Nitro mechanistic analysis

Substitutions by the SRn 1 mechanism (substitution, radical-nucleophilic, unimolecular) are a well-studied group of reactions which involve SET steps and radical anion intermediates (see Scheme 10.4). They have been elucidated for a range of precursors which include aryl, vinyl and bridgehead halides (i.e. halides which cannot undergo SN1 or SN2 mechanisms), and substituted nitro compounds. Studies of aryl halide reactions are discussed in Chapter 2. The methods used to determine the mechanisms of these reactions include inhibition and trapping studies, ESR spectroscopy, variation of the functional group and nucleophile reactivity coupled with product analysis, and the effect of solvent. We exemplify SRN1 mechanistic studies with the reactions of o -substituted nitroalkanes (Scheme 10.29) [23,24]. 

While the analysis of intermediates and/or products in the preceding examples unambiguously demonstrates the participation of zinc-coordinated nucleophiles in the chemical transformation, it generally is not possible to demonstrate unequivocally that the activation of a coordinated water molecule assumes the same mechanistic significance in metal ion-catalyzed hydrolytic reactions. This happenstance, as pointed out by Breslow et al. 19), is due (a) to the extremely labile nature of most aquo complexes, (b) to the invariance of water concentration in aqueous milieu, and (c) to the mechanistic ambiguity arising from the kinetic equivalance of the two mechanisms, A and B, illustrated below (here p-nitro-phenyl picolinate is used as the hypothetical example). 

The crystal and molecular structure of 2-(a-p-bromophenyl-P-nitro)-ethyl-5-methylcyclohexanone (6), a product from reaction of l-morpholino-3-methylcyclo-hexene with p-bromo-P-nitrostyrene, has been determined. Although a cis-configu-ration was anticipated on a mechanistic basis, the n.m.r. spectral analysis did not provide an unequivocal answer. X-Ray analysis indicated a chair cyclohexanone ring with the methyl group axial and the substituted ethyl group equatorial. 

---