Cinnolines electrochemical reduction

It was previously mentioned1 that cinnoline and 3-substituted cinnolines (94) could be prepared from the condensation products (95) between an o-nitrobenzaldehyde and a nitroalkane by electrochemical reduction. The reaction has been further studied,138 and it was noticed that when the reduction was carried out stepwise, anthranils (96) were formed, especially at elevated temperatures. The final ring closure was catalyzed by traces of oxygen, whereas too much oxygen produced the cinnoline JV-oxide (97) the ring closure was believed to be a radical chain reaction where the formation of the aromatic cinnoline was part of the driving force [Eq. (76)]. 

Fig. 14 Reduction of PhjCSPh by electrochemically generated aromatic anion radicals (in DMF at 25°C). Variation of the rate-determining step rate constant, A , with the standard potential of the aromatic anion radical, p,g (from left to right azobenzene, benzo[c]cinnoline, 4-dimethylaminoazobenzene, terephthalonitrile, naphthacene, phlhalonitrile, perylene, fluoranthene, 9,10-diphenylanthracene). The dotted lines are the theoretical limiting behaviours corresponding to the concerted (right) and stepwise (left) pathways. (Adapted from Severin et al 1988.)...

The reduction of compounds suitably substituted with reducible functional groups can be advantageously used for the preparation of heterocyclic ring systems 15 5 The guiding principle here is to form a nucleophilic center in the molecule by cathodic reduction of a functional group and then have this to react with an electrophilic center (which may also be formed in an electrochemical process or be present in the starting material) suitably positioned for ring closure. Thus, 2,2-dinitrobiphenyl can be reduced to benzo[c] cinnoline in acid solution ... 

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