Pinacolate, boryl

The most recent catalysts that operate under thermal conditions were then based on the premise that a Cp M fragment with ligands that dissociate under thermal conditions could be a catalyst for alkane borylation. After a brief study of Cp IrH4 and Cp Ir(ethylene)2, Dr. Chen studied related rhodium complexes. Ultimately, he proposed that the Cp Rh(ri" -C6Me6) complex would dissociate CeMce as an iimocent side product, and that Cp Rh(Bpin)2 from oxidative addition of pinBBpin (pin=pinacolate) would be the active catalyst. The overall catalytic... 

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. 

Aromatic carbon—hydrogen bonds, catalzyed borylation, 9, 172 Aromatic carbonyl compounds, in samarium pinacol couplings,... 

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 reaction of pinacolborane with styrenes 127 in the presence of bis(chloro-l,5-cyclooctadienylrhodium) at room temperature provides styrenyl pinacol boronate 128 <1999TL2585, 2002BCJ825>. While hydroboration of alkenes is the predominant reaction with phosphine-containing rhodium catalysts such as Wilkinson s catalyst and Rh(PPh3)2COCl, dehydrogenative borylation dominates over hydroboration in the presence of phosphine-free... 

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. 

Silyl(pinacol)borane added to terminal alkenes in the presence of Pt(GH2=CH2)(PPh3)2 to give l-boryl-2-silylalk-anes 247 Platinum-catalyzed silylboration of methylene cyclopropanes provided alkenylboron derivatives via a proximal G-G bond cleavage (Equation (41))250 Nickel(0)-catalyzed silylboration of vinyl cyclopropanes and cyclobutanes provided allylsilane derivatives via analogous G-G bond cleavage (Equation (42)).251... 

The reaction of HBpin in toluene in the presence of RhCl P(/-Pr)3 2(N2) (1 mol%) at 140 °C resulted in a mixture of (borylmethyl)benzene (69%) and bis(boryl)methyl benzene (7%), along with several products arising from aromatic C-H borylation (ca. 15%).345 Rhodium-bpy complexes catalyzed the borylation at the benzylic C-H bond.351 Pd/C was found to be a unique catalyst for selective benzylic C-H borylation of alkylbenzenes by B2pin2 or HBpin (Equation (70)).360 Toluene, xylenes, and mesitylene were all viable substrates however, the reaction can be strongly retarded by the presence of heteroatom functionalities such as MeO and F. Ethylbenzene resulted in a 3 1 mixture of pinacol 1-phenylethylboron and 2-phenylethylboron derivatives. 

The intramolecular allylboration of an aldehyde function leads selectively to cir-disubstituted cyclic ethers. It has been shown that both the reactive aldehyde and the allylboronate moiety can be initially generated in situ in a masked form and then liberated simultaneously by hydrolysis of the precursor functions <1997JA7499>. This methodology was successfully applied to the one-pot synthesis of the oxocene 82, a precursor of (-l-)-laurencin (Scheme 13). A DIBAL reduction of the Weinreb amide 80, metalation with r f-butyllithium, borylation with the pinacol borate ester, and, finally, liberation of both the aldehyde and the allylboronate function by aqueous pH 7 buffer solution generated the reactive 81, which cyclized in 38% overall yield to the oxocene 82. Only the all-cis-diastereomer is formed, which means that the cyclization proceeds under high asymmetric induction from the resident stereogenic center present in 80. 

Ruthenium boryl complexes containing catecholate, pinacolate and dithi-olate substituents have been analyzed as potential alkane functionalization... 

Borylation of benzyl chloride with 3 in toluene in the presence of AcOK, Pd(dba)2, and tri(4-anisyl)phosphine gives pinacol benzylboronate 9 in high yield [17]. More conveniently benzylboronate 9 can be prepared by borylation of toluene with 3 by the use of Pd on carbon as a catalyst [18]. 

Table 10. Iridium-catalysed borylation of arenes with pinacol ester of diboron (350)...

Ligands 5 and 6 are particularly efficacious for the borylation of aryl chlorides to pinacol boronate esters, thus allowing the direct one-pof synthesis of unsymmetrical biaryls from two aryl chlorides (Equation 2.23) [41]. Due to the wide breadth and scope that this family of biaryldialkylphosphine Hgands offers for palladium-catalyzed couplings of aryl chlorides, many are commercially available and aU are highly crystalline, air-stable solids, and hence easy to use. 

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