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B Values as a Sensitive Measure of Steric Effects



Someone who says “A” should be prepared to also say “B”: In contrast to cyclohexane model-based A values, biphenyl model-derived B values are powerful tools to quantify steric repulsion in and conformational behavior of ortho-substituted aromatic compounds.

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Torsional barriers of 15 ortho-substituted biphenyls have been determined computationally using the B3LYP density functional and experimentally by variable-temperature (“dynamic”) nuclear magnetic resonance. Taking advantage of the 3′-isopropyldimethylsilyl group as a novel and superior diastereotopicity probe and tracking coalescence temperatures down to −173 °C (100 K), activation energies of aryl–aryl rotation as small as 5 kcal mol−1 can be assessed. The 2-X/2′-H repulsion increments thus derived are powerful parameters for rationalizing and predicting the conformational behavior of aromatic compounds carrying ortho substituents.