Charges delocalisation

Kildahl-Andersen G, Bruas L, Lutnaes BF, and Liaaen-Jensen S. 2004. Nucleophilic reactions of charge delocalised carotenoid mono- and dications. Organic Biomolecular Chemistry 2(17) 2496-2506. 

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. 

At the reaction centre C effects appear to indicate the symmetry of an 5 n transition state, being low for S nI reactions but high for 5n2 reactions, particularly those with little bonding to leaving group or nucleophile but charge delocalisation away from the reaction centre. 

Birch reduction of aromatic heterocycles is equally rewarding if more challenging mechanistically. The pyridine diester 138 is reduced to an intermediate that could be drawn as 139. Both anions are extended enolates but one has the charge delocalised onto the nitrogen atom and so is less reactive than the other. Alkylation occurs at the a-position38 to give 140. 

Pyridine has all the disadvantages of benzene plus some special ones of its own. Attack at the 2- or 4-positions 4 gives intermediate cations such as 5 which have part of the positive charge delocalised onto nitrogen 5c. This would be no bad thing if the nitrogen atom were tetravalent like stable NH4+, but it is actually divalent like the unknown, electron-deficient, and presumably very unstable NH2+. 

The three amino-pyridines are all more basic than pyridine itself and form crystalline salts by protonation at the ring nitrogen. The a- and y-isomers are monobasic only, because charge delocalisation over both nitrogen atoms, in the manner of an amidinium cation, prevents the addition of a second proton. The effect of the delocalisation is strongest in 4-aminopyridine 9.1) and much weaker in 2-aminopyridine (p/ aH 7.2). Delocalisation is not possible for the P-isomer, which thus can form a di-cation in strong acid (p/faHS 6.6 and -1.5). " ... 

The VIS spectrum of the yellow alkali treated product of the blue oxonium salt also contained products with octaene-, heptaene- and hexaene like chromophores [80], compatible with conjugate addition and charge delocalisation in the blue oxonium ion. 

The detailed structure of the charge delocalised P-carotene dication (11) was established by COSY, HSQC, HMBC and ID and 2D ROESY NMR techniques (600 MHz, CDC13, -20°C), leading to complete... 

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