The quantitative electrophilic reactivity of annulenes. Part 5. Trithiadiazepine, its 6-bromo- and 6-nitro-derivatives, and trithiatriazepine

Abstract

The quantitative electrophilic reactivities of the 10π-annulenes viz. 1,3λ⁴δ²,5,2,4-trithiadiazepine (1), 6-bromo-1,3λ⁴δ²,5,2,4-trithiadiazepine (2), 6-nitro-1,3λ⁴δ²,5,2,4-trithiadiazepine (3), and 1,3λ⁴δ²,5,2,4,6-trithiatriazepine (4) have been determined via acid-catalysed hydrogen exchange (protiodetritiation). Partial rate factors (for exchange in trifluoroacetic acid at 70 °C) are: 2.78 × 10⁵(1), 3.15 × 10⁴(2), ca. 0.038 (3), and ca. 0.006 (4), the corresponding σ⁺-values being –0.62, –0.515, ca. 0.16, and ca. 0.25; the former two values are corrected for the effect of hydrogen bonding which reduces the reactivity of heteroaromatics in this medium. 1,3λ⁴δ²,5,2,4-trithiadiazepine is, thus, slightly more reactive than the 3-position of thiophene. Deactivation by the ortho-bromo substituent (ninefold) is much less than in benzene (37-fold) due to the higher order C–C bond in 1,3λ⁴δ²,5,2,4-trithiadiazepine which facilitates transmission of the conjugative electronreleasing (+M) effect of bromine; the small deactivation is comparable to that found across the 1,2-bond in naphthalene which is of similar bond order to that in 1,3λ⁴δ²,5,2,4-trithiadiazepine.