Alkanes borylation

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... 

Borylation is the photochemically or thermally promoted conversion of substrates with C-H bonds into boronate esters using bis(pinacol)diborane(4) (B2pin2) or pinacolborane (HBpin) reagents (Scheme 1) [16-35]. The selectivity for methyl C-H bonds and the versatility of organoboron reagents [36, 37] give alkane borylation potential for use in synthetic contexts. 

The mechanism of thermal borylation of alkanes seems to be similar to that of photochemical borylation of alkanes. The most active precatalysts for thermal alkane borylation, Cp Rh( 4-C6Me6) and Cp Rh(j/2-C2H4), are converted to mixtures of Cp Rh(Bpin)3(H) and Cp Rh(Bpin)2(H)2 in the catalytic reactions [48], Reaction of HBpin or B2pin2 with Cp Rh(Bpin)2(H)2 yields Cp Rh(Bpin)3(H). Furthermore, reaction of Cp Rh(Bpin)3(H) with alkanes and arenes as solvent yields Cp Rh(Bpin)2(H)2. Both species are kinetically capable of being catalysts of the borylation of alkanes. 

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. 

Table 1. Substrate scope of Re-catalyzed photochemical alkane borylation.

Table 2. Substrate scope of Rh-catalyzed thermal alkane borylation.

Table 3. Functional group tolerance of alkane borylation under substrate-limiting conditions.

Very recently, a promising new reaction has been discovered, alkane borylation, illustrated by Eq. 2.41 [123]. This can be driven by light, or, being exothermic, it can be carried out as a thermal process. Photoextrusion of CO from the tungsten carbonyl precursor is believed to be followed by oxidative addition of the alkane CH bond, followed by reductive elimination of the R-BR2product (Eq. 2.42) [123a]. 

The catalytic alkane borylation whose mechanistic scheme is detailed in section 2.6 also works according to the same mechanism with benzene in the sense that it can be borylated by pinB-Bpin to form the phenylboronate ester in 82% yield using only 0.25 mol% of the same catalyst, [Cp Rh(T]" -C6Me6)]. This efficient reaction is extremely useful in view of the Suzuki coupling leading to aryl functionalization using such boron derivatives (see Chap. 19.3.8) ... 

Hartwig and coworkers extended the rhodium-catalyzed thermal borylation of alkane C—H bonds for regioselective functionalization of polyethylethylene (PEE) (Fig. 29 A).Similar to the observed regioselectivity of alkane borylation, only the terminal C—H bonds of... 

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. 

In contrast to the borylation of alkane C-H bonds, the coupling of aryl halides with amines was based on a literature precedent from another group published about a decade before our initial studies. Kosugi, Kameyama and Migita published the coupling of aryl halides with tin amides." Mechanistic studies we conducted on this process led us to the perhaps obvious realization that the reaction" could be conducted with amines and a silylamide base instead of tin amides (equation 4)." Surveys of bases with similar p a values led Janis Louie to conduct reactions with alkoxide bases. Similar studies were conducted at nearly the same time by Steve Buchwald and coworkers."... 

A series of Irv boryl polyhydrides, cp IrH2XBR2 (X = H and BR2 = Bpin (1) (pin = 02C2Me4), Beat (2) (cat = 02C6H4), B02C6H2-3,4-Me2 (3) Bey (4), X = Bpin and BR2 = Bpin (5)) have been prepared and isolated by Hartwig and Kawamura according to Reaction Scheme 1. The X-ray crystal characterization of (1) is reported. (1) to (5) react regiospecifically with alkanes to produce free functionalized products. 

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 rhodium-catalyzed borylation of alkanes is applied to regiospecific functionalization of polyolefines.165,165a The reaction of polypropylenes (atactic, isotactic, and syndiotactic) with B2pin2 in the presence of Cp Rh( 74-C6Me6) catalyst at 200 °G affords the borylated polymers, which are treated with basic hydrogen peroxide in a mixture of THF and H20 to oxidize the boronate esters to the corresponding alcohols (Scheme 20). The hydroxylated polymers contain 0.2-1.5% hydroxymethyl side-chains. 

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... 

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. 

Keywords Alkane metathesis Borylation C-H bond activation Dehydrogenation Hydroarylation Iridium catalyst Silylation... 

Functionalization of alkanes to produce fine chemicals uses many different reaction types that cannot be listed completely within this introduction. An overview of typical reactions, for example halogenation, dehydration, borylation, oxidation, and hydroxyalkylation, is given in Table 6. 

Scheme 1. Borylation of alkanes by B2pin2 and HBpin reagents.

---