Iridium Catalyst Allylic coupling

As demonstrated in recent work by Obora and Ishii, alkynes serve as allyl donors in carbonyl allylations from the alcohol oxidation level [277]. Specifically, upon exposure to an iridium catalyst generated in situ from [lr(OH)(cod)]2 and P( -Oct)3, l-aryl-2-methylalkynes couple to primary alcohols to furnish homoallylic alcohols with complete branched regioselectivity and excellent levels of diastereoselectivity (Scheme 17). 

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). 

Siloxide complexes of rhodium and iridium are much more effective than the respective chloro-complexes in hydrosilylation of a variety of olefins such as 1-hexene, vinylsilanes, and allyl alkyl ethers (81). [ Rh(cod)()u-OSiMe3) 2] and [Rh(cod)(PCy3)3(OSilV[e3)] in particular are very active and selective catalysts for the hydrosilylation of glycidyl ether by triethoxysilane yielding commercially available silane coupling agent. 

Another application of this reaction is in a synthesis of the alkaloid xenovenine 9.150 (Scheme 9.46), one of many isolated from tropical frogs, which acquire the compound from their diet. While asymmetric allylation provided the original stereogenic centre, Suzuki coupling, the Wittig reaction and reductive amination were each employed for key bond-forming reactions. Additional syntheses of xenovenine can be found in Section 6.2. The readily available allylic carbonate 9.152 was reacted with the diacylammo-nia derivative 9.153 in the presence of an iridium complex and the chiral pre-catalyst (Scheme 9.47). The... 

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