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dibromo-5-chloro-2,3-dihydrobenzofuran and anti eliminations from cis- 2,3-dichloro-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 trans2,3-dibromo-2,3-dihydrobenzofuran is significantly affected by base association, but not by medium basicity. trans-2,3-Dihalogeno-2,3-dihydrobenzofurans are very suitable substrates for the study of elimination processes owing to their tendency to be converted by bases into the aromatic benzofuran system. Recently we have investigated the reactions of trans-2,3-dibromo-2,3-dihydrobenzofuran (I) with bromide and chloride ions in dipolar aprotic so1vents.l With bromide ions I is dehalogenated to benzofuran, whereas with chloride ions it gives mainly trans- 2,3-dichloro-2,3dihydrobenzofuran (11)2 together with ca. 10% of 3-bromobenzofuran presumably formed in a syn dehydrobromination process. Syn dehydrohalogenation, however, is the only reaction observed when I or I1 reacts with an alkoxide in the corresponding alcohol. Owing to the increasing importance of syn elimination reactions, it seemed interesting to carry out a detailed kinetic investigation, including the determination of deuterium kinetic isotope effect and substituent effect of the reactions of I and I1 in different alkali alkoxide-alcohol systems. For comparison purposes some anti eliminations from cis -2,3dichloro-2,3-dihydrobenzofuran (111) have also been studied.3 Results and Discussion The eliminations from 1-111, trans-2,3-dibromo-3-deuterio-2,3-dihydrobenzofuran (IV), and trans-2,3-dibromo5-chloro-2,3-dihydrobenzofuran (V) have been studied in EtOK-EtOH, n-BuOK-n-BuOH, t-BuOK-t-BuOH, nRuOK-18C6-n-BuOH (18C6 is 18-crown-6 ether), and t BuOK-18C6-t-BuOH. Some experiments for the reactions of 1-111 in EtONa-EtOH have been also carried out. In each

Les β-eliminations syn et anti a partir d'une serie de 31 dihalogeno-2,3 dihydro-2,3 benzofurannes sont etudiees du point de vue cinetique dans t-BuOK-t-BuOH, en presence et en l'absence de 18-crown-6, et dans EtOK-EtOH