B2pin2

Hartwig and Hillmyer have recently reported the Rh-catalyzed borylation of polyolefins to yield boronate-functionalized polymers (58) (Fig. 37) in a single step.83 The number-average molecular weight of the borylated polymer that was obtained at a B2pin2 polyethylethylene ratio of 0.3 was found to be the highest (Mn = 52,000 and PDI = 1.09) among the developed boronate-functionalized polyolefins. 

The rhodium-catalyzed borylation of methyl C-H bonds is compatible with several moieties containing oxygen, nitrogen, and fluorine.154 For example, the reaction of pinacol acetal of 2-hexanone with bis(pinacolato)diborane (B2pin2) in the presence of Cp Rh(774-C6Me6) catalyst gives the alkylboronate ester in 74% yield (Equation (111)). The rhodium-catalyzed C-H activation and borylation occur at the least hindered and least electron-rich methyl group. 

The rhodium-catalyzed borylation of alkanes is applied to regiospecific functionalization of polyolefines.165,165a The reaction of polypropylenes (atactic, isotactic, and syndiotactic) with B2pin2 in the presence of Cp Rh( 74-C6Me6) catalyst at 200 °G affords the borylated polymers, which are treated with basic hydrogen peroxide in a mixture of THF and H20 to oxidize the boronate esters to the corresponding alcohols (Scheme 20). The hydroxylated polymers contain 0.2-1.5% hydroxymethyl side-chains. 

B2Pin2 [79] or HBpin [80] at room temperature in nonpolar solvents. The reactions tolerate a wide range of functional groups including Br, I, C02Me, and CN. 

A convenient direct route has recently been described for obtaining regioregular polyalkylthiophenes using a tandem iridium-catalyzed borylation to produce the monomer, and a palladium-mediated coupling to produce the polymer [68]. The treatment of substituted thiophenes with B2pin2 in the presence of [lrCl2(COD)]2/ 4,4 -di-tert-butyl-2,2 -bipyridine (DTBPY) provided the expected monomer in 97% yield (Scheme 7.35). 

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. 

The Miyaura borylation reaction enables the synthesis of boronates by cross-coupling of bis(pinacolato)diboron (B2pin2) with aryl halides and vinyl halides. 

Borylated products derived from B2pin2 allow normal work up including chromatographic purification and are stable towards air. Pinacol esters are difficult to hydrolyze, but they may serve as coupling partners in the Suzuki Coupling and similar reactions without prior hydrolysis. 

The mild reaction conditions allow the preparation of boronates which are not accessible via lithium or Grignard intermediates followed by borylation. The use of HBPin instead of B2Pin2 allows similar reactions in large scale synthesis, and also tolerates various reducible functional groups, although side products may arise due to dehalogenation of the aryl halide. 

The mechanism proposed for aromatic C-H borylation of aromatic compounds 1 by B2pin2 3 catalyzed by the Ir-bpy complex is depicted in Scheme 3 [6-9]. A tris(boryl)Ir (III) species [5, 6, 11] 6 generated by reaction of an Ir(I) complex 5 with 3 is chemically and kinetically suitable to be an intermediate in the catalytic process. Oxidative addition of 1 to 6 yields an Ir(V) species 7 that reductively eliminates an aromatic boron compound 4 to give a bis(boryl)Ir(III) hydride complex 8. Oxidative addition of 3 to 8 can be followed by reductive elimination of HBpin 2 from 9 to regenerate 6. 2 also participates in the catalytic cycle via a sequence of oxidative addition to 8 and reductive elimination of H2 from an 18-electron Ir(V) intermediate 10. Borylation of 1 by 2 may occur after consumption of 3, because the catalytic reaction is a two-step process - fast borylation by 3 then slow borylation by 2 [6],... 

Table 2. Stoichiometric borylation of aromatic compounds 1 by HBpin 2 or B2pin2 3 catalyzed by 1 /2[lr(OMe)(cod)]2-dtbpy in hexane at room temperature a 5equiv. of to 2, b lOequiv. of 1 to 3, c 0.45 equiv. of to 2, d 1 equiv. of to 3.

Borylation is the photochemically or thermally promoted conversion of substrates with C-H bonds into boronate esters using bis(pinacol)diborane(4) (B2pin2) or pinacolborane (HBpin) reagents (Scheme 1) [16-35]. The selectivity for methyl C-H bonds and the versatility of organoboron reagents [36, 37] give alkane borylation potential for use in synthetic contexts. 

Scheme 1. Borylation of alkanes by B2pin2 and HBpin reagents.

The mechanism of thermal borylation of alkanes seems to be similar to that of photochemical borylation of alkanes. The most active precatalysts for thermal alkane borylation, Cp Rh( 4-C6Me6) and Cp Rh(j/2-C2H4), are converted to mixtures of Cp Rh(Bpin)3(H) and Cp Rh(Bpin)2(H)2 in the catalytic reactions [48], Reaction of HBpin or B2pin2 with Cp Rh(Bpin)2(H)2 yields Cp Rh(Bpin)3(H). Furthermore, reaction of Cp Rh(Bpin)3(H) with alkanes and arenes as solvent yields Cp Rh(Bpin)2(H)2. Both species are kinetically capable of being catalysts of the borylation of alkanes. 

Computational studies performed on model complexes in collaboration with Hall and coworkers suggest that alkane borylation may occur by a ej-bond metathesis pathway (Scheme 3) [48]. The proposed mechanism for the borylation of alkanes by 1 begins with elimination of HBpin to generate the 16-electron complex Cp Rh(Bpin)2. This complex then forms a <7-complex (3) with the alkane. The vacant p-orbital on boron then enables cr-bond metathesis to generate a o-borane complex (4). Reductive elimination of the alkylboronate ester product and oxidative addition of B2pin2 then regenerate 1. 

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