Atropisomers of Hindered Triarylisocyanurates: Structure, Conformation, Stereodynamics, and Absolute Configuration

The syn and anti diastereoisomers
of some 1,3,5-triarylisocyanurate derivatives were isolated and their
configuration assigned by NOE experiments and by X-ray diffraction.
The kinetics of the syn/anti interconversion
were determined, and the experimental activation energies matched
satisfactorily the values predicted by DFT computations. Low-temperature
NMR spectra were employed to determine the rotation barrier of N-bonded
unhindered aryl substituents: these barriers, too, are satisfactorily
reproduced by DFT computations. In the case of racemic diastereoisomers,
the two expected enantiomers (atropisomers) were isolated by enantioselective
HPLC and the absolute configuration established by DFT simulation
of the electronic and vibrational circular dichroism spectra.