Conformational consequences of the dynamic processes in the stereolabile atropisomers of acyl-substituted m-terphenyl derivatives.

By means of low-temperature NMR spectroscopy, 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 positions 2 and 6 by the 3-isopropylphenyl moiety (compounds 1-3, R=H, Me, and 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 to allow their separation by means of the enantioselective HPLC technique at ambient temperature and to obtain the corresponding CD spectra.

Unexpected stereodynamic consequences of the restricted rotations in ortho-acyl- and ortho-vinyl biphenyls.

Surprisingly, the aryl-aryl rotation barriers of biphenyl derivatives ortho-substituted by the "small" HC=O and HC=CH2 groups (10.0 and 8.4 kcal mol-1, respectively) were found greater than those observed in biphenyls ortho-substituted by the "large" t-BuC=O and t-BuC=CH2 groups (6.7 and 6.9 kcal mol-1, respectively).

Rotation in biphenyls with a single ortho-substituent.

Barriers to rotation in a range up to 15.4 kcal mol(-1) were determined by dynamic NMR spectroscopy for a series of biphenyl compounds 1a-1h and 2a-2d with a single ortho-substituent. Ab initio calculations reproduce these barriers satisfactorily and indicate the ground-state conformation of these molecules. Results are discussed in terms of the contribution of individual substituents to the barrier and of the buttressing of adjacent positions in a benzene ring by substituents.

A general procedure for the synthesis of stereochemically pure conduritol derivatives practical also for solid-phase chemistry.

A straightforward synthetic protocol apt to synthesize a library constituted by all conduritol stereoisomers in solution phase is described and successfully applied to some polymer-supported substrates. During the solid-phase sequence, an unprecedented rearrangement of a resin-bound sulfone performed under the Ramberg-Bäcklund conditions appears of particular interest. Upon treatment with Me(3)Si-I, thiepanes supported on resin are found to undergo regio- and stereospecific ring contraction to a six-membered ring system with traceless cleavage from the solid support.

Static and dynamic stereochemistry of the conformational atropisomers of tetra(o-tolyl)benzene.

[graph: see text] Whereas only one atropisomer of 1,2,4,5-tetra(o-tolyl)benzene was observed by X-ray diffraction in the solid, five conformational atropisomers were detected by low-temperature NMR in solution. Their structures were assigned by a combination of NOE experiments, solvent effect, and ab initio calculations. Variable temperature dynamic NMR and bidimensional EXSY experiments allowed the barrier for the interconversion of these atropisomers to be determined (deltaG(double dagger) = 15.3 kcal mol(-1)).

Structure, conformation, and dynamic processes of the stereolabile atropisomers of hindered terphenyl hydrocarbons.

[reaction: see text] Ortho-substituted p-terphenyl hydrocarbons exist as trans and cis atropisomers that were identified by low-temperature NMR spectroscopy. The interconversion barriers increase with the dimensions of the ortho substituents, the experimental values being matched by ab initio calculations. X-ray diffraction shows that only the trans atropisomer is present in the solids. Spectra of a tert-butyl derivative in nonequilibrium conditions indicate that the cis is more populated than the trans atropisomer in solution, favored by attractive interactions.

Stereomutations of two-bladed propeller derivatives: ortho-substituted diaryl ethylene and diaryl ketone.

Stereochemical analysis, supported by ab initio computations, predicts the existence of three possible stable helical conformers for o,o'-diisopropyl-1,1'-diphenylethylene (1) and o,o'-diisopropylbenzophenone (2). At low temperature the NMR spectra of 1 showed distinct sets of signals for these conformers, thus allowing the measurement of the three barriers involved in the related stereomutation processes to be obtained (DeltaG=6.45, 4.65, and < or =4.0 kcal mol(-1)). The NMR spectra also indicate that the asymmetric conformer (C1 point group) is the most stable one in solution, as anticipated by calculations. X-ray diffraction confirmed that this structure is that adopted in the crystalline state. On the other hand, o,o'-diisopropylbenzophenone (2) is predicted by calculations to exist essentially as a C2-type conformer, a result that was confirmed by the low-temperature NMR spectra. The interconversion barrier for the enantiomeric forms of this conformer was also measured (DeltaG=6.3(5) kcal mol(-1)).