NMR studies and conformational analysis: the use of Lanthanide Shift Reagents
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1975
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The use of Lanthanide Shift Reagents for the NMR-based conformational analysis of three different substituted cyclohexanones and their tertiary alcohol derivatives is described. The three cyclohexanones, 5,5-dimethyl-3-(l-naphthyl) cyclohexanone, 3,5,5-trimethyl-3-(2-naphthyl)-cyclohexanone and 5,5-dimethyl-3-(p-chlorophenyl)-3-trimethylsilyl-methylcyclohexanone and their cis- and trans- tertiary methyl alcohol derivatives were prepared by standard methods and characterized by both ¹H and ¹³C NMR spectroscopy. Lanthanide-induced shift data allowed a detailed determination of the preferred conformations of these compounds to be made in a rather simple and straightforward fashion. It was found that in all of the compounds studied the cyclohexane ring has a basic chair structure, with only small distortions present depending upon the substituents. Additionally, the C₃ substituents (l-naphtyl, 2-naphthyl and trimethylsilymethyl) were shown to exist as unequal populations of rotational isomers (for ration about the bond joining them to the cyclohexane ring). Two of the trans-alcohols (2-naphthyl and silyl) were shown to exist as equilibrium mixtures of significant amounts of two rapidly interconverting chair isomers. An x-ray crystal structure was determined for one of the alcohols of the l-naphthyl series and it was found that the l-naphthyl substituent exists in a different rotational state from that determined to be important in solution.
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Chemistry