Delocalized zwitterions

The calculations further show that the initially formed siliranimine 106 is less stable than the isomeric silaaziridine 107 (see Eq. 38) by about 6 kcal mol-1. The mechanism for the conversion suggested by the authors206 involves a four-membered carbene 108 rather than the open but delocalized zwitterion 109 suggested by Brook.204 Further experimental work is desirable to clarify the reaction mechanism. 

Rhodium (Il)-catalysed 3 + 2- and 3 + 4-cycloaddition of diazocarbonyl compounds (7) with conjugated dienes (e.g. 2,3-dimethylbuta-l,3-diene) provides a simple and rapid route to dihydrofurans (8) and dihydrooxepins (9) in high yields. A stepwise mechanism involving delocalized zwitterions has been proposed for the formation of the cycloadducts (Scheme 2).3... 

X-ray analysis of 2-methoxy-4-hydroxy-5//-l-benzazepin-5-one (a benzazatropolone), prepared by methylation of the corresponding 4-hydroxy-l-benzazepin-2,5-dione with Meerwein s reagent, demonstrates the presence of a planar seven-membered ring but, in contrast to tropolone, little 71-electron delocalization.17 Likewise, ll//-dibenz[f>,e]azepin-ll-ones display no significant aromatic character.18 In contrast, 7-chloro-8//-thieno[3,2-c]azepin-8-one (12) has azepine ring hydrogen resonances at 8.7 and 9.02 ppm that indicate a substantial contribution from the polar zwitterionic mesomer 13.19... 

The facile thermal isomerization (17) of norbornadiene derivatives [71]-[77] to cycloheptatrienes in polar solvents has been explained in terms of the initial heterolytic cleavage of the strained C(l)-C(7) bond (Hoffmann and Hauser, 1965 Lemal et al., 1966 Hoffmann, 1971, 1985 Lustgarten and Richey, 1974 Hoffmann et al., 1986 Bleasdale and Jones, 1993). The resulting zwitterion intermediates are stabilized by the cation-stabilizing groups attached to C(7) and the cyclohexadienyl-type delocalization of the negative charge. 

Tertiary amines too are found to produce cr-complexes 104 with 1,3,5-trinitrobenzene and related compounds. Complexes 104 are studied by spectroscopic and kinetic methods. In the case of l,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU) and 1,5-diazabicyclo[4,3,0]non-5-ene (DBN), the delocalization of the positive charge of the nucleophile moiety in the usaturated system of DBN and DBU enhances the stability of the zwitterionic complex 105 and, of course, the nucleophilic power of these bases, as well as their basicity244. 

All studies show the zwitterionic nature of the spiropyran merocyanine beyond any doubt and we can further see how the acid-base characteristics of the C9 oxygen and indoline nitrogen are delocalized as alternating charges on the methine bridge, leading to the labile properties of the bridge itself. 

The structure of tetraphenylhexaazanthracene was confirmed as 3 by X-ray crystallography and revealed a monoclinic unit cell, with the zwitterionic molecules arranged in stacks along the shortest crystallographic c axis. The charges are delocalized as depicted in 3 <1998JA2989>. 

The transient zwitterion (15), obtained from the /(-hydroxy acid precursor by laser flash photolysis, has been characterized 46 it reacts with nucleophiles more slowly than does the 9-fluorenyl cation itself. The parent acid was also characterized.46 Evidence has been presented that (16) undergoes substantial El elimination via a primary carbocation.47 An analysis of solvolysis results for (17) is indicative of extensive charge delocalization throughout the fluorenyl ring at the transition state apparently... 

In summary, we emphasize that homoaromatic stabilization appears to be more important for radical cations than for their neutral diamagnetic precursors. There are several reasons why radical cations may assume cyclic conjugated structures with 4n + 17i-electrons, one electron shy of the magic 4n + 2 7i-electrons which achieve aromaticity, when the parent molecules opt for a less delocalized nonaromatic structure. The principal cause might lie in the strength of the carbon-carbon single bond, which generally disfavors biradical or zwitterionic... 

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