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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.

The characteristics of back-bonding in metal–olefin complexes have been studied by using the NBO analyses. In metal–olefin complexes, the evidence for back-bonding is only possible from the d M–C and/or d CC bond length variations. However, these bond lengths might be ambiguous in explaining the extents of back-bonding, since both bond lengths are dependent upon the extents of the σ-bonding as well as the π-back-bonding. Instead, the explanation and prediction using the NBO analyses of electron populations in π * -orbitals of olefin ligands and of the second-order charge transfer interactions between metal d-orbitals and olefins π * -orbitals could be more efficient and correct.

Kinetic studies of the reactions of alpha-chloroacetanilides (YC6H4NRC(=O)CH2Cl; R = H (5) and CH3 (6)) with benzylamines (NH2CH2C6H4X) were carried out in dimethyl sulfoxide at 55.0 degrees C. The Brønsted betaX values were in the range from 0.6 to 0.9 and cross-interaction constants phoXY were positive: phoXY = +0.21 and +0.18 for 5 and 6, respectively. The rates were faster with 6 than with 5 and inverse secondary kinetic isotope effects involving deuterated benzylamine (ND2CH2C6H4X) nucleophiles, kH/kD1.0, were obtained. Based on these and other results, a stepwise mechanism with rate-limiting expulsion of the chloride leaving group from a zwitterionic tetrahedral intermediate, T+/-, is proposed. In this mechanism, a prior carbonyl addition to T+/- is followed by a bridged type transition state to expel the chloride. An enolate-like transition state in which the developing negative charge on C(alpha) delocalizes toward the carbonyl group (nC--pi*(C=O) interaction) is not feasible for the present series of reactions due to a stronger charge transfer involving the lone pair on the anilino nitrogen (nAN--pi*(C=O) interaction).

Facilitated olefin transport has been observed through membranes containing AgBF4 dissolved in poly(ethylene oxide) (PEO), but not through ones containing AgNO3 according to the previous studies. The difference in facilitated transport behavior between the two systems was theoretically investigated in terms of the mechanism on facilitated transport, in particular, the interaction of the silver cation with olefin using the density functional theory (DFT) method. It was found that the reaction of silver cation with ethylene occurred through two steps: (i) addition of one ethylene molecule to a silver cation dissolved in PEO, (ii) replacement of the ethylene molecule coordinated on the silver cation with another ethylene molecule. In the two-step mechanism of the addition and replacement reactions, the addition reaction is a nonactivated, preequilibrium process and the replacement reaction occurred through a transition state of two ethylene molecules coordinated to a silver cation dissolved in PEO. It was a...

Characterization of the stationary points in a flat transition structure region of the potential energy surface for the HCOCl+Cl − reaction has been carried out at the MP2/6-311+G∗∗ level. Two stationary points are found: an intermediate with C S symmetry and all positive eigenvalues in the Hessian and a transition state with C 1 symmetry and only one negative eigenvalue in the Hessian. The MP2/6-311+G∗∗ electronic energies of these two only differ by δΔ E (=Δ E TS −Δ E Int )=0.01 kcal mol −1 . The zero-point and thermal energy corrections to the electronic energy lead to an inversion of levels: δΔ E ZPE =−0.06 and δΔ H =−0.59 kcal mol −1 , which becomes normal when the − T Δ S term is added to obtain δΔ G =+0.90 kcal mol −1 (+0.80 kcal mol −1 at the G2(+)MP2 level). This inversion arises solely from one less vibrational mode in the TS on the flat transition structure region.

The gas-phase reactions of OH- and SH- with NCCH2CH2Cl have been studied with ab initio methods using the 6-31+G* basis sets. The electron correlation effect was accounted for at the MP2 level of theory. Both the SN2 and E2(anti) reactions are exothermic and have negative activation barriers when Cl- is the leaving group. Overall, E2 is preferred to SN2, and E2(anti) is favored over E2(gauche) for both OH- and SH-, indicating the preference of an antiperiplanar transition state in the gas phase. Since SH- is a weaker gas-phase nucleophile than OH-, the transition state occurs later on the reaction coordinate. The results of this work are consistent with those of gas-phase experimental studies which indicated that first-row nucleophiles undergo both SN2 and E2 reactions, whereas second-row nucleophiles have lower abilities to induce elimination reactions. Substitution of a CN group for a β-hydrogen of CH3CH2Cl to NCCH2CH2Cl is found to enhance both the SN2 and E2 reactivities and makes the β-H-elimination ...

Understanding the origin of the regioselectivity of inositol derivatives is essential to the synthetic design of these biologically important molecules. We have undertaken a study of the regioselectivities displayed by the myo-inositol derivatives toward electrophiles using various levels of quantum mechanical calculation: Hartree−Fock (HF) theory, Moller-Plesset second-order perturbation (MP2) theory, density functional theory (DFT), and self-consistent reaction field (SCRF) theory. Various basis sets were also tested. The systems investigated here are 1,2:4,5-di-O-methylidene-myo-inositol and its probable intermediates (deprotonated and sodium-chelated species). In experiments the O3 position was the major site for derivatization of the molecule. However, the MP2 and DFT results could not explain the experiments. Taking into account that the myo-inositol derivatives were highly insoluble with the exception of some polar solvents, we performed the SCRF calculations using the dielectric constant (e) of 4...

The ring opening reaction of 3,4-epimino-1-butene (EIP), 3,4-epoxy-1-butene (EOB) and 3,4-epithio-1-butene (ETB) initiated by the attack of hydroxide ion at three carbons (Cα, Cγ, and Cδ) have been investigated using MP2/6-31+G*//MP2/6-31+G* ab initio molecular orbital theory. The CαCβCγCδX process by the α-attack is favored by the strong frontier orbital interactions due to the low π* orbital, but is disfavored by a lower degree of progress in the developing π orbital as well as in the ring opening. In contrast, the processes by the γ-and δ-attacks are disfavored by the high σ* orbitals, but are favored by a greater strain energy release in a greater degree of ring opening in the transition state. The barriers to the ring opening processes are found to depend on the strain energy of the three-membered heterocycles, bond strength of the C−X bonds, frontier orbital (π*, σ*(1), and σ*(2)) levels, exothermicity of the reaction, and steric and entropy effects.

Ab initio studies on the SN2 identity exchange reactions RCH2X + X− X− + RCH2X for R = CH2CH with X− = H, NH2, OH, F, PH2, SH and Cl, and for R = CH3 and CHC with X− = Cl were carried out at the HF and MP2 levels using the 6–31 ++ G** basis sets. The activation barriers, ΔE≠, and major structural changes, Δd≠ (C–X), in the activation process are closely related to the electronegativity of the R and X groups. The effect of electronegativity is twofold: a stronger electronegativity of R and/or X leads to a lesser electronic as well as structural reorganization required in the activation and to a greater correlation energy in the transition state. The former effect lowers the energy barriers at both the HF and MP2 levels whereas the latter lowers only the correlated (MP2) activation energies. Results with R = CH2CH, as a model for R = C6H5, indicate that ‘benzylic effect’ arises mainly from the relatively stronger electron acceptor ability of the phenyl group.

Semiempirical MO (PM3) studies are reported on the reactions of 18 substituted phenolate anions with m-(MNPF), p-(PNPF) and 3,4-dinitrophenyl formates (DNPF). The mechanistic change from rate-limiting formation to breakdown of the anionic intermediate (T−) can occur in the gas phase, especially for MNPF, at approximately an equal proton affinity (PA) of the nucleophile and leaving group phenolates, i.e. ΔPA = 0 as the PA of the phenolate anion is decreased. In solution, however, owing to the low stability of T and a greater stabilization by solvation of the transition state for the breakdown step (TS2), all three formate esters are predicted to proceed by a concerted mechanism, which is in agreement with the experimental results. The low stability of T can be ascribed to the relatively high electron affinity of the phenolate anions.

Theoretical studies on the acid-catalysed dehydration of 15-substituted dihydrodiols were carried out using the AM1 method. In agreement with the experimental results, the rate-limiting step is dehydration of the protonated diols, R+, and the o-phenol-forming pathway (path 1) is favoured both kinetically and thermodynamically over m-phenol product formation. The transition state for path 1 (TS 1) is found to be at an early position (ca 23% progress) along the reaction coordinate and a better correlation of the rate constant is obtained with σ+. Three resonance forms can exist for the benzenonium intermediate but the contribution of one form is dominant in the later TS, which resembles the cationic intermediate. The contribution of the other two, however, weakens the σ+ effect of +M subsituents. The σP correlation of the rate constant in acid solution is therefore suggestive of a later TS resembling the cationic intermediate which is stabilized by solution.

Substituent effects on the thermal elimination reactions of carbonates and thiocarbonates were investigated MO theoretically using the AM1 method. For both substrates, an electron-withdrawing group is found to facilitate the decomposition. A two-step process is favoured by carbonates, whereas one- and two-step processes can compete in the pyrolysis of thiocarbonates. The ability to transmit substituent effect is greater with oxygen (carbonates) than with sulphur (thicarbonates) in contrast to the experimental results of Taylor and co-workers. Modelling studies with replacement of the phenyl ring with a vinyl group are found to be justified in this type of work.

The reactions of phosphoranes with anions were studied by ab initio calculations at the HF/6-311 + +G(d,p) level. The reaction PH 5 + H − proceeds by equatorial H or axial H deprotonation, or adduct formation, depending on the orientation of the entering anion, the former prevailing overall. In the reaction PH 4 X + H − (X = F, OH, NH 2 , CN), the orientation of ligand X and the acidic hydrogen atoms in X = OH and NH 2 groups has a strong influence on deprotonation pathways but little effect on the addition process, with a strong trans effect of ligand X in ax -PH 4 X. The reaction mode for PH 5 X − → PH 4 X. + H − was found to depend largely on the apicophilicity of X in PH 4 X, giving axial phosphoranes for X = F and CN and the equatorial form for X = OH. PH 5 NH 2 − has a C s structure and both axial H and equatorial H deprotonation lead to an eq -PH 4 NH 2 product due to the strong preference for equatorial NH 2 .

The base-promoted nitrile-forming elimination reactions of YCH=CHCβH=NOCH=CHZ (Y=OCH3, H or Cl and Z=H or NO2) were studied by the AM1 MO theoretical method with Cl− as a base. The reaction is found to proceed by an E1cB-like E2 mechanism in which Cβ—H bond cleavage is more advanced than N—O bond breaking. The syn-elimination has a more E1cB-like transition state (TS) than the anti elimination, which is attributed to the structurally favourable nN—o* (Cβ—H) charge-transfer interaction. An electron-withdrawing Y substituent lowers the activation barrier by stabilizing negative charge developed on Cβ in the TS. An electron-withdrawing substituent in the leaving group (Z = NO2) tends to enhance the anti relative to the syn elimination process by depressing the δ*(N—O) level, which in turn makes the nc—δ*(N—O) interaction more effective. The YCH=CH— and —CβH=N fragments are perpendicular in the TS, which is stabilized by delocalization of negative charge developed on the Cβ atom.

The nucleophilic substitution reactions of Y-O-aryl methyl phosphonochloridothioates with substituted anilines (XC(6)H(4)NH(2)) and deuterated anilines (XC(6)H(4)ND(2)) are investigated kinetically in acetonitrile at 55.0 degrees C. The Hammett and Brønsted plots for substituent (X) variations in the nucleophiles are biphasic concave downwards with a break region between X = H and 4-Cl. The deuterium kinetic isotope effects (DKIEs) are primary normal (k(H)/k(D) = 1.03-1.30) for stronger nucleophiles (X = 4-MeO, 4-Me and H), and extremely large secondary inverse (k(H)/k(D) = 0.367-0.567) for weaker nucleophiles (X = 4-Cl, 3-Cl and 3-NO(2)). The cross-interaction constants are negative (rho(XY(H)) = -0.95 and rho(XY(D)) = -1.11) for stronger nucleophiles, while positive (rho(XY(H)) = +0.77 and rho(XY(D)) = +0.21) for weaker nucleophiles. These kinetic results indicate that the mechanism changes from a concerted process involving frontside nucleophilic attack for stronger nucleophiles to a stepwise process with a rate-limiting leaving group expulsion from the intermediate involving backside attack for weaker nucleophiles. A hydrogen-bonded, four-center-type transition state (TS) is suggested for a frontside attack, while a trigonal bipyramidal pentacoordinate TS is suggested for a backside attack. The unusually small DKIEs, as small as or equal to 0.4, for weaker nucleophiles seem to be ascribed to severe steric congestion in the TS.

Gas-phase Smiles rearrangements and SNi processes of ions C6H5X(CH2)nY− (X, Y = 0 or S, and n = 2–4) have been studied by the AM1 MO method. The Smiles rearrangement to an ortho position of the phenyl ring is diffcult owing to involvement of a high-energy barrier process involving a 1,2-hydrogen shift. The reactivity of the Smiles rearrangement to the ipso position increases in the order X O and Y S < X S and Y O < X Y 0 as a result of steric and electronic effects in the transition state (TS). The reactivity order with respect to the side-chain length, n = 3 < 4 < 2, is mainly determined by ring strain in the TS. For the SNi process, in which Y attacks the β-carbon (β to phenyl), the enthalpy (δH≠) factor becomes more favourable but the entropy (δS≠) factor becomes unfavourable with an increase in the side-chain length, in agreement with a general trend for cyclization processes. The product ratio in the Smiles rearrangement and the SNi process is largely controlled by the thermodynamic stabilities of the products, as observed in gas-phase experiments.

Gas-phase reactions of vinylformate with the hydroxide ion and its hydrate have been studied using both the AM1 and ab initio MO calculations. Four competitive reaction pathways (α-H + abstraction, vinyl-H + abstraction, B AC 2, and nucleophilic vinyl substitution (NVS)) are found to be possible. In the reactions with OH − the four processes are all exothermic, whereas in the reactions with hydrated OH − only the nucleophilic reactions proceed exothermically. The NVS reaction has the greatest exothermicity in both reactions with bare and hydrated OH − . Solvation of the hydroxide ion leads to decreases in reactivities and reactivity differences between B AC 2 and NVS paths due to the smaller energy gaps between intermediates formed along the reaction coordinates, in addition to a mechanistic change in the vinyl-H + abstraction. The reactivities of nucleophilic reactions are discussed with the HOMO (nucleophile)-LUMO (substrate) interactions in the initial states. The AM1 results tend to describe anions incorrectly and show substantial disagreement in the heats of reaction with those obtained from ab initio calculations.

The A2 hydrolysis of acetamide was investigated using the AM1 method and the results were compared with those of MNDO. The two methods agree in the general mechanism but differ in detail; MNDO assigns an unduly high energy for a complex with long bonds in contrast to AM1 and ab initio (at the 3–21G and 6–31G levels) methods, which predict a stable structure for such a complex. Inclusion of solvate water molecules has the effect of lowering activation barriers in general. An increase in the number of solving molecules up to four results in narrowing of the activation energy gap between the two rate determining steps of the A2 hydrolysis of the N- and O-protonated acetamide, but no reversal of the relative order of the barrier occurs; the A2 hydrolysis proceeding through the less stable N-protonated tautometer is therefore likely to remain as a major reaction path in solution-phase reactions, in agreement with experimental results.

The gas-phase thermal decomposition of diacetyl compounds, (CH3CO)2X with X = O and S, was investigated theoretically using the semiempirical MO methods MNDO and AM1. The initial decomposition of the diacetyl compounds proceeded through a six-membered ring transition state involving the keto form with a slightly lower activation enthalpy for diacetyl sulphide (X = S); the process via an enolic form of the transition state was kinetically unfavourable. In the initial decomposition of the diacetyl compounds and in the subsequent pyrolysis of acetic and thioacetic acid, ketene formation was found to be the most preferred path, where the ease of CαX bond cleavage is relatively more important than nucleophilic attack on the β-hydrogen in determining the overall reactivity. In the methane formation process, the reactivity was entirely dependent on the XH bond strength in CH3COXH where X = S, NH and O.

The reactions of ethyl Y-phenyl chloro (1) and chlorothio (2) phosphates with X-anilines in acetonitrile at 55.0 degrees C are studied kinetically and theoretically. Kinetic results yield the primary kinetic isotope effects (k(H)/k(D) = 1.07-1.80 and 1.06-1.27 for 1 and 2, respectively) with deuterated aniline (XC(6)H(4)ND(2)) nucleophiles, and the cross-interaction constants rho(XY) = -0.60 and -0.28 for and , respectively. A concerted mechanism involving a partial frontside attack through a hydrogen-bonded, four-center-type transition state is proposed. The large rho(X) (rho(nuc) = -3.1 to -3.4) and beta(X) (beta(nuc) = 1.1-1.2) values seem to be characteristic of the anilinolysis of phosphates and thiophosphates with the Cl leaving group. Because of the relatively large size of the aniline nucleophile, the degree of steric hindrance could be the decisive factor that determines the direction of the nucleophilic attack to the phosphate and thiophosphate substrates with the relatively small-sized Cl leaving group.

The pi-donating effects of pi-accepting X-substituents in substituted benzylic cations, X-C(6)H(5)-CHR(+) where R = CF(3), H and OCH(3), and X = p-NH(2), p-OCH(3), p-CH(3), H, p-F, p-Cl, p-CHO, m-CN, p-CN, m-NO(2) or p-NO(2), have been studied theoretically by using isodesmic hydride transfer reactions at various levels of theory. It might be difficult to determine the pi-donating effects of pi-acceptors using the simple Hammett-type linear equation, because it is not sensitive enough to include small pi-donating effects. Therefore, this effect was estimated using the NBO deletion energy (DeltaE(D)) of the second-order charge-transfer interaction (DeltaE(ct)) between the pi-orbitals (or lone pair orbitals) of the X-substituent and the pi-orbitals of phenyl ring. The extents of pi-donating effects increased in the order X = p-NO(2) < p-CHO < p-CN << p-Cl for both neutral and cationic species, and these effects were found to be more important for para- than for meta-substituents. Moreover, this could represent a general trend for pi-donation by pi-acceptors. On the other hand, the effects of R-substituents on this pi-donating effect were found to be in the order R = OCH(3) < H congruent with CF(3), as predicted by natural resonance theory (NRT) analyses.

The relative partial molal enthalpies of component solvents have been measured at 25 °C in CH3OH–(CH2OH)2, CH3OH–CH3CN, CH3OH–C6H5NO2 and CH3OH–CH3NO2 binary mixtures using an LKB-2277 microcalorimeter, and the molar excess enthalpies have been determined. The structure of binary mixtures is discussed by calculating local mole fractions around an added component solvent based on derived non-random two-liquid type expressions.