Aromatic azides tricyclic

Rearomatization is much more common in bi- and tricyclic azides than ring expansion although ring expansion may be achieved employing sodium methoxide in methanol-dioxan as solvent. This dichotomy again may be resolved by invoking an aziridine intermediate vide supra). The formation of aminoketals from 2-azidoanthracene and 48 is of particular interest as similar products are commonly formed from aliphatic azirines on treatment with mildly basic methanol. At the moment, the intermediacy of benzazirine in phenyl azide photolysis at room temperature cannot be ruled out. In the case of bi- and tricyclic aromatic azides and azidouracil decompositions the nature of the products strongly supports azirine involvement. Only further experimental work will resolve this mechanistic dilemma. With this in mind, benzazirine intermediacy will be assumed for the purposes of this discussion. 

The photochemical reaction of azide-functionalized tetrazole derivatives such as 38 leads to the formation of the 5-5 bicyclic ring system 40 (Scheme 5) in very moderate yields <1999JHC863>. This reaction is believed to proceed via the singlet nitrene intermediate 39. Attack at the aromatic substituent in ortho position leads to product 40 <1974JOG2546> by subsequent cyclization. This intermediate is deprotonated during the workup conditions to the mesoionic tricyclic derivative 41. 

To put this working hypothesis on a more quantitative basis aromatic nucleophihc substitutions with azide as a nucleophile were tested for catalytic effects with 24 and 25. The corresponding transition states are even more delocalized than their aliphatic counterparts and thus higher rate augmentations should be expected. All or part of this favourable effect, however, can be anihilated due to an unfavourable activation entropy because the formation of the large transition state may suffer from the severe steric restrictions in the cavity of the host. As a corollary it was no surprise to find that the smaller tricyclic host 24 inhibited the substitution of 39 with azide in aqueous methanol The same reaction experienced a big rate acceleration in the... 

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