ON THE TAUTOMERISM OF MALEIMIDE AND PHTHALIMIDE DERIVATIVES

The tautomerism of maleimide and phthalimide and their derivatives was examined by means of ab initio MO theory. The molecular geometry of t he various tautomers was completely optimized employing the 6-31G bas is set and the semiempirical AM1 method, respectively. Correlation ene rgy effects were estimated using the MP2 formalism. The solvent influe nce on the tautomeric equilibria was considered by means of the quantu m chemical PCM formalism within the ab initio MO theory and the Cramer -Truhlar method (C.J. Cramer and D.G. Truhlar, J. Am. Chem. Soc., 113 (1991) 8305) incorporated into the semiempirical AM1 calculations. The results indicate the diketo tautomers of maleimide and phthalimide as the most stable ones both in the gas phase and in solution. The stepw ise replacement of the oxygen atoms by nitrogen causes significant cha nges. Whereas the lactam-lactim tautomerism can still be neglected in all compounds, the amino-imino tautomerism gains considerable importan ce, in particular in the maleic and phthalic imidine systems, where th e amino-imino forms are of comparable stability with the diimino forms .