Benzo cinnolines halogenation

Benzo[c]cinnoline is a weak base with a of 2.2 in water and 1.6 in 50% aqueous ethanol.Cryoscopic measurements show that it is dipro-tonated to some extent in 100% sulfuric acid. Well-defined quaternary salts result from heating benzo[c]cinnoline with alkyl halides and sul-fates. These are reported to decompose on treatment with ammonia, regenerating benzo[c]cinnoline, but apparently nucleophilic displacements of halogen by amines have been carried out on certain highly substituted examples in the synthesis of cationic dyestuffs. Quaternizations of benzo[c]cinnoline with a,a)-dibromo-propane and -butane are accompanied by hydrogen bromide elimination to give the cyclic iminium salts 40 and 41. When 40 is dehydrogenated, or merely refluxed in ethanol. 

Benzo[c]cinnoline forms molecular complexes with halogens in organic solvents thus attempts to brominate it with molecular bromine have been unsuccessful. Using a source of positive bromine, bromine/silver sulfate in sulfuric acid, Corbett and Holt found that reaction occurred at room temperature to give 27% of a monobromo and 4% of a dibromo derivative. These compounds were at first identified as 1-bromo- and l,4-dibromo-(or l,7-dibromo-)benzo[c]cinnolines, but the former was subsequently shown to be the 4 isomer. A reinvestigation of the reaction has shown that both 1- and 4-bromobenzo[c]cinnolines are primary products, formed in the ratio of ca. 2.3 1 at room temperature. The lower isomer ratio as compared with nitration in sulfuric acid probably reflects the greater steric demand of the attacking species. The dibromo compound formed is the 1,4-isomer. The formation of the octachloro derivative by chlorination of benzo[c]cinnoline in the presence of aluminum chloride has been mentioned,but no details are available. 

Halogenation reactions are known for quinoxalino[2,3-c]cinnolines (3,4- and 6,7-bisbenzo-an-nulatcd pyrazino[2,3-c]pyridazines)69 and for pyridazino[3,4-6]quinoxalines (6,7-benzo-annu-lated pyrazino[2,3-c]pyridazines).77... 

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