A Cationic High‐Valent Cp*Co^III Complex for the Catalytic Generation of Nucleophilic Organometallic Species: Directed CH Bond Activation

Abstract

The nucleophilic addition of organometallic reagents to polar electrophiles, such as aldehydes, imines, and Michael acceptors, is a fundamental CÀC bond-forming reaction in organic synthesis. The generation of nucleophilic organometallic reagents, however, generally requires stoichiometric amounts of strong bases and/or reducing metals, such as Mg and Li, and stoichiometric salt waste is therefore inevitably produced. Thus, the development of atom-economical processes [1] involving the catalytic generation of nucleophilic organometallic species and their addition to polar electrophiles without additional activating reagents is highly desirable. Transition-metal-catalyzed C À H bond functionalization could be an attractive method for addressing these issues, [2] but addition reactions of CÀH bonds to polar CÀX multiple bonds (X = N, O, or C) have been investigated much less [3] than related reactions with nonpolar alkenes and alkynes. [2] Quite recently, it was disclosed that Cp*Rh III complexes (Cp* = pentamethylcyclopentadienyl) [4] catalyze addition reactions of arene C À H bonds to imines, [5] aldehydes, [6] Michael acceptors, [7] and other polar electrophiles. [8] Although Cp*Rh III -catalyzed processes are useful and versatile, the need for expensive and precious rhodium sources is economically and environmentally disadvantageous. Therefore, studies are needed for the development of an inexpensive base metal catalyst as an alternative to the cationic Cp*Rh III complexes. [9] Herein, we describe the utility of a cationic high-valent cobalt complex and the structure-activity rela-tionship of various Cp*Co III complexes (Scheme 1) for the catalytic generation of nucleophilic organometallic species. We found that the [Cp*Co III (arene)](PF 6 ) 2 complex 1 a (5-10 mol %) promoted the addition of 2-aryl pyridines to imines, enones, and a,b-unsaturated N-acyl pyrroles as ester and amide surrogates.

Rh(III)- and Ir(III)-Catalyzed C–C Bond Cross Couplings from C–H Bonds

“…Moreover, the use of cheaper first-row transition metals is also an important goal. Thus, the use of CoCp* instead of RhCp* is getting increasing attention since the first pioneering report by Kanai in 2013 [168][169][170][171][172][173][174].…”

High-Valent Co(III)- and Ni(II)-Catalyzed C–H Activation

“…In their seminal report, 2-phenylpyridine is coupled to sulfonylimines to furnish secondary amines [40]. While neutral [Cp*CoCl 2 ] 2 is ineffective as a catalyst, the cationic [Cp*Co(C 6 H 6 )](PF 6 ) 2 provides the desired product in 80 % yield.…”

Studies on the Nature of Active Cobalt Species for the Production of Methane and Propylene in Catalytic Dehydrogenation of Propane

“…Low-valent cobalt catalysts are known to be efficient in C-H functionalization, such as hydroarylation of olefins [7,8], C-H/electrophiles coupling [9,10], addition of arylzinc reagents to alkynes [11,12]. For these reactions, it is speculated that the mechanism proceeds through a Co 1? intermediate and an oxidative addition/reductive elimination pathway.…”