Transition-metal-catalyzed borylation

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). 

Perhaps the most interesting development of these types of reactions, the formation of a single product from terminal borylation of linear alkanes by a transition metal catalyzed reaction, comes from Hartwig. Cp Rh(jj -C6Me6) catalyzes the high-yield formation of linear alkylboranes from commercially available borane reagents under thermal conditions. Iridium catalysts are also effective. ... 

Although group 5 organometallic systems have been found to be of relevance in transition-metal catalyzed hydroboration reactions, structurally authenticated group 5 boryl complexes remain relatively few in number. Smith and co-workers, for example, have probed the mechanisms for the formation of niobium and tantalum mono- and bis(boryls) from propylene complex precursors, with concomitant formation of propyl boronate esters [31,32]. Of particular interest from a structural viewpoint are the relative merits of alternative bonding descriptions for metal(V) boryl bis(hydrides) as borohydride complexes or as mono(hydride) a-borane systems [31-34]. 

Another strategy that bears potential is the direct borylation of alkanes, alkenes, and-most importantly - arenes by transition metal-catalyzed CH activation [39],... 

Scheme 22.7 Direct bo lation via transition metal-catalyzed C-H activation of arenes to furnish potential SPC monomers, (a) Reaction between benzene and 18 in the presence of a catalyst precursor Cp Rh( j — CeMee), leading to a mixture of mono-, m-, and p-di-, and 1,3,5-tri-borylated compounds. The relative yield varies...

A regioselective transition metal-catalyzed cycloisomerization reaction of boron-containing alkynyl epoxides toward C-2- and C-3-borylated furans was developed. It was found that the copper catalyst as well as the gold catalyst with a relatively more basic triflate counterion favored boryl migration toward C-3-borylated furans, whereas employment of the cationic gold hexafluoroantimonate afforded C-2-borylated furans via a formal 1,2-hydrogen shift (14JA13146). 

Transition-metal-catalyzed borylation of organic halides with hydrobor-anes 12H(85)1795. 

Boryl complexes participate in a variety of catalytic processes, including transition-metal-catalyzed hydroboration and diboration of olefin and acetylene C-C ir-bonds (Chapter 16) and tihe functionalization of alkane and arene C-H bonds (Chapter 17). The chemistry of these complexes has been developed primarily since 1990. A body of literature on metal-boiyl complexes was published in the 1960s, but the structures of these compounds were apparently misassigned in the absence of modem X-ray crystallographic and NMR methods. Boryl complexes of all transition metals except for group 3 (Sc, Y, and La) and group 4 (Ti, Zr, Hf) metals - have been isolated. A majority of transition-metal boryl-complexes contain late metals. 

The DreM of 0-carbamates can be efficiently linked to transition metal-catalyzed cross-couplings like the Negishi and Suzuki-Miyaura couplings, eventually performed after DoM protocols [175]. For example, the synthesis of alkaloid schumanniophytine 79 has been accomplished involving DoM of 0-carbamate 78 followed by borylation, Suzuki-Miyaura cross-coupling, and an ort/jo-silicon-induced 0-carbamate remote anionic Snieckus-Fries rearrangement (Scheme 26.24). 

Suginome M, Ohmura T (2011) Transition metal-catalyzed element-boryl additions to unsaturated organic compounds. In Hall DG (ed) Boronic acids preparation and applications in organic synthesis, medicine and materials. Wiley, Weinheim, pp 171-212... 

Transition-metal-catalyzed ring-opening functionalizations of donor - acceptor cyclopropanes have also been developed (Table 2.2). Addition of carbon nucleophiles to activated VCPs 69b and 69c in the presence of iron catalysts induced ring opening of the three-membered ring [81, 82]. A palladium pincer complex catalyzed ring-opening borylation of VCP 69c with 62(011)4 to produce... 

The boryl groups installed on the aromatic polymers are useful synthetic intermediates for conversion to other polar functional groups. For example, they can be oxidized to give hydroxylated polymers and a variety of polar group-containing polymers via either direct functional group transformations or palladium-catalyzed Suzuki cross-coupHng reactions with appropriate phenyl bromides (Fig. 25). This transition metal-catalyzed polymer... 

In addition to the Grignard/RLi approach to the preparation of arylboronates, transition metal-catalyzed coupling and C—H borylation reactions have grown in popularity due in part to their ability to tolerate a wide variety of functional groups that are often incompatible... 

Scheme 3.18 Synthesis of functionalized 37 boronic esters by transition-metal-catalyzed C-H borylation of arenes.

Scheme 3.19 Synthesis of heteroaryl boronates by transition-metal-catalyzed borylation of heteroarenes via C-H activation.

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>. 

Direct borylation of hydrocarbons catalyzed by a transition metal complex has been extensively studied by several groups and has become an economical, efficient, elegant, and environmentally benign protocol for the synthesis of a variety of organoboron compounds. The Rh-, Ir-, Re-, and Pd-catalyzed C-/H borylation of alkanes, arenes, and benzylic positions... 

Transition-metal complexes containing the boryl (-BX2) ligand first appeared in the chemical literature in the 1960s in an extensive series of papers reported by Noth and Schmid [1]. Subsequent developments (principally in the past 15 years) have been driven by the implication of boryl complexes in metal-catalyzed processes leading to the transfer of the BX2 fragment to or-... 

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. 

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