Effect of Pincer Methylation on the Selectivity and Activity in (PNP)Cobalt-Catalyzed C(sp²)–H Borylation

Abstract

Cobalt complexes supported by a tetramethylated PNP pincer ligand (Me 4 iPr PNP = 2,6-( i Pr 2 PCMe 2 ) 2 (C 5 H 3 N)) have been synthesized and structurally characterized. Examples include cobalt(I)−choride, −methyl, −aryl, and −benzofuranyl derivatives. The performance of these compounds was evaluated in the catalytic borylation of fluorinated arenes using B 2 Pin 2 as the boron source. While P−C bond cleavage, a known deactivation pathway in [(PNP)Co]-catalyzed borylation, was suppressed, the overall activity and selectivity of the borylation of fluoroarenes was reduced as compared to the previously reported [(PNP)Co] catalyst lacking isopropylene spacers. Stoichiometric reactions support an increased barrier for oxidative addition to cobalt(I), a result of the increased steric profile and decreased conformational flexibility of the pincer resulting from methylation distal to the active site. With a more activated substrate such as benzofuran, catalytic borylation with cobalt(I) precatalysts and HBPin was observed. Monitoring the progress of the reaction by NMR spectroscopy revealed the presence of cobalt(III) intermediates during the course of the borylation, supporting a cobalt(I)-(III) redox cycle.Article pubs.acs.org/Organometallics

Homogeneous catalysis with polyhydride complexes

“…194 A similar effect of the fluorine atom has been observed in the reactions mediated by the cobalt(III)-trihydride 37. 195 A detailed mechanistic study, including stoichiometric isotopic experiments and the kinetic analysis of all main elemental steps of the catalysis, strongly support the two cycles defined by the trihydride 104 and the hydride-boryl derivatives IrH 2 (Bpin){k 3 pinBH to give H 2 and complexes 111 and 112 in a sequential manner. Compound 112 activates a C-H bond of two molecules of arene to form the functionalized arene and regenerate 111 and 104, also in a sequential manner.…”

“…194 A similar effect of the fluorine atom has been observed in the reactions mediated by the cobalt( iii )-trihydride 37 . 195…”

Exploring the Effect of Pincer Rigidity on Oxidative Addition Reactions with Cobalt(I) Complexes

“…Structures, features, steric maps, and percent buried volumes (% V bur ) , of (A) ( iPr PNP)­Co­(CH 2 SiMe 3 ), (B) (Me 4 iPr PNP)­Co­(CH 2 SiMe 3 ), and (C) 1-CH 2 SiMe 3 . The Co atom defines the center of the xyz coordinate system.…”

Development of Cobalt Catalysts for the meta-Selective C(sp2)–H Borylation of Fluorinated Arenes

“…These observations established that borylation activity with 6-CH 2 SiMe 3 trends in the order meta (to fluorine) > para > ortho . Additional substrate exploration demonstrated that borylations using 6-CH 2 SiMe 3 suffered from many of the same functional group incompatibilities as those catalyzed by ( iPr PNP)­Co­(H) 2 BPin (see the Supporting Information for complete details), as nitriles, chlorides, bromides, and iodides were not tolerated …”

“…Cobalt-catalyzed borylation methods have been discovered that offer regioselective chemistry distinct from iridium catalysts . Use of bis­(phosphino)­pyridine (PNP) pincers produced catalysts with high ortho to fluorine selectivity including examples that are functionalized indiscriminately by iridium catalysts to yield statistical mixtures of products. , Mechanistic studies established that the ortho to fluorine selectivity arises from rapid C­(sp 2 )–H oxidative addition to the cobalt­(I) pincer that generates cobalt­(I)-aryl intermediates arising from a thermodynamically controlled “ortho-to-fluorine” effect .…”