Hydrogenation of cyclohexene

The reactivity of cyclohexene is much higher than that for benzene hydrogenation and cyclohexane dehydrogenation, particularly using noble metals as catalysts. Good results has been reported over 0.35 wt% Pt, Ir, Rh, Re, U, Ptir, PtRe, or PtU/y-Al203 catalysts in a pulsed MSR at 250 °C [24] or in a continuous MSR (thiophene, pyridine, and cyclohexene conversions of 94.3, 100, and 90.3%, respectively) [25]. 

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If has long been known that the enthalpy of hydrogenalion of benzene (49.8 kcal moU Conant and Kistiakowsky, 1937) is not the same as three times the enthalpy of hydrogenation of cyclohexene (3 x 28.6 kcal moU ). Evidently, the double bonds that w e write in the Kekule structure of benzene... 

Cyclooctatetraene is relatively stable but lacks the special stability of benzene Unlike benzene which we saw has a heat of hydrogenation that is 152 kJ/mol (36 kcal/mol) less than three times the heat of hydrogenation of cyclohexene cycloocta tetraene s heat of hydrogenation is only 26 kJ/mol (6 kJ/mol) less than four times that of CIS cyclooctene... 

Reasoning backward, however, we know that we can prepare cyclohexane by hydrogenation of cyclohexene. We ll therefore use this reaction as the last step in our proposed synthesis. 

FIGURE 11.2 Heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene, a hypothetical 1,3,5-cyclohexa-triene, and benzene. All heats of hydrogenation are in kilojoules per mole. 

Solvents influence rate as well as selectivity. The effect on rate can be very great, and a number of factors contribute to it. In closely related solvents, the rate may be directly proportional to the solubility of hydrogen in the solvent, as was shown to be the case for the hydrogenation of cyclohexene over platinum-on-alumina in cyclohexane, methylcyclohexane, and octane 48). Solvents can compete for catalyst sites with the reacting substrates, change viscosity and surface tension (108), and alter hydrogen availability at the catalyst surface. 

The first successful hydrogenation reactions in ionic liquids were studied by the groups of de Souza [45] and Chauvin [46] in 1995. De Souza et al. investigated the Rh-catalyzed hydrogenation of cyclohexene in l-n-butyl-3-methylimidazolium ([BMIM]) tetrafluoroborate. Chauvin et al. dissolved the cationic Osborn complex [Rh(nbd)(PPh3)2][PFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions (e.g., [PFg] , [BFJ , and [SbF ] ) and used the obtained ionic catalyst solutions for the biphasic hydrogenation of 1-pentene as seen in Scheme 5.2-7. 

Assume that you are in a laboratory carrying out the catalytic hydrogenation of cyclohexene to cyclohexane. How could you use a mass spectrometer to determine when the reaction is finished ... 

The catalytic hydrogenation of alkenes by mixed NHC/phosphine complexes of rhodinm was also stndied. Initial resnlts of the hydrogenation of cyclohexene by /ranx-[RhCl(ICy)(L)2] (ICy = M/V -(dicyclohexyl)imidazol-2-ylidene, L = PPhj. 

Finke has reported remarkable catalytic lifetimes for the polyoxoanion- and tetrabutylammonium-stabi-lized transition metal nanoclusters [288-292]. For example in the catalytic hydrogenation of cyclohexene, a common test for structure insensitive reactions, the lr(0) nanocluster [296] showed up to 18,000 total turnovers with turnover frequencies of 3200 h [293]. As many as 190,000 turnovers were reported in the case of the Rh(0) analogue reported recently. Obviously, the polyoxoanion component prevents the precious metal nanoparticles from aggregating so that the active metals exhibit a high surface area [297]. 

Using the titanocene-catalyzed co-hydrogenation of cyclohexene, we have studied the kinetics of the polymerization of a number of primary silanes ( 20 ). The rate law was found to be ... 

Another interesting comparison of the homogeneous Wilkinson catalyst with AHC-Wilk is in the high TON hydrogenation of cyclohexene in 10%... 

Fig. 5.14 Catalytic activities ( ) for the hydrogenation of cyclohexene over catalysts with various Au/Pd composition and average sizes (O) of loading noble metal. The reaction was carried out in a conventional closed system at 23 0.5°C. Conditions initial H2 pressure of 1 atm., cyclohexene of 1.18 mmol, 1-propanol solution of 30 mL [44]...

Price and Schiewetz Ind Eng. Chem. 49 (807), 1957] have studied the catalytic liquid phase hydrogenation of cyclohexene in a laboratory scale semibatch reactor. A supported platinum catalyst was suspended in a cyclohexene solution of the reactant by mechanical... 

Fig. 1. Mechanistic scheme for hydrogenation of cyclohexene (A) catalyzed by RhCl(PPh3)3 in benzene (P = PPh3) the major catalytic pathway is shown in the rectangle...

Muetterties has suggested that the dimeric hydride [RhH(P OiPr 3)2]2 catalyzes alkene and alkyne hydrogenation via dinuclear intermediates [91]. However, no kinetic evidence has been reported to prove the integrity of the catalysts during the reactions. On the other hand, studies of the kinetics of the hydrogenation of cyclohexene catalyzed by the heterodinuclear complexes [H(CO) (PPh3)2Ru((u-bim)M(diene)] (M = Rh, Ir bim=2,2 -biimidazolate) suggested that the full catalytic cycle involves dinuclear intermediates [92]. 

Catalytic homogeneous hydrogenation of cyclohexene has been claimed for simple systems such as nickel(II) acetylacetonate [39] or a nickel-chloride complex with two monodentate amines [40]. The latter complex was used as comparison for a heterogeneous catalyst obtained by impregnation of the complex on y-alu-mina [40]. SCRs of 100 were used at 30 atm. H2 and temperatures up to 100°C, resulting in conversions of only 20-35% after 1 h. 

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