Abstract:

Product evolution upon hydrogenating 3 diene derivs. of p-(Me3C)2C6H4 (I) is compared with the formation of 1,4-di-tert-butylcyclohexene (II) and cis- (III) and trans-1,4-di-tert-butylcyclohexane from I on a Rh/Al2O3 catalyst to det. which diene(s), upon adsorption on the catalyst, best represent(s) the structure of the intermediate formed in the rate-detg. surface reaction of I. Of the dienes 1,4-di-tert-butyl-1,4- (IV) and -1,3- (V) and 2,5-di-tert-butyl-1,3-cyclohexadiene (VI), VI exhibits best the properties expected if it forms the same absorbed intermediate as I at the same H pressure. Unlike I, however, the dienes do not yield the cis-satd. isomer III as an initial product at low H pressures. Instead cis-3,6-di-tert-butylcyclohexene (VII) is formed, along with II and I; the dienes tend to exclude the cycloalkenes from the catalyst, their effectiveness increasing in the order IV \textless V \textless VI. At low pressures, \textless5% of I is transformed directly to III; instead, II and VII are produced initially. Because the concn. of the intermediate adsorbed dienes remains low during I hydrogenation, the cis ene VIII is converted rapidly to III in the presence of I. [on SciFinder(R)]