Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl esters. Part 17. The reactivity of the 2- and 3-positions of benzo[b]furan and of all positions in benzo[b]thiophen

Abstract

All six 1-(benzo[b]thienyl)ethyl acetates, 1-(benzo[b]furan-2-yl)ethyl acetate, and 1-(benzo[b]furan-3-yl)ethyl acetate have been prepared and their rates of gas-phase elimination of acetic acid measured between 620.2 and 686.7 K. The positional order of reactivity in benzo[b]thiophen is 3 > 2 > 6 > 5 > 4 > 7, which is both theoretically predicted, observed in solvolysis of 1-(benzo[b]thienyl)ethyl chlorides, and (in part) in electrophilic aromatic substitutions. The derived σ⁺-values viz.–0.54, –0.39, –0.32, –0.29, –0.155, and +0.10 respectively, differ markedly for some positions from those obtained in the solvolysis, the values being more positive for all positions in the elimination. For benzo[b]furan the positional reactivity order is 3 > 2 in the elimination (the σ⁺-values being –0.495 and –0.225, respectively) but changes to a 2 > 3 order in reactions possessing transition states closer in structure to that of the Wheland intermediate; this change in positional order with transition state structure is predicted by calculations. The data confirm that the extended selectivity relationship is a meaningless concept for parameterizing the reactivities of π-excessive heterocycles.

For each molecule the 3-position is more reactive, and the 2-position less reactive than the corresponding position in furan and thiophen. This order, the overall positional order in benzo[b]thiophen and the relative positional order in the benzenoid rings of benzo[b]thiophen, and dibenzothiophen are accounted for in terms of the structure of the most stable canonical form of the resonance hybrid representing the transition state.