Crabtree catalyst homogenous hydrogenation

The stereochemistry of reduction by homogeneous catalysts is often controlled by functional groups in the reactant. Delivery of hydrogen occurs cis to a polar functional group. This behavior has been found to be particularly characteristic of an iridium-based catalyst that contains cyclooctadiene, pyridine, and tricyclohexylphosphine as ligands, known as the Crabtree catalyst 6 Homogeneous iridium catalysts have been found to be influenced not only by hydroxy groups, but also by amide, ester, and ether substituents.17... 

The efficiency of Crabtree s catalyst as a catalyst for small molecule hydrogenation has been known for many years. Unlike many homogeneous hydrogenation catalysts, Crabtree s catalyst is able to reduce hindered olefins at favourable rates.7 It has never been reported as a catalyst for the hydrogenation of rubber except for its use in the hydrogenation of bulk PBD.8 This paper describes the first use of Crabtree s catalyst in the hydrogenation NBR. Kinetic data are presented and analyzed to understand the underlying chemistry. 

Crabtree described the use of dibenzo[a,e]cyclooctatetraene, a potent selective poison of homogeneous hydrogenation catalysts, as a tool to distinguish between homogeneous and heterogeneous catalysis in the hydrogenation of hexene with a range of catalysts [24]. 

Enamide 88 proved to be unreactive under homogeneous hydrogenation conditions using either Wilkinson s or Crabtree s catalysts even at high temperature and pressure suggesting that the trisubstituted double bond was too hindered to participate in such a process. 

The earliest homogeneous hydrogenation catalyst was Wilkinson catalyst [Rh(PPh3)3Cl] 1 which was active at 1 atm of H2 pressure at room temperature for monosubstituted and cw-disubstituted alkenes such as cyclohexene. The second, [Rh(COD)(PPh3)2]PF6 2, and third, [Ir(COD)(PCy3)(pyr)]PF6 3, generation catalysts were developed by Osbom-Schrock and Crabtree, respectively. 

The development of homogeneous olefin hydrogenation catalysts based on soluble rhodium and iridium complexes, in particular Wilkinson s catalyst (33) [27], Crabtree s catalyst (34) [41], and the Schrock-Osborn catalyst (35)... 

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