Borylations arenes, pinacolborane

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. 

Smith and Marder reported the dehydrogenative borylation of arenes, yielding arylboronates, with pinacolborane in the presence of rhodium and iridium catalysts such as Cp Rh(7/ -C6Me6), CpIrPMes, and [RhClP( Pr)3]2N2 (eq 22). Toluene and other methyl substituted arenes react with pinacolborane in the presence of [RhClP(Tr)3]2N2 and furnish benzylboronates via benzylic C-H activation and dehydrogenative borylation (eq 23). 

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],... 

The functionalization of benzylic or arene C-H bonds with boranes leads to synthetically useful boranates. Shimada and co-workers showed that [RhCl(P/Pr3)2(N2)] is an effective catalyst precursor for the borylation of aromatic and benzylic C-H bonds with the use of pinacolborane, resulting in high selectivity for benzylic C-H functionalization. ... 

The ready availability of arylboronates by an aromatic C-H borylation provides a synthetic link to the well-established palladium-catalyzed cross-coupling reactions, rhodium-catalyzed 1,4-addition to a,p-unsaturated carbonyl compounds, and other bond forming reactions using arylboronic esters (Scheme 2.12). Borylation of 1,3-dichlorobenzene with pinacolborane is followed directly by a cross-coupling reaction with methyl p-bromobenzoate for the synthesis of a biaryl product in 91% yield [60]. Pinacol esters of arylboronic acids react much slower than the free acids [62], but both derivatives achieve high isolated yields and comparable enantioselectivities (91% ee) in asymmetric 1,4-addition to N-benzyl crotonamides [63]. Borylation of arenes followed by oxidation of the C-B bond is synthetically equivalent to an aromatic C-H oxidation to phenols [64]. Oxidation of the resulting arylboronates with Oxone in a 1 1 acetone-water solution is completed within 10 min at room temperature. 

As an alternative to the electrophific functionalization of PS, the iridium-catalyzed borylation of aromatic C—H bonds to functionalize PSs with different tacticities (atactic, isotactic, and syndiotactic Fig. 24A) was reported. Because the borylation did not occur at the C—H bonds of sp carbons, it did not affect the tacticity of PSs. Similar to the case of molecular arenes, this C—H borylation of PS was achievable with bis(pinacolato) diboron (B2pin2) or pinacolborane (HBpin) and generated a mixture of meta- and para-substituted borylated PSs. However, the ratio of the two isomers was different than that of isopropylbenzene because a phenyl ring was attached to every other carbon in the PS backbone, the meta C—H bonds of PS were more sterically hindered than the para C—H bonds. Thus, while the ratio of meta- and para-substituted isomers in the C—H borylation of isopropylbenzene was 7 3, the C—H borylation of PS under identical... 

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