Borylations with pinacolborane

Aryl chlorides typically do not react under these conditions. However, Miyaura has been able to extend the scope of this reaction to include aryl chlorides by changing the catalyst system to Pd(dba)2 and PCys and replacing pinacolborane with bis(pinacolato)diboron. Vinyl iodides and trifiates undergo borylation with pinacolborane under similar conditions in the presence of triphenylarsine (AsPEb) (eq 17). Benzylic halides react with pinacolborane in the presence of PdCl2, PPhs, and N,N-diisopropylethyl amine (eq 18). Borylation of allylic halides in the presence of Pt(dba)2, AsPhs, and EtsN leads to highly regio-and stereoselective allylboronates (eq 19). 

Scheme 3.9 Synthesis of aryl boronic esters by palladiumot-alyzed borylation with pinacolborane.

Murata, M., Oyama, T., Watanabe, S., Masuda, Y. Synthesis of alkenylboronates via palladium-catalyzed borylation of alkenyl trifiates (or iodides) with pinacolborane. Synthesis 2000, 778-780. 

Caballero A, Sabo-Etienne S (2007) Ruthenium-catalyzed hydroboration and dehydrogenative borylation of linear and cyclic alkenes with pinacolborane. Organometallics 26 1191... 

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],... 

Borylated pyrrole 88 was prepared by Oestreich by treatment of the corresponding pyrrole with pinacolborane and a ruthenium(II) thiolate complex. The direct synthesis of 88 promises to find wide utility in medicinal chemistry and was applied to a variety of substituted indoles (13JA10978). N-Methylpyrrole was directly arylated at room temperature by photoredox catalysis with diaryliodonium salts to furnish 89 in 84% yield (13SL507). 

Iridium(I)-catalyzed aromatic C-H bond borylation widi pinacolborane (HBpin) and its mechanism have been studied extensively by Smith [57]. Iridium complexes (27, 28) themselves are inefficient, but addition of a small electron-donating phosphine such as PMej or chelating dmpe [l,2-bis(dimethylphosphino)ethanej to give an iridi-um(I)-phosphine complex (29, Scheme 2.8) substantially increases catalyst activity and turnover number [57aj. The maximum turnover number achieved in the borylation of benzene with HBpin at 150 °C in a sealed ampoule is 4500 [57aj. 

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. 

Palladium-catalyzed substitution of an allylic acetate such as 37 with diboronyl reagent 24 gives good yields of allylboronates in DMSO as solvent (Equation 21) [57]. This reaction is a steieoconvergent process the boron tends to add to the least substituted end of unsymmetrical allylic units, and both E and Z isomers of acetate 37 yield the E-isomer of 38. Another drawback to this process is that the product is often formed with variable yields of the allylic dimer (e.g., 39). Allylic halides were also employed as substrates with pinacolborane as the borylating agent and a platinum catalyst [58]. 

Murata, M. Watanabe, S. Masuda, Y. 2000. Regio- and stereoselective synthesis of allylboranes via platinum(0)-catalyzed borylation of allyl halides with pinacolborane. Tetrahedron Lett. 41 5877-5880. 

Alternatively, the borylation can be conducted with pinacolborane (HBpin) that takes place smoothly for aryl and 1-alkenyl halides or triflates (Scheme 5-166). For this reaction, sometimes named the Matsuda reaction, both carbocyclic and heterocyclic systems can be used. 

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. 

Pinacolborane is extensively used in the borylation of aryl halides 114 in the presence of a base (typically pyridine or Et3N or KOAc) and catalytic amount of PdCl2(DPPF) to furnish aryl boronates 115 (DPPF = l,l -bis(diphenyl-phosphino)ferrocene Equation 7) <1997JOC6458, 2000JOC164>. Pinacolborane is compatible with esters, ketones,... 

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. 

In terms of C-3 substitution reactions, the regioscleclive borylation of 1-triisopropylsilyl-pyrrole (77) was achieved providing a valuable reagent for cross-coupling reactions <01OL2831>. Thus, reaction of 77 with the rhodium precatalyst shown in the presence of pinacolborane affords the C-3 substituted derivative 78. 

---