Conformational studies by dynamic NMR. 47. Conformation, stereodynamics, and chiral separation of the rotational enantiomers of hindered naphthyl ketones

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Abstract

A no. of α-naphthyl ketones bearing a Me group in the 2-position of the naphthalene ring adopt a twisted conformation, yielding a pair of rotational enantiomers owing to the restricted rotation about the naphthalene-carbonyl bond. NOE measurements indicate that the twisted conformation is almost exactly orthogonal. The enantiomerization barriers were detd. by variable-temp. dynamic NMR spectroscopy, with monitoring of either the anisochronous signals of prochiral substituents or the pairs of signals of the enantiomers themselves, made detectable at low temp. by the addn. of a chiral auxiliary agent. The free energies of activation cover a range of 8-20 kcal mol-1, depending on the steric hindrance of the various acyl groups. Mol. mechanics calcns. suggest that enantiomerization occurs through a pathway where the carbonyl function passes position 2, rather than position 8, of the naphthalene ring. In the case of a 1,5-disubstituted acyl deriv. contg. the tert-Bu moiety (I), the expected meso and racemic conformers were both obsd. in soln., whereas only the meso structure was present in the crystals (obtained by the usual slow crystn. procedure) as judged by CP MAS solid-state NMR spectra and by low-temp. soln. spectra in nonequil. conditions. Low-temp. (-15°) chiral high-performance liq. chromatog. (LT-ChirHPLC) was employed to sep., identify, and det. the yields of the two enantiomers of 2-methyl-1-naphthyl tert-Bu ketone (II) as well as the three conformers (meso, SS, and RR) of I. The low-temp. CD spectra of the rotational enantiomers of II and I were obtained and the abs. configurations assigned by connecting online a CD spectrometer to the LT-ChirHPLC app. Resp., the S and SS configurations were found to correspond to the conformational enantiomers with the shorter retention times [(R,R)-DAC-DNB column].

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