Cram Rules in the Kingdom of Grignard Reactions

This Spring we revisited an experiment we had done a couple of years ago that involved adding a phenylmagnesiumbromide Grignard reagent to a series methylcyclohexanone positional isomers. This time I performed an NMR analysis that I had not done before. A 1971 Journal of Organic Chemistry article entitled “Sterochemical Considerations of the Reactions of Phenylmagnesium Bromide and Phenyllithium with Isomeric Methylcyclohexanones” reports limited data on the melting point, refractive index (of liquid products), IR, proton NMR, and MS for all 6 diastereomers obtained from 2, 3, and 4-methylcyclohexanone. The two diastereomers of 3-methyl-1-phenyl-1-cyclohexanol and 4-methyl-1-phenyl-1-cyclohexanol are difficult to cleanly separate with GC. Therefore, I performed an NMR of two student samples to determine whether the same diastereomers could be clearly distinguished with NMR. The methyl groups were easily detected on both pairs of diastereomers. The 3-methyl-1-phenyl-1-cyclohexanol spectrum seems to have an impurity: possibly 3-methyl-1-phenyl-1-cyclohexene. The J coupling values, in Hz, for the 3-methyl-1-phenyl-1-cyclohexanol pair were 6.0 (0.983 ppm) and 6.4 (0.961 ppm), while the values for the 4-methyl-1-phenyl-1-cyclohexanol pair were 5.6 (1.054 ppm) and 6.8 (0.986 ppm).


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