Crabtree ’ s catalyst

Complex 5 was more active than the well-known precious-metal catalysts (palladium on activated carbon Pd/C, the Wilkinson catalyst RhCl(PPh3)3, and Crabtree s catalyst [lr(cod)(PCy3)py]PFg) and the analogous Ai-coordinated Fe complexes 6-8 [29] for the hydrogenation of 1-hexene (Table 2). In mechanistic studies, the NMR data revealed that 5 was converted into the dihydrogen complex 9 via the monodinitrogen complex under hydrogen atmosphere (Scheme 4). 

The isomerisation of aUylic alcohols to saturated ketones usually has a strong thermodynamic driving force. The ruthenium NHC complex 62 has been used to catalyse the isomerisation of allylic alcohol 59 which gives ketone 60 as the principal product along with some of the reduction product 61 [32]. The catalyst was water-soluble and the aqueous phase could be re-used for several runs (Scheme 11.15). NHC analogues of Crabtree s catalyst, [IrlPCyjKpyridineXcod)] PFg, were found to be less efficient for the isomerisation of allylic alcohols than... 

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 s catalyst was obtained from Strem Chemicals. The NBR grade used throughout was Krynac 38.50 (NBR containing 38% acrylonitrile units) from Bayer Rubber Inc. Solvents employed (chlorobenzene, acetone and 2-butanone) were used as received from Fisher Chemical Co. The purity of H2 and D2 (02 free) from Linde-Union Carbide was reported to be 99.99%. 

The product viscosity of HNBR produced using Crabtree s catalyst was studied in relation to how it varies with catalyst concentration. The results are... 

Figure 6 A catalytic pathway of NBR hydrogenation with Crabtree s catalyst.

Crabtree s catalyst is an efficient catalyst precursor for the selective hydrogenation of olefin resident within nitrile butadiene rubber (NBR). Its activity is favorably comparable to those of other catalyst systems used for this process. Under the conditions studied the process is essentially first order with respect to [Ir] and hydrogen pressure, implying that the active complex is mononuclear. Nitrile reduces the catalyst activity, by coordination to the metal center. At higher reaction pressures a tendency towards zero order behavior with respect to catalyst concentration was noted. This indicated the likelihood of further complexity in the system which can lead to possible formation of a multinuclear complex that causes loss of catalyst activity. 

Scheme 22.17 Exhaustive hydrogenation of diene- and enyne-containing reductive coupling products using Crabtree s catalyst.

Just as Wilkinson s catalyst gave rise to the bisphosphine ligands, Crabtree s catalyst [304] spawned the family of phosphorus-nitrogen ligands for simple al-kenes. Subsequently, Pfaltz developed the Phox family, which provides high ee-values with nonfunctionalized alkenes [305-310]. Other analogues are also illustrated in Figure 23.5. 

Hydrogenation of 8-terpinene (Fig. 2) proceeded in higher conversion with Crabtree s catalyst with BArp counter ion rather than the normal PFs. The BArp counter ion performed better in all instances where the more coordinating PFg salt failed to reach complete hydrogenation [20]. 

The complexes [lr(COD)(NCMe)(PR3)]BF4 (PR3 = P Pr3, PMe3), which are closely related to Crabtree s catalyst, also contain mixed ligands, very basic phosphines, P Pr3 or PMe3, and a nitrogen donor ligand, acetonitrile. These mixed-ligand pair combinations have been shown to be very useful for spectroscopic observations and have provided detailed information on the mechanism of reactions performed by those complexes [9, 10]. 

MLX plots and electron count, 1, 36 MLX plots and ligand bond number, 1, 42 MLX plots and reactivity patterns, 1, 42 MLX plots and valence, 1, 40 multicenter bonding, 1, 32 pi- and delta-backbonding, 1, 29 pi-donation considerations, 1, 29 Covalent character, dn configuration compounds, 1, 20 Covalent interaction studies, and bis(arene)chromium, 5, 345 CPPL, see Circularly polarized photoluminescence Crabtree s catalyst, for iridium dihydride complexes,... 

Rhodium-bisphospholane catalysts have been used effectively in directed hydrogenation. For example, careful choice of catalyst in the hydrogenation of the cyclopent-l-enecarboxylic ester 62 allows access to either diastereoisomer of the hydrogenation product 63 (Scheme 13.20). Crabtree s catalyst... 

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 use of NHCs has also found application in the catalytic hydrogenation of olefins. By the simple ligand exchange reaction of [Ir(COD)2(py)2]PF6 with SIMes in toluene, Nolan and coworkers have prepared the SIMes analog (27) of Crabtree s catalyst (26) [67]. The reactivity of this complex was tested for catalytic activity in the hydrogenation of several olefins. While the complex did show activity, it was less efficient than Crabtree s catalyst at ambient temperature and atmospheric H2 pressure. However, the SIMes complex did display greater activity at 60 psi of H2 pressure and 50 °C. 

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