Origin of Apparent Stereoelectronic Effects in Structure and Reactivity of Benzoquinone Monooximes

The origin of some unusual "sidedness" effects in 1,4-benzoquinone oximes is reinvestigated. It had been observed that (1) the NMR coupling constant J(23) is larger than J(56), (2) substituents at the 2 position affect the position of the E/Z equilibrium, and (3) addition reactions show a syn stereoselectivity. It was proposed that the nitrogen lone pair is delocalized into the sigma orbital of the antiperiplanar C3-C4 bond, thereby lengthening that bond and increasing the contribution of resonance forms with a C2-C3 double bond. Semiempirical MO calculations now show that the C3-C4 bond is not lengthened in benzoquinone oximes and analogous molecules. Protonated 1,4-benzoquinone 4-hydroxymethide is synthesized as a comparison lacking the nitrogen lone pair, and the H-H coupling constants are measured. Again J(23) is larger than J(56). Therefore the difference in the coupling constants is not due to the nitrogen lone pair. The effect of 2-substituents on the E/Z equilibrium is attributed to electrostatic interactions. According to calculations, the hydroxyl oxygen is within van der Waals contact with H3. We therefore conclude that steric effects are primarily responsible for the observed sidedness, both in the stereoselectivity and the coupling constants in both quinone oximes and quinone methides.