Cobalt-catalysed sp(2) C-H bond functionalization has attracted considerable attention in recent years because of the low cost of cobalt complexes and interesting modes of action in the process. In comparison, much less efforts have been devoted to the sp(3) carbons. Here we report the cobalt-catalysed site-selective dehydrogenative cyclization of aliphatic amides via a C-H bond functionalization process on unactivated sp(3) carbons with the assistance of a bidentate directing group. This method provides a straightforward synthesis of monocyclic and spiro β- or γ-lactams with good to excellent stereoselectivity and functional group tolerance. In addition, a new procedure has been developed to selectively remove the directing group, which enables the synthesis of free β- or γ-lactam compounds. Furthermore, the first cobalt-catalysed intermolecular dehydrogenative amination of unactivated sp(3) carbons is also realized.

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Cobalt-catalysed site-selective intra- and intermolecular dehydrogenative amination of unactivated sp(3) carbons.

Cobalt-catalysed sp2 C–H bond functionalization has attracted considerable attention in recent years because of the low cost of cobalt complexes and interesting modes of action in the process. In comparison, much less efforts have been devoted to the sp3 carbons. Here we report the cobalt-catalysed site-selective dehydrogenative cyclization of aliphatic amides via a C–H bond functionalization process on unactivated sp3 carbons with the assistance of a bidentate directing group. This method provides a straightforward synthesis of monocyclic and spiro β- or γ-lactams with good to excellent stereoselectivity and functional group tolerance. In addition, a new procedure has been developed to selectively remove the directing group, which enables the synthesis of free β- or γ-lactam compounds. Furthermore, the first cobalt-catalysed intermolecular dehydrogenative amination of unactivated sp3 carbons is also realized.

Cobalt-catalysed site-selective intra- and intermolecular dehydrogenative amination of unactivated sp3 carbons

The activation and conversion of hydrocarbons is one of the most important challenges in chemistry. Transition-metal ions (V, Cr, Fe, Co, etc.) isolated on silica surfaces are known to catalyze such processes. The mechanisms of these processes are currently unknown but are thought to involve C–H activation as the rate-determining step. Here, we synthesize well-defined Co(II) ions on a silica surface using a metal siloxide precursor followed by thermal treatment under vacuum at 500 °C. We show that these isolated Co(II) sites are catalysts for a number of hydrocarbon conversion reactions, such as the dehydrogenation of propane, the hydrogenation of propene, and the trimerization of terminal alkynes. We then investigate the mechanisms of these processes using kinetics, kinetic isotope effects, isotopic labeling experiments, parahydrogen induced polarization (PHIP) NMR, and comparison with a molecular analog. The data are consistent with all of these reactions occurring by a common mechanism, involving heter...

C–H Activation on Co,O Sites: Isolated Surface Sites versus Molecular Analogs

The dearomatized complex cis-[Re(PNPtBu*)(CO)2] (4) undergoes cooperative activation of C≡N triple bonds of nitriles via [1,3]-addition. Reversible C–C and Re–N bond formation in 4 was investigated in a combined experimental and computational study. The reversible formation of the ketimido complexes (5–7) was observed. When nitriles bearing an alpha methylene group are used, reversible formation of the enamido complexes (8 and 9) takes place. The reversibility of the activation of the nitriles in the resulting ketimido compounds was demonstrated by the displacement of p-CF3-benzonitrile from cis-[Re(PNPtBu–N═CPhpCF3)(CO)2] (6) upon addition of an excess of benzonitrile and by the temperature-dependent [1,3]-addition of pivalonitrile to complex 4. The reversible binding of the nitrile in the enamido compound cis-[Re(PNPtBu–HNC═CHPh)(CO)2] (9) was demonstrated via the displacement of benzyl cyanide from 9 by CO. Computational studies suggest a stepwise activation of the nitriles by 4, with remarkably low ac...

Activation of nitriles by metal ligand cooperation. Reversible formation of ketimido- and enamido-rhenium PNP pincer complexes and relevance to catalytic design.

The reaction of the cobalt(0) alkene complex [(IMes)Co(η2:η2-dvtms)] (1) (IMes = 1,3-bis(1′,3′,5′-trimethylphenyl)imidazol-2-ylidene, dvtms = divinyltetramethyldisiloxane) with 2 equiv of DippN3 (Dipp = 2,6-diisopropylphenyl) afforded the cobalt(IV) imido complex [(IMes)Co(NDipp)2] (2), which could be oxidized by [Cp2Fe][BArF4] (ArF = 3,5-di(trifluoromethyl)phenyl) to give the cobalt(V) imido species [(IMes)Co(NDipp)2][BArF4] (3). The molecular structures of all these complexes were established by single-crystal X-ray diffraction studies. Characterization data and theoretical calculations suggest ground spin states of S = 1/2 and S = 0 for the cobalt(IV) and cobalt(V) species, respectively. When heated, the cobalt(IV) imido species was converted to a cobalt(II) diamido complex via an intramolecular C–H bond amination reaction, but the cobalt(V) species was stable under similar conditions. The different outcomes suggest that a high oxidation state does not guarantee C–H bond activation reactivity of late-t...

Three-Coordinate Cobalt(IV) and Cobalt(V) Imido Complexes with N-Heterocyclic Carbene Ligation: Synthesis, Structure, and Their Distinct Reactivity in C–H Bond Amination

Stephen P. George

Papers

Synthesis of a cobalt(IV) ketimide with a squashed tetrahedral geometry

Development of [CpRu(η 6 -benzylcarbamate)]X salts as two-stage photobase generators