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Tautomerizations of mesomeric betaines

This review tries to shed light on the early history of different methods for the generation of N-heterocyclic carbenes (NHCs), i.e., the extrusion of heterocumulenes (decarboxylations) from suitable mesomeric betaines, deprotonations of hetarenium salts, a-eliminations, tautomerizations of mesomeric betaines, and reductive desulfurizations of cyclic thioureas. Selected examples of acyclic and three- to eight-membered NHCs are presented, as well the generation of selected five- and six-membered anionic NHCs. [Pg.143]

Betaine—carbene interconversions can be achieved by decarboxylations or deprotonations from suitable betaines. Deprotonations of mesomeric betaines result in the formation of anionic N-heterocyclic carbenes. Some mesoionic compounds, for example, five-membered representatives of the conjugated mesomeric betaines, are in equilibrium with their tautomeric normal N-heterocyclic carbenes according to general Scheme 32 (2013ARK(i)424). [Pg.231]

The preceding sections demonstrate two important general routes to six-membered heterocyclic mesomeric betaines. These are (i) deprotonation of appropriate quaternary salts and (ii) valence tautomerism of bicyclic isomers. Both approaches have been used to prepare isoquinolinium-4-olates (170) (Scheme 7). [Pg.30]

When the diphenyl betaine 310 (R = Ph) is generated in acetic anhydride at 120°C in the absence of dipolarophile, a quantitative yield of 1,2-diphenylacenaphthylene (318 R = Ph) is obtained. This desulfurization occurs by initial valence tautomerism to the episulfide (317 R = Ph) which can be isolated in 40% yield if the reaction is performed at 100°C.2 The episulfide 317 (R = Ph) and the hydrocarbon 318 (R = Ph) are also produced by treating the sulfoxide 312 (R = Ph) with phenyllithium. In the presence of oxygen, 1,8-dibenzoylnaphthalene is also a major product, and these observations suggest that the mesomeric betaine 310 (R = Ph) is a common intermediate in these reactions. ... [Pg.57]

The structure of the reaction product of 2-aminopyridine and diethyl malonate, described by Chichibabin as 2,4-dioxo-3,4-dihydro-2//-pyrido-[l,2-<7]pyrimidine,96 was first questioned by Snyder and Robison253 on the basis of the high melting point and poor solubility of the compound. They suggested the tautomeric 2-hydroxy-4-oxo-4H-pyrido[l,2-a]pyrimidine structure. The problem was solved by Katritzky and Waring273 who compared the UV spectrum of the product with that of fixed tautomers and found that the product may best be described as anhydro- 2-hydroxy-4-oxo-4/f-pyrido[l,2- ]pyrimidinium)hydroxide (63). Because of the chemical behavior of these compounds, however, the contribution of other mesomeric forms to the structure has also been considered.122 Thus, PPP-SCF quantum chemical calculations suggest that 1,4-dipolar cycloadditions to the C-3 and C-9a atoms are to be expected.352 This type of reaction does in fact occur (see Section III,C,10). Katritzky and Waring273 estimated the ratio of the mesomeric betaine (63 R = H) and the 2-hydroxy-4-oxo tautomers to be about 20 1. [Pg.321]

For most compounds, including mesomeric betaines which cannot be satisfactorily represented by a Lewis formula, all the molecules have the same structure. On the other hand, for many compounds there is a mixture of two or more structurally distinct tautomeric compounds that are in rapid equilibria. Since the subject of this chapter is so wide, it covers both compounds the structure of which is usually represented by several dipolar structures (mesomeric betaines) and compounds which are present as a mixture of tautomers. Both aspects are treated in this subsection. [Pg.763]

Until very recently, it has not been recognized that the tautomerism of suitable mesomeric betaines is a source of NHCs. l,2,4-Triazolium-3-aminide 48 is a mesoionic compound and it has been used as analytical reagent for the detection of nitrate anions for a period of almost 100 years ( nitron, Busch s reagent) (1905CB861), before it was realized that nitron is in equilibrium with its NHC (Scheme 13). Numerous trapping reactions of the latter have been carried out (2012CC227). Rhodium complexes 49 were prepared, as well as thione formations and trapping reactions with carbon disulfide. [Pg.152]

Ylides derived from imidazolium- and triazolium-substituted indole anions belong to the class of conjugated mesomeric betaines. Their tautomers are N-heterocyclic carbenes, which undergo trapping reactions with sulfur, selenium, and triethylborane, respectively. Deprotonation of the salts 45a—c with NaOH/EtOH produces the ylides 46a—c which are in tautomeric equihbrium with carbenes 47a—c (Scheme 36) (2014T8672). [Pg.233]

In 2014, the first examples of cross-conjugated HMBs which are in tautomeric equilibrium with their N-heterocyclic carbenes were published (2014OBC2737). Deprotonation ofimidazolium uracilates 58a—e, prepared from 6-chlorouracil and imidazoles, afforded the mesomeric betaines 59a—e, which are in tautomeric equilibrium with the corresponding carbenes 60a—e (Scheme 40) (2014OBC2737). [Pg.235]


See other pages where Tautomerizations of mesomeric betaines is mentioned: [Pg.143]    [Pg.146]    [Pg.152]    [Pg.143]    [Pg.146]    [Pg.152]    [Pg.40]    [Pg.40]    [Pg.85]    [Pg.120]    [Pg.28]    [Pg.85]    [Pg.120]    [Pg.28]    [Pg.85]    [Pg.120]    [Pg.85]    [Pg.120]   


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Betaine tautomerism

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