Solvent and structure effects on rotamerization in 3,4‐alkylenedioxy‐ and 3,4‐dialkoxy‐2,5‐bis[di(tert‐butyl) hydroxymethyl]thiophenes: an NMR, IR, molecular mechanics and single‐crystal x‐ray study

Solvent effects on the rotational equilibria of two‐rotor systems consisting of 3,4‐alkylenedioxy‐ and 3,4‐dialkoxy‐2,5‐bis[di( tert ‐butyl)hydroxymethyl]thiophenes have been investigated. In these systems there are three rotational isomers: syn , syn ( SS ); anti , syn ( AS or SA ) and anti , anti ( AA ) ( syn , [free] hydroxy group; anti , intramolecularly hydrogen‐bonded hydroxy group). In non‐hydrogen‐bonding solvents, such as chloroform and benzene, the AA rotamer is preferred for the larger bridges (three or four methylenes) and the dialkoxy derivatives, AS/SA being the minor component and SS hardly detectable. In contrast, the 3,4‐methylenedioxythiophene derivative is largely SS with AS/SA minor and just perceptible. The 3,4‐ethylenedioxythiophene (EDOT) derivative shows all three isomers in similar amounts. In hydrogen bond acceptor solvents, such as pyridine and DMSO, the two equilibria AA ⇋ AS/SA and AS/SA ⇋ SS shift in favour of the AS/SA and SS forms, respectively. The equilibrium constants, K 1 and K 2 , show different sensitivities to solvent hydrogen bond basicity, the latter being about 25% (log/log) less sensitive than the former. Work on the BiEDOT derivative suggests that this is probably due to the proximity of the two OH groups, which inhibits solvation of the SS form. Comparison of rotation barriers (mainly in the range 20–22 kcal mol −1 ) with those for the monosubstituted compounds indicates that the SS → AS/SA barrier is unusually low in pyridine and DMSO. The structure of the AA isomer of the 3,4‐propylenedioxythiophene derivative has been determined by single‐crystal x‐ray diffraction.