Reduction with Iridium-Containing Catalysts

A recently discovered reduction procedure provides a convenient route to axial alcohols in cyclohexyl derivatives (5). The detailed mechanism of the reaction remains to be elucidated, but undoubtedly the reducing agent is an iridium species containing one or more phosphate groups as ligands. In any case, it is clear that the steric demands of the reducing agent must be extraordinary since the stereochemical outcome of the reaction is so specific. The procedure below is for the preparation of a pure axial alcohol from the ketone. 

To a solution of 1.0 g (0.003 mole) of iridium tetrachloride in 0.5 ml of concentrated hydrochloric acid is added 15 ml of trimethylphosphite. This solution is added to a solution of 7.7 g (0.05 mole) of 4-/-butylcyclohexanone in 160 ml of isopropanol in a 500-ml flask equipped with a reflux condenser. The solution is refluxed for 48 hours, then cooled, and the isopropanol is removed on a rotary evaporator. The residue is diluted with 65 ml of water and extracted four times with 40-ml portions of ether. The extracts are dried with anhydrous magnesium sulfate, filtered, and the ether is removed on the rotary evaporator. The white solid residue is recrystallized from 60 % aqueous ethanol affording cis alcohol of greater than 99% purity, mp 82-83.5°. 

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This Chapter is devoted to the development of cobalt-, rhodium- and iridium-based catalysts that contain N-heterocyclic carbene (NHC) ligands. It will cover their most relevant catalytic applications, along with stoichiometric model reactions, except for catalytic oxidations and reductions such as hydrogenations, hydrosilylations and hydroborations, which are treated in detail in Chapters 12 and 13. Since the NHC chemistry of Group 9 metals is one of the most developed areas in this field, this overview will only cover the chemistry of classical NHCs, namely cyclic diaminocarbenes. Chapter 5 reviews the chemistry of the non-classical NHCs, to which the reader is referred. Finally, this Chapter does not pretend to be exhaustive and further details may be found in previous overviews. ... 

Sample C, containing iridium and iron, exhibits the same reversible oxidation-reduction behavior, indicating the incorporation of the iron into the iridium clusters. The isomer shift for sample C, however, was not in good agreement with the value of 0.38 mm sec- (58) expected for dilute iron in iridium alloys. This difference may reflect an unusual chemical state for iron which is associated with surface iridium atoms in clusters. A similar situation has recently been reported and discussed for iron-ruthenium catalysts (59). 

The iridium(I) salt containing 185 has also been employed as catalyst in hydrogenation of imines. In the hydrogenation of 2-substitufed quinolines to provide chiral tetrahydro derivatives iridium complexes of SYNPHOS and DIFLUORPHOS prove to he effective catalysts. Oximes undergo enantioselective reduction (and N—O bond cleavage) on treatment with borane and spirocychc boronate 186. ... 

Itsimo [25] has also shown that polymer-supported OPEN monosulfonamides containing sulfonated pendent group (Scheme 16) are able to catalyze the HTR reduction of ketones in water with sodium formiate as hydrogen donor (S/C = 100). However, TsDPEN immobilized on polystyrene crosslinked or not, polymer 30 and 31 respectively, shrank in water. Sodium /j-styrene sulfonate was copolymerized with chiral A-(vinylbenzene-p-sulfonyl)-DPEN (20) imder radical polymerization conditions with or without DVB leading respectively to ligand 32 and 33. Control of the balance hydrophilicity/hydrophobieity of the polymer support is carried out by changing the salt from Na to quaternary ammonium. All of these polymers swelled in water, and their respective ruthenium, rhodium or iridium complexes were prepared. Compared to sodium salt polymer-supported catalyst from 32a and 33a, ammonium... 

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