KINETIC STUDY OF THE BASE-INDUCED ANTI AND SYN ELIMINATIONS FROM 2,3-DIHALOGENO-2,3-DIHYDROBENZOFURANS IN DIFFERENT BASE-SOLVENT SYSTEMS

Syn eliminations from trans-2, 3-dibromo-, trans-2, 3-dichloro-, trans-2, 3-dibromo-3-deuterio-, and trans-2, 3-dibromo-5-chloro-2, 3-dihydrobenzofuran and anti eliminations from ds-2, 3-dichIoro-2, 3-dihydrobenzofuran have been investigated in different base-solvent systems. Anti elimination is favored over the syn by a factor of ~33 000 in EtONa-EtOH, ~10 000 in t-BuOK-t-BuOH, and >10 000 in t-BuOK-t-BuOH, in the presence of 18-crown-6 ether. The rate of both the anti and syn pathway increases with increasing the medium basicity, the syn path being also favored, however to much less extent, by base association. From the values of the deuterium kinetic isotope effect and substituent effect it can be concluded that the transition state structure for the syn elimination from trans-2, 3dibromo-2, 3-dihydrobenzofuran is significantly affected by base association, but not by medium basicity. © 1979, American Chemical Society. All rights reserved.