Rhodium-catalyzed borylation

The rhodium-catalyzed borylation of methyl C-H bonds is compatible with several moieties containing oxygen, nitrogen, and fluorine.154 For example, the reaction of pinacol acetal of 2-hexanone with bis(pinacolato)diborane (B2pin2) in the presence of Cp Rh(774-C6Me6) catalyst gives the alkylboronate ester in 74% yield (Equation (111)). The rhodium-catalyzed C-H activation and borylation occur at the least hindered and least electron-rich methyl group. 

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. 

Rhodium-catalyzed borylation with B2pin2 at 150 °C provides functionalized polyolefins340 (Equation (66)). One methyl group per main chain is hydroxylated by borylation-oxidation sequence. 

SCHEME 6.31 Rhodium-catalyzed borylation through C—O bond cleavage [62]. 

An extensive array of chiral phosphine ligands has been tested for the asymmetric rhodium-catalyzed hydroboration of aryl-substituted alkenes. It is well known that cationic Rh complexes bearing chelating phosphine ligands (e.g., dppf) result in Markovnikoff addition of HBcat to vinylarenes to afford branched boryl compounds. These can then be oxidized through to the corresponding chiral alcohol (11) (Equation (5)) ... 

Fig. 2.8 A d irect route to vinylboranes in rhodium-catalyzed hydroborations with phos-phine-free catalysts (including oxidative degradation of a rhodium phosphine). The key intermediate is a rhodium hydride, capable of reversible insertion into the alkene (step A), followed by addition of borane in step B. This leads to reductive elimination of RH in step C followed by boryl migration in step D. A further...

Bis(pinacolato)diboron 14 and bis(neopentyl glycolate)diboron 15 have been used for rhodium-catalyzed conjugate addition to a,/ -unsaturated ketones giving /9-boryl ketones, though the asymmetric version has not been reported [15]. 

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

Recently, rhodium-catalyzed direct borylation of alkane with diboron was reported [50], where Cp Rh(ri -CgMeg) (Cp =C5Me5) was used as a catalyst. This reaction has drawn considerable interest because the C-H o-bond activation and the introduction of some functional group into alkane were achieved in one pot. The similar reaction (Eq. 8) was theoretically investigated with the DFT method [51] ... 

Rhodium- and Ir-catalyzed C-H borylation of arenes are nowadays important pathways. The regioselectivity of the first successful reactions were chiefly under steric control (to the least hindered positions), often offering a simple access to positions (for instance position 5 of 1,3-dimethoxy-benzene) where installation of a halide group for Pd-catalyzed borylation is difficult. These successes prompted a promising development of strategies for directed C-H borylations. ... 

The ready availability of arylboronates by an aromatic C-H borylation provides a synthetic link to the well-established palladium-catalyzed cross-coupling reactions, rhodium-catalyzed 1,4-addition to a,p-unsaturated carbonyl compounds, and other bond forming reactions using arylboronic esters (Scheme 2.12). Borylation of 1,3-dichlorobenzene with pinacolborane is followed directly by a cross-coupling reaction with methyl p-bromobenzoate for the synthesis of a biaryl product in 91% yield [60]. Pinacol esters of arylboronic acids react much slower than the free acids [62], but both derivatives achieve high isolated yields and comparable enantioselectivities (91% ee) in asymmetric 1,4-addition to N-benzyl crotonamides [63]. Borylation of arenes followed by oxidation of the C-B bond is synthetically equivalent to an aromatic C-H oxidation to phenols [64]. Oxidation of the resulting arylboronates with Oxone in a 1 1 acetone-water solution is completed within 10 min at room temperature. 

Scheme 17 A plausible mechanism for rhodium-catalyzed decyanative borylation of nitriles...

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... 

Recently, Bae and coworkers synthesized hydrophilic graft copolymers of polyolefins with a combination of rhodium-catalyzed C—H borylation... 

Figure 30 Synthesis of graft copoiymers of isotactic poiy(1-butene) with rhodium-catalyzed C—H borylation.

The rhodium-catalyzed conversion of aryl pyridyl ethers into arylboronates has been achieved using an NHC-supported rhodium catalyst (Scheme 6.31) [62]. The main theme of this work was the use of rhodium complexes to promote the cleavage of the pyridyl ether fragment and borylation of the arene. The reaction was carried out at elevated temperatures and afforded moderate to good yields of the arylboronates. One of the most impressive aspects of this chemistry was its tolerance to a wide range of functional groups. Heteroaryl ethers as well as substrates bearing esters, amides, and even a free amine were successfully converted into arylboronates. If the substrate is appropriately functionalized, this would be a reasonable approach to the formation of arylboronates. 

The same reaction produced different alkenes 58-6563 (Scheme 13). These reactions were not catalyzed by rhodium(I) or palladium(0). Tetra(methoxo)- and bis(pinacolato)diboron(4) added to both terminal and internal alkynes in the presence of a catalytic amount of Pt(PPh3)4 to provide stereo defined pinacolato r/s-bis(boryl)alkenes 58-65 in excellent yields. Because reagents and reaction conditions were sufficiently mild,... 

The reaction of HBpin in toluene in the presence of RhCl P(/-Pr)3 2(N2) (1 mol%) at 140 °C resulted in a mixture of (borylmethyl)benzene (69%) and bis(boryl)methyl benzene (7%), along with several products arising from aromatic C-H borylation (ca. 15%).345 Rhodium-bpy complexes catalyzed the borylation at the benzylic C-H bond.351 Pd/C was found to be a unique catalyst for selective benzylic C-H borylation of alkylbenzenes by B2pin2 or HBpin (Equation (70)).360 Toluene, xylenes, and mesitylene were all viable substrates however, the reaction can be strongly retarded by the presence of heteroatom functionalities such as MeO and F. Ethylbenzene resulted in a 3 1 mixture of pinacol 1-phenylethylboron and 2-phenylethylboron derivatives. 

Complexes of the group 9 metals, especially rhodium and iridium, represent one of the more numerous families on which a systematic appraisal of struc-ture/bonding properties for the boryl ligand can be based. In part, this reflects the involvement of such systems not only in earlier work on metal-catalyzed hydroboration chemistry [2-5,35,113-123], but in more recent studies of di-boration [124,125], and the activation of C-H bonds in both saturated [9,10, 126-135] and unsaturated hydrocarbons [9,10,50,51,127,129,134,136-159]. 

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