Nuclear magnetic resonance studies of methyl-5, 6-dicarboxy-2-norbornene derivatives

A simple procedure is provided for the quantitative analysis of Diels-Alder adduct prepared from commercial methylcyclopentadiene and maleic anhydride by NMR spectra without further separation of its components. The adduct is considered as a mixture of endo norbornene derivatives (C_n), (D_n), (E) and (F_n) which are obtained from 1-, 2- and 5-methylcyclopentadiene and cyclopentadiene, respectively. The quantitative analysis of the adduct can be made on a basis of the ratio of the signal intensities of the olefinic protons in the adduct. The result shows that the adduct prepared under mild conditions mainly consists of three norbornene derivatives (C_n), (D_n) and (F_n), with a negligibly small amount of (E). When the adduct prepared under mild conditions is heated, it contains the exo isomers (Cx), (Dx) and (Fx). The ratio of the endo and the exo norbornene derivatives, i.e. ((C_n) + (D_n) + (F_n)):((C_x) + (D_x) + (F_x)), may be estimated from the signal intensities of the 5- and 6-protons. As the result of the analyses of the adducts which are obtained by heating at different temperatures the adduct prepared under mild conditions, the mole fraction of 2-methyl isomers (D_n) and (D_x), which is found to be 58 % in the adduct prepared at ? 15°C, keeps on increasing with rising temperature and shows almost the same value, ～86%, when heated above 150°C. Therefore, the equilibrium between 1-methyl and 2-methyl isomers does not change above 150°C. On the other hand, the isomerization from endo to exo isomers is not found below 110°C, and then exo isomers keep on increasing above 140°C.