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B Values as a Sensitive Measure of Steric Effects
Author(s) -
Ruzziconi Renzo,
Spizzichino Sara,
Lunazzi Lodovico,
Mazzanti Andrea,
Schlosser Manfred
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200801963
Subject(s) - steric effects , cyclohexane , chemistry , aryl , a value , biphenyl , coalescence (physics) , computational chemistry , crystallography , alkyl , stereochemistry , organic chemistry , physics , astrobiology
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.