A SERIES OF N-(2,3-DIHYDROXYBENZILIDENE)AMINES - DETERMINATION OF TAUTOMERIC EQUILIBRIA

The crystal structures of five N-(2,3-dihydroxybenzylidene)amine deriv atives, C7H6NO2-R [R = phenyl(1), p-methylphenyl (2), o-chlorophenyl ( 3), isopropyl (4) and cyclopropyl (5)] are presented and discussed. [I UPAC names: 3-(phenyliminomethyl)- (1), 3-(4-tolyliminomethyl)- (2), 3 -(2-chlorophenyliminomethyl) -(3) and 3-(cyclopropyliminomethyl)- 1,2 -benzenediol (5), and (isopropylaminomethylene)-2,4-cyclo-hexadien-1-o ne (4).] All the molecules are characterized by the presence of a stro ng intramolecular hydrogen bond, O-H ... N, which determines the forma tion of a six-membered pseudocycle in the same plane as the phenolic m oiety. When R is an aromatic ring, the molecules are either planar or make a dihedral angle which does not exceed 10 degrees: Except for (2) , all the molecules are associated as dimers with two intermolecular O -H ... O hydrogen bonds involved in a ten-membered pseudocycle. In com pound (5), an intermolecular hydrogen bond with a third molecule is al so observed. In (2), the cohesion of the crystal is mainly secured by the intermolecular hydrogen bond between the hydroxyl group and the ar omatic p-methylphenyl ring. Unlike the N-(2-hydroxybenzylidene)amines for which the phenolic tautomer largely prevails, in the present compo unds the quinonic form is present in significant amounts and is even d ominant for compound (4). Hence the presence of a second vicinal hydro xyl group determines an important shift in the tautomeric equilibrium.