Conjugated Betaines

The spectra of these highly conjugated betaines are solvent sensitive. The absorption maximum of 144 is 450 nm in carbon tetrachloride and 392nm in dimethylsulfoxide. The spectra of a number of substituted diaryl thiobetaines correlated well with Kossower s Z solvent polarity indicators.227... 

C. Heterocyclic Betaines Isoelectronic with Reducible Anions (Cross-Conjugated Betaines)... 

Further examples of cross-conjugated betaines (535-537) have been reported. 88-394 Particularly notable is the thermal ring opening of compounds of the general type 538 to give the ketene intermediates 539. ... 

A wide variety of methods for the synthesis of these compounds are available the following examples are intended only to be illustrative. Pyrazoles 153 react with reactive malonic acid derivatives, e.g., dichlorides in the presence of triethylamine, yielding pyrazolo[l,2-cross-conjugated betaines 154 (Scheme 93) <1980JA3971, 1994JOM(481)109>. Likewise, treatment of malonic acid dihydrazide 155 with a 1,3-diketone in ethanol at room temperature provides 156 in good yield (Scheme 94) <2005JHC287>. 

These elegant transformations (Scheme 19) clearly demonstrate that cross-conjugated betaines are in no way the poor relations of the betaine family. 

Figure 1 Neutral and charge-separated resonance structures of two vinylogue merocyanine dyes (Ml, M2) and a conjugated betaine dye (stilbazolium betaine) (M3).

The fructofurano-oxazoline 16 was obtained by reaction of 1,6-di-O-trityl-D-fructosc with cyanamide, and used in the construction of nucleoside analogues (see Chapter 20). The imidazoline 17 formed through solvent incorporation when l,3,4,6-tetra-0-acetyl-2-allyloxycarbonylamino-2-deoxy- 3-D-glucose was used as a glycosylating agent in acetonitrile. Synthesis of the cross-conjugated betaines 18 from glucosylamine has been reported. ... 

Clearly, in the case of (66) two amide tautomers (72) and (73) are possible, but if both hydroxyl protons tautomerize to the nitrogen atoms one amide bond then becomes formally cross-conjugated and its normal resonance stabilization is not developed (c/. 74). Indeed, part of the driving force for the reactions may come from this feature, since once the cycloaddition (of 72 or 73) has occurred the double bond shift results in an intermediate imidic acid which should rapidly tautomerize. In addition, literature precedent suggests that betaines such as (74) may also be present and clearly this opens avenues for alternative mechanistic pathways. 

Tliis chapter covers nitrogen-containing fulvalenes that can be obtained by replacement of CH=CH and/or CH, for example, types 1-3 starting from compounds 1-6. Compounds in which nitrogen atoms are arranged on the periphery of the cross-conjugated system as in 15 or 16, as well as derivatives in which the central double bond contains heteroatoms as in 17, are not included. For azoniafulvalenes of type 17 and related heterocyclic betaines see (94AFIC197). 

D. Pseudo-cross-conjugated Heterocyclic Mesomeric Betaines. 131... 

In contrast to the alkaloids mentioned so far, heterocyclic mesomeric betaines are defined as neutral conjugated molecules which can be represented only by dipolar structures in which both the negative and the positive charges are delocalized within the rr-electron system (38JCS824, 85T2239). The first heterocyclic mesomeric betaine was prepared unknowingly by Emil Fischer (1882LA316). The real structure remained unknown... 

The type of conjugation is also reflected in the frontier orbital profile, the charge distribution, and the permanent dipole moments. The results of semiempirical calculations on l-methylpyridinium-3-olate (16), Malloapeltine (17), Trigollenine (18), and Homarine (19) are presented in Scheme 7. Characteristically for the class of conjugated mesomeric betaines, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are distributed over the entire molecule as examplifled for l-methylpyridinium-3-olate. It was shown that 90% of the... 

Trigollenine (18) and the pseudo-cross-conjugated Homarine (19) are isoconjugate with the isopropenylbenzene anion, which is an odd alternant hydrocarbon anion. Therefore, these betaines belong to class 9 and 13, respectively. 

Dehydrodiscretamine (72), Thalifedine (73), Thalidastine (74), Fissisaine (75), Stepharanine (76), and Dehydrocorydahnine (77) are additional examples of alkaloids, which possess the 7-hydroxy-isoquinoline moiety which on deprotonation yields conjugated mesomeric betaines (Scheme 27). In either case, the rr-electron system is extended by substitution... 

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