Determination of the Atropisomeric Stability and Solution Conformation of Asymmetrically Substituted Biphenyls by Means of Vibrational Circular Dichroism (VCD)

✍ Scribed by Teresa B. Freedman; Xiaolin Cao; Laurence A. Nafie; Monica Kalbermatter; Anthony Linden; Andreas Johannes Rippert

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: German
• Weight: 249 KB
• Volume: 88
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.200590165

✦ Synopsis

Vibrational-circular-dichroism (VCD) studies of the solution conformations of three 2,2'-substituted biphenyls are reported. Biphenyls with only two substituents at the peri-position normally show rotation about their central axis of chirality at room temperature in solution. We previously found no evidence for rotation of ( 1) in CDCl 3 about its 1,1'-axis, due to stabilization by intramolecular H-bonding and the presence of the i-Pr substituent, but two conformers were found in solution that result from rotation of the heterocycle in 1 between OH ¥¥¥ N (the form present in the solid state) and the OH ¥¥¥ O H-bonded forms, with no rotation of the i-Pr group or (P) 3 (M) twist [1]. For (P,S)-2, where the i-Pr substituent of 1 has been replaced by a Ph group, rotation of the heterocycle takes place in CDCl 3 solution. For (P,S)-3 and (M,S)-4, where Me substitution at the two 6,6'-positions of (P,S)-1 prevents rotation about the central axis of chirality, rotation of the heterocycle is observed for the (P)configuration ((P,S)-3), but not for the (M)-configuration ((M,S)-4). Only one rotamer involving the i-Pr group, which was found in the solid state of (P,S)-3, was also observed in solution, but (M,S)-4, obtained as an oil, was found to be a mixture of three rotamers of the i-Pr group.