Borylation reactions

Further insight into the P-borylation reaction of a,P-enones (Scheme 2.32) showed that the reaction can be carried out in THF, and the catalyst generated in situ from CuCl (5 mol%), the imidazolium salt (5 mol%), and NaO Bu (10 mol%), to form the [Cu(O Bu) (NHC)] as the catalysis initiating species. In this case, stable boron enolate products are formed which need to be hydrolysed by methanol to the ketone products [114]. 

In 2011, Hartwig and coworkers reported the total synthesis of taiwaniaquinol B (55, Scheme 11.9), a member of a family of diterpenoids that are derived from the abietane skeleton [36]. A key aspect of the Hartwig synthesis of taiwaniaquinol B was the use of the iridium-catalyzed borylation reaction to accomplish the C(5) functionalization of resorcinol derivative 53. This regioselectivity for the overall bromination is complementary to that which would be obtained using a standard electrophilic aromatic substitution (EAS) reaction. In the transformation of 53 to 54, a sterically controlled borylation was first accomplished, which was then followed by treatment of the boronic ester intermediate with cupric bromide to... 

Although the borylation reaction of a-hydroxyalkylphosphine derivatives can be considered autocatalized, the catalyst is incorporated into the final reaction product. 

In addition to amines, alkali metal alkoxides, hydroxides, and metals themselves can be used in borylation reactions [Eq. (69)] (90IZV886). 

Compounds with P—C—N—B fragments Compounds containing the P—C—N—B fragment can be obtained in the borylation reactions of functionally substituted phosphines with boranes (88IZV2190 89IZV1375). As a rule, the first stage is characterized by the formation of complexes in all cases. 

Bis(a-aminomethyl)phosphines having a mobile hydrogen atom could be expected to undergo a borylation reaction with borane, hydrogen being evolved. In fact, bis(Af-phenylaminomethyl)-phenylphosphine (199) and its sulfide (200) appeared to interact with borane under mild conditions, yielding a new type of phosphorus-boron-containing heterocycle—2-... 

The formation of a strong intramolecular dative P—B bond in the hydro-borylation reaction of diallylphenylphosphine with triethylaminophenyl-borane results in a bicyclic product, l-borata-5-phosphoniabi-cyclo[3,3,0]octane (204) [Eq. (145)] (64JA5045). The presence of the dative bond in this compound is indicated by an NMR study and by its stability to hydrolysis and oxidation. 

A masked allylic boron unit can be revealed through a transition-metal-catalyzed borylation reaction. For example, a one-pot borylation/allylation tandem process based on the borylation of various ketone-containing allylic acetates has been developed. The intramolecular allylboration step is very slow in DMSO, which is the usual solvent for these borylations of allylic acetates (see Eq. 33). The use of a non-coordinating solvent like toluene is more suitable for the overall process provided that an arsine or phosphine ligand is added to stabilize the active Pd(0) species during the borylation reaction. With cyclic ketones such as 136, the intramolecular allylation provides cis-fused bicyclic products in agreement with the involvement of the usual chairlike transition structure, 137 (Eq. 102). 

The Miyaura borylation reaction enables the synthesis of boronates by cross-coupling of bis(pinacolato)diboron (B2pin2) with aryl halides and vinyl halides. 

Crucial for the success of the borylation reaction is the choice of an appropriate base, as strong activation of the product enables the competing Suzuki Coupling. The use of KOAc and KOPh is actually the result of a screening of different reaction conditions by the Miyaura group. 

The first synthesis of arylboronic esters 209-215 via the coupling of bis(pinacolato)diborane(4) with easily prepared aryldiazonium tetra-fluoroborate salts was reported. The palladium-catalyzed borylation reaction proceeds efficiently under mild reaction conditions in the absence of a base to afford various functionalized arylboronic esters in moderate to high yields (Scheme 34).121... 

In the past 20-25 years, the use of transition metal catalysts has opened up many new possibilities in the area of borylation reactions. Catalytic hydroboration with diboranes is an exciting new strategy for... 

Highly electron-rich aromatic compounds are known to undergo direct electrophilic borylation reactions with the strong electrophiles BI3, BBrs, BCI3, or HB(C6p5)2 under thermal conditions. As an example, the dibromoborylation of ferrocene in refluxing hexanes is shown in equation (27). Facile intramolecular borylation reactions have also been... 

Although not a palladium-catalyzed reaction, the Ir(I)-catalyzed C-H borylation reaction developed independently by Smith and Malezcka [58] and Hartwig and Miyaura [59] deserves some mention in the context of indole and pyrrole functionalization. Based on the original studies, indoles and pyrroles can be borylated (and hence cross coupled under Suzuki conditions) to form either the C2 or C3 functionalised products (Scheme 35) [60, 61]. Free (NH)-indoles and pyrroles react exclusively at the C2, whereas /V-TIPS indole and pyrroles are borylated at the C3 positions. Interestingly, Smith, Maleczka and co-workers also demonstrated that when the C2 position of indole is blocked, then the borylation reaction takes place at the C7-position of the indole nucleus [62]. They propose that an A-chelation to Ir (or B) is responsible for the observed selectivity. 

Lam KC, Lin Z, Marder TB (2007) DFT studies of /3-boryl elimination processes Potential role in catalyzed borylation reactions of alkenes. Organometallics 26 3149... 

Abstract A variety of structural types of transition metal complexes containing a-borane ligands are reviewed. Their structure and bonding are discussed. Compared with other types of a-complexes, a-borane complexes display quite different structural and bonding properties because of the electron unsaturated boron center in the a-borane ligands in the precoordination state. The availability of an sp3-hybridized orbital at the boron center allows stronger back-donation interaction without breaking the coordinated B-H bond. The role played by a-borane complexes in hydroboration and borylation reactions has also been reviewed. 

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. 

Independently, in 2001 Marder and his coworkers reported a high degree of benzyl selectivity in the borylation reactions of C-H bonds in toluene, p-xylene, and mesitylene with HBpin (pin = 0CMe2CMe20) using the catalyst precursor [(P Pr3)2Rh(Cl)(N2)] (Scheme 11) [78]. Oxidative addition of HBpin to [(P Pr3)2Rh(Cl)(N2)] yields frans-[(P/Pr3)2Rh(Cl)(H)(Bpin)], the structure of which was determined by low temperature single-crystal X-ray... 

In an interesting footnote to these alkane and aromatic borylation reactions, photolysis of Cp M(CO)3Me, M = Mo or W, and HsBPMe3 resulted in the formation of methane and Cp M(CO)3BH2PMe3.i ... 

Undoubtedly, the synthetic value of the Rh-catalyzed alkyl borylation reaction follows from the fact that it can selectively deliver a functional group, such as an alcohol, an amine or an alkene, at the terminus of an alkyl chain during the synthesis of more complex structures. With the proper substituent at boron, this process can extrude the same borane reagent that was used in the alkane functionalization step to allow recycling of the main group reagent for large-scale synthesis. 

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