Preparation of Alkyl-Substituted Biphenylenes by the Pyrolytic Extrusion of Nitrogen from Benzocinnolines

Article abstract of DOI:10.1021/jo00253a027

Flash vacuum pyrolysis of 1,10-dimethylbenzocinnoline (1a) at 730 deg C (0.12 Torr) provides 1,5-dimethylbiphenylene (6) in 71percent (53percent isolated) yield, whereas pyrolysis of 1,10-diethylbenzocinnoline (1b) produces a complex mixture containing some 1,8-diethylbiphenylene (8) (11.3percent) but more phenanthrene (9) (12.9percent) as well as a myriad of other side products.The preparation of the benzocinnolines and an investigation of the mechanism of phenanthrene formation are described.A straightforward correspondence between the mass spectrum of 1b and the pyrolysis pathway leading to phenanthrene is noted.The enhanced rates of side reactions in the pyrolysis of 1b are rationalized by estimating activation barriers for the processes competing with N2 extrusion and with ring closure.