Borylation catalytic

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 stoichiometric insertion of terminal alkenes into the Cu-B bond of the (NHC)Cu-B(cat) complex, and the isolation and full characterisation of the p-boryl-alkyl-copper (I) complex has been reported. The alkyl complex decomposes at higher temperatures by P-H elimination to vinylboronate ester [67]. These data provide experimental evidence for a mechanism involving insertion of alkenes into Cu-boryl bonds, and establish a versatile and inexpensive catalytic system of wide scope for the diboration of alkenes and alkynes based on copper. 

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

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... 

Thus, the initial result of the stoichiometric boiylation of arenes was discovered serendipitously. However, the development of this initial observation into the catalytic borylation of alkane C-H bonds was largely based on the design of complexes for the stoichiometric functionalization of alkanes and then the catalytic functionalization of alkanes. 

Formal hydration of the double bond appeared by the hydroboration-oxidation sequence. Desymmetrization reactions with catalytic asymmetric hydroboration are not restricted to norbornene or nonfunctionalized substrates and can be successfully applied to meso bicyclic hydrazines. In the case of 157, hydroxy derivative 158 is formed with only moderate enantioselectivity both using Rh or Ir precatalysts. Interestingly, a reversal of enantioselectivity is observed for the catalytic desymmetrization reaction by exchanging these two transition metals. Rh-catalyzed hydroboration involves a metal-H insertion, and a boryl migration is involved when using an Ir precatalyst (Equation 17) <2002JA12098, 2002JOC3522>. 

Cross-coupling to form carbon heteroatom bonds occurs by oxidative addition of an organic halide, generation of an aryl- or vinylpalladium amido, alkoxo, tholato, phosphido, silyl, stannyl, germyl, or boryl complex, and reductive elimination (Scheme 2). The relative rates and thermodynamics of the individual steps and the precise structure of the intermediates depend on the substrate and catalyst. A full discussion of the mechanism for each type of substrate and each catalyst is beyond the scope of this review. However, a series of reviews and primary literature has begun to provide information on the overall catalytic process.18,19,22,23,77,186... 

The regioselective borylation of alkanes can be achieved either catalytically or stoichiometrically using a host of metal complexes such as rhenium analogs. Such processes can be photochemically or thermally induced (Equation (19)).30 30a 30c... 

The regiospecific functionalization of the terminal alkyl group of simple amines or ethers with bis(pinacolato)-diborane leads to organoboranes. The latter have manifold applications in organic synthesis since the catalytic borylation process can be combined with a functional group transformation step, including Suzuki-Miyaura couplings, for the synthesis of elaborated molecules. Curiously, functionalization of the C-H atoms a to the heteroatom was not observed (Equation (22)). a... 

Silaboration of 3-substituted 1,2-dienes takes place smoothly at the internal double bond in the presence of the catalytic Pd(acac)2-2,6-xylyl isocyanide complex and the boryl group is regioselectively introduced to the central carbon atom of an allene (Scheme 16.55) [59, 60]. The same regioselectivity is observed with the catalytic system Pd2(dba)3-P(OCH2)3CEt [59]. 

For each case we will also present catalytic analogues, namely (1) the activation of methane to form methanol with platinum, the reaction of certain aromatics with palladium to give alkene-substituted aromatics, and (2) the alkylation of aromatics with ruthenium catalysts, and the borylation of alkanes and arenes with a variety of metal complexes. 

Iridium-catalyzed formation of B-C bonds from arene C-H bonds was first reported by Smith and coworkers [73]. They demonstrated that the archetypal C-H activation products, Cp lr(PMe3)(H)(R), could mediate B-C bond formation (R = Ph, cyclohexyl) and were able to effect the catalytic borylation of benzene with HBpin (8) to produce CgHsBpin and H2 at 150°C (8). 

One of the most active and well-studied catalytic borylation systems is that generated from iridium(l) precursors such as [lr(COD)Cl]2 or [lr(COD)(OMe)]2 and bipyridine type ligands such as 2,2 -bipyridine or 4,4 -di-ferf-butyl-2,2 -bipyridine (dtbpy). In 2002, Ishiyama, Miyaura, and Hartwig et al. reported that the combination of [lr(COD)Cl]2 and 2,2 -bipyridine catalyzes arene borylation in the presence of excess arene under mild conditions (80°C). When the catalyst is generated from [lr(COE)2Cl]2 and dtbpy, the reaction proceeds even at room temperature [78, 79]. The same groups optimized conditions and found that the combination of [Ir(COD) (OMe)]2 and dtbpy (10) is a highly effective catalyst in the borylation of arenes so that reactions can be successfully performed with equimolar ratio of arenes and... 

The [Ir(OMe)(COD)]2/dtbpy catalytic system borylates indole selectively at the 2-position (Scheme 6). Smith and coworkers reported that borylation of N-unprotected 2-substituted indoles exclusively occurs at 7-position (Scheme 6) [85, 86]. It has been suggested that nitrogen interaction with the iridium center or possibly the empty p-orbital of boron in a boryl ligand induces the observed regioselectivity. Borylation of other heteroarenes have been reported using the same or similar Ir(I) and bipyridine combination [85, 87-90]. 

The beneficial effects of chelating ligands were also demonstrated by Hartwig, Ishiyama and Miyaura [62]. This group isolated the iridium(I) complex [lr(Bpin)3(COE)(DTBPY) modified with simple 2,2 -bipyridine ligands (such as 4,4 -di-tert-butyl-2,2 -bipyridine DTBPY), which seemed to be responsible for the first catalytic C—H borylation at room temperature (Scheme 7.30). An extension... 

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. 

Optically active cyanohydrins are obtained in good selectivity by the nucleophilic attack of cyanating reagents to chiral acetals.(21) However, the chiral auxiliaries are destroyed, and not recovered. In catalytic processes with chiral boryl compounds,(22) D-oxynitrilase,(23) and synthetic peptides,(24) the optical purities of the resulting cyanohydrins are generally not sufficient. 

Interestingly, formation of (23) was accomplished by cyclization of (210) on a catalytically fixed N2 complex <920M4ll6>. An unconventional cyclization to the azasilacyclopentene system was achieved in high yields by amination of cis l-boryl-2-silylethylenes (219) with sodamide <81AG1009, 87CB669, 88CB597). 

Benzyl esters, cyclization-hydrosilylation, 11, 386-387 Benzyl ethers, cyclization-hydrosilylation, 11, 386-387 Benzyl groups, C-H bond silylation, 10, 240 Benzylic alcohols, catalytic alkylation, 11, 146 Benzylic carbon-hydrogen bonds borylation, 9, 174... 

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 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],... 

Most current regioselective, catalytic functionalization of saturated C-H bonds [1-5] occurs by directed processes [6-13]. In these processes the catalyst or reagent docks at a functional group or reacts at C-H bonds that are located a to a heteroat-om, because these bonds are weaker than more distal ones [14]. Borylation of methyl C-H bonds complements the directed chemistry because it occurs at unactivated C-H bonds with regioselectivity that is independent of the position of the functional group in the reagent [15]. 

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