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Homogeneous catalysts are also used in asymmetric synthesis by the hydrogenation of prochiral substrates. An early demonstration of this approach was the Rh-catalyzed hydrogenation of enamides as precursors to the drug L-DOPA . [ 10 ]
It is a homogeneous catalyst for hydrogenation and hydrogen-transfer reactions, developed by Robert H. Crabtree. This air stable orange solid is commercially available and known for its directed hydrogenation to give trans stereoselectivity with respective of directing group. [2] [3]
The hydrogenation of alkynes is troublesome to control since alkynes tend to be reduced to alkanes, via intermediacy of the cis-alkene. [14] Ethylene reacts with Wilkinson's catalyst to give RhCl(C 2 H 4 )(PPh 3 ) 2 , but it is not a substrate for hydrogenation.
Hydrogenation of exocyclic methyl imine This key step to synthesize a HIV integrase inhibitor, Crixivan, is one of the few known homogeneous heteroarene hydrogenation reactions. Bulky R groups increase the catalyst's performance, with 97% e.e. and TON and TOF of 1k and 8 min −1, respectively. [18] [19]
In contrast, the hydrogenation reaction with Palladium on carbon only achieves a selectivity ratio of 20:80. The chelation of the alcohol to the catalyst is evident from the identification of a catalyst-substrate complex involving norbornene-2-ol. [11] [12] Selective Hydrogenation of terpinen-4-ol utilizing Crabtree's Catalyst.
This early work provided the insights that led to the flowering of the area of homogeneous catalysis. NiCl 2 (PPh 3) 2 is a tetrahedral (spin triplet) complex of Ni(II). In contrast PdCl 2 (PPh 3) 2 is square planar. Stryker's reagent, [(PPh 3)CuH] 6, PPh 3-stabilized transition metal hydride cluster that used as a reagent for "conjugate ...
The Shvo catalyst is an organoruthenium compound that catalyzes the hydrogenation of polar functional groups including aldehydes, ketones and imines. The compound is of academic interest as an early example of a catalyst for transfer hydrogenation that operates by an "outer sphere mechanism". [1]
Such complexes are used to analyze mechanistic aspects of homogeneous catalysts. [3] For example, rhodium forms complexes with CH 3 C[CH 2 PPh 2] 3 like [(triphos)RhCl(C 2 H 4)], [(triphos)RhH(C 2 H 4)], and [(triphos)Rh(C 2 H 5)(C 2 H 4)], provide model intermediates in the catalytic cycle for hydrogenation of alkenes. [4] Triphos sometimes ...