Heterolytic decarboxylation, mechanisms

Applying pulse radiolysis to the formation of Cuni(GlyGlyHis) only two consecutive reactions are observed experimentally during its decomposition. A volume of activation of Ah = +14 cm3 mol-1 was measured for the first observable step, which is in accord with the decarboxylation process being the rate determining step in this mechanism. AV = + 8 cm3 mol-1 was obtained for the second observable process in agreement with expectations for the heterolytic. in... 

Dichlorprop and 2,4-D are structurally related dichlorinated phenoxy-acetic acid pesticides (see Table 1). The photochemistry of 2,4-D has been studied some time ago 2,4-dichlorophenol was found as the the main product [75]. More recent photoproduct analysis studies are available for dichlorprop [76,77]. Climent et al. identified several products corresponding to dechlorination on the one hand, and to decarboxylation on the other ho-molytic bond cleavage steps were proposed to take place [76]. In a later study, Meunier et al. were able to demonstrate that 4-chloropyrocatechol is the major primary photoproduct [77]. A heterolytic cleavage of the ether bond and of the C - Cl bond was put forward as a possible mechanism (Scheme 9). 

Decarboxylation reactions may be induced (depending on the acid) in a variety of ways thermally, bacteriologically, photochemically, or even by electrolysis as in the anodic reaction of the Kolbe synthesis. Thermally induced decarboxylation of many carboxylic acids in solution proceeds by a bimolecular mechanism involving addition of a nucleophilic solvent molecule to an electrophilic carbon atom on the root molecule - preferably at a carbon adjacent to (a) or one removed from (P) the carboxyl carbon (Fraenkel et al. 1954 Clark 1958, 1969). An electrophile-nucleophile pair is formed in the transition state, which subsequently undergoes heterolytic fission (i.e., decomposition of a molecule into two ions of opposite charge) to yield CO2, a proton, and a carbanion the latter two species are reactive intermediates, which then combine rapidly (Brown 1951). The solvent molecule departs unaffected and in this sense the solvent may be considered... 

The photodecarboxylation mechanism of simple arylacetic acids is not completely understood due to the relatively low quantum yields of reaction (O < 0.06). The formation of benzyUc radicals (which show a transient absorption spectrum, with -315 nm) has been taken as evidence for the homolytic cleavage of the undissociated acid. By contrast, photolysis of carboxylate forms gives rise to carbanion intermediates -350 nm) via heterolytic cleavage (Scheme 1). However, the possibility of initial electron photoejection and subsequent homolytic decarboxylation (eventually followed by recombination of the electron with the benzyl radical) cannot be ruled out, as the hydrated electron has been observed as a transient species. ... 

---