Stereochemie der halogensubstituierten Bernsteinsäuren

Solvolytic Displacement Reactions At Saturated Carbon Atoms

“…In the hydrolysis of chlorosuccinate ion Holmberg (339,340,341) (382,383) was able to isolate the p-lactone intermediates of the solvolyses of p-bromocarboxylate salts. By analogy with a set of related reactions, the formation of the p-lactone should be considered as a displacement reaction accompanied by complete inversion of configuration.…”

The Walden cycle revisited: a computational study of competitive ring closure to α- and β-lactones

“…The TS bond orders suggest that { a 4 has progressed y80% towards the product; the barrier height represents y70% of the ring strain energy expected for an a-lactone. The difference in barrier heights for the two reactions (23 kJ mol 21 ) is only 27% of the difference in strain energy between the a-and b-lactone products (86 kJ mol 21 ) as determined by the difference in overall reaction energies in PCM water. Disparity between the ring-strain energies of small rings and of the TSs leading to them has been noted before.…”

“…The energy barrier for a-lactone formation is 108 kJ mol 21 , and the angle O nuc C a Cl involving the nucleophile and leaving group atoms in { a 4 is 139u in PCM water (Table 1). In contrast, the energy barrier for b-lactone formation in PCM water is only 85 kJ mol 21 However, it is of interest to note that the respective energy barriers under vacuum are 43 and 61 kJ mol 21 : this implies that formation of the a-lactone would be the kinetically preferred channel for this reaction in the gaseous phase, despite b-lactone formation being more exothermic by 70 kJ mol 21 . In PCM water, a-lactone formation is predicted to be endothermic by 64 kJ mol 21 as compared with the exothermic (222 kJ mol 21 ) formation of the b-lactone.…”

“…In contrast, the energy barrier for b-lactone formation in PCM water is only 85 kJ mol 21 However, it is of interest to note that the respective energy barriers under vacuum are 43 and 61 kJ mol 21 : this implies that formation of the a-lactone would be the kinetically preferred channel for this reaction in the gaseous phase, despite b-lactone formation being more exothermic by 70 kJ mol 21 . In PCM water, a-lactone formation is predicted to be endothermic by 64 kJ mol 21 as compared with the exothermic (222 kJ mol 21 ) formation of the b-lactone. The dianionic reactant is solvated much more strongly than the two separate monoanionic products, lactone and chloride; thus solvation energy contributes endothermically to the overall energy change for the reaction, and serves to increase the reaction barriers relative to the gas phase.…”

“…We have estimated the ring strain energy of unsubstituted oxiranone, the parent a-lactone, at 150 kJ mol 21 by means of similar calculations applied to the same isodesmic reaction used previously to estimate the strain energy under vacuum. 20 Pauling bond orders n = exp[(r 1 2 r n )/0.6 may be determined for the making and breaking of bonds in each TS, where r n is the length of a bond with order n, based upon the C-Cl bond in the chlorosuccinate reactant and the C-O bond in the b-lactone product having unit bond order.…”