Cobalt‐Catalyzed Reductive Cross‐Coupling Between Styryl and Benzyl Halides

Abstract

A simple and efficient protocol for the direct reductive cross-coupling between alkenyl and benzyl halides using a Co/Mn system has been developed. This reaction proceeds smoothly in the presence of [CoBr (PPh ) ] as the catalyst, with NaI as an additive in acetonitrile with a broad scope of functionalized alkenyl and benzyl halides. Different functional groups are tolerated on both coupling partners, thus, significantly extending the general scope of transition-metal-catalyzed benzylation of alkenyl halides. Moderate to excellent yields were also obtained. From a mechanistic point of view, a radical chain mechanism was proposed. This reaction is stereospecific and some studies suggest the retention of the double-bond configuration.

Reductive Cross-Coupling of Vinyl Electrophiles

“…The cobalt-catalyzed Csp 2 -Csp 3 vinylation reaction was reported by Gosmini and co-workers in 2017 (Scheme 4c). 16 They found that -bromostyrene and benzyl chlorides coupled well in the presence of CoBr 2 (PPh 3 ) 2 . The reaction proceeded with high stereospecificity and broad substrate scope.…”

Cobalt-Catalyzed Cross-Coupling Reactions of Arylboronic Esters and Aryl Halides

“…In comparison to nickel and copper, , much less has been achieved with iron and cobalt as catalyst in the Suzuki–Miyaura reaction, presumably due to the difficulty associated with transmetalation from boron to these earlier transition metals . Indeed, iron- and cobalt-catalyzed cross-coupling reactions are generally accomplished with more reactive organometals in Kumada- and Negishi-type reactions. , To facilitate transmetalation, organoboron compounds can be activated by highly reactive organolithium and organomagnesium reagents for cross-coupling reactions under iron catalysis (Scheme a) . Recently, Chirik et al developed a bis­(phosphino)­pyridine cobalt catalyst for the cross-coupling of aryl triflates and (benzo)­furanylboronic esters in the presence of an alkaline alkoxide (Scheme b) .…”

“…To probe any potential reactivity of organoboron compounds under cobalt catalysis, we started out to explore the reactions of different arylboron reagents with 2-chloropyridine ( 1a ), an activated substrate that had been well-studied in iron- and cobalt-catalyzed Kumada and Negishi cross-coupling reactions (Table ). , After some experimentation with different alkoxides, we realized that 3a could be formed in a moderate yield in the reaction of 1a with PhB­(neo) ( 2a ) in the presence of 12 mol % of CoCl 2 and 1.5 equiv of KOMe (Table , entry 1). Since bis­(phosphino)­pyridine, pyridine diimine, and terpyridine pincer ligands are known to support cobalt complexes in C­(sp 2 )–H borylation and cross-coupling reactions, , we evaluated some easily accessible or commercially available ligands belonging to these families (entries 3–5).…”

Preparation of Polyfunctional Naphthyridines by Cobalt-Catalyzed Cross-Couplings of Halogenated Naphthyridines with Magnesium and Zinc Organometallics

“…We also found that 1d smoothly reacts with benzylic zinc reagents. 11 Thus, the couplings of 1d with benzylzinc chlorides 8a and 8b provided 1,5-naphthyridines 9a and 9b in 62–75% yield (Scheme 2).…”