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Organic Lab Reactions 247

Organic Lab Reactions 247

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242 THE PREPARATION OF UNSYMMETRICAL BIARYLS isomers formed. 38 - Moreover, itls difficult or impossible to prepare many compounds by direct substitution. Certain compounds which are not available through direct substitution in biphenyl itself have been pre- pared by deamination of substituted ammobiphenyls and benzidines. 28 Grignard Reaction. By addition of an arylmagnesium halide to a eyclic ketone, a carbinol is formed which can be dehydrated and dehydro- genated to a biaryl compound. 39 - 40 - 41 - 42 - 43 - 43a For example, m-tolyl- magnesium bromide and cyclohexanol yield 1-m-tolylcyclohexanol, which on dehydration and dehydrogenation gives 3-methylbiphenyl. When applicable, this method is excellent. However, only those biaryls can be prepared which contain.substituents that do not enter into reac- tion readily with Grignard reagents and that can withstand the condi- tions of dehydrogenation. Diels-Alder Reaction. By means of the Diels-Alder reaction a con- siderable number of hydrobiphenyls have been prepared, 436 from which it should be possible to obtain unsymmetrical biaryls by dehydrogenation.
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