The Torsional Barriers of 2-Hydroxy- and 2-Fluorobiphenyl: Small but Measurable

By making use of a novel diastereotopicity probe, namely C(CF3)2OH, it has been possible to measure by very low temperature 19F NMR spectroscopy the elusive aryl–aryl rotation barriers of biphenyls bearing an OH or F group in one ortho position. The experimental values (5.4 and 4.4 kcal mol−1, respectively) are matched by those from ab initio calculations (5.3 and 4.3 kcal mol−1, respectively). Unmatched performance as a diastereotopicity probe is shown by the α‐hydroxyhexafluoroisopropyl group. Using this probe as a detector made it possible to determine experimentally, by dynamic NMR spectroscopy measurements, the very small barriers to aryl–aryl torsion of 2‐fluorobiphenyl and 2‐hydroxybiphenyl (see picture) and to elucidate other important features of the stereomutation as well.