Anti and Syn Eliminations from 2,3-Dihalo-2,3-dihydrobenzofurans. The Role of the Substrate Structure and the Base-Solvent System on the Reaction Mechanism

Article abstract of DOI:10.1021/ja00357a023

The anti and syn β-eliminations from a series of 31 2,3-dihalo-2,3-dihydrobenzofurans (to give 3-halobenzofuran) have been kinetically investigated in t-BuOK-t-BuOH, in the presence and in the absence of 18-crown-6 ether (18C6), and in EtOK-EtOH.Reaction mechanisms have been assigned on the basis of leaving group, kinetic deuterium isotope, ring substituent (5-chlorine), and β-halogen effects.These data have provided information concerning structure and solvent effect on the mechanism of β-elimination reactions that lead to the following conclusions: (a) an E1cB_I mechanism is likely to be operating, regardless of stereochemistry, with chlorine as β-activating atom and fluorine as the leaving group and (b) an E2 reaction is likely to be operating for the opposite structural situation, i. e., with β-fluorine activation and chlorine as the leaving group.The mechanism is likely to change from E2 to E1cB_I as the reaction stereochemistry changes from anti to syn and as we move from EtOK-EtOH to t-BuOK-t-BuOH and from here to t-BuOK-t-BuOH-18C6.