Borylation proposed mechanism

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. 

Scheme 5 Proposed mechanism for borylation by bipyridine ligated iridium complexes...

Scheme 2. Proposed mechanism of photochemical C-H borylation catalyzed by Cp Re(CO)3.

Computational studies performed on model complexes in collaboration with Hall and coworkers suggest that alkane borylation may occur by a ej-bond metathesis pathway (Scheme 3) [48]. The proposed mechanism for the borylation of alkanes by 1 begins with elimination of HBpin to generate the 16-electron complex Cp Rh(Bpin)2. This complex then forms a <7-complex (3) with the alkane. The vacant p-orbital on boron then enables cr-bond metathesis to generate a o-borane complex (4). Reductive elimination of the alkylboronate ester product and oxidative addition of B2pin2 then regenerate 1. 

Scheme 2 A general proposed mechanism for the rhodium-catalyzed dehydrogenative borylation of alkenes using HBcat...

Investigation of the reactivity of these borylpalladium complexes demonstrated that the electron-deficient borylpalladium 39d (Ar= 3,5-(CF3)2C,5H3) smoothly reacted with (F)- 3-methylstyrene to give (Z)-p-boryl-(3-methylstyrene 42 quantitatively whereas normal 39a (Ar = Ph) caused no reaction (Scheme 9.10) [27]. Thus, electron-withdrawing nature of the PSiP-Hgand clearly accelerates the borylpalladation step, leading to high catalytic activity of 39d. Additionally, the stereospecific formation of (Z)-alkenylboronic ester 42 from ( )-P-methylstyrene supports the proposed mechanism for borylation of alkenes via syn-insertion/syn-ehmination. 

SCHEME 10.28 Hartwig s proposed mechanism for the borylation of 3-picoline. 

A mechanism involving Ir(m) and Ir(v) has been proposed (Scheme 19).84,84a 84c Similar rhodium-mediated borylations are known (Equation (90)).85... 

The reaction of silylborane with 1-halo-l-lithio-l-alkenes yields 1-boryl-l-silyl-l-alkenes via borate formation followed by 1,2-migration of silyl group (Equation (90)).76,240 The mechanism seems to be closely related to that proposed for the silaboration of isocyanide (Figure 2). Vinyl-substituted carbenoids, l-chloro-l-lithio-2-alkenes, react with silylpinacolborane to give l-boryl-l-silyl-2-alkanes in good yield (Equation (91)).241 This methodology is applied to the synthesis of l-boryl-l-silyl-2-cyclobutene.2 2 Similar reactions are carried out with other carbenoid... 

Extensive studies of kinetics and isotope effects by Hartwig and coworkers support the mechanism shown in Scheme 5 for the lr(I)/dtbpy catalyzed borylation [81]. In particular, these studies indicate that the iridium(III) trisboryl bipyridine complex (10) is the species that activates the arene C-H bond this is in agreement with DFT calculations by Sakaki et al. predicting the key intermediacy of the trisboryl complex and the seven-coordinated Ir(V) species resulting from C-H addition [82]. C-H addition to Ir(III) was also proposed in the (Ind)Ir(COD)/ phosphine-catalyzed borylation by Smith et al. [76]. 

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],... 

Intensive studies also showed that many transition metal complexes are able to catalyze aromatic C-H borylation of various arenes (Scheme 7), e.g., Cp Ir(H)(Bpin)(PMe3) [64,65], Cp Rh(Ti4-C6Me6) [65,66], ( 75-Ind)Ir(COD) [67], (776-mesitylene)Ir(Bpin)3 [67], [IrX(COD)]2/bpy (X = Cl, OH, OMe, OPh) [68-70]. A very recent study by Marder and his coworkers showed that [Ir(OMe)(COD)]2 can also catalyze borylation of C-H bonds in N-containing heterocycles [71]. For the Ir-catalyzed borylation reactions, it is now believed that tris(boryl)iridium(III) complexes [67,69], 40c, [72] are likely the reactive intermediates and a mechanism involving an Ir(III)-Ir(V) catalytic cycle is operative [67,69]. A recent theoretical study [73] provided further support for this hypothesis. A mechanism, shown in Scheme 8, was proposed. Interestingly, there are no cr-borane complexes involved in the Ir-catalyzed reactions. The very electropositive boryl and hydride ligands may play important roles in stabilizing the iridium(V) intermediates. 

The reactions conducted with ligand-less cationic systems are proposed to occur through the rhodium(I) boryl complexes shown in Scheme 16.12, but fewer data are available to support this mechanism. Theoretical studies have implied that insertion of the olefin into the metal-boryl linkage occurs and that this step is followed by oxidative addition of the borane and reductive elimination to form the C-H bond of the product and the starting rhodiiun(I) boryl complex. 

Information on the mechanism of the borylation of alkanes and arenes has been reported by Hartwig. Catalytic cycles proposed for the functionalization of alkanes and arenes by the Cp Rh and (dfbpy)Ir complexes are shown in Schemes 18.15 and 18.16. Both cycles involve the formation of 16-electron metal-boryl intermediates and the reactions of these intermediates with C-H bonds, followed by the formation of the B-C bond in the final product. Details on the C-H bond cleavage step were gained by studies on isolated metal-boryl complexes. 

Scheme 8.8 Proposed catalytic mechanism for C-H borylation by an FeCu heterobimetallic.

Through the DPT calculations, a mechanism for the Pt(0)-catalyzed borylation of acrolein was proposed (Scheme 10), which involves oxidative addition of diboron reagents, acrolein insertion into the Pt-B bond, acrolein coordination, and reductive elimination [106]. A similar mechanism has also been proposed for the Pt(0)-catalyzed borylation of methylacrylate (Scheme 11) [106]. 

This result represents the first example of a synthetic cycle for arene borylation facihtated by an f-element. The authors investigated the reaction mechanism and proposed a concerted direct B—H attack on an aromatic C—H bond, which resulted in the formation of the B—C bond and the release of one molecule of H2. This type of reactivity resembles a-bond metathesis if boron is considered a metal center. Albeit uranium was not directly involved in the borylation of the arene C—H bond, the formation of the diuranium arene inverse-sandwich complex plays an important role. The arene, i.e., benzene or naphthalene, is partially reduced upon coordination to uranium,which makes it more susceptible to attack by an electrophile such as borane. Therefore, although uranium is not direcdy involved in the C—H bond activation step, this example illustrates that f-elements can render arenes reactive in nonmetal-mediated transformations by forming activated arene metal complexes. 

The activation of B2PUI2 could also be carried out in the presence of Cu(ll) salts to promote P-boration of a,P-unsaturated carbonyl compounds in water as solvent. Santos has proposed a mechanism to activate the B2pin2 where amine/H20 could contribute to the preactivation of the diboron to form the Cu(lI)-boryl species (Scheme 23A). In a similar context,... 

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