Iridium complex boryl

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-... 

The remarkable stability of iridium-boryl complexes, as a function of the substituents on boron, is most likely responsible for the unique behavior of iridium in metal-promoted B-addition to unsaturated molecules. 

Table 10 Selected data for structurally characterized iridium boryl 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 beneficial effects of chelating ligands were also demonstrated by Hartwig, Ishiyama and Miyaura [62]. This group isolated the iridium(I) complex [lr(Bpin)3(COE)(DTBPY) modified with simple 2,2 -bipyridine ligands (such as 4,4 -di-tert-butyl-2,2 -bipyridine DTBPY), which seemed to be responsible for the first catalytic C—H borylation at room temperature (Scheme 7.30). An extension... 

An iridium(l) complex, generated from l/2[Ir(OMe)(COD)]2 and 4,4 -di-/-butyl-2,2 -bipyridine (dtbpy), catalyzed the direct borylation of 2-substituted pyrroles in stoichiometric amounts relative to 2,2 -bi-l,3,2-dioxaborolane 785 in hexane at room temperature (Equation 188) <2003ASC1103>. The pyrrolylborates 786 from regioselective C-H activation at the 5-position were formed in high yields. Similar borylation of unsubstituted pyrrole with an equimolar amount of borolane 785 regioselectively provided 2,5-bis(boryl)pyrrole 787 (Equation 189). 

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...

Intensive studies also showed that many transition metal complexes are able to catalyze aromatic C-H borylation of various arenes (Scheme 7), e.g., Cp Ir(H)(Bpin)(PMe3) [64,65], Cp Rh(Ti4-C6Me6) [65,66], ( 75-Ind)Ir(COD) [67], (776-mesitylene)Ir(Bpin)3 [67], [IrX(COD)]2/bpy (X = Cl, OH, OMe, OPh) [68-70]. A very recent study by Marder and his coworkers showed that [Ir(OMe)(COD)]2 can also catalyze borylation of C-H bonds in N-containing heterocycles [71]. For the Ir-catalyzed borylation reactions, it is now believed that tris(boryl)iridium(III) complexes [67,69], 40c, [72] are likely the reactive intermediates and a mechanism involving an Ir(III)-Ir(V) catalytic cycle is operative [67,69]. A recent theoretical study [73] provided further support for this hypothesis. A mechanism, shown in Scheme 8, was proposed. Interestingly, there are no cr-borane complexes involved in the Ir-catalyzed reactions. The very electropositive boryl and hydride ligands may play important roles in stabilizing the iridium(V) intermediates. 

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 similar Ir-catalyzed direct borylation of benzene with diboron was recently reported experimentally [52] and its full catalytic cycle was theoretically investigated with the DFT(B3LYP) method [53]. In this reaction, the iridium(III) complex, Ir(bpy)(Beg)3 (bpy=2,2 -bipyridine eg=ethyleneglycolate),is an active... 

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... 

For other neutron diffraction smdies of coordinated B—H bonds, see (a) Lam WH, Shimada S, Batsanov AS, et al. Accurate molecular strucmres of 16-electron rhodium hydrido boryl complexes low-temperature single-crystal X-ray and neutron diffraction and computational smcbes of [(PR3)2RhHCl(Boryl)] (Boryl=Bpin, Beat). Organometallics. 2003 22 4557-4568 (b) Hebden TJ, Denney TJ, Pons V, et al. O-Borane complexes of iridium syntliesis and structural characterization. J Am Chem Soc. 2008 130 10812-10820. 

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. ... 

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)). 

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