Iridium-Complex-Catalyzed Hydrogenations

The mechanistic basis of iridium-complex-catalyzed enantioselective hydrogenation is less secure than in the rhodium case. It is well known that square-planar iridium complexes exhibit a stronger affinity for dihydrogen than their rhodium counterparts. In earlier studies, Crabtree et al. investigated the addition of H2 to their complex and observed two stereoisomeric intermediate dihydrides in the hydrogenation of the coordinated cycloocta-1,5-diene. The observations were in contrast to the course of H2 addition to Ms-phosphine iridium complexes [69]. 

In an earlier report, Maitlis et al. showed that 1 could be easily converted into a hydrido complex [Cp lrHCl]2 (2) under ambient conditions by treatment with alcohol and a weak base (Scheme 5.1) [19], probably accompanied by the formation of carbonyl compounds. This fact means that the hydrogen atom in an alcohol can be rapidly transferred to the iridium center in the form of a hydride but then, if the hydride on the iridium could be re-transferred to another hydrogen acceptor, a new catalytic system using alcohols as substrates might be realized. In fact, a wide variety of Cp Ir complex-catalyzed hydrogen transfer systems using alcohols as substrates, and based on the above hypothesis, have been reported to date [20]. 

I 5 Catalytic Activity of Cp Iridium Complexes in Hydrogen Transfer Reactions Table 5.3 Transfer hydrogenation of ketones and imines catalyzed by ll. "... 

Hydrogen transfer reactions are catalyzed by several iridium complexes, including the dimethyl sulfoxide (DMSO) complexes cis- and trans-[Ir(Cl)4(DMSO)2]", [Ir(Cl)3(DMSO)3] and [lr(H)-(Cl)2(DMSO)3], as well as the cyclooctadiene (cod) complexes [Ir(Cl)(cod)]2 and [Ir(3,4,7,8-Me4phen)(cod)], and tra 5-[Ir(Cl)(CO)(PPh3)2]. Vaska s complex catalyzes the conversion of p-methoxybenzoyl chloride to the corresponding aldehyde. The dimethyl sulfoxide iridium(III) complexes catalyze hydrogen transfer from propan-2-ol to unhindered cyclohexanones to yield cyclohexanols, while the cod complexes serve as catalysts in the transfer of hydrogen from propan-2-ol to alkenes, ketones and a,/3-unsaturated ketones. ... 

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 mechanism of iridium-catalyzed hydrogenation remains unclear. Although several experimental [31, 53, 54] and computational [53, 55, 56] studies have been reported recently, further investigations will be necessary to establish a coherent mechanistic model. Until now, most studies have dealt with simple test substrates hence, it will be important to explore more complex and also industrially important substrates, in order to determine the full scope and limitations of iridium catalysis. 

In situ-generated iridium complexes containing bulky 3,3 -substituted H8-phosphoroamidites 20 as ligands catalyzed the asymmetric hydrogenation of... 

There have been many reports of the use of iridium-catalyzed transfer hydrogenation of carbonyl compounds, and this section focuses on more recent examples where the control of enantioselectivity is not considered. In particular, recent interest has been in the use of iridium A -heterocyclic carbene complexes as active catalysts for transfer hydrogenation. However, alternative iridium complexes are effective catalysts [1, 2] and the air-stable complex 1 has been shown to be exceptionally active for the transfer hydrogenation of ketones [3]. For example, acetophenone 2 was converted into the corresponding alcohol 3 using only 0.001 mol% of this... 

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... 

In contrast, 1,5-cyclo-octadiene remains coordinated during the catalytic cycle of hydrogenation of phenylacetylene to styrene, catalyzed by the related iridium complex [Ir(C0D)( Pr2PCH2CH20Me)]BF4. This complex, which contains an ether-phosphine-chelated ligand, catalyzes the selective hydrogenation reaction via a dihydrido-cyclo-octadiene intermediate. The reaction is first order in each of catalyst, phenylacetylene and hydrogen [11] the proposed catalytic cycle is shown in Scheme 2.3. 

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