Nickel-catalyzed electrochemical carboxylation of unactivated aryl and alkyl halides with CO2

Abstract

Electrochemical catalytic reductive cross couplings are powerful and sustainable methods to construct C-C bonds by using electron as the clean reductant. However, activated substrates are used in most cases. Herein, we report a general and practical electro-reductive Ni-catalytic system, realizing the electrocatalytic carboxylation of unactivated aryl chlorides and alkyl bromides with CO₂. A variety of unactivated aryl bromides, iodides and sulfonates can also undergo such a reaction smoothly. Notably, we also realize the catalytic electrochemical carboxylation of aryl (pseudo)halides with CO₂ avoiding the use of sacrificial electrodes. Moreover, this sustainable and economic strategy with electron as the clean reductant features mild conditions, inexpensive catalyst, safe and cheap electrodes, good functional group tolerance and broad substrate scope. Mechanistic investigations indicate that the reaction might proceed via oxidative addition of aryl halides to Ni(0) complex, the reduction of aryl-Ni(II) adduct to the Ni(I) species and following carboxylation with CO₂.

Fig. 1. Background and synopsis of our work.

a Selected examples of bioactive compounds containing carboxylic acid moiety. b A general electrocatalytic carboxylation of unactivated organo (pseudo)halides with CO₂.

Fig. 2. Scope of aryl chlorides, bromides, iodines, and sulfonates.

^aReaction conditions, see more details in “Supplementary information”. ^b5 Å molecular sieve was added. ^cNiBr₂•DME (5 mol%), dtbbpy (5 mol%), DMAP (10 mol%). ^dNi(acac)₂ (5 mol%), dtbbpy (5 mol%), KO^tBu (0.5 equiv), I = 8 mA. ^e50 °C, 8 h. ^fRoom temperature, 12 h. ^g3-formylbenzoic acid was obtained as product. ^hI = 4 mA.

Fig. 3. Scope of alkyl bromides.

^aReaction conditions, see more details in “Supplementary information”. ^bI = 4 mA.

Fig. 4. Scope of organo(pseudo) halides in non-sacrificial electrode manner.

^aReaction conditions, see more details in Table S2 in “Supplementary information”. ^b0.5 mmol scale. ^c60 °C. ^dNiI₂ (10 mol%), dtbbpy (10 mol%), DMAP (20 mol%), MgBr₂ (1.5 equiv). Anodic chamber: Et₃N (0.6 mmol), 3 Å MS (100 mg), NMP/NaI (0.2 M). ^eDMAP (20 mol%) was added. ^fNiBr₂•DME (10 mol%), dmbby (20 mol%), CsF (1.0 quiv.), NMP/LiClO₄ (0.2 M), RT. Anodic chamber: Et₃N (0.6 mmol), NMP/LiClO₄ (0.2 M). MS = molecule sieve.

Fig. 5. Control experiments.

a Investigation of the effect of nickel complex LNi(acac)₂. b Investigation of the effect of Ni(0) catalyst. c Preparation of oxidative addition adduct 1-1. d Investigation of the effect of adduct 1-1. e HRMS detection of oxidation addition adduct. L = dtbbpy. DMAP = N, N-dimethyl-4-aminopyridine. HRMS = High resolution mas spectrometry.

Fig. 6. Electrochemical and spectroscopic investigations.

a CV tests of [Ni(acac)₂ + L] and LNi(acac)₂. Testing conditions: working electrode, glassy carbon electrode; counter electrode, Pt wire; reference electrode, Ag/AgNO₃ electrode (10 mM AgNO₃ in CH₃CN). Scan rate: 100 mV/s. Solvent: NMP/ⁿBu₄NPF₆ (0.1 M). Samples were tested under N₂ atmosphere. Ni(acac)₂, 3 mM; L, dtbbpy, 3 mM, LNi(acac)₂, 3 mM. b UV-vis spectra of Ni(acac)₂, 3 × 10⁻⁵M; L, 3 × 10⁻⁵M; [Ni(acac)₂ + L], 3 × 10⁻⁵M + 3 × 10⁻⁵M and LNi(acac)₂, 3 × 10⁻⁵M. NMP is chosen as the testing solvent. L = dtbbpy, 4,4′-di-tert-butyl-2,2′-bipyridine; NMP = 1-methyl-2-pyrrolidinone.

Fig. 7. Performance of nickel catalysts in CV tests.

Testing conditions: working electrode, glassy carbon electrode; counter electrode, Pt wire; reference electrode, Ag/AgNO₃ electrode (10 mM AgNO₃ in CH₃CN). Scan rate: 100 mV/s. Solvent: NMP/ⁿBu₄NPF₆ (0.1 M). Samples were tested under N₂ atmosphere except some cases under CO₂ atmosphere. a CV tests of [Ni(acac)₂ + dtbbpy] and aryl bromide 3 h. Ni(acac)₂, 3 mM; L, dtbbpy, 3 mM; LNi(acac)₂, 3 mM; 3 h, ethyl 4-bromobenzoate, 60 mM; CO₂ (bubbled in the solution for 10 min). b CV tests for the role of DMAP. [Ni], NiBr₂(DME), 3 mM; L, dtbbpy, 3 mM; DMAP, 3 mM, 3b, 4-bromophenyl, 5 mM. c CV tests of adduct 1-1. adduct 1-1, 5 mM; CO₂ (bubbled in the solution for 10 min). L, dtbbpy, 4,4′-di-tert-butyl-2,2′-bipyridine; DMAP, 4-dimethylaminopyridine.

Fig. 8. Proposed mechanistic cycle.

Proposed reaction pathway is started up via the electro-generated Ni(0) species.