Iridium complexes arene

Remarkable carbon-boron bond-forming reactions are catalyzed by iridium complexes and proceed at room temperature with excellent regioselectivity, governed by steric factors. Heteroarenes are borylated in the 2-position and this reaction is generally tolerant of halide substituents on the arene (Equations (87) and (88)). 

The iridium complex composed of l/2[ Ir(OMe)(cod)2 ] and 4,4 -di-/ r/-butyl-2,2 -bipyridine (dtbpy) shows a high catalytic activity for aromatic G-H silylation of arenes by l,2-di-/z r/-butyl-l,l,2,2,-tetrafluorodisilane.142 The reaction of 1,2-dimethylbenzene with l,2-di-/< r/-butyl-l,l,2,2,-tetrafluorodisilane in the presence of l/2[ Ir(OMe)(cod)2 ] and dtbpy gives 4-silyl-l,2-dimethylbenzene in 99% yield (Equation (103)), which can be utilized for other functionalizations such as arylation and alkylation. 

Several arylations involving reactive alkenes such as norbomene or allenes have been reported. Togni and coworkers have shown that norbomene is selectively added to the ortho positions of phenols to produce a mixture of 30 and 31 in 69% and 13% yield, respectively, after 72 hours at 100°C (22) [108, 109]. 1,1-dimethylallene also reacts with aromatic carboxamides (33) to produce prenylation products (34) in the presence of cationic iridium complexes (23) [110]. In both cases, initial ortho C-H bond activation in arenes directed by coordinating groups followed by olefin insertion has been proposed. 

The behavior shown in Schemes 2.22 and 2.23 suggests that these hydrido arene-iridium complexes might act as hydrogenation catalysts. Thus, the [(T -C6H6)IrH2(P Pr3)]BF4 complex has been shown to be an efficient hydrogenation catalysts for alkenes and alkynes, as well as some carbonyl groups [22]. 

Perhaps most dramatically of all, for the first time, bis(carbene)-substituted iridium complexes, such as [Ir(cod)(NHC)2] (NHC = 1,3-dimethyl- or 1,3-dicyclohexylimidazolin-2-ylidene] were successfully used by Herrmann and coworkers as C—H-activation catalysts in the synthesis of arylboronic acids starting from pinacolborane and arene derivatives [46]. 

The direct borylation of arenes was catalyzed by iridium complexes [61-63]. Iridium complex generated from [lrCl(cod)]2 and 2,2 -bipyridine (bpy) showed the high catalytic activity of the reaction of bis (pinaco la to) diboron (B2Pin2) 138 with benzene 139 to afford phenylborane 140 (Equation 10.36) [61]. Various arenes and heteroarenes are allowed to react with B2Pin2 and pinacolborane (HBpin) in the presence of [lrCl(cod)]2/bipyridne or [lr(OMe)(cod)]2/bipyridine to produce corresponding aryl- and heteroarylboron compounds [62]. The reaction is considered to proceed via the formation of a tris(boryl)iridium(lll) species and its oxidative addition to an aromahc C—H bond. 

Among the latter group, iridium complexes (though less common than rhodium) and perhaps also ruthenium play crucial roles in many of the above-mentioned transformations of silicon compounds, leading to the creahon of sihcon-carbon bonds. Examples include the hydrosilylation or dehydrogenahve silylation of alkenes and alkynes, the hydroformylahon of vinylsilanes, and the silyhbrmylation of alkynes as well as activation of the sp C—H of arenes (by disilanes) and alkenes (by vinylsilanes). 

Silylation of arenes was reported to take place with rhodium and iridium complexes as the catalyst. The Vaska complex effects the reaction of benzene with pen-tamethyldisiloxane,230 whereas [RhCl(CO)(PMe3)2] catalyzes the formation of C—Si bonds under irradiation231 [Eq. (10.39)] 232... 

Similar effects are observed in -complexes Ag1 and Hgn coordinated to arenes exhibit bending angles of a 5° (which is symptomatic of high -character of the complex and a low degree of CT). On the other hand, in rhenium, rhodium, iridium, and nickel complexes, arenes exhibit bending angles of up to 45° [2]. 

An interesting example of C-H bond activation is the ferrocenium (le-oxidant) catalysis of monomethylaryliridium complexes [39], in which the oxidized (17-electron) iridium complex is coordinated to the arene substrate in a n(rj2) fashion. Complete charge transfer from the arene to the metal (strong acceptor) takes place upon coordination and results in the formation of the (T-arenium complex. The latter can readily transfer a proton to a methyl ligand to yield methane and the oxidized monomethyl complex, which is finally reduced by the original dimethyliridium complex to complete the ET chain process [39] ... 

Examples for o-phenylene scaffolds for bis-carbene ligands come from the research groups of Peris [344,345] and Herrmann [346]. Synthesis of the bis-imidazolium salt is achieved by reaction of a,a -xylene dichloride and the N-substituted imidazole. The rhodium(l) and iridinm(I) complexes can then be made by addition of the imidazolium salt to a solution of [M(cod)Cl]2 (M = Rh, Ir) in ethanol or acetonitrile (with NEtj as auxiliary base) (see Figure 3.108). The rhodium complexes were used successfully in the hydrosi-lylation of styrene [344] whereas both the rhodium and iridium complexes were used for the direct borylation of arenes making functionalised arylboronic acid esters accessible by a simple one-pot reaction [346]. 

The bridging chloride ligands in these [Ir(olefin)2Cl]2 compounds are susceptible to metathesis reactions, yielding new dimeric compounds of the form [Ir(olefin)2B]2 where B represents a new bridging ligand. AUcoxides, thiolates, and carboxylates have all been employed successfully in the replacement of chloride. The complexes with B = Br, I have also been prepared, both by metathesis reactions and by direct reaction of cyclooctene or cyclooctadiene with IrBrs or Iris The olefin complexes also provide excellent starting materials for the syntheses of arene and cyclopentadienyl iridium complexes, a subject that will be discussed in the next section. 

A similar iridium complex, [IrH2(acetone)2PPh3]SbF6 [73], catalyzes the selective dehydrogenation of cyclohexenes to arenes. In this case, the cyclohexenes work as the substrate and also as the hydrogen acceptor. 

Direct arylations of arenes are, however, not restricted to palladium-catalyzed transformations, but were also accomplished with, inter alia, iridium complexes. Thus, the direct coupling of various aryl iodides with an excess of benzene in the presence of [Cp IrHCl]2 afforded the corresponding biaryl products, but usually in moderate yields only (Scheme 9.30) [69]. The reaction is believed to proceed via a radical-based mechanism with initial base-mediated reduction of iridium(III) followed by electron transfer from iridium(II) to the aryl iodide. Rather high catalyst loadings were required and the phenylation of toluene (90) under these reaction conditions provided a mixture of regioisomers 91, 92, and 93 in an overall low yield (Scheme 9.30) [69]. 

The pentamethylcyclopentadienyl complex of iridium relative to the iridium complexes that activate alkanes via typical oxidative addition, also reacts [33] with arenes by an unknown mechanism ... 

Pentamethylcyclopentadienyl-Rhodium and -Iridium Complexes as Catalysts for Olefin and Arene Hydrogenation... 

It is also noteworthy that the iridium complex (lb) is a very much poorer catalyst than the rhodium analog. This point and the fact that the more highly alkyl-substituted benzenes are only hydrogenated to a small degree, even by [RhC5Me3Cl2]2, can be explained by the observations that (i) iridium forms more stable complexes of the type [M(CoMe5)-(arene )] than rhodium does and (ii) within a series of alkylbenzenes, the more highly substituted ones form the more stable arene complexes for both Rh and Ir (20,21). Presumably the more stable complexes are reduced less easily. 

In addition to transfer hydrogenation reactions, arene ruthenium complexes also display excellent activity in the catalytic hydrogenation of olefins and alkynes including asymmetric reduction [40]. Remarkably, this process occurs under milder conditions, than required for catalysis with the dissociation of arene-metal bond. Lately, arene iridium complexes have also been found to be effective hydrogenation catalysts [41 ]. It is noteworthy that iridium can also promotes addition to the carbon-nitrogen double bond. 

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