Iridium -catalyzed heterocyclization activation

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... 

Although not fitting exactly into the scope of this book, the iridium catalyzed borylation of five membered heterocycles through C-H bond activation also deserves mentioning. A recent report by Miyaura disclosed the reaction of bis(pinacolato)diboron with heteroaromatic systems, where thiophene, fiirane and pyrrole were converted to their 2-boryl derivatives with good selectivity (6.86.), The yields presented refer to the diboron compound since the heterocycles were used in excess in all cases. Indole, benzofurane and benzothiophene were monoborylated with similar efficiency.116... 

Driver and coworkers reported on iridium-catalyzed Af-heterocyclization by benzylic C-H bond activation [154]. In this case, treatment of an ort/ro-substituted aryl azide with a catalytic amount of [lr(OMe)(cod)]2 (2mol%) at 25 C gave a mixture of the corresponding aniline, indole, and indoline in 19, 13, and 58% yield, respectively, for a total yield of 90% (Scheme 11.6). The introduction of the electron-withdrawing CF3 group on the aryl substituent led to indoline derivatives in high yield with high selectivity. 

In conclusion, this chapter described some representative examples of the iridium-catalyzed synthesis of heterocycles involving C-H bond activation. The reactions include oxidative ort/ro-aryl C-H, benzyl, and sp -CH activation... 

In contrast, reactions catalyzed by la were typically conducted with added [Ir (C0D)C1]2 to trap the K -phosphoramidite ligand after dissociation, and thereby, to leave the unsaturated active catalyst. Under these conductions, as much as half of the iridium in the system is present in an inactive acyclic species. In contrast, when ethylene adduct lb is used as the catalyst, all of the iridium belongs to the active metalacyclic species. Hartwig and coworkers have recently taken advantage of the increased availability of the active catalyst generated from lb to develop new allylic substitution reactions. These new processes include the reactions of carbamates, nitrogen heterocycles, and ammonia. 

Tejel C, Ciriano MA (2007) Catalysis and Organometallic Chemistry of Rhodium and Iridium in the Oxidation of Organic Substrates. 22 97-124 Tekavec TN, Louie J (2006) Transition Metal-Catalyzed Reactions Using N-Heterocyclic Carbene Ligands (Besides Pd- and Ru-Catalyzed Reactions). 21 159-192 Theopold KH (2007) Dioxygen Activation by Organometallics of Early Transition Metals. 22 17-37... 

Alternatively, they were also found to be good racemization catalysts. Iridium complexes, but also rhodium compounds, catalyzed the racemization step in the enzymatic dynamic kinetic resolution of secondary alcohols, and excellent results were reported for alkyl-aryl as well as dialkyl secondary alcohols. Finally, picolyl and pyridine functionalized N-heterocyclic carbene iridium complexes [(C N)Ir(Cp )Cl]Cl were moderately active catalysts for the polymerization of norbomene in the presence of methylaluminoxane as cocatalyst. ... 

While major advances in the area of C-H functionalization have been made with catalysts based on rare and expensive transition metals such as rhodium, palladium, ruthenium, and iridium [7], increasing interest in the sustainability aspect of catalysis has stimulated researchers toward the development of alternative catalysts based on naturally abundant first-row transition metals including cobalt [8]. As such, a growing number of cobalt-catalyzed C-H functionalization reactions, including those for heterocycle synthesis, have been reported over the last several years to date (early 2015) [9]. The purpose of this chapter is to provide an overview of such recent advancements with classification according to the nature of the catalytically active cobalt species involved in the C-H activation event. Besides inner-sphere C-H activation reactions catalyzed by low-valent and high-valent cobalt complexes, nitrene and carbene C-H insertion reactions promoted by cobalt(II)-porphyrin metalloradical catalysts are also discussed. 

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