Synthesis of Quaternary Salts from 2 , 3 ' - B ip y r id in e and Dibromoalkanes

There has been considerable interest in the pro­ ducts of the reaction of 2 ,2 '-bipyridine with di­ bromoalkanes. The bridged diquaternary salt (1) from 2 ,2 '-bipyridine and 1 ,2 -dibromoethane is the well-known herbicide diquat dibromide [1]. Similar bridged salts are obtained with 1,3-dibromopropane and 1,4-dibromobutane [2] but reaction of 2,2'-bipyridine with 1,5-dibromopentane gives instead the diquaternary salt (2) [3]. Related systems have also received attention [2]. In the 4,4'-bipyridine series, reaction of 4,4'-bipyridine with dibromoalkanes usu­ ally results in the formation of polyviologens [2, 4] or diquaternary salts such as 3 [5] depending on reac­ tion conditions. The monoquaternary salt (4) has also been isolated, but in an impure state [6 ], from the reaction of 4,4'-bipyridine with 1,2-dibromoethane. We now report the results of work on the products obtained by the reaction of 2,3'-bipyridine with dibromoalkanes. Reaction of 2,3'-bipyridine with excess of 1,2-di­ brom oethane, 1,3-dibromopropane, 1,4-dibromo­ butane and 1,5-dibromopentane in boiling ethanol afforded in each case a mixture of two products which were separated by fractional crystallisation. The pre­ dominant products (40—50% yield) comprised the 1'bromoalkyl m onoquaternary salts (5; n = 2), (5; n = 3), (5; n = 4) and (5; n = 5) respectively. The struc­ ture of these salts was confirmed by microanalyses and by their NMR spectra. The latter in each case showed a deshielded singlet at ö ~ 9.5 ppm which is attributed to the hydrogen at the 2'-position. This confirms that the nitrogen in the ß-pyridyl ring and not in the a-pyridyl ring has been quaternized. This