-bonded cyclic transition state.

KINETICS AND MECHANISM OF THE AMINOLYSIS OF O-ETHYL S-ARYL DITHIOCARBONATES IN ACETONITRILE

The kinetics and mechanism of the reactions of O-ethyl S-(Z)aryl thiocarbonates with (X)benzylamines in acetonitrile at 45.0°C are studied. Relatively small values of βX(βnuc) = 0.6 ∼ 0.8 and βZ (βlg) = −0.5 ∼ −0.7 together with a negative cross-interaction constant ρXZ (= −0.47) and failure of the reactivity–selectivity principle (RSP) are interpreted to indicate a concerted mechanism. The normal kinetic isotope effects (kH/kD = 1.3 ∼ 1.8) involving deuterated benzylamine nucleophiles suggest a hydrogen-bonded, four-center-type transition state.

Kinetics and mechanism of the aminolysis of thiophenyl methylacetates in acetonitrile

and cross-interaction constant ρXZ is relatively large and positive (0.90). These trends are consistent with the rate-limiting breakdown of a tetrahedral intermediate, T ± . The proposed mechanism is also supported by adherence of the rate data to the reactivity-selectivity principle (RSP). The kinetic isotope effects, kH/kD, are greater than unity (1.3-1.4) suggesting a possibility of hydrogen-bonded four-centered transition state. The activation parameters, ΔH ≠ and ΔS ≠ , are consistent with this transition-state structure.

Kinetics and Mechanism of the Aminolysis of Aryl N-Allyl Thiocarbamates in Acetonitrile

The aminolysis reactions of phenyl N-benzyl thiocarbamate with benzylamines in acetonitrile at are investigated. The reactions are first order in both the amine and the substrate. Under amine excess, pseudo-first coefficient () are obtained, plot of vs free amine concentration are linear. The signs of ( and with respect to the sustituent in the substrate and large value indicate that the reactions proceed concerted mechanism. The normal kinetic isotope effects ( = 1.3 ~ 1.5) involving deuterated benzylamine nucleophiles suggest a hydrogen-bonded, four-centered-type transition state. The activation parameters, and , are consistent with this transition state structure.

Kinetics and mechanism of benzylamine additions to ethyl α-acetyl-β-phenylacrylates in acetonitrile

Kinetic studies of the addition of benzylamines to a noncyclic dicarbonyl group activated olefin, ethyl α-acetyl-β-phenylacrylate (EAP), in acetonitrile at 25.0 °C are reported. The rates are lower than those for the cyclic dicarbonyl group activated olefins. The addition occurs in a single step with concurrent formation of the Cα–N and Cβ–H bonds through a four-center hydrogen bonded transition state. The kinetic isotope effects (kH/kD > 1.0) measured with deuterated benzylamines (XC6H4CH2ND2) increase with a stronger electron acceptor substituent (δσX > 0) which is the same trend as those found for other dicarbonyl group activated series ( 1–4), but is in contrast to those for other ( noncarbonyl) group activated series (5–9). For the dicarbonyl series, the reactivity-selectivity principle (RSP) holds, but for others the anti-RSP applies. These are interpreted to indicate an insignificant imbalance for the former, but substantial lag in the resonance delocalization in the transition state for the latter series.

Kinetics and mechanism of the aminolysis of S-phenyl cyclopropanecarboxylates in acetonitrile

The kinetics and mechanism of the aminolysis of S-phenyl cyclopropanecarboxylates [cyclo-C3H5C(O)SC6H4Z] with benzylamines (XC6H4CH2NH2) were investigated in acetonitrile at 35.0 °C. The large magnitudes of the Bronsted coefficients βX (1.7–2.3) and βZ (−0.9 to −1.3), together with a large positive cross-interaction constant ρXZ = +1.4, are interpreted to indicate a stepwise mechanism with rate-limiting breakdown of the zwitterionic tetrahedral intermediate, T±. The proposed mechanism is supported by adherence of the rates to the reactivity-selectivity principle (RSP). The kinetic isotope effects involving deuterated benzylamines (XC6H4CH2ND2) are greater than unity (kH/kD > 1.0), suggesting the possibility of a hydrogen-bonded four-center transition state. The activation parameters, ΔH≠ (≅5 kcal mol−1) and ΔS≠ (=−45 to −54 e.u.), are consistent with this transition state structure.

Kinetics and mechanism of the addition of benzylamines to β-nitrostilbenes and β-cyano-4′-nitrostilbenes

Nucleophilic addition reactions of benzylamines (BA) to β-nitrostilbenes (NSB) and β-cyano-4′-nitrostilbenes (CNS) have been studied in acetonitrile at 25.0 and 30.0 °C, respectively. The rate is first order with respect to BA and substrate. The rate of reaction with CNS is much lower than that expected from the rate sequence observed in aqueous solution indicating that the mechanisms of BA addition in acetonitrile and in water are different. The major factor determining reactivity of the amine addition in acetonitrile is the direct resonance effect (σ− or R−) while that in aqueous solution is the polar electron-withdrawing effect (σ) of the activating groups. Due to steric inhibition the β-phenyl rings in NSB and CNS are prevented from π-overlap with the anionic center in the TS so that the reduced resonance effect leads to unduly low addition rates. The kinetic isotope effects and activation parameters are in line with the one step addition mechanism in which N–Cα and H–Cβ bonds are formed concurrently with a hydrogen bonded four-center cyclic transition state. The cross-interaction constant ρXY is negative and the magnitude is somewhat larger than those for other similar addition reactions.

Kinetics and mechanism of the aminolysis of phenyl dithiobenzoates

The kinetics and mechanism of the reactions of phenyl dithiobenzoates with anilines in acetonitrile at 55.0 °C have been studied. The large magnitude of βX(βnuc) and the signs of cross-interaction constants, ρeXY > 0, ρYZ 0, are all consistent with the carbonyl addition mechanism in which the breakdown of the tetrahedral intermediate, T ±, is rate limiting. The thiocarbonyl group (CS) is found to favour amine expulsion in contrast to the carbonyl group (CO) which favours the S-bonded nucleofuge expulsion from T±. The signs of cross-interaction constants, ρXY, ρYZ and/or ρXZ, are shown to provide useful mechanistic criteria for distinguishing, especially, the carbonyl addition mechanism involving the rate-limiting breakdown of the tetrahedral intermediate (T±) from the concerted SN2 mechanism.

Nucleophilic substitution reactions of allyl arenesulphonates with anilines and N,N-dimethylanilines

Kinetic studies of the reactions of allyl arenesulphonates, I, with anilines and N,N-dimethylanilines in acetonitrile at 45.0 °C are reported. The sign and magnitude of the cross-interaction constants ρxz(andβxz) between substituents in the nucleophile (X) and leaving group (Z) indicate that the transition state (TS) for I is relatively tight and is similar to that for the corresponding reactions of ethyl systems, rather than for benzyl systems. Variations of the simple Hammett (and Bronsted) coefficients ρx(βx) and ρx(βx) with substituents Z and X, respectively, are consistent with the trend expected from a positive ρxzi.e., that predicted by the potential energy surface diagram. The kinetic isotope effects involving N-deuteriated anilines support the mechanism proposed based onρxz(βxz) for the reactions of I, i.e., an associative SN2 process with an earlier TS for a stronger nucleophile and/or a better leaving group.