Iridium complexes borylation

One most important observation for the mechanistic discussion is the oxidative addition/insertion/reductive elimination processes of the iridium complex (31) (Scheme 1-10) [62]. The oxidative addition of catecholborane yields an octahedral iridium-boryl complex (32) which allows the anti-Markovnikov insertion of alkyne into the H-Ir bond giving a l-alkenyliridium(III) intermediate (34). The electron-... 

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)). 

Scheme 5 Proposed mechanism for borylation by bipyridine ligated iridium complexes...

It is no coincidence that the first X-ray structure analysis of a transition metal-boryl complex [8] was made on the iridium complex jac-[IrH2(BC8Hi4)(PMe3)3] (2) [9] derived from the B—H activation of the 9-borabicyclo[3.3.1]-nonane (9-BBN)... 

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. 

Iridium complexes generated from [IrCl(COD)]2 and 2,2 -bipyridine (bpy) catalyze the borylation of pyrrole and indole derivatives to yield the borylated products 788 in moderate to good yields (Equation 190) <2004JM021>. 

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]. 

Fig. 31 Crystallographically characterized octahedrally coordinated iridium(III) boryl complexes 9.22-9.27...

The structure of a further iridium bis(boryl) complex, the cationic species [(tbbpy)(cod)Ir(Bpin)2]+[OTf] (9.33), together with that of a related neutral tris(boryl) system (vide infra), has been reported as part of a study to elucidate the mechanism and intermediates of arene borylation chemistry [136]. 9.33 features an approximately octahedral coordination geometry with mutually cis pairs of bipyridyl, diene and Bpin ligands. The two Ir-B bond lengths [2.06(2) and 2.12(1) A] are effectively identical within the standard 3a limit. 

Tris(boryl) complexes have been shown to have a central role in the iridium-catalyzed borylation of arenes [136,137]. The syntheses of the half-sandwich iridium(III) systems ( 6-toluene)Ir(Bcat)3, 9.34, and rf-mesitylene)Ir(Bcat)3, 9.35, were reported by Marder and co-workers in 1993, by exploiting the reaction of ( 5-Ind)Ir(cod) with excess HBcat using the arene as a solvent. The organic products of the reactions were found not only to include species resulting from the hydroboration or hydrogenation of the cod or indenyl ligands, but also compounds derived from borylation... 

Fig. 32 Crystallographically characterized iridium tris(boryl) complexes 9.34-9.37...

Recently, it was also shown [74] that pentamethylcyclopentadienyl rhodium and iridium complexes with labile dative ligands catalyze the borylation of alkanes at the terminal position under thermal conditions (Scheme 9). It was proposed that Ir(V) and Rh(V) boryl complexes act as intermediates in these reactions. Reaction of Cp IrH4 with HBpin gives Cp Ir(Bpin)(H)3 and Cp Ir(Bpin)2(H)2. Both of the two Ir(V) boryl complexes react with octane to give octylboronate ester [75], though it is still unclear regarding the role... 

The direct borylation of arenes is an attractive strategy for accessing synthetically useM arylboron reagents. Iridium complexes have emerged as the catalyst of choice for the selective borylation of arenes using HB(pin) or Bj(pin)j [67], Extensive studies by the Hartwig, Ishiyama, and Miyanra groups have led to the identification of the Ir OMe(cod)j/dtbpy as the optimal catalyst system for these transformations [68-70], As illustrated in Scheme 24.59, electronically diverse arenes are borylated at room temperature to afford the products in excellent yields. The site-selectivity of these... 

Iridium(I)-catalyzed aromatic C-H bond borylation widi pinacolborane (HBpin) and its mechanism have been studied extensively by Smith [57]. Iridium complexes (27, 28) themselves are inefficient, but addition of a small electron-donating phosphine such as PMej or chelating dmpe [l,2-bis(dimethylphosphino)ethanej to give an iridi-um(I)-phosphine complex (29, Scheme 2.8) substantially increases catalyst activity and turnover number [57aj. The maximum turnover number achieved in the borylation of benzene with HBpin at 150 °C in a sealed ampoule is 4500 [57aj. 

Theoretical calculations, using the DFT method, examined the iridium-catalyzed borylation of benzene with diboron source B2Cg2 (eg — ethyleneglycolato, —OCH2CH2O—). This study found that oxidative addition of the B—B bond occurred at a lower energy than the corresponding C—H bond activation step. A tris(boryl)iridium(III) complex was also concluded to be catalyticaUy active, and an unusual seven-coordinate iridium(V) species is beheved to be involved as a key intermediate in the catalytic cycle.A tris(boryl)iridium(III) complex has been previously prepared and structurally characterized by Marder from the reaction between [Ir(T] -indenyl) (r " -COD)] (COD = cyclooctadiene) and excess of catecholborane. ... 

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