Iridium-Catalyzed Hydroformylation

A comparison of Rh and Ru catalysts in the hydroformylation of linear butenes [4] or the strong electron-deficient substrate 3,3,3-trifluoropropene led to the conclusion that the latter are less active [5]. Moreover, in the hydroformylation of propene in comparison with Co and Rh catalysts, an inferior selectivity was noted [6]. In a competition experiment with the iridium-catalyzed hydroformylation of several a-olefins at 13 bar syngas pressure and 100 C, a related PPhj-modified Ru complex revealed no activity [7]. On the other hand, unmodified ruthenium based catalysts were shown to be more active than osmium complexes [8], thus the following rough order of reactivity results ... 

The mechanism of the iridium-catalyzed hydroformylation has been studied by NMR spectroscopy. Species including iridium acyl and alkyl dihydride intermediates were detected. The results confirmed that the CO-deficient atmosphere favors hydrogenation over carbonylation [76]. 

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... 

Discussion Point DPS hint Using the concepts behind the successful iridium catalyzed methanol carbonylation as guide which would you expect the difficult steps in a hydroformylation to be. 

Anderson s group combined this iridium-catalyzed generation of syngas with the rhodium-catalyzed hydroformylation of styrene [45]. As alcohol components. 

Aqueous organometalHc catalysis allows the use of NH3-solutions in water for the direct synthesis of amines from olefins in a combined hydroformylation/reductive amination procedure (Scheme 4.19). The hydroformylation step was catalyzed by the proven Rh/TPPTS or Rh/BINAS (44) catalysts, while the iridium complexes formed from the same phosphine ligands and [ IrCl(COD) 2] were found suitable for the hydrogenation of the intermediate imines. With sufficiently high NH3/olefin ratios (8/1) high selectivity towards the formation of primary amines (up to 90 %) could be achieved, while in an excess of olefin the corresponding... 

Hydroformylation of olefins was also shown to be catalyzed by rhodium and iridium carbonyls in zeolites. Mononuclear as weU as polynuclear carbonyls are selective towards hydroformylation versus hydrogenation and rhodium carbonyls are more selective. 

The intramolecular insertion of a hydride into a coordinated olefin is a crucial step in olefin hydrogenation catalyzed by late transition metal complexes, such as those of iridium, rhodium, and ruthenium (Chapter 15), in hydroformylation reactions catalyzed by cobalt, rhodium, and platinum complexes (Chapter 16), and in many other reactions, including the initiation of some olefin polymerizations. The microscopic reverse, 3-hydride elimination, is the most common pathway for the decomposition of metal-alkyl complexes and is a mechanism for olefin isomerizations. 

The presented series deals with the average catalytic activity of many compounds. Of course, one can find cobalt compounds which show lower activity than iridium compounds. Rhodium complexes are 10 -10" more active than [Co2(CO)g] which is most commonly utilized in industry. Therefore, it is possible to obtain the same amounts of aldehydes with considerably smaller amounts of rhodium complexes and under milder conditions, because rhodium compounds catalyze the hydroformylation reaction at lower pressures and temperatures, even at room temperature and atmospheric pressure. The price of the rhodium catalyst is comparable to the cost of [Co2(CO)g]. Therefore,... 

In the literature it is stated that chromium [101], iridium [173, 178, 742, 980], manganese [180, 181], copper [109, 182, 183], magnesium [191], calcium [192], sodium [193], rhenium [742], osmium [742] and platinum also catalyze the hydroformylation reaction. However, most of the results given in these papers are not reproducible (see also ref. [980]). 

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