Selective dehydrogenation of dipentene (R-(+)-limonene) into paracymene on silica supported palladium assisted by α-olefins as hydrogen acceptor

Paracymene is a well known material used in polymers and perfume chemistry. It can be obtained by the dehydrogenation and the aromatization of dipentene (R-(+)-limonene). To be useful, this reaction must be fully selective to avoid further purification processes. Dipentene is readily dehydrogenated with a palladium catalyst, but hydrogenation simultaneously takes place leding to cis and trans paramenthanes which drastically decreases the yield in paracymene. To avoid this hydrogenation which occurs by hydrogen transfer, α-olefin has been used as hydrogen acceptor. Using silica supported palladium and a hydrogen acceptor (e.g. l-decene) paracymene is obtained from dipentene with a very high yield (95%) at a moderate temperature (145°C).