Asymmetric C–H Dehydrogenative Allylic Alkylation by Ternary Photoredox-Cobalt-Chiral Primary Amine Catalysis under Visible Light

Abstract

We report herein an asymmetric C–H dehydrogenative allylic alkylation by a synergistic catalytic system involving a chiral primary amine, a photoredox catalyst, and a cobaloxime cocatalyst. The ternary catalytic system enables the coupling of β-ketocarbonyls and olefins with good yields and high enantioselectivities. Mechanism studies disclosed a cooperative radical addition process with a chiral α-imino radical and Co­(II)-metalloradical wherein the chiral primary aminocatalyst and the cobaloxime catalyst work in concert to control the stereoinduction.

Photoinduced Three-Component Carboarylation of Unactivated Alkenes with Protic C(sp3)–H Feedstocks

“…Recently, oxidant-free photoredox catalysis on the functionalization of protic C­(sp 3 )–H bonds has received widespread attention . Under photoredox conditions, the protic C­(sp 3 )–H feedstocks can be converted into electrophilic radicals, , which were readily coupled with unactivated alkenes, offering a new opportunity for carboarylation of unactivated alkenes.…”

Hydrogen-Evolution Allylic C(sp3)–H Alkylation with Protic C(sp3)–H Bonds via Triplet Synergistic Brønsted Base/Cobalt/Photoredox Catalysis

“…This method represents an alternative approach to oxidant-free allylic C­(sp 3 )–H alkylation without the use of specialized electrochemical equipment. It should be also mentioned that, during our preparation of this paper, an elegant protocol of asymmetric hydrogen-evolution allylic C­(sp 3 )–H alkylation of α-methylstyrenes by ternary photoredox/cobalt/chiral primary amine catalysis was reported by Luo et al…”

“…This method represents an alternative approach to oxidant-free allylic C(sp 3 )−H alkylation without the use of specialized electrochemical equipment. It should be also mentioned that, during our preparation of this paper, an elegant protocol of asymmetric hydrogen-evolution allylic C(sp 3 )−H alkylation of α-methylstyrenes by ternary photoredox/cobalt/chiral primary amine catalysis was reported by Luo et al 48 At the onset of our investigation, α-methylstyrene 1a and malonate 2a were chosen as the model coupling partners for the proposed hydrogen-evolution allylic C(sp 3 )−H alkylation. After extensive optimization (for details, see Tables S1−S4), we found that hydrogen-evolution allylic C(sp 3 )−H alkylation can be achieved through the combination of a catalytic amount of Ru(bpy) 3 Cl 2 •6H 2 O, Co(dmgH) 2 PyCl (Co-1), and Cs 2 CO 3 in acetonitrile at 30 °C under blue LED light irradiation, giving the desired coupling product 3 exclusively in 64% yield (Table 1, entry 1).…”

Strategies That Utilize Ion Pairing Interactions to Exert Selectivity Control in the Functionalization of C–H Bonds

“…Luo and co-workers recently reported an enantioselective dehydrogenative allylic alkylation of β-ketocarbonyls utilizing a ternary catalytic system involving a chiral primary amine bearing a protonated morpholine unit, a photoredox catalyst, and a cobaloxime cocatalyst (Figure 13a,b). 53 Although a carbonyl functionalization, it constitutes a C−H bond functionalization on the alkene component. The authors propose a mechanism whereby, after chiral enamine formation, the iridium photocatalyst oxidizes the enamine to give a prochiral carbon-centered radical.…”