Diels-Alder adducts from X-substituted diphenylbutadienes (X = p-NMe,, p-OMe, p-Me, m-Me, H, m-OMe, p-CI, m-CI, p-CN, and rn-NO,) and maleic anhydride, and from diphenylbutadiene and some dienophiles (Nmethylmaleimide, N-phenylmaleimide, and 1 ,4-benzoquinone), have been prepared. Second-order rate constants and activation parameters for these reactions have been measured in m-xylene as solvent. The effects of some aprotic solvents (toluene, anisole, chlorobenzene, bromobenzene, benzonitrile, nitrobenzene, and NN-dimethylformamide) on the kinetics of diphenylbutadiene and maleic anhydride have been investigated. The stereospecificity of the endo-addition and the small substituent and solvent effects on the reaction rate rule out the presence of a zwitterionic intermediate along the reaction co-ordinate. These results, together with the low activation enthalpies and the large negative entropies, can be interpreted in terms of the diradical or concerted mechanism, even though the latter seems to be more acceptable.
Y =0, NMc, NPh, CH=CH
RESULTS AND DISCUSSIONThe addition of maleic anhydride to a trans,trans-1,4diphenylbutadiene solution yields the cyclohexene derivative (111; X = H) quantitatively, in which both phenyl groups are cis to the anhydride moiety,13 because efido-addition is energetically more f avourable as shown by MO treatment.14 The eutdo-structure of the adducts W (111)t "C = K -273.16.
