RING-CHAIN TAUTOMERISM IN 2-(2,2-DICYANO-1-METHYLETHENYL)BENZOIC ACIDAND RELATED-COMPOUNDS

Ring-chain tautomerism with slow interconversion (compared with the NM R timescale) was observed in solutions of 2-(2,2-dicyano-1-methylethen yl)benzoic acid (3e), obtained by Knoevenagel condensation of 2-acetyl benzoic acid with malononitrile, forming the ring tautomer 3-dicyanome thyl-3-methylphthalide (4e) in admixture with 3e. Similar condensation s of 2-formylbenzoic acid with methyl cyanoacetate or malononitrile gi ve 2-(2-cyano-2-methoxycarbonyl-ethenyl)benzoic acid (3b) and 2-(2,2-d icyanoethenyl)benzoic acid (3d), respectively, which in solution also exhibit the same tautomerism to give the ring tautomers, 3-(cyanometho xycarbonylmethyl)phthalide (4d) and 3-(dicyanomethyl)phthalide (4d), r espectively. Condensation of 2-formylbenzoic acid with dimethyl malona te gave only the ring compound, 3-(dimethoxycarbonylmethyl)phthalide ( 4a). Attempts to synthezise (2-cyano-2-methoxycarbonyl-1-methylethenyl )benzoic acid (3c) by methylation of the trimethyl silylester of 3b wi th diazomethane led to the ring form of 3c, viz. 3-cyanomethoxycarbony lmethyl-3-methylphthalide (4c) as an equimolar mixture of two diastere omers. No tautomerism was observed when the benzene ring was replaced by a thiophene ring (7a, 7b and 8) or an aliphatic double bond (9). So lid state spectra (IR and NMR) indicated that all compounds carrying t wo cyano groups at the double bond, except the aliphatic compound 9, w ere in the open-chain form, while all the others were in the ring form . Equilibrium studies for compound (3e reversible arrow 4e) indicated increased stability for the chain form 4e with increasing solvent pola rity. Determination of the free energy change, Delta G degrees, and of the free energy of activation, Delta G(double dagger), for the tautom erization in deuteriochloroform (using H-1 NMR spectroscopy) indicated that, in this solvent, a concerted process from the starting material 3e to the anion of 4e is taking place. It is also postulated that a s imilar reaction path is followed in the other solvents used in this in vestigation, all belonging to the solvent class 'protophobic dipolar a protic solvents'.