Relative reactivities of 1,2-cyclohexadiene with conjugated dienes and styrene

Generation of 1,2-cyclohexadiene in the presence of conjugated dienes leads to (2 + 2) and/or (2 + 4) cycloaddition products. Methods used to generate 1,2-cyclohexadiene were: (1) treatment of 6,6-dibromobicyclo3.1.0]hexane with methyllithium in tetrahydrofuran-ether at 0° and 60°; (2) treatment of 1,6-dichlorocyclohexene with magnesium in tetrahydrofuran at 60°; and (3) treatment of 1-bromocyclohexene with t-BuOK in THF at 60° or dimethyl sulfoxide at 40°. Comparison of relative reactivities with various dienes and styrene in ether solvents at 60° confirmed that the same intermediate, uncomplexed 1,2-cyclohexadiene, was involved in these reactions. Relative reactivities at 0° and 60° were found to be: 2-methylfuran (0·12, 0·14); furan (0·17, 0·16); 2,4-hexadiene (0·17,—); cis-pentadiene (0·53, 0·53); 2,3-dimethylbutadiene (2·35, 1·9); 1,3-cyclohexadiene (1·85,—); styrene (2·35, 1·9); and 1,3-cyclopentadiene (47, 14).