Chain-ring

An interesting ring-chain tautomerism between 2-azidothiazole (328) and thiazolotetrazole (328a) has been reported (597, 618, 619), the 328 structure predominating (Scheme 190). The solvent polarity and basicity influences this equilibrium constant significantly (1592). 

The thermal stability of polymers of types (1) and (2) is also dependent on the nature of the substituents on phosphoms. Polymers with methoxy and ethoxy substituents undergo skeletal changes and degradation above about 100°C, but aryloxy and fluoroalkoxy substituents provide higher thermal stability (4). Most of the P—N- and P—O-substituted polymers either depolymerize via ring-chain equilibration or undergo cross-linking reactions at temperatures much above 150—175°C. 

The ring-opening polymerization of is controUed by entropy, because thermodynamically all bonds in the monomer and polymer are approximately the same (21). The molar cycHzation equihbrium constants of dimethyl siloxane rings have been predicted by the Jacobson-Stockmayer theory (85). The ring—chain equihbrium for siloxane polymers has been studied in detail and is the subject of several reviews (82,83,86—89). The equihbrium constant of the formation of each cycHc is approximately equal to the equihbrium concentration of this cycHc, [(SiR O) Thus the total... 

Some 4,5-disubstituted pyridazines exhibit ring-chain isomerism involving heterospiro compounds. For example, 5-(o-aminophenylcarbamoyl)pyridazine-4-carboxylic acid exists in a zwitterionic form in the solid state, but in a solution of DMSO it is almost exclusively 3, 4 -dihydro-3 -oxospiro[pyridazine-5(2//),2 (l //)-quinoxaline]-4-carboxylic acid (134). The equilibrium is strongly influenced by the nature of the solvent, the substituents on the pyridazine ring and the nucleophilicity of the group attached to the phenyl ring (Scheme 48) <80JCS(P2)1339). 

Thiol-thione tautomers have not been extensively studied, but UV and IR evidence show that 5-phenylisothiazole-3-thiol exists in the SH form. Ring-chain tautomerism of 2,3-dihydro derivatives of 1,2-benzisothiazole can occur (26a 26b) and the position of equilibrium depends very much on the solvent, physical state and nature of the substituents (69JOC919, 81KGS1209). 

Benzoxadiazines ring-chain tautomerism, 3, 1056 ring contraction, 3, 1066... 

Ring transformations heterocyclic compounds reviews, 1, 70 Ring-chain tautomerism polyheteroatom six-membered rings, 3, 1056 Ripariochromene A synthesis, 3, 751, 755 Robinson-Gabriel synthesis oxazoles, 6, 216... 

These cover the following topics (a) theoretical methods, with emphasis on the utility of such methods b) molecular dimensions, as determined by X-ray, electron diffraction and microwave spectra (c) molecular spectra, covering NMR, IR, UV, mass and photoelectron spectra [d) thermodynamic aspects, such as stability, ring strain, aromaticity, shape and conformation of saturated and partially saturated rings (c) tautomerism, including prototopic and ring-chain (/) betaine and other unusual structures. 

However, a critical examination of the enormous amount of the experimental material that has piled up over more than 80 years leads to the conclusion that the three tautomeric forms postulated by Gada-mer have not been proven in one single case. The so-called evidence is based on chemical reactions, " which are noncompelling, or on physical constants that have been interpreted in a biased manner. For the simultaneous existence of both forms of the ring-chain proto-tropic system, i.e., the carbinolamine and the amino-aldehyde, evidence is available only for one single case. ... 

The situation is similar for the derivatives obtained from the bases by the action of various nucleophilic agents. These derivatives can also be considered as ring-chain prototropic systems. There is also... 

Thus the following problems are unsolved (1) Does the threefold tautomerism assumed by Gadamer really exist for heterocyclic pseudo bases and which factors determine the stability of the various forms (2) Does ring-chain tautomerism occur in the derivatives of these bases when this is formally possible, and if not, do these derivatives possess the cyclic or open-chain structure, and by what path have they been formed ... 

The reaction products formed by other nucleophilic reagents (e.g., amino compounds or carbanions derived from carbonyl or nitaro compounds) can be considered as ring chain prototropic systems. The newly formed C—or C—bond in the cyclic form of these compounds is more or less polar (evidence for this is the sensitivity to... 

As an over-all conclusion, it can be stated that the assumption of ring-chain tautomerism in the pseudo bases derived from the heterocyclic quaternary ammonium salts is quite unnecessary as an explanation of the formation of two (cyclic and open-chain) types of deriva-... 

The hydrochloride of (3) holds water rather tenaciously, and the infrared spectrum indicates that the water is covalently bound. Mild oxidation of the cation (3) gives 4-hydroxyquinazoline in high yield and ring-chain tautomerism is excluded on the grounds that quinazo-line does not give a positive aldehyde test in acid solution, 2-Methyl-quinazoline also has an anomalous cationic spectrum and a high basic strength (see Table I), but 2,4-dimethylquinazoline is normal in both these respects, which supports the view that abnormal cation formation entails attack on an unsubstituted 4-position. ... 

Despite the increasing information on the photochemistry of 2,4-dienones and other unsaturated ketones, as well as on the ring-chain valence isomerism of halogen-substituted pyran and dihydi opyran systems,the data are still very scarce. The intermediate formation of pyrans valence-isomeric with unsaturated carbonyl compounds in the pyridine syntheses based on reactions of ammonia with aldehydes or ketones, advocated by various authors (cf. Section II,B,2,f), is still rather speculative. (See also Section II,B,2,e for the valence isomerism of 5-chloro-2,4-dienones with pyrylium chlorides.)... 

The spectra of 1,3,5-, 1,3,7-, and 1,3,8-triazanaphthalerie cations revealed that they were predominantly hydrated, and mild oxidation to the corresponding 4-oxo compounds indicated that hydroxylation took place on C-4. Ring-chain tautomerism in the cation was excluded, at least during the first 45 min after mixing, because negative aldehyde tests were obtained with p-nitrophenylhydrazine. 1,3,6-Triazanaphthalene, under the same conditions, gave a yellow precipit-... 

The phenomenon of tautomerism comprises many different types of which the prototropic tautomerism that we consider here is only one. Prototropic tautomerism exists when the two tautomers differ only in the position of a proton (this is, of course, an approximation there are other differences between two tautomers, for example, in precise bond lengths). Other important types of tautomerism include the following (1) anioniotropy, where the two tautomers differ only in the position of an anion, which moves from one place to another in the molecule (2) cationiotropy, where the two tautomers differ in the position of a cation (other than a proton), which moves from one place to another in the molecule (3) ring-chain tautomerism and (4) bond-valence tautomerism. 

C. Ligands Susceptible to Ring-Chain and Azido-Tetrazole Tautomerism. 294... 

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