Zwitterion phases mechanism

A mechanism has been proposed recently by O Neal and Blumstein for the gas-phase ozone-olefin reaction. This mechanism postulates that molozonide-biradical equilibrium is reached fast and postulates a competition between a-, 8-, and y-hydrogen abstraction reactions and the classical mechanism proposed by Criegee for the liquid-phase reaction. The main features of the Criegee mechanism (Figure 3-9) are the formation, from the initial molozonide, of the major carbonyl products and a second biradical intermediate, the zwitterion. The decomposition pathways of the zwitterion comprise unimolecular re-... 

The concerted nature of these reactions is also supported by the pyrolytic cleavage of cis-2,3-dimethylcyclobutanone and rra i-2,3-dimethylcyclobutanone at a temperature of 325 °C under a pressure of 10 Torr, from which approximately 99% retention of configuration in the resulting but-2-enes is observed.92,93 Moreover, the results obtained from the thermolysis of cis- and m -2,4-dimethylcyclobutanone also lend support to the concertedness of this reaction.92,93 However, as substantiated by Arrhenius parameters, a diradical pathway is the most likely competing reaction in the pyrolysis of 2-chlorocyclobutanone.94 On the other hand, the kinetics of the gas-phase pyrolysis of bicyclo[3.2.0]hept-2-en-6-one (14),95 bicyclo[3.2.0]heptan-6-one (IS)96 and 2,2-dimethyl-3-ethoxycyclobutanone (16)97 are commensurate with a concerted cycloreversion mechanism involving four-center auasi-zwitterionic transition structures. 

The cycloaddition of formaldehyde and ketene has been studied by ab initio methods.22 A two-step zwitterionic mechanism is suggested for dichloromethane solvent, while the gas-phase reaction is concerted but asynchronous. 

A systematic study of the impact of geminal a-fluorine substitution upon the rate of decarboxylation of /1-lactones has included investigation of the thermolysis of oc,oc-difluoro /1-lactones, to give CO2 and 1,1-difluoroalkenes, in the gas phase and in solution.48 The gas-phase results have been interpreted, with reference to ab initio calculations on the fluoro- and non-fluorinated /1-lactone systems, in terms of a probable concerted, asynchronous, non-polar mechanism. However, a polar mechanism which probably involves formation of an intermediate zwitterion has been invoked to explain the solvent dependence observed. 

DFT was employed to study the mechanism of ammonolysis of phenyl formate in the gas phase, and the effect of various solvents on the title reaction was assessed by the polarizable continuum model (PCM). The calculated results show that the neutral concerted pathway is the most favourable one in the gas phase and in solution.24 The structure and stability of putative zwitterionic complexes in the ammonolysis of phenyl acetate were examined using DFT and ab initio methods by applying the explicit, up to 7H20, and implicit PCM solvation models. The stability of the zwitterionic tetrahedral intermediate required an explicit solvation by at least five water molecules with stabilization energy of approximately 35 kcalmol-1 25... 

Quantum mechanical calculations in the gas phase and DMSO solution at different temperatures can highlight the hazards of standard 0 K gas-phase calculations.259 For the Wittig reaction, a small barrier in the potential energy curve is transformed into a significant entropic barrier in the free energy profile, and the formally neutral oxaphosphetane intermediate is displaced in favour of the zwitterionic betaine in the presence of DMSO. 

W-C4H9) [106, 107]. Ab initio MO calculations have shown that the [2 - - 2] cycloaddition between cis- or Jraw-alkenes and isocyanates to yield y9-lactams takes place by a concerted mechanism in the gas phase, with retention of the alkene configuration in the product. However, increasing solvent polarity contributes to the asynchronicity of the reaction to such an extent that the mechanism changes from a concerted to a two-step process involving a zwitterionic intermediate, with a consequent loss of alkene stereospecificity, as observed in the reaction between chlorosulfonyl isocyanates and vinyl ethers [794]. 

This part of the chapter is devoted to the structural analysis of some flexible molecules in condensed phase, for which a quantum mechanical approach becomes mandatory even for a qualitative study (e.g. zwitterions, radicals). 

The system examined in this study was that of a simple phenylalanine /chloride counter-transport system in which the interior phase was concentrated in chloride anion and the exterior phase contained concentrations of phenylalanine in the range of commercial fermentation titers. Since phenylalanine (like all a-amino acids) is predominantly a zwitterion at pH s between 3.5 and 9.0 (and is thus unable to be transported through the membrane), the pH of the exterior phase was kept above pH 10.0 to insure that the amino acid was present in its transportable anionic form. As mentioned above, the mechanism for this system is shown in Figure 2. 

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