A bis(imino)pyridine cobalt-catalyzed hydroboration of terminal alkynes with HBPin (Pin = pinacolate) with high yield and (Z)-selectivity for synthetically valuable vinylboronate esters is described. Deuterium labeling studies, stoichiometric experiments, and isolation of catalytically relevant intermediates support a mechanism involving selective insertion of an alkynylboronate ester into a Co-H bond, a pathway distinct from known precious metal catalysts where metal vinylidene intermediates have been proposed to account for the observed (Z) selectivity. The identity of the imine substituents dictates the relative rates of activation of the cobalt precatalyst with HBPin or the terminal alkyne and, as a consequence, is responsible for the stereochemical outcome of the catalytic reaction.

α,β-Unsaturated O-pivaloyl oximes are coupled to alkenes by Rh(III) catalysis to afford substituted pyridines. The reaction with activated alkenes is exceptionally regioselective and high yielding. Mechanistic studies suggest that heterocycle formation proceeds via reversible C-H activation, alkene insertion and a C-N bond formation/N-O bond cleavage process.

α,β-Unsaturated oxime pivalates are proposed to undergo reversible C(sp2)-H insertion with cationic Rh(III) complexes to furnish five-membered metallacycles. In the presence of 1,1-disubstituted olefins, these species participate in irreversible migratory insertion to give, after reductive elimination, 2,3-dihydropyridine products in good yields. Catalytic hydrogenation was then used to convert these molecules into piperidines, which are important structural components of numerous pharmaceuticals.

α,β-Unsaturated carboxylic acids undergo Rh(III)-catalyzed decarboxylative coupling with α,β-unsaturated O-pivaloyl oximes to provide substituted pyridines in good yield. The carboxylic acid, which is removed by decarboxylation, serves as a traceless activating group, giving 5-substituted pyridines with very high levels of regioselectivity. Mechanistic studies rule out a picolinic acid intermediate, and an isolable rhodium complex sheds further light on the reaction mechanism.

Among the fundamental transformations that comprise a catalytic cycle for cross coupling, transmetalation from the nucleophile to the metal catalyst is perhaps the least understood. Optimizing this elementary step has enabled the first example of a cobalt-catalyzed Suzuki–Miyaura cross coupling between aryl triflate electrophiles and heteroaryl boron nucleophiles. Key to this discovery was the preparation and characterization of a new class of tetrahedral, high-spin bis(phosphino)pyridine cobalt(I) alkoxide and aryloxide complexes, (iPrPNP)CoOR, and optimizing their reactivity with 2-benzofuranylBPin (Pin = pinacolate). Cobalt compounds with small alkoxide substituents such as R = methyl and ethyl underwent swift transmetalation at 23 °C but also proved kinetically unstable toward β–H elimination. Secondary alkoxides such as R = iPr or CH(Ph)Me balanced stability and reactivity. Isolation and structural characterization of the product following transmetalation, (iPrPNP)Co(2-benzofuranyl), established a planar, diamagnetic cobalt(I) complex, demonstrating the high- and low-spin states of cobalt(I) rapidly interconvert during this reaction. The insights from the studies in this elementary step guided selection of appropriate reaction conditions to enable the first examples of cobalt-catalyzed C–C bond formation between neutral boron nucleophiles and aryl triflate electrophiles, and a model for the successful transmetalation reactivity is proposed.

Cobalt Catalyzed Z-Selective Hydroboration of Terminal Alkynes and Elucidation of the Origin of Selectivity. -Anti-Markovnikov hydroboration of terminal alkynes in the presence of a bis(imino)pyridine cobalt catalyst affords (Z)-vinylboronate esters. Deuterium labeling studies, stoichiometric experiments, and isolation of catalytically relevant intermediates support a mechanism which involves selective insertion of an alkynylboronate ester into a Co-H bond, a pathway distinct from known precious metal catalysts where metal vinylidene intermediates seem to be responsible for the observed (Z)-selectivity. The identity of the imine substituents dictates the relative rates of activation of the cobalt precatalyst with HBPin or the terminal alkyne and influences the stereochemical outcome of the reaction. -(OBLIGACION, J. V.; NEELY, J. M.; YAZDANI, A. N.; PAPPAS, I.; CHIRIK*, P. J.; J. Am. Chem. Soc. 137 (2015) 18, 5855-5858, http://dx.

A variety of C–N bond-forming methods are enabled by the [2 + 2] cycloaddition reaction of a transition metal imide complex and an alkyne substrate to generate an azametallacyclobutene intermediate. This type of reactivity has been primarily limited to early transition metals like zirconium and titanium. Herein, we describe the preparation of an iron azametallacyclobutene complex by [2 + 2] cycloaddition of a β-diketiminate iron imide complex and an internal alkyne, 1-phenyl-1-propyne. The metallacycle reacts further upon exposure to a terminal alkyne, phenylacetylene, by a proposed protonation pathway that is distinct from the chemistry of its group 4 congeners and is in line with formation of an azametallacyclobutene intermediate in iron-catalyzed alkyne carboamination. The iron azametallacyclobutene complex also undergoes migratory insertion of aldehyde and nitrile substrates to the metal–nitrogen bond, in contrast to the exclusive metal–carbon insertion that has been observed for zirconium and titanium analogs.

Transition metal imide-mediated C–N bond formation is a powerful strategy for the introduction of nitrogen into organic compounds. We have discovered that the reaction of N-mesityl­(β-diketiminato)­iron imide complex tBuLFeNMes ( tBuL = 3,5-bis­(2,6-diisopropylphenylimino)-2,2,6,6-tetramethylheptyl and Mes = 2,4,6-trimethylphenyl) with a terminal alkyne substrate gives a β-alkynyl enamine product by a novel alkyne carboamination process. Stoichiometric experiments revealed a catalyst deactivation pathway involving generation of the acetylide complex, tBuLFeCCPh, and mesityl amine (MesNH2) from the acetylene complex, tBuLFe­(HCCPh), and mesityl azide (MesN3). This reactivity is suppressed in the presence of coordinating additive 4-tert-butylpyridine ( t BuPy), likely through formation of the four-coordinate complex tBuLFe­(HCCPh)­( t BuPy). These insights were instrumental in identifying reaction conditions that allow for turnover of the iron catalyst.