Reduction with iridium complexes

A mixture of the ketone (4.62 g), iridium tetrachloride (1.23 g), trimethyl phosphite (15 ml), propan-2-oI (200 ml) and water (50 ml) is heated under reflux for 21 hr. Much of the solvent is then distilled off ca. 215 ml) and the organic products remaining are isolated by extraction with ether. If reduction is essentially complete, the product at this stage may be sufficiently pure for most preparative purposes. Pure components can be obtained by chromatography over alumina, a representative experiment (on the above scale) gives unchanged ketone (0.13 g), cw-alcohol (4.36 g) and tmns-2 co o (0.16 g) (eluted in this order by pentane, and then by pentane containing ether). 

A solution of cholestanone (0.39 g), chloroiridic acid (0.13 g), trimethyl phosphite (2 ml) and water (4 ml) in propan-2-ol (30 ml) is boiled under reflux for 16 hr and worked up in the usual way. Chromatography of the product on deactivated alumina (40 g) gives cholestanone (0.16 g), cholestan-3a-ol (0.2 g) and cholestan-3jS-ol (19 mg). 

Fajkos and F. Sorm, Coll. Czech. Chem. Comm., 24, 766 (1959). 

Fieser and M. Fieser, Reagents for Organic Synthesis , John Wiley and Sons, Inc., New York, 1967, p. v. 

Vitali, A. Ercoli and W. Klyne, Tetrahedron Letters, 189(1962). 

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The main methods of reducing ketones to alcohols are (a) use of complex metal hydrides (b) use of alkali metals in alcohols or liquid ammonia or amines 221 (c) catalytic hydrogenation 14,217 (d) Meerwein-Ponndorf reduction.169,249 The reduction of organic compounds by complex metal hydrides, first reported in 1947,174 is a widely used technique. This chapter reviews first the main metal hydride reagents, their reactivities towards various functional groups and the conditions under which they are used to reduce ketones. The reduction of ketones by hydrides is then discussed under the headings of mechanism and stereochemistry, reduction of unsaturated ketones, and stereochemistry and selectivity of reduction of steroidal ketones. Finally reductions with the mixed hydride reagent of lithium aluminum hydride and aluminum chloride, with diborane and with iridium complexes, are briefly described. 

Iridium-catalyzed transfer hydrogenation of aldehyde 73 in the presence of 1,1-dimethylallene promotes tert-prenylation [64] to form the secondary neopentyl alcohol 74. In this process, isopropanol serves as the hydrogen donor, and the isolated iridium complex prepared from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS is used as catalyst. Complete levels of catalyst-directed diastereoselectivity are observed. Exposure of neopentyl alcohol 74 to acetic anhydride followed by ozonolysis provides p-acetoxy aldehyde 75. Reductive coupling of aldehyde 75 with allyl acetate under transfer hydrogenation conditions results in the formation of homoallylic alcohol 76. As the stereochemistry of this addition is irrelevant, an achiral iridium complex derived from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and BIPHEP was employed as catalyst (Scheme 5.9). 

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-... 

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... 

The cationic iridium complex [Ir(cod)(PPh3)2]OTf, when activated by H2, catalyzes the aldol reaction of aldehydes 141 or acetal with silyl enol ethers 142 to afford 143 (Equation 10.37) [63]. The same Ir complex catalyzes the coupling of a, 5-enones with silyl enol ethers to give 1,5-dicarbonyl compounds [64]. Furthermore, the alkylation of propargylic esters 144 with silyl enol ethers 145 catalyzed by [Ir(cod)[P(OPh)3]2]OTf gives alkylated products 146 in high yields (Equation 10.38) [65]. An iridium-catalyzed enantioselective reductive aldol reaction has also been reported [66]. 

Computational and catalytic studies of the hydrosilylation of terminal alkynes have been very recently reported, with the use of [ Ir( r-Cl)(Cl)(Cp ) 2] catalyst to afford highly stereoselectively P-Z-vinylsilanes with high yields (>90%) [35]. B-isomers can be also found among the products, due to subsequent Z —> E isomerization under the conditions employed. The catalytic cycle is based on an lr(lll)-lr(V) oxidahve addition and direct reductive elimination of the P-Z-vinylsilane. Other iridium complexes have been found to be active in the hydrosilylation of phenylacetylene and 1-alkynes for example, when phenylacetylene is used as a substrate, dehydrogenative silylation products are also formed (see Scheme 14.5 and Table 14.3). 

Bis[iV,iV -di(2-pyridyl)-imidazol-2-ylidene]aurate(I) tetrafluoroborates, preparation, 2, 292-293 Bis[iV,iV -di(2-pyridyl-methyl)-imidazol-2-ylidene]aurate(I) tetrafluoroborates, preparation, 2, 292-293 Bis(diselenolate) complexes, dinuclear iron compounds, 6, 242 Bis(dithiolene) compounds, in tungsten carbonyl and isocyanide complexes, 5, 644 Bis(enolato) complexes, with bis-Cp Ti(IV), 4, 589 Bis(enones), in reductive cyclizations, 10, 502 Bis(ethanethiolato) complexes, with bis-Cp Ti(IV), 4, 601 Bis(ethene)iridium complexes, preparation, 7, 328-329 -Bis(fluorenyl)zirconocene dichlorides, preparation,... 

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