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. 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. 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.