Pyridine-based iridium complexes

Catalyst 58, in which the oxazoline ring has been replaced with an imidazoline, gave ee-values in the low 90% region for substrates 36 and 38-40 [42]. However, for certain substrates (see Section 30.5), replacement of the oxazoline by an imidazoline has resulted in significantly higher enantioselectivity. Recently, a number of pyridine- and quinoline-derived iridium complexes 59-62 have been developed, which gave promising enantioselectivities with substrates 36-39 [43, 44]. However, these catalysts cannot yet compete with the most efficient oxazoline-based complexes and complex 14. 

Cyclometallated iridium complexes, for OLEDs, 12, 145 Cyclometallated palladium(II) complexes from amines and pyridines, 8, 280 with C,C-chelating ligands, 8, 291 enantioselective synthesis, 8, 296 ferrocene-based palladacycles, 8, 292 four-membered palladacycles, 8, 297 imine- and oxime-based complexes, 8, 285 with N-N and N=N bonds, 8, 288 palladacycle catalysis, 8, 297... 

Development of pyridine-based iridium N,P ligand complexes... 

Some metal-pyridine complexes have promise as anticancer treatments. Cationic iridium (III) complexes with pyridine-based ligands have been shown to localize in the endoplasmic reticulum resulting in stress-induced apoptosis. This effect, useful as a potential cancer treatment, can be tuned by changing the size of the ligands (13JMC3636). 

The double bonds of NBR have been selectively hydrogenated for the same purpose, i.e. in order to improve the resistance of the vulcanisates to ageing in oils and hot air. Pyridine-cobalt complexes and complexes of rhodium, ruthenium, and iridium " have been described as hydrogenation catalysts. In some cases the hydrogenation is incomplete. The main obstacle to homogeneous catalytic hydrogenation of olefinic structures is the difficulty in obtaining adequate selectivity, but there are special catalysts based on transition metals which are almost entirely satisfactory in this respect. 

The complexes of iridium(IV), fewer in number than those of iridium(III), often contain pyridine as a ligand. An impure substance with the reported composition Ir(C6H6N)2Cl4, prepared by the action of pyridine on an iridium (IV) chloride solution, was described first by Renz and later by Gutbier and Hoyermann. Since the composition was based entirely on iridium content without any allusion to isomerism and since the color was inconsistent with his findings, Delepine concluded that the product was not a pure substance. 

The synthesis of chiral racemic atropisomeric pyridines by cobalt-catalyzed [2 + 2 + 2] cycloaddition between diynes and nitriles was reported in 2006 by Hrdina et al. using standard CpCo catalysts [CpCo(CO)2, CpCo(C2H4)2, CpCo(COD)] [34], On the other hand, chiral complexes of type II were used by Gutnov et al. in 2004 [35] and by Hapke et al. in 2010 [36] for the synthesis of enantiomerically enriched atropisomers of 2-arylpyridines (Scheme 1.18). This topic is described in detail in Chapter 9. It is noteworthy that the 2004 paper contains the first examples of asymmetric cobalt-catalyzed [2 - - 2 - - 2] cycloadditions. At that time, it had been preceded by only three articles dealing with asymmetric nickel-catalyzed transformations [37]. Then enantioselective metal-catalyzed [2 -i- 2 - - 2] cycloadditions gained popularity, mostly with iridium- and rhodium-based catalysts, as shown in Chapter 9. 

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