Lanthanide hydrogenation catalysts

Such lanthanide catalysts were also used in hydroamination/cyclization strategies for the synthesis of the alkaloid (+)-xenovenine. This reaction of enantiomerically pure 147 leading to 148 via two C-N bond formations was used in a late step of the synthesis after a hydrogenation, the natural product was isolated (Scheme 15.46) [100]. 

Moreover, MPVO reactions are traditionally performed with stoichiometric amounts of Al(III) alkoxides. Some improvements came from the use of dinuclear AI(III) complexes that can be used in catalytic amount [6, 7]. This is why there has been an ever-increasing interest in catalytic MPVO reactions promoted by lanthanides and transition-metal systems [8]. In these cases, it is believed that reaction proceeds via formation of a metal hydride, in contrast with the mechanism accepted for traditional aluminum alkoxide systems, which involves direct hydrogen transfer by means of a cyclic intermediate [9]. As well as La, Sm, Rh and Ir complexes, Ru complexes have been found to be excellent hydrogen transfer catalysts. The high flexibility of these systems makes them very useful not only for MPVO-type reactions, but also for isomerization processes [10]. 

Hydrogenations with Early Transition Metal and Lanthanide Catalysts... 

Mikami, K., et al., Nanoflow microreactor for dramatic increase not only in reactivity but also inselectivity Baeyer-Villiger oxidation by aqueous hydrogen peroxide using lowest concentration of a fluorous lanthanide catalyst. Journal of Fluorine Chemistry, 2006,127 592-596. 

Core of the mechanism of hydrogenation by lanthanide catalysts through a-bond metathesis steps. 

Chiral lanthanide catalysts studied for the asymmetric hydrogenation of unfunctionalized olefins. 

A series of high-cis B-I-B triblock copolymers were prepared with lanthanide catalysts by incremental monomer addition. TTie hydrogenated products lead to E-(E/P)-E triblock copolymers with hard (semi-crystalline) ethylene block at both chain ends and soft (amorphous rubbery) alternating E/P block in the center segment. 

Poly(2,4-hexadiene) with a trans-1,4 content greater than 98% was obtained with lanthanide catalysts. The completely hydrogenated product will be a head-to-head polypropylene. 

In this reaction, a rhodium atom complexed to a chiral diphosphine ligand ( P—P ) catalyzes the hydrogenation of a prochiral enamide, with essentially complete enan-tioselectivity and limiting kinetic rates exceeding hundreds of catalyst turnovers per second. While precious metals such as Ru, Rh, and Ir are notably effective for catalysis of hydrogenation reactions, many other transition-metal and lanthanide complexes exhibit similar potency. 

Airco A modification of the Deacon process for oxidizing hydrogen chloride to chlorine. The copper catalyst is modified with lanthanides and used in a reversing flow reactor without the need for external heat. Developed by the Air Reduction Company from the late 1930s. U.S. Patents 2,204,172 2,312,952 2,271,056 2,447,834. 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

Hydrogenation Catalysts Based on Croup III, Lanthanide, and Actinide Complexes... 

Scheme 6.11 Enantioselective hydrogenation Group III and lanthanide metallocene catalysts.

---