Fused protonated species

A saturated ketone is obtained when an a, unsaturated ketone is treated with lithium in liquid ammonia, followed by water or a similar protonating species. Most steroidal A -3-ketones and similar compounds give products with the trnns-fused all-chair structure of minimum energy, which led Barton [32] to the generalisation that the more stable product is produced by protonation of an intermediate carbanion (14) in its thermodynamically preferred conformation (15) (see p. 54). Further studies, including the reduction of substituted... 

In most cases protonation of azafulvalenes leads to a hypsochromic shift in their electronic spectra as exemplified by the blue 90a, which gives its orange dication (90JPC949). But some derivatives are able to form cyaninelike structures and therefore absorb bathochromically (68CB1287 82CB3756 97LAR617). For example, on treatment of imidazo-fused TAFs 137 with acids the color immediately turns from red to deep blue. The protonated species can be considered to be an antiaromatic... 

The question of aromaticity versus antiaromaticity and delocalized versus localized double bonds in pentalene (2) dates back to 1922, when Armit and Robinson compared it with naphthalene and postulated that the former might be similarly aromatic [32, 33]. While the first synthesis of a non-fused hexaphenylpentalene (38) [30] provided only some clues as to the non-aromatic reactivity of the pentalene skeleton, the tri-tert-butyl derivative 39, prepared and studied by Hafner et al. in great detail [31], gave a better insight. The ring-proton signals of this alkyl-substituted pentalene 39 are shifted upfield compared to those of fulvene (27) and other cyclic polyenes. This observation led to the conclusion that the pentalene derivative 39 should be an antiaromatic species. However, the results did not permit a distinction... 

Gust, Moore, Moore and coworkers covalent cartenoid-porphyrin-quinone molecular triads 55-60 contain a cyclized hydrogen bond within the quinone acceptor framework [143], The naphthaquinone moiety of 55 is fused to a norbomene system whose bridgehead position bears a carboxylic acid, which can hydrogen bond to an adjacent quinone. Photoinduced electron transfer from the porphyrin to the quinone leads to a marked p/fg increase of the latter, resulting in a fast proton transfer ( pt 10 s ) to form the semiquinone. Back electron transfer from the semiquinone is attenuated as a consequence of the proton-stabilized charge-separated species. This leads to a two-fold increase in the quantum yield of the charge-separated state of 55, as compared to those of the reference triads 56 and 57 (see Volume III, Part 2, Chapter 2). 

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