Dienes as Possible Intermediates in the Catalytic Hydrogenation of Aromatic Hydrocarbons. 1. Dienes Derived from 1,4-Di-tert-butylbenzene and a Rhodium Catalyst

The evolution of the products upon hydrogenating three diene derivatives of 1,4-di-tert-butylbenzene (1) is compared with the formation of 1,4-di-tert-butylcyclohexene (2) and cis- and trans-l,4-di-terr-butylcyclohexane (3 and 4) from 1 on an alumina-supported rhodium catalyst to determine which diene (or dienes), upon adsorption on the catalyst, best represents the structure of the intermediate formed in the rate-determining surface reaction of 1. Of the dienes l,4-di-tert-butyl-l,4-cyclohexadiene (6), l,4-di-tert-butyl-l,3-cyclohexadiene (7), and 2,5-di-tert-butyl-l,3-cyclohexadiene (8), the last exhibits best the properties expected if it forms the same adsorbed intermediate as does 1 at the same hydrogen pressure. Unlike the arene, however, the dienes do not yield the cis-saturated isomer 3 as an initial product at low hydrogen pressures. Instead, cis-3,6-di-tert-butylcyclohexene (5) is formed along with ene 2 and arene 1; the dienes tend to exclude the cycloalkenes from the catalyst, their effectiveness increasing in the order 6 < 7 < 8. The result indicates that at low pressures little of arene 1 (less than 5%) is transformed directly to cis-1,4-di-tert-butylcyclohexane (3); instead enes 2 and 5 are produced initially, and because the concentration of the intermediate adsorbed dienes remains low during the hydrogenation of the arene, cis ene 5 is rapidly converted to 3 in the presence of the arene. At high hydrogen pressures, the cis ane 3 is an initial hydrogenation product of the dienes as it is of 1.