Iridium complexes carbon bond activation reactions

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

In addition to transfer hydrogenation reactions, arene ruthenium complexes also display excellent activity in the catalytic hydrogenation of olefins and alkynes including asymmetric reduction [40]. Remarkably, this process occurs under milder conditions, than required for catalysis with the dissociation of arene-metal bond. Lately, arene iridium complexes have also been found to be effective hydrogenation catalysts [41 ]. It is noteworthy that iridium can also promotes addition to the carbon-nitrogen double bond. 

In 1970 two conflicting reports on the stereochemistry of the addition of chiral alkyl halides to square-planar iridium(I) complexes appeared. In one report it was claimed that the reaction of /ra/w-[IrCI(CO)(PPh2Me)2] with optically active CHjCHBrCOjEt occurred with retention of configuration as shown in Scheme 5 (Pearson and Muir, 1970). This result is consistent with a six-coordinate intermediate , as is the lack of incorporation of any free halide into the product (Section 8). However, the conclusions should again be treated with care since the study employs the cleavage of the iridium-carbon bond by halogen, and without knowing the stereochemistry of this reaction little can be said about the stereochemistry of the displacement. 

The fourth chapter gives a comprehensive review about catalyzed hydroamina-tions of carbon carbon multiple bond systems from the beginning of this century to the state-of-the-art today. As was mentioned above, the direct - and whenever possible stereoselective - addition of amines to unsaturated hydrocarbons is one of the shortest routes to produce (chiral) amines. Provided that a catalyst of sufficient activity and stabihty can be found, this heterofunctionalization reaction could compete with classical substitution chemistry and is of high industrial interest. As the authors J. J. Bmnet and D. Neibecker show in their contribution, almost any transition metal salt has been subjected to this reaction and numerous reaction conditions were tested. However, although considerable progress has been made and enantios-electivites of 95% could be reached, all catalytic systems known to date suffer from low activity (TOP < 500 h ) or/and low stability. The most effective systems are represented by some iridium phosphine or cyclopentadienyl samarium complexes. 

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