A novel approach to synthesis of the cinnoline ring system via organoiron(cyclopentadienyl) complexes

An efficient synthesis of 3-mono or 3,4-disubstituted cinnolines from (o-dichlorobenzene)(cyclopentadienyl)iron hexafluorophosphate in 3 or 4 steps is reported. o-Chlorophenylalkyl or alkylaryl ketone complexes obtained from the o-dichlorobenzene complex upon treatment with enolate anions, react with hydrazine forming 3-mono or 3,4-disubstituted 1,4-dihydrocinnoline complexes. Treating the latter with NaNH2 forms cinnolines, i.e., 3-methyl-, 3-phenyl- and 3,4-dimethylcinnoline, via an aromatization-demetalation reaction. The influence of substituents bonded to the C atom adjacent to the complexed benzene ring in o-chlorophenylalkyl or -alkylaryl ketone prior to cyclization on the cyclization reaction is discussed.