Electrophilic aromatic substitution. Part 32. Partial rate factors for detritiation of dithieno[2,3-b:3′,2′-d]thiophen, dithieno[3,2-b:2′,3′-d]-thiophen, and dithieno[2,3-b:2′,3′-d]thiophen
Abstract
Dithieno[2,3-b:3′,2′-d]thiophen (I), dithieno[3,2-b:2′,3′-d]thiophen (II), and dithieno[2,3-b:2′,3′-d]thiophen (III), specifically labelled with tritium in each position have been prepared, and their rates of detritiation measured either in pure trifluoroacetic acid, or mixtures of acetic and trifluroacetic acids, at 70 °C. The variation in rate with acidity shows that these compounds are more strongly hydrogen bonded in trifluoroacetic acid than are the thienothiophens (which are in turn more strongly bonded than thiophen); the overall extent of bonding appears to be greater than in anisole. Partial rate factors (corrected for hydrogen bonding) are calculated as (position and compound in parentheses): 3.8 × 109[2-(I)]; 3.63 × 106[3-(I)]; 2.07 × 109[2-(I)]; 1.94 × 106[3-(II)]; 2.67 × 109[2-(III)]; 6.31 × 106[3-(III)]: 1.62 × 109[5-(III)]; 3.41 × 106[6-(III)], the corresponding σ+-values being –1.095, –0.75, –1.065, –0.72, –1.08, –0.78, –1.05, and –0.75. These compounds are thus very reactive towards electrophilic substitution and the reactivity of each position is increased by 0.05–0.1 σ units compared to the corresponding position in the thienothiophens, i.e. slightly less than the amount by which the latter are more reactive than the corresponding positions in thiophen. The reactivities of the α-positions of a given ring are increased more by addition of a thiophen ring which has sulphur on the same side of the molecule as in the ring undergoing substitution; this was true also for the thiophen to thienothiophen transformation. The converse is true for the β-position reactivities and this is the reverse of the observation for the thiophen to thienothiophen transformation. Hückel localization energies correctly predict the relative order of reactivities of the α-positions, and are only marginally incorrect for the β-positions despite the fact that the d-orbital electrons are not taken into account.