Retentive Friedel-Crafts Alkylation of Benzene with Optically Active 2-Chloro-1-phenylpropane and 1-Chloro-2-phenylpropane

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Abstract
Alkylations of benzene with both (−)-2-chloro-1-phenylpropane (1) and (+)-1-chloro-2-phenylpropane (2) in the presence of Lewis acid gave the same products: (−)-1,2-diphenylpropane (3), 1,1-diphenylpropane, and polymeric materials. In these reactions, (−)-3 was obtained in 45–100% optical yield and was not racemized under the conditions used. These results reveal that the reaction from 1 to 3 proceeds with retention of configuration and that from 2 to 3 with inversion. The stereochemistry of the alkylation with 1 is elucidated by the mechanism involving a neighboring phenyl π-assisted cation; benzene attacks only the β-carbon of 1 from the side on which the previously attached chloride anion is located. The result of the reaction with 2 can be explained by a process involving the 1,2-shift of phenyl group in the ionization step of 2, followed by the formation of the same intermediate as in the reaction with 1. The mechanism of the overall reaction is discussed.