Annulenes, bridged oxidation

The n.m.r. spectrum of (XV) indicates that all the charge resides on the periphery, with little, if any, on the central double bond [192]. This in turn suggests that the best description of this compound is as a [12]annulene bridged by a carbon-carbon double bond. Treatment with deuterium oxide causes addition of deuterium at the 2- and 6-positions of the pentalene moiety. 

Porphycenes 51 bearing substituents at ethylenic carbons have been prepared from 5,5 -acyl bipyrroles 53 (05OL1887) (Scheme 26). However, oxidation of the intermediate annulene 54 is difficult, owing to the presence of the alkyl chains on the alkene bridges. These reduce conformational flexibility, resulting in a near-nonplanar geometry, which resists oxidation and/or aromatization (08CSR215). 

The benzannulation technique is also applicable to the benzene homologation and functionalization of annulenes, as well to a quadruple arene modification of dendritic cores. The reaction of chromium carbene functionalized l,6-methano[10]annulene 82 with 3-hexyne under standard conditions afforded a fair yield of the benzannulation product 83 after protection and oxidative work-up (Scheme 32) [75]. The chromium complex 84 evidently partly survived the oxidation conditions using Feln a syn-stereochemistry with respect to the Cr(CO)3 fragment and the methano bridge was suggested on the basis of NMR data, which is in contrast to the preferred formation of anti-annulation products bearing cydophane skeletons [75b]. 

The black coloured -oxo-bridged Ni-tetraaza (14) annulene (69) was formed by heating Ni(OH)2Taa (oxidized NiTaa) at 473 K in bulk (Eq. 29) But the structure was only confirmed by elementary analysis and tte amount of eliminated H2O. 

Yet monosubstitution at C (11) does not suffice and the product is the open bridged annulene, e.g. 27. Its preparation illustrates many other cases in which [4.4.1]propellane intermediates are formed and exist as propellanes at lower oxidation states (cf. 26). But once the tetraenic one is arrived at, the product more often than not prefers to exist as the more stable aromatic bridged annulene. 

The photo-oxidation of some annulenes has been reported.188 For example, the bridged [10]annulene (87) produces the 1,4-emfo-peroxide (89), possibly by... 

---