Theoretical studies on nitrile-forming elimination reactions

The base-promoted nitrile-forming elimination reactions of YCH=CHCβH=NOCH=CHZ (Y=OCH3, H or Cl and Z=H or NO2) were studied by the AM1 MO theoretical method with Cl− as a base. The reaction is found to proceed by an E1cB-like E2 mechanism in which Cβ—H bond cleavage is more advanced than N—O bond breaking. The syn-elimination has a more E1cB-like transition state (TS) than the anti elimination, which is attributed to the structurally favourable nN—o* (Cβ—H) charge-transfer interaction. An electron-withdrawing Y substituent lowers the activation barrier by stabilizing negative charge developed on Cβ in the TS. An electron-withdrawing substituent in the leaving group (Z = NO2) tends to enhance the anti relative to the syn elimination process by depressing the δ*(N—O) level, which in turn makes the nc—δ*(N—O) interaction more effective. The YCH=CH— and —CβH=N fragments are perpendicular in the TS, which is stabilized by delocalization of negative charge developed on the Cβ atom.