Delocalisation/aromaticity

A further significant feature of the ethyl-vinyl complexes Tp Ir(C2H4R)(CH = CHR)(NCR ) (R = Me (292), (473), (390), R = Bu (387)) and, in acetonitrile, 198, is their propensity for intramolecular [3 + 2] cycloaddition of the alkenyl and nitrile ligands.This affords iridapyrrole complexes 474, 475, 545, 546 and 547 (Chart 9 discussed in Section II-E) for which fully delocalised aromatic structures have been assigned on the basis of NMR spectroscopic data. Besides this study the hydrides 201 and 204 were found to react with a series of aldehydes, resulting in formation of the alkoxide-alkene chelates 244-247 (Scheme 13, Section III-B.l). 

The diazonium ion is a very weak electrophile since the positive charge can be delocalised into the aromatic ring, and its reactivity is modified by substituents... 

Carbon-carbon single bonds, p. 6 1.3.2 Carbon-carbon double bonds, p. 8 1.3.3 Carbon-carbon triple bonds, p. 9 1.3.4 Carbon-oxygen and carbon-nitrogen bonds, p. 10 1.3.5 Conjugation, p. 11 1.3.6 Benzene and aromaticity, p. 14 1.3.7 Conditions necessary for delocalisation, p. 18. 

With alcohols there is no such factor stabilising the alkoxide anion ROe, relative to the alcohol itself, and alcohols are thus very much less acidic than carboxylic acids. With phenols, however, there is again the possibility of relative stabilisation of the anion (2), by delocalisation of its negative charge through interaction with the n orbitals of the aromatic nucleus ... 

Arynes present structural features of some interest. They clearly cannot be acetylenic in the usual sense as this would require enormous deformation of the benzene ring in order to accommodate the 180° bond angle required by the sp1 hybridised carbons in an alkyne (p. 9). It seems more likely that the delocalised 7i orbitals of the aromatic system are left largely untouched (aromatic stability thereby being conserved), and that the two available electrons are accommodated in the original sp2 hybrid orbitals (101) ... 

Thus when Na is dissolved, in the absence of air, in ethereal solutions of aromatic ketones, a blue colour is seen, due to the presence of the delocalised (over Ar as well as over the C=0 system) species (58), a sodium ketyl ... 

The operation of (d) is seen in cyclopentadiene (14) which is found to have a pKa value of 16 compared with 37 for a simple alkene. This is due to the resultant carbanion, the cyclopentadienyl anion (15), being a 6n electron delocalised system, i.e. a 4n + 2 Hiickel system where n = 1 (cf. p. 18). The 6 electrons can be accommodated in three stabilised n molecular orbitals, like benzene, and the anion thus shows quasi-aromatic stabilisation it is stabilised by aromatisation ... 

The hetero radicals that have already been referred to—(9, p. 301), (10, p. 302), (14, p. 302) and (15, p. 302)—owe their relative stability [with respect to their dimers—apart from l,l-diphenyl-2-picrylhydrazyl (10)] to a variety of factors (a) the relative weakness of N—N, S—S and 0—0 bonds, (b) the delocalisation through the agency of aromatic nuclei, and (c) steric inhibition of access to the atom with the unpaired electron, or to an aryl p-position, cf. (50). The latter factor bulks large (in addition to the weakness of O—O bonds) in the great stability of (15, cf. p. 302) and all three factors operate to stabilise (51), which is wholly dissociated in solution ... 

Radical attack on methylbenzene (toluene, 60) results in preferential hydrogen abstraction by Cl leading to overall substitution in the CH3 group, rather than addition to the nucleus. This reflects the greater stability of the first formed (delocalised) benzyl radical, PhCH2 (61), rather than the hexadienyl radical (62), in which the aromatic stabilisation of the starting material has been lost ... 

Structure 6.8 demonstrates a most extreme example of anisotropy. In this unusual metacyclophane, the predicted chemical shift (Table 5.8) of the methine proton that is suspended above the aromatic ring would be 1.9 ppm. In fact, the observed shift is -4 ppm, i.e., 4 ppm above TMS The discrepancy between these values is all down to the anisotropic effect of the benzene ring and the fact that the proton in question is held very close to the delocalised p electrons of the pi cloud. 

Electrochemical studies indicate that the redox behaviours of the different derivatives are dominated by the nature of the chalcogen at position 2, with the selenium containing cations more readily reduced (by more than 0.1 V) than the sulfur analogues.82 The difference has been ascribed to the extent of delocalisation of the positive charge away from the heterocycle, onto the fused aromatic ring. 

For phenylnitrenium ions, we85 and others 72,78,102 104 have computed that there is extensive positive charge delocalisation into the aromatic ring and arylnitrenium ions are best described as 4-imino-2,5-cyclohexadienyl-l-yl carbenium ions 18. 

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