Iridium-catalyzed transfer hydrogenation

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). 

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... 

Iridium-Catalyzed Transfer Hydrogenation for C-C Bond Formation.119... 

Scheme 11 Carbonyl tert-prenylation, crotylation, and allylation from the aldehyde or alcohol oxidation level under the conditions or iridium-catalyzed transfer hydrogenation...

Under the conditions of iridium-catalyzed transfer hydrogenation employing isopropanol as reductant, 1,3-cyclohexadiene couples to aryl aldehydes to provide... 

Scheme 17 Carbonyl arylallylation from the alcohol oxidation level via iridium-catalyzed transfer hydrogenation employing alkynes as allyl donors...

Scheme 25 Diastereo- and enantioselective tinti-(hydroxy)allylation employing allylic gem-dicarboxylates as allyl donors via iridium-catalyzed transfer hydrogenation...

New catalytic allylation methodologies continue to emerge. For example, iridium-catalyzed transfer hydrogenation of a-(trimethylsilyl)allyl acetate in the presence of aldehydes mediated by isopropanol and employing the iridium catalyst... 

McFarland JM, Francis MB. Reductive alkylation of proteins using iridium catalyzed transfer hydrogenation. J. Am. Chem. Soc. 2005 127 13490-13491. 

Fig. 10.3-11 Reductive alkylation of proteins using iridium catalyzed transfer hydrogenation, (a) The iridium(lll) catalyst shown reacts with formate ion to form a water-stable hydride. This species reduces imines formed in situ, (b) This reduction process proceeds readily on proteins, affording multiple alkylated products.

Scheme 12.70 Various diastereo- and enantioselective allylations by iridium-catalyzed transfer hydrogenation [143d].

Iridium-catalyzed transfer hydrogenation of alcohol substrates also provides a useful method of C-C bond formation. 

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