Eliminations from (E)-O-arylbenzaldehyde oximes promoted by hydroxide in 60% aq. Dimethyl sulphoxide. Mechanism and transition-state characteristics of nitrile-forming eliminations

Reactions of the (E)-O-arylbenzaldehyde oximes (1; Ar = 2,4-dinitrophenyl), (2; Ar =p-nitrophenyl), and (3; Ar = phenyl) with OH⁻ in 60% aq. DMSO have been investigated. The eliminations are quantitative, producing only benzonitriles and aryloxides. The observation of second-order kinetics, k₂^OD-/k₂^OH-= 1.15-1.84, and β_lg =-0.59 is consistent with an E2 mechanism. The Hammett ρ and k₂^OD-/k₂^OH- values increased with poorer leaving groups. For a given substrate the k₂^OD-/k₂^OH- value remained (within experimental error) roughly constant, in spite of changes in the β-aryl substituents. These changes in transition state parameters can be interpreted in terms of the differing nitrite-forming transition states involved.