Four-membered Cyclic Transition State Research Articles

Kinetic Studies on the Nucleophilic Addition Reactons of Vinylic β-Diketones

The kinetics of the addition of X-substituted benzylamines (BA) to Y-substituted Benzylideneacetylacetones (BAA) have been investigated in acetonitrile at <TEX>$25.0^{\circ}C$</TEX>. The reaction is studied under pseudo-first-order conditions by keeping a large excess of BA over BAA. The addition of BA to BAA occurs in a single step in which the addition of BA to <TEX>$C_\alpha$</TEX> of BAA and proton transfer from BA to <TEX>$C_\beta$</TEX> of BAA take place concurrently with a four-membered cyclic transition state structure. The magnitude of the Hammett (<TEX>$p_X$</TEX>) and Bronsted (<TEX>$\beta_x$</TEX>) coefficients are rather small suggesting an early tansition state (TS). The sign and magnitude of the cross-interaction constant, <TEX>$p_{XY}$</TEX> (= -0.49), is comparatible to those found in the normal bond formation processes in the <TEX>$S_N2$</TEX> and addition reactions. The normal kinetic isotope effect (<TEX>$K_H/K_D$</TEX> > 1.0) and relatively low <TEX>$\Delta</TEX><TEX>$H^{</TEX>${\neq}$</TEX>}$</TEX> and large negative <TEX>$\Delta</TEX><TEX>$S^{<TEX>${\neq}$</TEX>}$</TEX> values are also consistent with the mechanism proposed.

Kinetics and mechanism of oxidation of dimethyl sulphoxide by sodium bromate-sodium bisulphite reagent in aqueous medium

Rates of oxidation of dimethyl sulphoxide (DMSO) by HOBr producedin situ from sodium bromate-sodium bisulphite reagent have been studied iodometrically in aqueous medium. The order in [DMSO] is one when [DMSO] < 001 mol dm-3, fractional when [DMSO] is between 0.01 and 0.5mol dm-3 and zero when (DMSO) > 0.5 mol dm-3. Different rate laws are operative under these three conditions though HOBr is the effective oxidizing species in all the cases. A mechanism involving an intermediate four-membered cyclic transition state between DMSO and HOBr (formation constantK), which decomposes in a slow step with a rate constant(k) has been proposed. Thermodynamic parameters for the adduct formation step and activation parameters for the first-order decomposition of the adduct step have been evaluated in the temperature range 308–323 K. Activation parameters have also been determined while the orders in [DMSO] are unity and zero. The reaction product has been identified as dimethyl sulphone (DMSO2).

Elimination kinetics of β-hydroxynitriles in the gas phase

The gas-phase elimination kinetics of primary, secondary and tertiary β-hydroxynitriles were examined in static seasoned vessels over the temperature range 360–450 °C and pressure range 47–167 Torr (1 Torr = 133.3 Pa). These reactions are homogeneous, unimolecular and follow a first-order rate law. The rate coefficients are given by the Arrhenius equation: for 3-hydroxypropionitrile log k1 = (14.29 ± 0.47) − (234.9 ± 6.3) kJ mol−1 (2.303 RT)−1; for 3-hydroxybutyronitrile log k1 = (13.76 ± 0.10) − (222.6 ± 0.7) kJ mol−1 (2.303RT)−1; and for 3-hydroxy-3-methylbutyronitrile log k1 (s−1) = (13.68 ± 0.68) − (212.5 ± 8.7) kJ mol−1 (2.303RT)−1. The decomposition rates of the β-hydroxynitriles increase from primary to tertiary carbon containing the OH group. The rates for the β-hydroxynitriles are found to be slower than those for the corresponding β-hydroxyacetylene analogs. The value of log A from 13.7 to 14.4 and the small positive ΔS ≠ indicate a mechanism different from a six-centered cyclic transition state. These data appear to indicate that a four-membered cyclic transition state or a quasi-heterolytic mechanism is conceivable. Copyright © 1999 John Wiley & Sons, Ltd.

Energy dependence of the fragmentation of haloanisole molecular ions

Breakdown graphs have been constructed for the isomeric fluoro-, chloro-, and dichloroanisoles from the results of charge exchange experiments. When a halogen atom is ortho or para to the methoxy function, the dominant primary fragmentation reaction involves loss of CH 3 from the molecular ion, while when the halogen atom is meta to the methoxy group, the dominant primary fragmentation reaction involves elimination of formaldehyde from the molecular ion. This differing behaviour arises from a lowering of the critical energy for CH 3 loss by an ortho or para halogen through resonance stabilization of the resulting phenoxy ion. Metastable ion studies show that elimination of formaldehyde occurs by two mechanisms, one involving a four-membered cyclic transition state and one involving a five-membered cyclic transition state. A halogen meta to the methoxy function strongly favours the reaction involving the five-membered cyclic transition state.

Mechanism of formation of carbene complexes by reaction of cis-[PdCl 2(PPh 3)(CNR)] with secondary amines

A kinetic study is reported for the reactions of secondary aromatic amines p-YC 6H 4NHR (Y = MeO, Me, H; R = Me, Et) with the isocyanide complexes cis-[PdCl 2( p-XC 6H 4NC)(PPh 3)] (X = Me, H, Cl) leading to the carbene derivatives cis-[PdCl 2 {C(NH- p-C 6H 4X)NR- p-C 6H 4Y} (PPh 3)] in 1,2-dichloroethane at 25°C. A stepwise mechanism is proposed which involves a direct nucleophilic attack of the entering amine on the isocyanide carbon followed by proton transfers to the final carbene complexes. These take place both intramolecularly in a four-membered cyclic transition state and by the agency of one further amine molecule serving as a proton acceptor-donor in a six-membered transition state. Competition experiments with primary amines and trends in rate parameters are discussed to support the mechanism.

The Pyrolysis of some Neopentyl Xanthates: a Competitive Formation of Olefins and Dithiolcarbonates

A study of the pyrolysis of a series of neopentyl-type xanthate esters was carried out at 250 °C. The formation of olefins and dithiolcarbonates were found to be two competing processes. The olefin formation process became more important when the number of β-phenyl groups increased and when there was an electron donating group substituted at the para-position of the β-phenyl ring. Kinetics studies supported the proposal that the transition state of the olefin formation process involved a stabilized phenonium ion. However the dithiolcarbonate formation became more important when there was the least number of β-phenyl substituents and this rearrangement appeared to proceed through a concerted four-membered cyclic transition state.

Studies of the Thiocarbonyl Compounds. III. The Mechanism of the Thermal Rearrangement of Aryl Thionocarboxylates

Abstract The thermal rearrangement of aryl thionocarboxylates was kinetically investigated in diphenyl ether. The rate constants of the rearrangement of aryl N,N-dimethylthionocarbamates were well correlated with the σ− values, but the plots of the electron-releasing para-substituents deviated slightly on the lower side of the meta correlation line. A good linear free-energy relationship existed between the rearrangement of aryl N,N-dimethylthionocarbamates and the bimolecular nucleophilic reaction of 4- or 5-substituted 1-chloro-2-nitrobenzenes with piperidine. By the use of the substituent constants obtained from the latter reaction for the electron-releasing para-substituent constants, a fairly good ρ–σ− relationship was obtained. Linear free-energy relationships also existed between the rearrangements of aryl N,N-dimethylthionocarbamates and aryl thionobenzoates, and between the rearrangements of aryl N,N-dimethylthionocarbamates and O-aryl S-phenyl dithiocarbonates. The order of the reaction constants was in accord with that of the inductive effects of the α-substituents of the thiocarbonyl group, so the electron-releasing conjugative effects of the α-substituents of the thiocarbonyl group did not play important roles in the rate-determining step. The present results indicate that the thermal rearrangement of aryl thionocarboxylates is an intramolecular SN–Ar which involves a four-membered cyclic transition state formed by a nucleophilic attack of the lone-pair electrons of the thiocarbonyl sulfur atom on the migrating aromatic ring.

Mass spectrometry of tautomeric compounds—II: A mechanism of ketene elimination from enol acetates

Comparisons of the mass spectral behaviour of the [M – 42] ions from enol acetates and that of the corresponding ketones indicate that the elimination of a ketene molecule from enol acetates proceeded through a four-membered cyclic transition state to afford fragment ions with an enolic structure.