Hydrogenations of Arenes

Nickel catalysts although less expensive than rhodium and platinum are also less active Hydrogenation of arenes m the presence of nickel requires high temperatures (100-200°C) and pressures (100 atm)... 

The hydrogenation of arenes is performed at room temperature and under 40 bar of H2. In all cases, the conversion is complete after a few hours. 3000 TTO are demonstrated for anisole hydrogenation in 45 h (Scheme 5). 

Mevellec, V., Roucoux, A., Ramirez, E., Philippot, K. and Chaudret, B. (2004) Surfactant-stabilized aqueous iridium(O) colloidal suspension an effident reusable catalyst for hydrogenation of arenes in biphasic media. Advanced Synthesis and Catalysis, 346 (1), 72-76. 

The hydrogenation of arenes also has attracted interest, but until recently only a few catalytic systems have been found effective.56-60 The catalytic activity of [Rh(MeOH)2(diphos)]X and of [RuH2(H2)(PPh3)3] in the hydrogenation of 9-trifiuoroacetylanthracene and 9-methylanthracene has been explored.61,62... 

Finally, these particles generated in ionic liquids are efficient nanocatalysts for the hydrogenation of arenes, although the best performances were not obtained in biphasic liquid-liquid conditions. The main importance of this system should be seen in terms of product separation and catalyst recycling. An interesting alternative is proposed by Kou and coworkers [107], who described the synthesis of a rhodium colloidal suspension in BMI BF4 in the presence of the ionic copolymer poly[(N-vinyl-2-pyrrolidone)-co-(l-vinyl-3-butylimidazolium chloride)] as protective agent. The authors reported nanoparticles with a mean diameter of ca. 2.9 nm and a TOF of 250 h-1 in the hydrogenation of benzene at 75 °C and under 40 bar H2. An impressive TTO of 20 000 is claimed after five total recycles. 

The scarce number of metal complexes capable of catalyzing the hydrogenation of arenes is a direct consequence of the tendency of these substrates to use all the available Ti-electrons for coordination, and hence to occupy three contigu-... 

Several excellent reviews on the selective hydrogenation of arenes and heteroaromatics by single-site metal catalysts have been published over the past few years [8-10]. Consequently, the reader is advised to become acquainted with these accounts in order to obtain a deeper insight into the subject. 

Whilst the metals of the cobalt group have provided valuable mechanistic information on the mechanism of homogeneous hydrogenation of arenes, there is little doubt that ruthenium forms the most active and versatile catalysts. 

The ruthenium cluster [Ru4H4(//6-C6H6)4]2+ was also employed for the hydrogenation of arenes in a biphasic water/l-butyl-3-methylimidazolium tetrafluoro-borate biphasic system. At 90°C and 60 bar H2, benzene was reduced to cyclohexane with a TOF of 364. 

Arene hydrogenation catalysts based on other metals than late transition ones are less numerous. Of particular relevance are the results reported by Rothwell, who found that Nbv and Tav hydride complexes containing bulky aryloxide ligands (Fig. 16.2) are active for the homogeneous hydrogenation of arenes [30]. 

---