Borylations pinacolborane

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

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. 

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... 

Schlecht and Hartwig reported in 2000 that both photolysis of MeCpMn(CO)3 in the presence of HBcat (catecholborane) or HBpin (pinacolborane) and borylation of [MeCpMn(CO)2H] by ClBcat, ClBpin or ClBCy2 (Cy = cyclohexyl) can give a-borane complexes of manganese 3 [20]. The synthesis... 

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. 

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). 

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. ... 

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