The Gallivanting Grignard

After last year’s “discovery” of the commercially available phenylmagnesiumbromide, I decided to branch out to more interesting reactant than benzophenone. (One redeeming feature of the Grignard synthesis of triphenylmethanol was the pink coloration of the reaction mixture.) This year we used the three positional isomers of methycyclohexanone as the ketone reagent. The resulting methylphenylcyclohexanols are not commercially available. The lack of commercial standards of the products is challenging but also adds a degree of novelty to the experiment. An interesting, somewhat unanticipated, outcome for this experiment was the unequal formation of two product diastereomers. The ratio of diastereomers varies for the three methylcyclohexanones. The ratios are 1:8, 1:3 and nearly 1:1 for the 2, 3, and 4-methyl-1-phenyl-1-cyclohexanols respectively. A 1971 article in the Canadian Journal of Chemistry by Rutherford, Wassenaar, Brien, and Fung “Preparation and Structural Elucidation of cis– and trans-1-phenyl-2-methylcyclohexanol” proposes that the “trans” isomer is the dominant one. In this case “trans” corresponds to the SR/RS enantiomer pair. Evidently, the position (and nearness) of the methyl group relative to the incoming phenyl group (as described by Cram) influences the stereochemical outcome of the reaction.
2-mpchol1

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  1. […] 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 […]

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