The kinetics, stereochemistry, and mechanism of hydrogenation of some tertbutylbenzenes on a rhodium catalyst

The kinetics and stereochemistry of the rhodium-on-alumina-catalyzed hydrogenation of 1,3- (1) and 1,4-di-t-butylbenzene (2) have been examined to seek clarification of the mechanism of hydrogenation of the benzene ring. The influence of structure upon the rate is illustrated through comparisons with the rate of hydrogenation of t-butylbenzene (11) and 4-t-butyltoluene (12). Near atmospheric pressure, the rates are zero order in arene, first order in hydrogen, and increase in the sequence 11, 1 < 12 < 2. In competitive experiments, the order of reactivity is 11 > 12 > 1 > 2 which yields the relative adsorption coefficients of 3 × 10², 45, 3, and 1, respectively. The possibility that a dissociatively adsorbed benzene is an important entity in the mechanism is discounted because 2 does not exhibit an unusual kinetic order in hydrogen. At low pressures (0.3-2 atm) about 80% of the initial products from 2 is 1,4-di-t-butyl-cyclohexene (3), the remainder being cis-1,4-di-t-butylcyclohexane (6). The latter may have formed from the hydrogenation of cis-3,6-di-t-butylcyclohexene (5) which is also observed. In the low pressure range, the maximum concentration of the intermediate increases with pressure presumably because the rate of hydrogenation of 3 is less dependent upon the hydrogen pressure than the hydrogenation of 2. The 1,4-ene, 3, is isomerized to 5 which adds hydrogen faster than does 3. At high hydrogen pressures, the direct conversion of 2 to saturated products is the dominant process and most of the intermediate 3 is hydrogenated without prior isomerization. Much smaller amounts of the intermediate cyclohexenes are formed from 1 although increasing the pressure increases the maximum amount which is observed (1% at 1 atm, 3.5% at 30 atm). At high pressures (>30 atm) both 1 and 2 yield small amounts of the intermediates. Apparently the orientation of the bulky t-butyl substituents in the arene not only affect the rate of hydrogenation but also determine the proportion of products which are formed via the subsequent alternative reaction paths.