Electrophilic aromatic substitution. Part 31. Partial rate factors for detritiation of thieno[2,3-b]thiophen and thieno[3,2-b]thiophen : weak hydrogen bonding to sulphur in trifluoroacetic acid

Abstract

Thieno[2,3-b]thiophen (I) and thieno[3,2-b]thiophen (II), specifically labelled with tritium in each position have been prepared, and their rates of protiodetritiation measured along with that for thiophen in either pure trifluoroacetic acid, or mixtures of trifluoroacetic acid and acetic acid, all at 70 °C. The dependence of exchange rate coefficients upon acid composition indicates that the ring sulphur is hydrogen bonded, and to an extent which depends upon the number of sulphur atoms in the heterocycle. Partial rate factors for detritiation of the non-hydrogen-bonded compounds are calculated as follows (position and compound in parentheses) : 7.18 × 10⁸[2-(I)]; 6.83 × 10⁸[2-(II)]; 9.75 × 10⁷(2-thiophen); 7.59 × 10⁵[3-(I)]; 5.54 × 10⁵[3-(II)]; 7.84 × 10⁴(3-thiophen), the corresponding σ⁺-values being –1.012, –1.010, –0.913, –0.672, –0.656, and –0.560. The results, taken along with those for acylation and chlorination show that (II) is more polarisable than (I), so that these compounds, like all others π-excessive heterocycles previously examined, are unsuitable for rigorous application of the Extended Selectivity Relationship. Annelation of thiophen by thiophen produces a comparable change in reactivities of the α- and β-positions in contrast to annelation by benzene, where the high resonance energy of benzene plays a crucial role in raising the reactivity of the β-position, relative to that of the α-position.