Catalyzed borylation of aliphatic and aromatic C-H bonds

The catalytic C-H borylation of hydrocarbons with B2pin2 or HBpin has been reviewed.2,201,335 

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. 

Iridium complexes such as Cp Ir(774-C6Me6) and Ir( 75-indene)(cod) themselves were inefficient, but addition of the small electron-donating phosphines such as PMe3 or the chelating l,2-bis(dimethylphosphino)ethane (dmpe) substantially increased the catalyst activity and TON (Equation (68)).344,346 The maximum TON achieved for benzene with HBpin at 150 °C was 4,500 TON. 

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. 

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