NMR spectra of biphenyl derivatives bearing a single CR2OH substituent in the ortho position indicate that they exist as sp (more stable ) and ap (less stable) conformers, due to the restricted rotation about the Ar-CR2OH bond. When R=Et (compound 2) the corresponding rotation barrier was determined (7.5 kcal mol-1) by line shape simulation of the low temperature NMR spectra. Introduction of the prochiral i-Pr group in the position 3’ of a biphenyl with the CMe2OH substituent in the position 2 (4) allowed the determination of the enantiomerisation barrier (due to the Ar-Ar bond rotation) for the stereolabile axially chiral atropisomers (13.95 kcal mol-1). DFT computations of these barriers were all in agreement with the experiments. Biphenyls bearing two CR2OH groups in the 2,2’ positions were found to exist as configurationally stable atropisomers: when R=Me (7) they were separated by enantioselective HPLC and the absolute configuration assigned on the basis of the corresponding CD spectra.
D. Casarini, L. Lunazzi, M. Mancinelli, A. Mazzanti, C. Rosini (2007). Structure, Conformation, Stereodynamics, Dimer Formation and Absolute Configuration of Axially Chiral Atropisomers of Hindered Biphenyl Carbinols. JOURNAL OF ORGANIC CHEMISTRY, 72, 7667-7676 [10.1021/jo701481m].
Structure, Conformation, Stereodynamics, Dimer Formation and Absolute Configuration of Axially Chiral Atropisomers of Hindered Biphenyl Carbinols
LUNAZZI, LODOVICO;MANCINELLI, MICHELE;MAZZANTI, ANDREA;
2007
Abstract
NMR spectra of biphenyl derivatives bearing a single CR2OH substituent in the ortho position indicate that they exist as sp (more stable ) and ap (less stable) conformers, due to the restricted rotation about the Ar-CR2OH bond. When R=Et (compound 2) the corresponding rotation barrier was determined (7.5 kcal mol-1) by line shape simulation of the low temperature NMR spectra. Introduction of the prochiral i-Pr group in the position 3’ of a biphenyl with the CMe2OH substituent in the position 2 (4) allowed the determination of the enantiomerisation barrier (due to the Ar-Ar bond rotation) for the stereolabile axially chiral atropisomers (13.95 kcal mol-1). DFT computations of these barriers were all in agreement with the experiments. Biphenyls bearing two CR2OH groups in the 2,2’ positions were found to exist as configurationally stable atropisomers: when R=Me (7) they were separated by enantioselective HPLC and the absolute configuration assigned on the basis of the corresponding CD spectra.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
