1,3-Dipolar Cycloadditions to Bicyclic Olefins. II. The Effects of Non-aromatic Polar Solvents on the exo/endo Product Ratios in 1,3-Dipolar Cycloadditions to Norbornadienes

• Published in: Bulletin of the Chemical Society of Japan Vol. 50; no. 12; pp. 3335 - 3337
• Main Authors: Taniguchi, Hisaji, Yoshida, Yoshihiro, Imoto, Eiji
• Format: Journal Article
• Language: English
• Published: The Chemical Society of Japan 01.12.1977
• ISSN: 0009-2673; 1348-0634
• DOI: 10.1246/bcsj.50.3335

The exo/endo product ratios for three types of 1,3-dipolar cycloadditions of phenylglyoxylonitrile oxide(1) to 2,3-bis(methoxycarbonyl)norbornadiene(2a), norbornadiene, and 2,3-dicyanonorbornadiene decrease with increasing ET value of the non-aromatic polar solvent used. The exo/endo ratio in the reaction of 1 with 2a is larger than unity, but the value of ΔHexo\eweq−ΔHendo\eweq is nevertheless positive. This phenomenon is attributed to the relatively large positive value of ΔSexo\eweq−ΔSendo\eweq, that is, ΔGexo\eweq−ΔGendo\eweq<0 at ordinary temperatures. On the basis of these results, the conditions, under which the exo/endo ratio in the reaction of 1 with 2a is smaller than unity, were determined. The results can be reasonably explained by the assumption that the dipole moment of the endo-form in the transition state is greater than that of the exo-form.