Ring-chain tautomerism and related studies

Abstract

The thesis is entitled “Ring-Chain Tautomerism and Related Studies” and is divided into three parts. Part I consists of two chapters.

Part I

Chapter I consists of a review on tautomerism containing:

A historical outline,

A discussion on its definition and classification,

A survey of chemical and physical methods employed in the study of tautomerism, illustrated with examples from recent literature, and

A discussion on structural and environmental effects on tautomeric equilibria.

In Chapter II, a systematic survey of ring-chain tautomerism has been attempted. Different types of ring-chain tautomeric equilibria are also discussed. A brief account at the end deals with examples of chemical reactions in which ring-chain tautomerism is involved.

Part II

In Part II, results of the present investigation on the structural and environmental influences on the ring-chain tautomerism of o-benzoylbenzoic acids are discussed. Nine acids with substituents in the non-carboxyl group-bearing ring in four solvents have been studied.

Infrared spectra of the acids show that all these acids, except ?, exist in the open form in the solid state.

In solution, an equilibrium exists between the predominant open form and the cyclic pseudo-acid form (I ? II). Tautomeric equilibrium was estimated by quantitative infrared spectroscopic analysis using the “absorption area” technique. The normal and pseudo esters of each acid were prepared and used as model compounds for the two tautomeric forms of the acid in the quantitative analyses.

Ring-chain tautomeric equilibria of these acids in four solvents—chloroform, dioxane, acetonitrile, and nitromethane—were estimated. A linear free-energy relationship with Hammett’s substitution constants has been found to exist in the ring-chain tautomeric equilibria of o-benzoylbenzoic acids. The effect of substituents in rings A and B of o-benzoylbenzoic acids and also the influence of the medium on the tautomeric equilibria have been evaluated.

Part III

Part III deals with the results obtained in the present investigation on the reactions of pseudo-acid chloride of o-benzoylbenzoic acid (VIa) with different types of nucleophiles.

Previous work on the reactions of pseudo-acid chloride is briefly surveyed and two possible reaction pathways are identified. The reagents employed in the present investigation include different types of alcohols, alkyl and aryl amines, urea, thiourea, diazoalkanes, sodium azide, sodium nitrite, potassium cyanide, and potassium thiocyanate.

Reactions of VIa with sodium iodide, diethyl sodiomalonate, methyl iodide, and diethyl malonate in the presence of pyridine were also investigated. The products obtained in all these reactions have been characterized, and their structures have been established.

Alcohols, irrespective of their nature, gave only the pseudo esters (VIb). Of the nitrogen bases, ammonia, monomethylamine, and dimethylamine gave open-chain amides (VII). The first two of these amides exist as the cyclic hydroxylactams (VIIIa and VIIIb) in chloroform solution.

Aniline, on the other hand, gave 3-anilino-3-phenylphthalide (VIc) and the normal anilide (VIId), while the weaker base p-nitroaniline gave only the cyclic 3-(p-nitroanilino)-3-phenylphthalide (VIe).

Urea and thiourea yielded only the phthalide derivatives (VIf and VIg). Diazomethane and phenyldiazomethane reacted with VIa to give a high-melting lactonic compound, which was characterized as 3,3?-diphenyl-3-biphthalidyl (IX). Normal and pseudo esters (VIg and Xa, and VIh and Xb) were also isolated from these reactions.

Diazoacetic ester, on the other hand, gave normal (Xc) and pseudo (VIIi) esters only. The reaction of pseudo-acid chloride with sodium azide, potassium cyanide, and potassium thiocyanate gave the corresponding 3-substituted 3-phenylphthalides (VIj, VIk, and VIl respectively). The reaction of sodium nitrite and VIa gave the ?-anhydride (XI) of benzoylbenzoic acid.

The same anhydride was obtained along with dilactone (IX) in the reaction of methyl iodide with pseudo-acid chloride (VIa). The reaction of VIa with sodium iodide gave the dilactone IX in almost theoretical yield, which was separated into two isomers: the higher-melting meso compound and the lower-melting racemic product.