Search

Kinetic studies on the reactions of O-methyl (2) and O-ethyl (3) phenyl phosphonochloridothioates with X-pyridines have been carried out in acetonitrile at 35.0°C. The pyridinolysis rates of 2 are slightly faster than those of 3. The substituent effects of X on the pyridinolysis rates of 2 and 3 are similar. The Hammett and Bronsted plots with X in the nucleophiles are biphasic concave upward with a break point at X = 3-Ph for both substrates. The stepwise mechanism with rate-limiting leaving group departure from the intermediate is proposed based on the βX values and biphasic concave upward free energy relationship for both substrates. The biphasic concave upward free energy relationships are rationalized by a frontside nucleophilic attack TSf with more basic pyridines and backside attack TSb with less basic pyridines for both substrates.

Kinetic studies for the reactions of dimethyl chlorophosphate (1), dimethyl chlorothionophosphate (2), diethyl chlorophosphate (3), and diethyl chlorothionophosphate (4) with substituted (X) pyridines have been carried out in acetonitrile. The pyridinolyses of the P = O systems (1 and 3) show linear free energy correlations for substituent X variations, while those of the P = S systems (2 and 4) show biphasic concave upward Hammett and Bronsted plots with a break region between 3-phenyl and 3-methoxy pyridines. A concerted mechanism with both frontside and backside nucleophilic attack is proposed for the reactions of 1 and 3, but the fraction of a frontside attacks is somewhat larger than that of a backside attack. A concerted mechanism with a dominant backside nucleophilic attack for weakly basic pyridines and a concerted mechanism with a dominant frontside nucleophilic attack for strongly basic pyridines are proposed for the reactions of 2 and 4. The pyridinolysis rate does not depend solely on one major factor. The concave upward free energy relationship can be diagnostic of a change in the direction of the nucleophilic attack. Copyright © 2010 John Wiley & Sons, Ltd.

Kinetic studies for the reactions of Z-N-aryl-P,P-diphenyl phosphinic amides with X-pyridines have been carried out in dimethyl sulfoxide at 85.0 °C. The two strong π-acceptor substituents, X = 4-Ac and 4-CN in the X-pyridine, exhibit positive deviations from both the Hammett and Bronsted plots. The Hammett plots for substituent Z variations are anomalously biphasic concave upwards with a break point at Z = H, and the sign of ρZ changes from negative for electron-donating to positive for electron-withdrawing substituents Z in the leaving group. The negative sign of the cross-interaction constants (ρXZ) for both electron-donating (large magnitude of ρXZ = −1.54) and -withdrawing (ρXZ = −0.27) substituents Z indicates that the reaction proceeds through a concerted mechanism. These results are indicative of frontside and backside nucleophilic attacks at the P reaction center for electron-donating and -withdrawing substituents Z, respectively. The poor leaving group mobility of NHPhZ yields the change of the nucleophilic attacking direction, resulting in biphasic concave upwards Hammett plots for substituent Z variations. Copyright © 2010 John Wiley & Sons, Ltd.

The reactions of dimethyl phosphinic chloride (1) and methyl phenyl phosphinic chloride (2) with X-anilines have been studied kinetically in acetonitrile at 15.0 and 55.0 °C, respectively. The deuterium kinetic isotope effects (KIEs) involving deuterated aniline nucleophiles (XC6H4ND2) are also reported for the same reactions. The obtained KIEs for 1 are secondary inverse (kH/kD = 0.703–0.899 1). A concerted mechanism involving predominantly backside nucleophilic attack is proposed for the anilinolysis of 1. A concerted mechanism involving predominantly frontside attack via a hydrogen-bonded four-center-type transition state is proposed for the anilinolysis of 2. The degree of steric hindrance is the major factor that determines both the reactivity of the phosphinates and the direction of the nucleophilic attack on the phosphinates. Copyright © 2008 John Wiley & Sons, Ltd.

The deuterium kinetic isotope effects (KIEs) involving deuterated aniline nucleophiles (XC6H4ND2) are reported for the reactions of dimethyl chlorophosphate (1), dimethyl chlorothionophosphate (2), diethyl chlorophosphate (3) and diethyl chlorothionophosphate (4) in acetonitrile at 55.0 °C. The obtained kH/kD values are 0.798–0.979, 0.945–1.06, 0.714–0.919 and 1.01–1.10 for 1, 2, 3 and 4, respectively. A concerted mechanism with dominant backside nucleophilic attack is proposed for the reactions of 1 and 3. A concerted mechanism involving partial frontside attack through a hydrogen-bonded four-centre-type transition state (TS) is proposed for the reactions of 2 and 4. Copyright © 2008 John Wiley & Sons, Ltd.

In this study, we examined the gas-phase pyrolysis of ethyl N,N-dimethylcarbamate theoretically at various theoretical levels. The reaction consists of a two-step mechanism, with N,N-dimethylcarbamic acid and ethylene as reaction intermediates. In the first step, the reaction proceeds via a six-membered cyclic transition state (TS), which is more favorable than that via a four-membered cyclic TS. Here, the contribution of entropy to the overall potential energy surface was found to play an important role in determining the rate-limiting step, which was found to be the second step when viewed in terms of the enthalpy of activation (ΔH≠), but the first step when entropy changes (−TΔS≠) were considered. These results are consistent with experimental findings. Moreover, the experimental activation entropy can be reproduced by using the hindered rotor approximation, which converts some low vibration frequencies that correspond to internal rotational modes into hindered rotors. © 2006 Wiley Periodicals, Inc. J Comput Chem 28: 625–631, 2007

The generalized interaction properties function (GIPF) methodology developed by Politzer and coworkers, which calculated molecular surface electrostatic potential (MSESP) on a density envelope surface, was modified by calculating the MSESP on a much simpler van der Waals (vdW) surface of a molecule. In this work, vdW molecular surfaces were obtained from the fully optimized structures confirmed by frequency calculations at B3LYP/6-31G(d) level of theory. Multiple linear regressions for normal boiling point, heats of vaporization, heats of sublimation, heats of fusion, liquid density, and solid density were performed using GIPF variables from vdW model surface. Results from our model are compared with those from Politzer and coworkers. The surface-dependent β (and γ) values are dependent on the surface models but the surface-independent α and regression coefficients (r) are constant when vdW surface and density surface with 0.001 a.u. contour value are compared. This interesting phenomenon is explained by linear dependencies of GIPF variables. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 2073–2079, 2004

The kinetics and mechanism of the aminolysis of aryl dithiobenzoates (RC(S)SC6H4Z; R = Ph) with pyridines (XC5H4N) in acetonitrile at 60.0°C have been studied. A biphasic Bronsted plot is obtained with a change in slope from a large value (βX ≅ 0.7–0.8) to a small value (βX ≅ 0.2) at pKa° = 5.2, which is interpreted to indicate a change of the rate-determining step from breakdown to formation of the zwitterionic tetrahedral intermediate, T±, at pKa = 5.2 as the pyridine basicity is increased. Rates are compared with those corresponding values for aryl dithioacetates (R = Me). The faster rates for R = Me than for R = Ph, for the rate-limiting formation of T±, can be attributed to the hyperconjugative charge transfer effect of the Me group, pseudo-πMe π*C=S. Clear-cut change in the cross-interaction constants, ρXZ, from +1.47 to −0.20 supports the proposed mechanistic change. The breakpoint at pKa° = 5.2 for R = Ph in the present work is in agreement with those for the pyridinolysis of R = Me and 2-furyl, and attests to the insignificant effect of acyl group, R, on the breakpoint. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 434–440, 2004

The kinetics and mechanism of the nucleophilic substitution reactions of p-chlorophenyl aryl chlorophosphates (2) with anilines are investigated in acetonitrile at 55°C. Relatively large magnitudes of ρX and βX values are indicative of a large degree of bond making in the TS. Smaller magnitudes of ρX (0.20 for X = H) and ρXY (−0.30) than those for the corresponding reactions with phenyl aryl chlorophosphates (1) (ρX = 0.54 for X = H and ρXY = −1.31) are interpreted to indicate partial electron loss, or shunt, towards the electron acceptor equatorial ligand (p-ClC6H4O-) in the bipyramidal pentacoordinated transition state. The inverse secondary kinetic isotope effects (kH/kD = 0.64–0.87) involving deuterated aniline (ND2C6H4X) nucleophiles, and small ΔH‡ and large negative ΔS‡ are obtained. These results are consistent with a concerted nucleophilic substitution mechanism. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 632–637, 2002

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.

Kinetic studies on the reversible reactions of benzyl bromides with N, N-dimethylanilines were carried out for both the forward (kf) and reverse (kr) directions. The equilibrium constants, K were calculated using the ratio kf/kr and the equilibrium cross-interaction constant, ρeXY was determined. The ρeXY value was shown to represent a maximum intensity of interactions between substituents X and Y through covalent bonds within a molecule. The normalized values of ρX (ρnuc) and ρXY indicate that in the transition state the fractional development or loss of polar and resonance interactions in the benzyl system are imbalanced or non-perfectly synchronized. In the forward reaction the fractional loss of resonance interaction becomes enhanced, whereas in the reverse reaction the fractional development of resonance interaction lags behind the corresponding changes of polar interactions by ca 45%.

The negative slope (ΔρY +

The solvolysis rates of 5-(Y)-substituted indan-2-yl (Z)-benzenesulfonates were determined in a variety of protic solvents at 45.0, 55.0 and 65.0 °C. Application of an extended Grunwald–Winstein equation indicates that kinetic responses to both solvent ionizing power, YOTs and solvent nucleophilicity, NOTs, are relatively low but increase in parallel with increasing leaving ability, m= 0.39–0.46 and l= 0.32–0.41 for Z=p-Me ∼p-NO2. The magnitude of cross-interaction constant. ρYZ, is also relatively small (0.16–0.32), which is, however, much more dependent on the nucleophilicity rather than the ionizing power of the solvent. These are consistent with the typical SN2 mechanism proceeding through a transition state in which solvent participates in both bondmaking and -breaking processes.

Nucleophilic substitution reactions of 1-(Y-phenyl)ethyl chlorides with X-anilines have been investigated in methanol at 65.0 °C. An isokinetic point is obtained at σY+=ca.– 0.23 with ρx= 0; the sign of ρx changes at this point from positive for the relatively strong electron donating Y substituents to negative for the more electron withdrawing Y substituents.The magnitude of the cross-interaction constant, ρxy, between the substituents X and Y is unusually large with the relatively small magnitude of ρx° resulting in an observable isokinetic point at σY+=–ρx°//ρxy=–0.23. These results are interpreted in terms of a transition state (TS) structure in which nearly complete bond formation between the nucleophile and cation formed in an ion-pair preequilibrium is coupled with a TS imbalance phenomenon. The results of kinetic isotope effect studies involving deuteriated anilines and methanol are also in line with this mechanism.

The nucleophilic substitution reaction of cumyl arenesulfonate with aniline has been investigated. The reaction in acetonitrile proceeds by the SN2 mechanism with probable frontside nucleophilic attack. The large magnitude of ρxz(=–0.75) obtained results in an observable sign reversal of ρz at x= 0.83, with a negative ρz value for σx > x. This rather unusual phenomenon can be rationalized by a strong interaction between the nucleophile and leaving group due to their close proximity in the transition state, which in turn is a result of the frontside nucleophilic attack. The reactions in methanol indicate that the SN1 channel competes with the SN2 pathway and ion-pair return is observed when the aniline nucleophile concentration is low.

Kinetic studies of the reaction of Z-phenyl cyclopropanecarboxylates with X-benzylamines in acetonitrile at 55.0 °C have been carried out. The reaction proceeds by a stepwise mechanism in which the rate-determining step is the breakdown of the zwitterionic tetrahedral intermediate, T±, with a hydrogen-bonded four-center type transition state (TS). These mechanistic conclusions are drawn based on (i) the large magnitude of ρX and ρZ, (ii) the normal kinetic isotope effects (kH/kD > 1.0) involving deuterated benzylamine nucleophiles, (iii) the positive sign of ρXZ and the larger magnitude of ρXZ than that for normal SN2 processes, and lastly (iv) adherence to the reactivity–selectivity principle (RSP) in all cases.

Kinetic studies have been carried out on the solvolysis and aminolysis with anilines of anilino thioethers, N-methyl-N-[(Z-phenylthio)methyl]-Y-anilines, YC6H4N(CH3)CH2SC6H4Z, I, in methanol at 45.0°C. In contrast to the iminium cations with significant lifetimes produced in water, the solvolysis proceeds by a direct displacement (SN2) mechanism in methanol. In the aminolysis, both bond formation and cleavage are well under way in a late SN2 transition state with high ρX (βX) and ρZ (βZ) values. However, cross interactions between the nucleophile and leaving group are extensive with large negative constants (ρXZ=−1.7, βXZ=−0.27), which are suggestive of an SN2 reaction with frontside attack. The inverse secondary kinetic isotope effects involving deuterated anilines (kH/kD=0.84–0.88) along with low ΔH≠ (4.2–5.2 kcal mol−1) and ΔS≠ (−46–−59 e.u.) values are consistent with the proposed mechanism.

Kinetic isotope effects (KIE) were determined for the reactions of 2-phenylethyl and 1-methyl-2-phenylethyl benzenesulphonates with deuterated aniline nucleophiles in acetonitrile at 65.0°C. The results are in good agreement with the transition state (TS) structures proposed based on the sign and magnitude of the cross-interaction constants, pxz, between the substituents in the nucleophile (X) and the leaving group (Z). In the reactions of 2-phenylethyl derivatives, all three reation pathways, K r , k f and k⊇, were found to be contributing competitively; the inverse secondary KIE observed with a stronger nucleophile changed into the primary KIE with a weaker nucleophile owing to the predominant contribution of the four-centre TS in the k f path. For the reactions of the 1-methyl-2-phenylethyl series, the k r path played a major role, the contribution from the front-side nucleophilic attack, k f , being negligible. In both reaction series, the aryl participation was important for the p-CH 3 O-substituted substrate

Kinetic studies on the reactions of methyl (MBS) and ethyl benzenesulphonates (EBS) with N,N-dimethylanilines (DMA) in methanol and acetonitrile are reported. The cross interaction constants ρXZ and βXZ, between the substituents in the nucleophile (X) and the leaving group (Z) indicated that the transition states (TS) are looser than those for the reactions with anilines, but the relative tightness between the two substrates was the same; the TS was tighter for EBS despite the increase in steric effect leading to looser TSs for MBS and EBS alike. The TS variation between two different reaction series expected from the simple Hammett and Bronsted coefficients, ρX, ρZ, βX and βZ, was incompatible with that predicted by the cross interaction constants, demonstrating again the unreliability of the simple parameters.

The kinetics of reactions between 2-phenylethyl benzenesulphonates (2-PEB) and benzylamines in acetonitrile at 65·0°C have been studied; the mechanism was examined on the basis of the sign and magnitude of cross-interaction constants ρij and βij. In contrast to the reactions of 2-PEB with anilines in methanol, participation of the aryl-assisted pathway was negligible, with a strong indication that the reaction proceeds largely by an intermolecular SNi mechanism with a four-centre transition state (TS). The effect of substituents on the TS variation was in accord with the predictions of the quantum mechanical model.

The second-order rate constants of the reactions between benzenesulphonyl chlorides and anionic nucleophiles, benzoates and cinnamates, in methanol at 30·0°C are reported. A marked increase in rate found with a p-nitro substituent in the substrate indicated the development of an electron-rich centre on the S atom in the transition state. The two types of cross-interaction constants, ρxy and λxy, suggested that bond formation in the SN2 transition state with anionic nucleophiles is greater than that for the corresponding reaction with aniline nucleophiles.

The aminolysis of Z-aryl cyclobutanecarboxylates (II) with X-benzylamines is investigated in acetonitrile at 55.0 °C. The rates are uniformly greater by 2.2 times than the corresponding rates of aryl cyclopropanecarboxylates (III). All the selectivity parameters, ρX (βX), ρZ (βZ) and ρXZ, are quite similar for the two substrates II and III. The kH/kD values involving deuterated benzylamines (XC6H4CH2ND2) are greater than unity, but are marginally smaller than those for the reactions of III. Low ΔH‡ and large negative ΔS‡ values are obtained and adherence to the reactivity–selectivity principle is observed. All these results are consistent with a stepwise mechanism with rate-limiting expulsion of a leaving group (aryl oxides) from a tetrahedral intermediate, T±, with a hydrogen-bonded, four-center transition state.

The aminolysis reactions of Z-aryl phenyldithioacetates (C6H5CH2C(S)SC6H4Z) with benzylamines (XC6H4CH2NH2) are investigated in acetonitrile at −25.0 °C. The reactions are predicted to proceed by a stepwise mechanism with rate-limiting expulsion of the thiophenolate anion from the zwitterionic tetrahedral intermediate, T±, based on (i) large magnitudes of selectivity parameters, βX (ρX) and βZ (ρZ), (ii) positive and large cross-interaction constants ρXZ (=2.05), and (iii) adherence to the reactivity–selectivity principle. The significant normal kinetic isotope effects (kH/kD = 1.14–1.81) involving deuterated amines and low activation enthalpies (ΔH‡ ≈ 3 kcal mol−1) together with large negative activation entropies (ΔS‡ = −37 to −53 e.u.) are consistent with concurrent proton transfer of amine hydrogen with the leaving group departure in the transition state.

The kinetics and mechanisms of the reactions between 1-phenylethyl benzenesulfonates (1-PEB) with N,N-dimethylanilines are investigated in methanol at 35·0°C. Reactivity and selectivity trends were found to be similar to those for the reactions of 1-PEB with anilines, but the magnitudes of cross interaction constants, ρXZ, between substituents X in the nucleophile and Z in the leaving group were substantially smaller indicating no hydrogen-bond bypass bridge formation in the transition state. However, the magnitude of ρXZ suggested a direct electrostatic interaction between the reaction centers in the nucleophile and leaving group in the frontside nucleophilic attack with a loose transition state structure.

The Kinetics of reactions of phenacyl benzenesulphonates with benzylamines were investigated in methanol at 45·0 °C and the cross-interaction constants λXY, λYZ and βXZ were determined in order to elucidate the transition-state structure. The unusually small magnitude of λXY can only be accounted for by the resonance ‘shunt’ effect of the α-CO group of the phenacyl system. Large |λYZ| values indicate a small degree of bond breaking whereas relatively large |βXZ| values compared with those for the dissociative SN2 reaction indicate a relatively tight transition state for the reactions. Further, the similar magnitudes of βXZ values compared with those of the corresponding aniline nucleophile series suggest a similar transition-state structure for the two armatic amine nucleophile series.

Effet des substituants des groupes phenyles: des anilines et des benzenesulfonates de phenyl-1 ethyle