Transition metal complexes are efficient catalysts for the C-H bond functionalization of heteroarenes to generate useful products for the pharmaceutical and agricultural industries. However, the costly need to remove potentially toxic trace metals from the end products has prompted great interest in developing metal-free catalysts that can mimic metallic systems. We demonstrated that the borane (1-TMP-2-BH2-C6H4)2 (TMP, 2,2,6,6-tetramethylpiperidine) can activate the C-H bonds of heteroarenes and catalyze the borylation of furans, pyrroles, and electron-rich thiophenes. The selectivities complement those observed with most transition metal catalysts reported for this transformation.

The FLP species 1-BR2-2-NMe2-C6H4 (R = 2,4,6-Me3C6H2, 2,4,5-Me3C6H2) reacts with H2 in sequential hydrogen activation and protodeborylation reactions to give (1-BH2-2-NMe2-C6H4)2. While reacts with H2/CO2 to give formyl, acetal and methoxy-derivatives, reacts with H2/CO2 to give C6H4(NMe2)(B(2,4,5-Me3C6H2)O)2CH2. The mechanism of CO2 reduction is considered.

New phosphine-based ligands and DFT calculations for defect-free conjugated polymers by direct (hetero)arylation polymerization (DHAP).

The mechanism of C­(sp2)–H borylation of fluorinated arenes with B2Pin2 (Pin = pinacolato) catalyzed by bis­(phosphino)­pyridine (iPrPNP) cobalt complexes was studied to understand the origins of the uniquely high ortho-to-fluorine regioselectivity observed in these reactions. Variable time normalization analysis (VTNA) of reaction time courses and deuterium kinetic isotope effect measurements established a kinetic regime wherein C­(sp2)–H oxidative addition is fast and reversible. Monitoring the reaction by in situ NMR spectroscopy revealed the intermediacy of a cobalt­(I)–aryl complex that was generated with the same high ortho-to-fluorine regioselectivity associated with the overall catalytic transformation. Deuterium labeling experiments and stoichiometric studies established C­(sp2)–H oxidative addition of the fluorinated arene as the selectivity-determining step of the reaction. This step favors the formation of ortho-fluoroaryl cobalt intermediates due to the ortho fluorine effect, a phenomenon whereby ortho fluorine substituents stabilize transition metal–carbon bonds. Computational studies provided evidence that the cobalt–carbon bonds of the relevant intermediates in (iPrPNP)­Co-catalyzed borylation are strengthened with increasing ortho fluorine substitution. The atypical kinetic regime involving fast and reversible C­(sp2)–H oxidative addition in combination with the thermodynamic preference for forming cobalt–aryl bonds adjacent to fluorinated sites are the origin of the high regioselectivity in the catalytic borylation reaction.