Concerted anionic pathway

The first step of the mechanism is the deprotonation of the O-acylated ketoxime at its a-position, which gives rise to the corresponding enolate. This enolate then can react via two possible pathways 1) a concerted anionic pathway in which the leaving group is directly displaced to give the isolable 2H-azirine or 2) a nitrene pathway that leads to the same 2/-/-azirine intermediate via nitrene Insertion. The nitrene pathway has not been disproved experimentally. 

The chemistry of radical sites adjacent to phosphatoxy centers elicited interest because of the involvement of such species in DNA degradation processes. These species can give rise to rearrangement, elimination, and substitution products, and for some time concerted eliminations and migrations as well as heterolysis to a radical cation and a phosphate anion were considered to be involved (Scheme 2). Recently, experimental studies of the l,2-dibenzyl-2-(diphenylphosphatoxy)-2-phenylethyl radical and complementary theoretical studies of l,l-dimethyl-2-(dimethylphosphatoxy)ethyl radical have been interpreted as indicating that a radical cation/anion pathway with initial formation of 49 is favored. ... 

Finally, it is increasingly clear that molecules confined in the zeolitic nanocavities see their electronic properties modified. It has been shown for instance (19) that the dipole moment of acetonitrile increases significantly upon its introduction in the side pockets of MOR compared to the linear channels of the same zeolite. The guest molecule is made more basic and is easily protonated in such a confined environment. Zeolites also act as solid solvents and the anionic framework acts as the conjugate base of the proton thereby stabilizing some charged intermediates along concerted catalytic pathways. 

A theoretical study of degenerate Boulton-Katritzky rearrangements concerning the anions of the 3-hydroxyimi-nomethyl-l,2,5-oxadiazole has been carried out by using semi-empirical modified neglect of diatomic overlap (MNDO) and ab initio Hartree-Fock procedures. Different transition structures and reactive pathways were obtained in the two cases. Semi-empirical treatment shows asymmetrical transition states and nonconcerted processes via symmetrical intermediates. By contrast, ab initio procedures describe concerted and synchronous processes involving symmetrically located transition states <1998JMT(452)67>. 

Aldehyde 54 and the hydroxamic acids 55 were generated together in an acid-catalysed elimination reaction (Scheme 7 pathway (ii)). A crossover experiment indicated that esters are formed in a concerted rearrangement concomitant with the likely formation of the hydroxynitrene 57 (Scheme 7 pathway (iii)) while there is no evidence to date for the formation of hydroxynitrene, joint solvolysis of equimolar quantities of /V-acetoxy-/V-butoxy-/>-chlorobenzamide 26e and N- acetoxy-/V-benzyloxybenzamide 27a afforded significant quantities of butyl p-chlorobenzo-ate (36%) and benzyl benzoate (54%) as the only esters. This is an example of a HERON reaction, which has been identified in these laboratories as a characteristic rearrangement of bisheteroatom-substituted amides.32,33,42 43 155 158 Since ester formation was shown to prevail in neutral or low acid concentrations, it could involve the conjugate anion of the hydroxamic acid (vide infra).158... 

From the rate data thus obtained it is difficult to conclude whether the reaction goes through the intermediacy of the polyhalogenoalkane anion radical or rather follows a concerted pathway. The fact that the anion radicals have been detected under quite different medium and driving force conditions (Brickenstein and Khairutdinov, 1985 Mishra and Symons, 1973) does not provide unequivocal evidence for the stepwise pathway, as discussed earlier. Some evidence in favour of the latter mechanism was. 

For the reduction of CCI4 by electrochemically generated aromatic anion radicals in DMF (Eberson et al., 1989), the kinetics were treated according to a reaction scheme in which CCl, " appears as an intermediate, but no strong evidence was offered that they would not fit a concerted pathway as well. As discussed on pp. 32, 33, it is indeed not possible to distinguish between the two pathways in such types of experiment when, in the stepwise pathway, the cleavage reaction is so fast that the electron-transfer step would be rate determining, a situation likely to be met in the present case. 

Fig. 14 Reduction of PhjCSPh by electrochemically generated aromatic anion radicals (in DMF at 25°C). Variation of the rate-determining step rate constant, A , with the standard potential of the aromatic anion radical, p,g (from left to right azobenzene, benzo[c]cinnoline, 4-dimethylaminoazobenzene, terephthalonitrile, naphthacene, phlhalonitrile, perylene, fluoranthene, 9,10-diphenylanthracene). The dotted lines are the theoretical limiting behaviours corresponding to the concerted (right) and stepwise (left) pathways. (Adapted from Severin et al 1988.)...

Thus, if the concerted pathway has a stronger driving force than that of the first step of the stepwise pathway, the cleavage of anion-radical is thermodynamically favorable. Stepwise mechanisms are considered viable when the intermediate (anion-radical in the preceding schemes) has a lifetime that is longer than the time for a bond vibration (ca. 10 s). Concerted mechanisms occur under... 

---