Oxidative addition and C–H activation chemistry with a PNP pincer-ligated cobalt complex

Abstract

The bis(phosphino)pyridine (PNP) cobalt(i) methyl complex, (iPrPNP)CoCH3is a rich platform for the oxidative addition of non-polar reagents such as H2, the C–H bonds of arenes and terminal alkynes.

Mechanistic Studies of Cobalt-Catalyzed C(sp2)–H Borylation of Five-Membered Heteroarenes with Pinacolborane

“…Pathways involving reductive elimination of either H 2 or HBPin seem the most plausible as Co(I)−Co(III) redox cycles are amply precedented in [(PNP)Co] chemistry. 8a,22,23 As presented in Scheme 2, the “hydride pathway” generates ( iPr PNP)CoH following reductive elimination of HBPin, whereas the alternative “boryl pathway” involves H 2 loss to access ( iPr PNP)CoBPin , analogous to the intermediate responsible for C−H activation during the borylation of 2,6-lutidine with B 2 Pin 2 . 8a Because of the trans disposition of the hydride ligands in trans -( iPr PNP)Co-(H) 2 BPin , isomerization to the cis isomer either by phosphine dissociation 24 or HBPin reductive coupling is required to achieve an appropriate geometry for H 2 loss.…”

“…Heating the THF- d 8 solution of the reaction mixture to 80 °C for 32 h produced 19% yield of the borylated product and a new cobalt compound identified as 4-BPin-( iPr PNP)Co(H)(PH i Pr 2 ) . This product, arising from P−C bond cleavage of the pincer, 22 was independently synthesized from addition of HP i Pr 2 to 4-BPin-( iPr PNP)CoCH 3 8a under an atmosphere of hydrogen gas (see complete experimental details on page S9) and crystallographically characterized (Figure 6). Performing the C−H borylation of 2,6-lutidine with HBPin in 0.55 M THF solution using 10 mol % of 4-BPin-( iPr PNP)Co(H)(PH i Pr 2 ) as the precatalyst resulted in no conversion to the borylated product, establishing that the formation of 4-BPin-( iPr PNP)Co(H)-(PH i Pr 2 ) by P−C bond cleavage is a catalyst deactivation pathway (see S10).…”

Insight into Transmetalation Enables Cobalt-Catalyzed Suzuki–Miyaura Cross Coupling

“…With these considerations in mind, the flexible diisopropyl-substituted bis(phosphino)pyridine pincer ligand ( iPr PNP) was selected for this study due to its known ability to support both tetrahedral (X = Cl) and planar (X = alkyl, aryl) cobalt(I) complexes. 50−54 Furthermore, the ( iPr PNP)cobalt platform promotes two-electron oxidative addition 53 and has been applied to catalytic C–H borylation. 50,52 …”

“…Thus, the catalytically relevant route to the ( iPr PNP)CoOR species was explored using the known cobalt(I) chloride ( iPr PNP)CoCl. 53 Addition of 1 equiv of NaOPh in THF- d 8 solution resulted in formation of ( iPr PNP)CoOPh as the major product observed by 1 H NMR spectroscopy (Scheme 6). Addition of 2-benzofuranylBPin to the resulting THF- d 8 solution produced a mixture containing ( iPr PNP)CoCl as the only cobalt species observable by 1 H NMR spectroscopy.…”

Theoretical studies of palladium-catalyzed cycloaddition of alkynyl aryl ethers and alkynes

“…During the last three decades, many successful applications of catalytic C-H functionalization reactions directed toward the construction of C-C or C-N bonds have been reported in synthetic communities. Many transition metals, such as manganese [6], iron [7], cobalt [8], nickel [9], ruthenium [10], rhodium [11,12], palladium [9,[13][14][15][16][17], iridium [18], and platinum [19] have been found to catalyze C-H functionalization reactions.…”