Iridium-catalyzed carbonylations proposed mechanism

Scheme 3. Proposed mechanism for the iridium-catalyzed carbonylations of methanol.

The reactivity of [Ir(GO)2l3Me] with other species has also been investigated, in particular, reactions leading to methane, a known byproduct of iridium-catalyzed carbonylation. Methane formation occurs on reaction of [Ir(GO)2l3Me] either with carboxylic acids or with H2 at elevated temperature. In both cases, the reaction is inhibited by the presence of GO, suggesting that GO dissociation from the reactant complex is required. For the protonolysis reaction with carboxylic acids, a mechanism was proposed (Scheme 8(a)) in which the acid coordinates to a vacant site created by GO loss, and methane is then liberated via a cyclic transition state. The hydrogenolysis reaction, which leads cleanly to [Ir(GO)2l3H] , could proceed via oxidative addition of H2 or an rf-Hz complex as shown in Scheme 8(b). 

Scheme 14 Proposed catalytic mechanism for carbonyl tert-prenylation from the alcohol oxidation level via iridium-catalyzed C-C bond-forming transfer hydrogenation...

The first example of direct enantioselective addition of a C—H bond to a ketone was reported by Shibata and co-workers in 2009 using the cationic Ir/ (S)-Hg-BINAP as the catalyst in the synthesis of a chiral 4-acetyl-3-hydroxy-3-methyl-2-oxindole with 72% ee. Recently, Yamamoto and co-workers developed a cationic Ir/(R,R)-Me-BIPAM catalyzed asymmetric intramolecular direct hydroarylation of a-keto amides 178 affording the chiral 3-substi-tuted 3-hydroxy-2-oxindoles 179 in high yields with complete regioselectivity and high enantioselectivity (84-98% ee). In their proposed reaction mechanism, the aryl iridium complex formed via C—H bond activation is coordinated with the two carbonyl groups of the amide (Scheme 5.65). 

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