Iridium-catalyzed Reactions

A class of iridium(I) complex (30) possessing 2,2 -bipyridine (bpy) or 4,4 -di-fe/t-butyl-2,2 -bipyridine (dtbpy) ligands exhibits excellent activity and selectivity for aromatic C-H borylation with Bzpinj [59] or HBpin [60], An Ir catalyst prepared from %[IrCl(COD)2l2 (COD = 1,5-cyclooctadiene) and dtbpy achieves a maximum turnover number (8000) with 0.02 mol% catalyst loading at 100 °C. The reaction was first demonstrated at 80-100 °C using an Ir-Cl complex, but was foimd to proceed smoothly even at room temperature when the catalyst is prepared from 

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Following on from stochiometric iridium-based C-H functionalizations, iridium-catalyzed reactions have been developed and can be combined with oxidation chemistry to enable the selective functionalization of aromatics. Moreover, these reactions can be performed under solventless conditions (Equations (91) and (92)). ... 

The most fundamental reaction is the alkylation of benzene with ethene.38,38a-38c Arylation of inactivated alkenes with inactivated arenes proceeds with the aid of a binuclear Ir(m) catalyst, [Ir(/x-acac-0,0,C3)(acac-0,0)(acac-C3)]2, to afford anti-Markovnikov hydroarylation products (Equation (33)). The iridium-catalyzed reaction of benzene with ethene at 180 °G for 3 h gives ethylbenzene (TN = 455, TOF = 0.0421 s 1). The reaction of benzene with propene leads to the formation of /z-propylbenzene and isopropylbenzene in 61% and 39% selectivities (TN = 13, TOF = 0.0110s-1). The catalytic reaction of the dinuclear Ir complex is shown to proceed via the formation of a mononuclear bis-acac-0,0 phenyl-Ir(m) species.388 The interesting aspect is the lack of /3-hydride elimination from the aryliridium intermediates giving the olefinic products. The reaction of substituted arenes with olefins provides a mixture of regioisomers. For example, the reaction of toluene with ethene affords m- and />-isomers in 63% and 37% selectivity, respectively. 

The iridium-catalyzed reaction of primary amines with diols gave cyclic amines. The reaction of amine 109 with diol 110 in the presence of [Cp lrCl2]/NaHC03 catalyst gave heterocyclization product 111 (Equation 10.26) [50]. 

This chapter describes the application of iridium-catalyzed reactions in the synthesis of molecular and macromolecular organosilicon compounds and related sihcon derivatives. Some mechanistic implications are introduced which illustrate the specific catalytic activation of organic and silicon compounds by iridium... 

Iridium complexes are known to be generally less active in hydrosilylation reactions when compared to rhodium derivatives, although iridium-based catalysts with bonded chiral carbene ligands have been used successfully in the synthesis of chiral alcohols and amines via hydrosilylation/protodesilylation of ketones [46-52] and imines [53-55], The iridium-catalyzed reaction of acetophenone derivatives with organosubstituted silanes often gives two products (Equation 14.3) ... 

Iridium-catalyzed reactions of organosilicon derivahves with various organic compounds involving C—C bond formation have been found suitable for the synthesis... 

A novel hydrazepine formation is observed in the iridium-catalyzed reaction of alkynyl hydrazone 304 with Bu Me2SiH in excess under slightly forcing conditions (Equation (53))/ The transformation leading to 305 can be explained by continuous interaction of Bu MceSiH with a silylformylation product of 304 under the reaction conditions/ ... 

Compared to the rhodium-catalyzed stereoselective reactions, studies on the iridium-catalyzed reactions have been limited until recently. Usually lower selectivities have been observed in the Ir(i)-catalyzed reactions.459,460 The asymmetric hydrosilylation of imines affords optically active secondary amines. These are very valuable compounds, but the studies on that reaction are quite limited.461 Close examinations of these reactions revealed that they proceed via a transfer hydrogenation. Other conditions such as the 2-propanol/base system in the presence of an appropriate metal complex have been employed as well, but only low selectivities were obtained.462... 

Although the studies at Monsanto indicated that an iodide promoted iridium catalyzed reaction was feasible, much more basic research was needed before a viable commercial procedure (the Cativa process) became possible. 

Butene is thought to be formed by the initial isomerization of the reactant to adsorbed 1,2-butadiene, followed by hydrogenation (see Fig. 26). This mechanism predicts a maximum at -d in the 2-butenes and at -d in 1-butene if the chance of surface species aquiring a deuterium atom, as opposed to a hydrogen atom, is high. Moreover, the appearance of trace quantities of 1,2-butadiene in the rhodium and iridium catalyzed reactions lends further support to this route. A check may be made, since, if the 1,2-butadiene hydrogenates to yield approximately equal proportions of cis-2-butene and 1-butene (see Section... 

It can be concluded that in the case of the iridium-catalyzed reactions, radicals must be generated from which allylic hydroperoxides can form but perhaps another pathway exists for hydroperoxide formation in the presence of the rhodium complex. 

Propyl-3-iodoquinoline has been transfonned into 2-propyl-3-(trifluoromethyl) quinoline by action of ClCF2C02Me, Cul, and KF on reflux in DMF [75], An interesting example of highly selective trifluoromethylation of 6-methylquinoline by means of the iridium-catalyzed reaction is presented in Scheme 49 [76],... 

Kinetic studies have allowed the optimum conditions to be defined for the synthesis of acetic anhydride by the carbonylation of methyl acetate using a variety of Group VIII metal catalysts. Such studies, complemented by IR and UV spectroscopic studies, have helped to elucidate the main catalytic pathways for the rhodium- and iridium-catalyzed reactions in the presence of iodide. Although complex, both mechanisms essentially involve oxidative addition of Mel to [M(CO)2l2] (M = Rh or Ir) followed by CO insertion into the metal-methyl bond and subsequent reductive elimination of MeCOI the latter reacts with acetate ion to give acetic anhydride and regenerate iodide. ... 

An iridium-catalyzed reaction process has successfully promoted the addition of indoles to unactivated alkenes with Markovnikov selectivity [23]... 

Several C—H borylation processes have been developed. One of the most practical is an itidium-catalyzed reaction [65,68] The iridium-catalyzed reaction enables the borylation of arenes with retention of existing aryl bromides that can be functionahzed in subsequent reactions [68]... 

Therefore, direct functionalization by iridium-catalyzed reactions involving C-H bond activation would provide an alternative protocol to the existing multi-step organic synthesis [116-130]. To date, iridium-catalyzed C-H activation of aromatic rings for reactions such as borylation [131 -136], alkylation/alkenylation [137-143] and silylation [144-146], and cross-coupling [147, 148] has been investigated. 

The major reason why the development of iridium-catalyzed reactions is far behind that of rhodium-, palladium-, and platinum-catalyzed reactions is connected to the high stability of the iridium complexes. 

A breakthrough in the study of iridium-catalyzed reactions was reported by Crabtree in 1977 regarding the hydrogenation reactions of alkenes [38, 39]. Only recently, iridium complexes have been utilized on alkynes for other reactions than the hydrogenation. 

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