Iridium-catalyzed hydrogenation olefins

Secondary phosphine oxides are known to be excellent ligands in palladium-catalyzed coupling reactions and platinum-catalyzed nitrile hydrolysis. A series of chiral enantiopure secondary phosphine oxides 49 and 50 has been prepared and studied in the iridium-catalyzed enantioselective hydrogenation of imines [48] and in the rhodium- and iridium-catalyzed hydrogenation functionalized olefins [86]. Especially in benzyl substituted imine-hydrogenation, 49a ranks among the best ligands available in terms of ex. 

Ligand 19 performs excellently with the wide variety of l,l -disubstituted olefins reported. Substrates 61a-m are efficiently reduced at 1 bar of hydrogen in high enantioselectivity with very little dependence on the bulk of the alkyl substituents. Strongly coordinating olefins such as 611 and 61m tyqrically perform poorly in iridium-catalyzed hydrogenations, but reduction with 19 clearly breaks this rule and the substrates are reduced in excellent selectivity and yield. 

Abstract Computational methods are an indispensible tool for the study of metal-organic reaction mechanisms. A particularly fruitful area is that of transition metal-catalyzed hydrogenations, including enantioselective versions that are extensively used at both the laboratory and the industrial scale. This review covers computational studies of rhodium-, ruthenium-, and iridium-catalyzed hydrogenation of enamides, acrylamides, carbonyls, and unactivated olefins. The evolution of the mechanistic models and the relationship of the computational studies to experimental studies are discussed. 

Scheme 22 Mechanism of iridium-catalyzed hydrogenation of olefins proposed by Andersson...

Iridium-catalyzed hydrogenations of olefins and ketmies have also been reported. These reactions have not yet been studied computationally at the level of detail of the rhodium- and ruthenium-catalyzed reactions, and comparisons to experiments have been less clear. As a result, only a very general description of the mechanism and rationalization of the stereoselective outcome is available at this time. The results do, however, clearly demonstrates that amidst such mechanistic diversity of the iridium- and the ruthenium-catalyzed reactions, they prefer either the direct transfer after migratory insertion, or the concerted transfer. 

Kallstrom, K. Hedberg, C. Brandt, P. Bayer, A. Andersson, P. G. Rationally designed ligands for asymmetric iridium-catalyzed hydrogenation of olefins. /. Am. Chem. Soc. 2004,126,14308-14309. 

Margalef, J. Caldentey, X. Karlsson, E. A. Coll, M. Mazuela, J. Ramies, O. Dieguez, M. Pericas, M. A. A theoretically-guided optimization of a new family of modular P, S-ligands for iridium-catalyzed hydrogenation of minimally functionalized olefins. Chem. 2014,20,12201-12214. 

Iridium-Catalyzed Asymmetric Hydrogenation of Olefins with Chiral N,P and C,N Ligands 33... 

Virtually every iridium catalyst of the formula [Ir(L )(COD)] [X] for asymmetric olefin hydrogenation that has appeared after the initial counterion effect studies was based on BArp as the preferred anion [14]. The anion effect is broadly applicable in iridium-catalyzed reductions as experiments with a direct analog of the Crabtree catalyst of the formula [Ir(pyridine)(Cy3P)(COD)]BArp indicates (Fig. 2). 

Iridium-Catalyzed Asymmetric Hydrogenation of Olefins with Chiral N,P and C,N Ligands 55 Table 4 Asymmetric hydrogenation of a-substltuted a,(3-unsaturated amides with ligand 24... 

Scheme 15 Iridium-catalyzed couplings of 1,3-cyclohexadiene under transfer hydrogenation conditions (ratio refers to 1,4-oIefinic versus 1,5-olefinic alcohols)...

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