The conformational effect of predominance for the axial conformer ( % A) has been observed for the title compounds and is discusse in terms of steric and orbital interactions.
Recently it has been demonstrated that the anomeric effect Is2 (predominance of axial conformation for the systems of type I_) can be imitated by the substitution of heteroatom Y by other groups having p-orbital (e.g. systems 2_ 3 and Z4).The phenomenologically analogous effect (see Ref.2) for ketone systems of type 4 has been studied in detail 5.
It seemed attractive70 imitate the carbonyl group by substituting for oxygen, a double-bonded carbon atom linked to electronegative substituents and to investigate the possibility of the existence of a conformational effect leading to axial preference in these systems.The 2-substituted cyclohexylidenemalononitriles zare good models for that purpose.
Here we report a 'H NMR study of the conformational equilibria of 6-11 . ..%which provide the evidence for the conformational effect discussed.BromideL was obtained by NBS bromination of cyclohexylidenemalononitrile;compounds A, ,7_ and ll_ were synthEsised by the condensation of the corresponding ketones with malononitrile .Compounds gand lkwere obtained as described elsewhere?
The 'H NMR spectra of 6-11 were recorded at 30' in five solvents (9 mols, T-60 Varian).Widths at ha;hzeight for the Hx signal were employed to calculate the amount of conformer &using the Eliel eqn: \Vx obs=nW~+(l-n)W~.