Thin-Layer Chromatography of the N-Substituted Maleimides on Alumina: Stereochemical Factors

The N-phenyl-; N-benzyl-; o, m, and p N-(methylphenyl)-; o, m, and p N-(chlorophenyl)-; o and p N-(methoxyphenyl)-; and o, m, and p N-(nitrophenyl) maleimides were prepared. Ultraviolet spectroscopic studies indicated that in all cases the o isomers were sterically hindered and nonplaner, whereas the m and p isomers were planer accompanied by increased delocalization of the π elections. Chromatography in four solvent systems on alumina-C showed the o-Me and o-Cl compounds to be retained more firmly, than the MeO and NO2 families. It was concluded that charge-transfer interaction of delocalized π electrons with the surface was secondary to specific group interactions, that the projecting imide carbonyl group of the o-Me and o-Cl isomers was favorable to retention, and that there was concerted action of the carbonyl and substituent groups of the MeO and NO2 families with the surface, so that the planer m and p isomers were better adsorbed because of a favorable posture on the surface.