242THE PREPARATION OF UNSYMMETRICAL BIARYLSisomers formed.38- Moreover, itls difficult or impossible to prepare manycompounds by direct substitution. Certain compounds which are notavailable through direct substitution in biphenyl itself have been pre-pared by deamination of substituted ammobiphenyls and benzidines.28Grignard Reaction.By addition of an arylmagnesium halide to aeyclic ketone, a carbinol is formed which can be dehydrated and dehydro-genated to a biaryl compound.39-40-41-42-43-43aFor 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 biarylscan 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,436from which itshould be possible to obtain unsymmetrical biaryls by dehydrogenation.
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