The reactions of nitrogen dioxide with dienes†

Abstract

The reaction of nitrogen dioxide with conjugated dienes (2,5-dimethylhexa-2,4-diene, hexa-2,4-diene and 2,3-dimethylbuta-1,3-diene) in organic solvents (mainly hexane, benzene and diethyl ether) results mainly in 1,4-addition giving, with the first two substrates, a mixture of 1,4-dinitro compounds and 1,4-nitro alcohols. The latter are presumably formed from hydrolysis of the intermediate nitronitrites. With 2,3-dimethylbuta-1,3-diene, only the cis and trans 1,4-dinitro compounds were isolated. The reactions effectively stop after the addition of two nitrogen dioxide radicals, only traces of tetrasubstituted products being obtained. With the unconjugated hepta-1,6-diene, addition can be made to occur to both double bonds but, when two equivalents of nitrogen dioxide are added very slowly, the main products derive from addition to one double bond and cyclisation to form cis and trans 1,2-bis(nitromethyl)cyclopentane. All the above reactions give a very complex mixture of products and the product composition depends markedly on the reaction conditions; however, with the conjugated dienes, the 1,4-dinitro compounds can be obtained in yields of ca. 50%. The crystal structure of trans-1,4-dinitro-2,3-dimethylbut-2-ene was determined.