Alkenes catalysts, iridium complexes

The classical notion has been that iridium complexes can be effective hydrogenation catalysts, with defined limitations. In this respect, Crabtree and Morris made the key breakthroughs [9], and their catalyst (Fig. 31.16) has been widely employed for the reduction of simple alkenes. It was widely successful in the di-... 

Diastereomerically pure iridium complexes of the formula [(ri -C5Me5)lr (/ )-Pro-phos (activated alkene)](SbF6)2 (activated alkene = enal, methacrylonitrile) are active, and selective catalysts for the DCR between one point binding activated alkenes and nitrones. Enals coordinate to the metal in a completely diastereoselec-tive way with a restricted geometry. From the point of view of the selectivity, a key point in enal coordination is the establishment of CH/n-attractive interactions between the CHO aldehyde proton and one (f )-Prophos phenyl group. This interaction fixes the methacrolein rotamer around the M-O bonds and renders the system enantioselective. 

The behavior shown in Schemes 2.22 and 2.23 suggests that these hydrido arene-iridium complexes might act as hydrogenation catalysts. Thus, the [(T -C6H6)IrH2(P Pr3)]BF4 complex has been shown to be an efficient hydrogenation catalysts for alkenes and alkynes, as well as some carbonyl groups [22]. 

Fig. 2.10 Applications of asymmetric hydroboration with diphosphine catalysts to meso-symmetric alkenes. For 10(a), iridium complexes reverse the sense of enantioselectivity in up to 54% enantiomeric excess.

Iridium complexes show very limited activity as hydroformylation catalysts compared with those of rhodium. [IrCl(CO)(PPh3)2] (43) has been shown to hydroformylate terminal alkenes but much higher temperature and pressure are required than with rhodium.348 349 Reviews on rhodium-catalyzed hydroformylation are available.350,351... 

As previously shown for platinum versus palladium complexes, iridium complexes are generally much less effective catalysts than rhodium ones for the ketonization of alkenes by 02, H202 or ROOH. 

Iridium complexes such as IrCl(COD)(L-L), [Ir(COD)(L-L)]+ and [Ir(COD)-(py)(PR3)2]+ are active catalysts for the hydrogenation of a variety of substrates, particularly tetrasubstituted alkenes, e.g. [Ir(COD)(py)(PCy3)] known as Crabtree s catalyst, hydrogenates tetramethylethylene. On the other hand, [Ir(COD)(py)2]+ is inactive, apparently since it does not add H2. The hydrido cation [Ir(H)2(COD)(i72-Pr,2PCH2CH2OMe)]+, with a chelating P—O ligand, selectively hydrogenates phenylacetylene to styrene, via Ir—CH=CHPh intermediates.2... 

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