Mechanism of Base-Promoted Eliminative Fragmentations of 2-Alkyl-3-Phenyloxaziridines

Article abstract of DOI:10.1021/jo00372a024

The base-promoted fragmentations of 2-benzyl-3-(4-substituted-phenyl)oxaziridines 1 and 2-(4-substituted-benzyl)-3-phenyloxaziridines 2 in anhydrous organic solvents give benzaldehydes and unstable benzylideneimines.The subsequent trimerization of the imines provides the benzylidene animals with the liberation of ammonia.The fragmentations can be regarded as an α,β-elimination and have been studied kinetically with triethylamine in acetonitrile at 40 deg C.The (Z)-oxaziridines react more rapidly than the corresponding (E)-oxaziridines.The Hammett ρ values for the E isomers of 1 and 2 are 0.68 and 0.89, respectively.The primary kinetic β-deuterium isotope effects (k_H/k_D) are 6.1 for (E)-2-benzyl-3-(4-nitrophenyl)oxaziridine <(E)-1a> and 6.9 for (E)-2-benzyl-3-(4-methoxyphenyl)oxaziridine <(E)-1e>.From these results and considerations on the magnitudes of the Broensted β (0.46), the activation parameters and the Arrhenius parameters for (E)-1a, the triethylamine-promoted fragmentations of 1 and 2 are best interpreted in terms of a near central E2 mechanism; for the transition state fragmentations of 1 and 2 are best interpreted in terms of a near central E2 mechanism; for the transition state of (E)-1a the Nα-O bond breaking is slightly ahead of the β-proton removal.The triethylamine-promoted fragmentations of (E)- and (Z)-2-methyl-3-(4-nitrophenyl)oxaziridines (9) in acetonitrile are slower than those of 1a, but give comparable primary isotope effects; k_H/k_D = 6.1 for (Z)-9 and 6.6 for (E)-9.The Arrhenius plot for (Z)-9 shows excellent linearity, suggesting neither change in mechanism nor the necessity for a tunneling correction.The fragmentation of (Z)-9 in chloroform is slightl y slower than that in acetonitrile, with k_H/k_D = 6.2.These data suggest that the tertiary amine promoted fragmentations of 2-alkyl-3-phenyloxaziridines with β-hydrogen exclusively proceed through an E2 mechanism.