Theoretical studies on nitrile‐forming elimination reactions

The base‐promoted nitrile‐forming elimination reactions of YCH=CHC β H=NOCH=CHZ (Y=OCH 3 , H or Cl and Z=H or NO 2 ) were studied by the AM1 MO theoretical method with Cl − as a base. The reaction is found to proceed by an E 1 cB ‐like E2 mechanism in which C β —H bond cleavage is more advanced than N—O bond breaking. The syn ‐elimination has a more E 1 cB ‐like transition state (TS) than the anti elimination, which is attributed to the structurally favourable n N—ó * (C β —H) charge‐transfer interaction. An electron‐withdrawing Y substituent lowers the activation barrier by stabilizing negative charge developed on C β in the TS. An electron‐withdrawing substituent in the leaving group (Z = NO 2 ) tends to enhance the anti relative to the syn elimination process by depressing the δ * (N—O) level, which in turn makes the nć—δ * (N—O) interaction more effective. The YCH=CH— and —C β H=N fragments are perpendicular in the TS, which is stabilized by delocalization of negative charge developed on the C β atom.