Borylation catalyst efficiency

Initial reports on the borylation of alkanes using isolated transition-metal-boryl complexes date back to 1995, when Hartwig showed that Cp Re(CO)2(Bpin)2 converts pentane to 1-borylpentane with high regioselectivity. " The catalytic C-H borylation of alkanes with Cp Re(CO)3 using photochemical activation was demonstrated soon thereafter (equation 25). Also, an efficient thermal process that involves the use of rhodium catalysts has since been developed (equation 26). It is interesting to note that this methodology is not restricted to small molecules, but has recently been exploited for the direct side-chain functionalization of polyolefins. ... 

Very recently, Ma and coworkers reported another bicyclic triazolium ligand 102 based on [2.2]paracyclophane and its applications in the asymmetric copper (I)-catalyzed p-borylation of a,p-unsaturated acyclic enones (Scheme 3.64) [93]. The catalyst generated in situ by the reaction of 102 and CU2O was remarkably efficient giving the p-boryl ketones in high yields (up to 99%) and enantioselectivities (up to 97%). A screening of the ligand revealed that the absolute... 

Notably, Rh catalysts have also been employed by Hartwig for efficient C—H borylations, which enable even C—H functionalizations of aliphatic C—H bonds in alkanes (42)." "" C—H borylations of alkanes are remarkably selective for functionalization at primary C—H bonds. 

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) ... 

Organoborane compounds are one of the most commonly used coupling partners for C-C couplings. In 2014, Shi et al. achieved a Pd(ll)-catalyzed selective borylation of methyl C-H bond with O2 as a benign oxidant (Scheme 1.36) [90]. Unambiguously, the produced C-B coupling products possess low reactivity in the presence of palladium catalyst. The direct borylation of unactivated Y-C(sp )-H bond also contribute an efficient access to alcohols, amines, and various functionalized molecules. 

Kobayashi has postulated a a-bond metathesis step between B2pin2 and Cu(OAc)2 to form a Cu(II)-boryl species by assistance of a chiral 2,2 -bipyridine hgand (Scheme 23B), which resulted in a very efficient catalyst for the asymmetric P-boration of a,P-unsaturated carbonyl compounds in water as solvent. ... 

Commercially available and inexpensive y-Fe203 magnetic nanoparticles (particle size 58 nm) also efficiently activate B2pin2 and promote a direct borylation of alkenes. " The mechanism of this unusual nano-Fc203-catalyzed aromatic borylation reaction is not clear. The kinetic isotope effect was measured to be 1.3, indicating that a C—H bond activation by oxidative addition to the iron catalyst is not likely. An electrophUic metalation by Fe—B species, followed by reductive ehmination, seems conceivable. 

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