Direct borylation

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. 

Direct borylation of hydrocarbons catalyzed by a transition metal complex has been extensively studied by several groups and has become an economical, efficient, elegant, and environmentally benign protocol for the synthesis of a variety of organoboron compounds. The Rh-, Ir-, Re-, and Pd-catalyzed C-/H borylation of alkanes, arenes, and benzylic positions... 

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). 

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]. 

Although direct borylation of arenes in specific cases is possible and metal-catalyzed C H and C-X activation reactions provide new possibihties, most arylboranes are prepared from organometalhc precursors. In particular, monoborylated... 

Group 4 Complexes. Borylated group 4 complexes have been primarily investigated with regard to their utility as single component olefin polymerization catalysts. The attachment of pendant boryl fimctionalities was envisioned to obviate the need for external Lewis acid activation (see Section 7.3.3). There are only a few cases of direct borylation of group 4... 

The phenalenyl derivative of boron seemed to be an intersting target molecule in our studies of cyclic polyunsaturated triorganoboranes. However, the attempted direct borylation of phenalenylithium 23 did not give the desired borane 24 but the "ate"-complex 25 instead, independent of the reagents ratio. Examination of the EXSY spectra of 25 showed that its two diastereomers are in a fast equilibrium (Eq. 10). Interconversion of 25a and 25b apparently proceeds via a dissociation-association mechanism with intermediate formation of 23 and 24. 

Recently, rhodium-catalyzed direct borylation of alkane with diboron was reported [50], where Cp Rh(ri -CgMeg) (Cp =C5Me5) was used as a catalyst. This reaction has drawn considerable interest because the C-H o-bond activation and the introduction of some functional group into alkane were achieved in one pot. The similar reaction (Eq. 8) was theoretically investigated with the DFT method [51] ... 

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... 

Although this is not a Pd-catalyzed reaction, direct borylation of arenes 15 with pinacolborane 10 to give arylboronate 16 can be achieved using several transition metal complexes as catalysts. Some Rh and Ir catalysts are known to be active [21,22],... 

Another strategy that bears potential is the direct borylation of alkanes, alkenes, and-most importantly - arenes by transition metal-catalyzed CH activation [39],... 

The cross-coupling protocol provides a valuable method for homologation of arylboronic acids or direct borylation of haloarenes. The coupling takes place at the Sn-C bond in preference to the boronic ester because the B-C bond is inert to biaryl coupling in the absence of a base (Eq. 11). Bulky 1,3-diphenylpropandiol protects boronic acid during... 

Recently, the group of Scott found that the direct borylation of PAHs proceeds in a reversible manner, finally delivering the thermodynamically most stable isomer [109]. With this methodology large PAHs can be selectively converted into boronates, which are suitable reaction partners for further transformations, such as Suzuki cross-coupling reactions (Scheme 45). 

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... 

In terms of atom-economy a very attractive strategy for accessing arylboronic acids is the direct boronylation of arenes through a transition metal promoted C-H functionalization. In addition to the catalyst, a suitable boron donor is required, and both diboronyl esters and dialkoxyboranes are very appropriate in this role. The concept of this type of direct borylation was first demonstrated on alkanes using photochemical... 

C-H borylation of alkanes was first demonstrated by Hartwig using photochemical activation of Cp RefCOjj (Scheme 2.3) [51]. With irradiation from a 450 W medium-pressure Hanovia mercury arc lamp, alkylboronates are obtained by direct borylation of alkanes with pin2B2 in the presence of Cp Re(CO)j (2.4-5.0 mol%) and CO (2 atm). Pinacolborane is not effective for the borylation thus, one of two boron atoms participated in the catalytic cyde. All reactions resulted in high regiospecifidty for the functionalization of terminal primary carbons. 

The recent developments on the metallation chemistry of oxazoles and benzoxazoles, isoxazoles and benzisoxazoles, pyrazoles and indazoles, thiazoles and benzo-thiazoles, and isothiazoles, benzo[c]isothiazoles, and benzoMisothiazoles have been reviewed. The two-decade history of catalytic carbon-carbon bond formation via direct borylation of alkane C-H bonds catalysed by transition metal complexes has been reported. The alkane functionalization via electrophilic activation has been underlined. " Recent advances of transition-metal-catalysed addition reactions of C-H bonds to polar C-X (X=N, O) multiple bonds have been highlighted and their mechanisms have been discussed. The development and applications of the transition metal-catalysed coupling reactions have been also reviewed. - ... 

Organoborane compounds are one of the most commonly used coupling partners for C-C couplings. In 2014, Shi et al. achieved a Pd(ll)-catalyzed selective borylation of methyl C-H bond with O2 as a benign oxidant (Scheme 1.36) [90]. Unambiguously, the produced C-B coupling products possess low reactivity in the presence of palladium catalyst. The direct borylation of unactivated Y-C(sp )-H bond also contribute an efficient access to alcohols, amines, and various functionalized molecules. 

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