Phenanthrenes dications

Besides the fluorene cyclization, the 1,2-ethylene dications are known to undergo another type of electrocyclization reaction to produce the phenan-threne ring system (see eqs 9 and 11 and Scheme 3).32a Although derived carbenium-carboxonium dications (70) will give the phenanthrene-type products (albeit in low yield eq 15), the bis-carboxonium dication 71 does not. Moreover, the fluorene cyclization occurs readily in 1,1-dihydroxylie systems (eq 10). 

Calculations at the 4-31G level of theory found that the 1,2-bis-carboxonium ion (72) is significantly more stable than the carbenium-carboxonium dication (73). Thus, when compared with the 1,1-dihydroxylie structure 73, the 1,2-dihydroxy lie structure 72 is more effective at stabilizing the 1,2-ethylene dication. This effect leads to the stabilization of dication 71 (eq 16), and not to phenanthrene cyclization. 

Fig. 14. 13C NMR chemical shifts for dications from cyclopenta[a]phenanthrene derivatives (A513C in parentheses).

Fig. 15. >H chemical shifts for dications from cyclopenta[a]phenanthrene derivatives, t and t denote interchangeable assignments within the two cations. means that specific assignments of the diastereotropic protons in the five-membered rings are unknown.

Side reactions of the dication (e.g. hydrolysis), and perhaps further oxidation of the carbazole at potentials where the dication forms, limits the yield of carbazole cation radical in some cases. A similar reaction sequence reported by Ronlan and Parker (1970) involves oxidation of . [7a] which produces a cyclized product, the 9,10-dihydrophenanthrene [7b]. On the CV time scale [7b] forms a stable cation radical, but on coulometric oxidation it shows an napp-value of two and produces the phenanthrene [7c]. A similar intramolecular coupling occurs upon oxidation of... 

---