ORIGIN OF APPARENT STEREOELECTRONIC EFFECTS IN STRUCTURE AND REACTIVITY OF BENZOQUINONE MONOOXIMES

The origin of some unusual ''sidedness'' effects in 1,4-benzoquinone o ximes is reinvestigated. It had been observed that (1) the NMR couplin g constant J(23) is larger than J(56), (2) substituents at the 2 posit ion affect the position of the E/Z equilibrium, and (3) addition react ions show a syn stereoselectivity. It was proposed that the nitrogen l one pair is delocalized into the sigma orbital of the antiperiplanar C3-C4 bond, thereby lengthening that bond and increasing the contribut ion of resonance forms with a C2-C3 double bond. Semiempirical MO calc ulations now show that the C3-C4 bond is not lengthened in benzoquinon e oximes and analogous molecules. Protonated 1,4-benzoquinone 4-hydrox ymethide is synthesized as a comparison lacking the nitrogen lone pair , and the H-H coupling constants are measured. Again J(23) is larger t han J(56) Therefore the difference in the coupling constants is not du e to the nitrogen lone pair. The effect of 2-substituents on the E/Z e quilibrium is attributed to electrostatic interactions. According to c alculations, the hydroxyl oxygen is within van der Waals contact with H3. We therefore conclude that steric effects are primarily responsibl e for the observed sidedness, both in the stereoselectivity and the co upling constants in both quinone oximes and quinone methides.