Conformational Studies by Dynamic NMR. 97.Structure, Conformation, Stereodynamics and Enantioseparation of Aryl Substituted Norbornanes.

CAN 140:163414 AN 2003:996217 CAPLUS (COPYRIGHT (C) 2008 ACS ON SCIFINDER (R)) ABSTRACT THE STRUCTURE OF A 1,7,7-TRIARYLNORBORNANE (I) HAS BEEN DETD. BY X-RAY DIFFRACTION AND WAS FOUND ESSENTIALLY EQUAL TO THAT PREDICTED BY MOL. MECHANICS CALCNS. RESTRICTED ROTATION OF THE ARYL GROUPS ALSO HAS BEEN OBSD. BY DYNAMIC NMR SPECTROSCOPY IN THIS COMPD. AND IN A NO. OF ANALOGOUSLY SUBSTITUTED NORBORNANES. THE ARYL-NORBORNANE BOND ROTATION BARRIERS WERE MEASURED BY LINE SHAPE ANAL. OF THE 13C NMR SPECTRA OBTAINED AT TEMPS. LOWER THAN -100 °C AND WERE FOUND TO COVER THE RANGE 6.0 TO 7.9 KCAL MOL-1. AN EXCEPTION WAS THE ROTATION INVOLVING THE O-ANISYL GROUP IN II, WHICH OCCURS NEAR AMBIENT TEMP. SINCE THE CORRESPONDING BARRIER IS MUCH HIGHER (14.4 KCAL MOL-1). IN ONE CASE (III) CONFIGURATIONAL ENANTIOMERS COULD BE SEPD. BY CHIRAL HPLC AND THE CORRESPONDING CD SPECTRA RECORDED

Conformational Studies by Dynamic NMR. 100. Enantiomerization Process of Stereolabile Atropisomers in Pyridine-Substituted Adamantane Derivatives.

CAN 141:224907 AN 2004:566825 CAPLUS (COPYRIGHT (C) 2008 ACS ON SCIFINDER (R)) ABSTRACT THE BARRIERS FOR INTERCONVERTING THE CONFORMATIONAL ENANTIOMERS (STEREOLABILE ATROPISOMERS) OF PYRIDINE-SUBSTITUTED ADAMANTANE DERIVS. HAVE BEEN DETD. BY DYNAMIC 13C NMR SPECTROSCOPY. THE TREND OF THESE VALUES PARALLELS THAT ANTICIPATED BY MM CALCNS. IN AT LEAST ONE CASE, THE COMPUTED STRUCTURE WAS FOUND TO AGREE WITH THAT OBTAINED BY SINGLE-CRYSTAL X-RAY DIFFRACTION. IN ADDN., IT HAS BEEN POSSIBLE TO ACHIEVE A PHYS. SEPN. OF A PAIR OF THESE STEREOLABILE ATROPISOMERS AT -60 °C BY MEANS OF THE ENANTIOSELECTIVE CRYOGENIC HPLC TECHNIQUE

The biphenyl-monitored effective size of unsaturated functional or fluorinated ortho substituents

The size of a series of typical substituents has been probed by dynamic NMR measurements of the barriers to aryl-aryl rotation of the corresponding biphenyls. The resulting B values are meaningful because only mono-ortho substituted compounds were investigated and thus the results are not compromised by the non-additivity of multiple steric effects. On the basis of the chosen model system ethynyl and cyano groups were found to be slightly smaller than a phenyl ring. In contrast, vinyl and, in particular, formyl groups proved to be larger than phenyl. The latter difference is due to the loss of conjugation forces at the planar transition state. alpha-Hydroxyhexafluoroisopropyl is slightly more bulky than tert-butyl. Pentafluorophenyl and trifluoromethoxy exhibit nearly the same effective size as phenyl and methoxy, respectively Trifluoromethyl is somewhat smaller than isopropyl

Rotational barriers of biphenyls having heavy heteroatoms as ortho-substituents: experimental and theoretical determination of steric effects

The free energies of activation for the aryl-aryl rotation of 17 biphenyl derivatives, bearing a heavy heteroatom (S, Se, Te, P, Si, Sn) as ortho substituent, have been measured by variable temperature NMR. These numbers, so called B values, represent a meaningful measure of the steric hindrance exerted by the selected substituents. DFT computations match quite satisfactorily the experimental barriers and the ground state geometries as well (determined, in two cases, by X-ray diffraction). The present values extend the available list of B values and thus provide an enlarged basis for the compilation of the space requirements of standard substituents, based solely on experimental determinations

Conformational Consequences of the Dynamic Processes in the Stereolabile Atropisomers of Acyl-Substituted Meta-Terphenyl Derivatives

By means of low temperature NMR spectroscopy the conformers (stereolabile atropisomers) due to the restricted rotation about the Ar-Ar and Ar-C(O)R bonds were detected in a number of acylphenyl derivatives, substituted in the positions 2 and 6 by the 3-isopropyl phenyl moiety (compounds 1-3, R = H, Me, t-Bu, respectively). The conformational assignment was accomplished on the basis of the symmetry of the low temperature 13C NMR spectra with the added support of ab-initio calculations. The interconversion barriers were also determined by complete line shape simulation of the NMR spectra and the experimental values were satisfactorily reproduced by ab-initio calculations. In the case of the asymmetric derivative 4, two enantiomers, generated by the restricted t-BuC(O)-Ar rotation, were found sufficiently stable as to allow their separation by means of the enantioselective HPLC technique at ambient temperature and to obtain the corresponding CD spectra