Borylation stoichiometric

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. 

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. 

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... 

Ishiyama, T., Takagi, J., Hartwig, J.F. and Miyaura, N., A stoichiometric aromatic C—H borylation catalysed by iridium(I)/2,2 -bipyridine complexes at room temperature, Angew. Chem., Int. Ed. Engl, 2002, 41, 3056-3058. 

Table 2. Stoichiometric borylation of aromatic compounds 1 by HBpin 2 or B2pin2 3 catalyzed by 1 /2[lr(OMe)(cod)]2-dtbpy in hexane at room temperature a 5equiv. of to 2, b lOequiv. of 1 to 3, c 0.45 equiv. of to 2, d 1 equiv. of to 3.

An iridium(l) complex, generated from l/2[Ir(OMe)(COD)]2 and 4,4 -di-/-butyl-2,2 -bipyridine (dtbpy), catalyzed the direct borylation of 2-substituted pyrroles in stoichiometric amounts relative to 2,2 -bi-l,3,2-dioxaborolane 785 in hexane at room temperature (Equation 188) <2003ASC1103>. The pyrrolylborates 786 from regioselective C-H activation at the 5-position were formed in high yields. Similar borylation of unsubstituted pyrrole with an equimolar amount of borolane 785 regioselectively provided 2,5-bis(boryl)pyrrole 787 (Equation 189). 

The mechanism of stoichiometric CH borylation by complexes of this type has been the subject of several recent studies designed, in particular, to probe the underlying factors responsible for the unique ability of boryl systems to bring about alkane functionalization [37-39,50,51]. The thermodynamics of B-C bond formation have previously been noted to be favourable [50,51], and... 

Despite these successes in stoichiometric reactions, productive (catalytic) functionalization of simple alkanes via oxidative addition is almost restricted to CO insertion and dehydrogenation by the RhCl(CO)(P(CH3)3)2- v system (9-11) (vide infra) and the dehydrogenative borylation (eq. (5)) (12). 

Interestingly, [IrCl(cod)]2 did not catalyze the reaction. The [Ir(OMe)(cod)]2]/dtbpy catalytic system in nonpolar solvents such as hexane was found to be very effective for the synthesis of arylboronates, using stoichiometric amounts of arene and pin2fi2 at room temperature (Table 1). The borylation t) ically occurs at the para or meta position with respect to the functional groups on the arene. The ortho C-H positions are less active due to the steric hindrance. In addition, the electronic effect plays a minor role, where the electron-poor carbon seems to be more active. This allows the reaction to occur regiospecifically (Table 1). For example, the borylation of 1,3-disubstituted arenes selectively occurs at the common meta position, while the borylation of heterocycles such as benzo[l)]thiophene, benzo[fc]furan, or indole occurs selectively at the 2-position (Table 1). 

Iron carbonyl fragments have been incorporated into many heterometallic borane and carborane compounds. Those compounds are covered in Chapter 3.05 and will not be discussed here. Simple boryl adducts have been reported from the reaction of Na2Fe(CO)4 and ClBCat" (Cat = catecholate or a Bu-substituted derivative. Equation (8)). The same products can be obtained upon irradiating Fe(CO)s and Cat B-BCat" in toluene at 0°C. Reaction of the diboryl 6 with 1 equiv. of lithium bis(trimethylsilyl)amide in toluene solvent leads to removal of one boryl unit and the formation of an anionic iron boryl complex (Equation (9)). The anionic species can also be obtained by stoichiometric addition of the ClBCat complex to Na2Fe(CO)4. Reaction of [Fe(CO)4 B(3,5-/-Bu2-Cat) ] with ClSnMes gives the stannylated derivative Fe(CO)4 B(3,5-/-Bu2-Cat) (SnMe3). 

Transition metal boryl complexes have been the subject of considerable recent research effort. These compounds find application as synthetically useful reagents in organic transformations such as hydroboration and diboration of carbon-carbon multiple bonds.More recently, the involvement of cyclopentadienyl transition metal boryl complexes in both stoichiometric and catalytic functionalization of alkanes and arenes has been demonstrated, notably by Hartwig ef Complexes CpFe(CO)2(Bcat) (cat = 02C6H4) 80, and GpFe(GO)2(BPh2), which have been... 

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