Ni(COD)(DMFU): A Heteroleptic 16-Electron Precatalyst for 1,2-Diarylation of Alkenes

Abstract

Electron-deficient olefin (EDO) ligands are known to promote a variety of nickel-catalyzed cross-coupling reactions, presumably by accelerating the reductive elimination step and preventing undesired β-hydride elimination. While there is a growing body of experimental and computational evidence elucidating the beneficial effects of EDO ligands, significant gaps remain in our understanding of the underlying coordination chemistry of the Ni–EDO species involved. In particular, most procedures rely on in situ assembly of the active catalyst, and there is a paucity of preligated Ni–EDO precatalysts. Herein, we investigate the 16-electron, heteroleptic nickel complex, Ni(COD)(DMFU), and examine the performance of this complex as a precatalyst in 1,2-diarylation of alkenes.

Key words

nickel - precatalysts - homogeneous catalysis - electron-deficient olefin ligand

• References and Notes


For representative reviews, see:
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DMFU has also been used as a stabilizing ligand in NHC–Ni(0) precatalysts:
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• 8 The transition state for reductive elimination with dimethyl fumarate (DMFU) displays significantly lower activation energy than transition states with ethylene- or solvent-bound nickel (ΔG^‡ = 2.6 kcal/mol compared to ΔG^‡ = 14.6 kcal/mol or ΔG^‡ = 37.4 kcal/mol, respectively; see ref. 6b for detail).
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• 12 For comparison, values for related complexes are as follows: +0.21065 for Ni(COD)(DQ) and +0.09120 for Ni(COD)₂ (see ref. 4c).
• 13 The sum of second order perturbations from Ni(LP) to π*(C=C) of both DMFU and COD is much greater than the corresponding value for Ni(COD)(DQ), which was 82.62 kcal/mol.
• 14 In the standard procedure in ref. 6c, 20 mol% Ni(COD)₂ and 15 mol% DMFU are used. In order to make a more direct comparison to Ni(COD)(DMFU), in conditions A we used 20 mol% loading of DMFU under otherwise identical conditions to ref. 6c. Empirically, we have found that 15–20 mol% DMFU leads to similar yields (±5%) in the extraneous period in situ coordination protocol.
• 15 Representative Example of 1,2-Diarylation of Alkenes (Table [1], Entry 2) To a 1-dram (4 mL) vial equipped with a Teflon-coated magnetic stir bar were added the alkene substrate (0.1 mmol) and the appropriate aryl boronic acid neopentylglycol ester (0.3 mmol). The vial was then equipped with a septum cap and brought into the glovebox. In the glovebox, anhydrous NaOH (0.3 mmol), the appropriate aryl iodide electrophile (0.3 mmol), a stock solution of 4,4′-di-tert-butylbiphenyl (internal standard; 0.2 mL of 0.25 mM solution in sec-butanol), and anhydrous sec-butanol (0.3 mL) were added. For conditions A, DMFU (20 mol%) and Ni(COD)₂ (20 mol%) were added, and for conditions B, Ni(COD)(DMFU) (20 mol%) was added, respectively. The vial was sealed with a screw-top cap, removed from the glovebox, and left to stir at room temperature for 16 h. After this time, the reaction mixture was quenched with sat. aq. NaHCO₃ (1 mL), diluted with diethyl ether (2 mL), and stirred vigorously for 20 min. Next, the organic and aqueous phases were separated, and the aqueous phase was further extracted with diethyl ether (3 × 2 mL). The combined organic layers were dried over MgSO₄, filtered, and concentrated under vacuum. The resulting crude mixture was analyzed by ¹H NMR spectroscopy in CDCl₃, displaying 49% formation of S21 (see the SI) from Conditions A, and 67% from Conditions B. ¹H NMR yields for diarylation were determined by integrating the ¹H resonances for the products relative to the resonances of internal standard (0.50 mmol; 18 H integrated to 0.90) at δ = 1.37 ppm in CDCl₃. Purification by preparative thin-layer chromatography (PTLC; 20% acetone in hexanes) afforded S21 as a colorless oil, which solidifies upon standing or cooling (33 mg, 70%; conditions B). ¹H NMR (600 MHz, CDCl₃): δ = 7.85 (d, J = 7.9 Hz, 2 H), 7.73 (d, J = 8.4 Hz, 2 H), 7.12 (dpd, J = 14.1, 7.9, 3.3 Hz, 3 H), 7.07–7.01 (m, 2 H), 6.85–6.78 (m, 2 H), 6.75–6.68 (m, 2 H), 4.29 (d, J = 6.0 Hz, 1 H), 3.81 (s, 3 H), 2.97–2.83 (m, 2 H), 2.77–2.65 (m, 2 H), 2.10 (ddd, J = 15.2, 10.2, 5.3 Hz, 1 H), 1.72 (ddd, J = 13.0, 10.3, 5.5 Hz, 1 H), 1.18 (s, 3 H). ¹³C NMR (150 MHz, CDCl₃): δ = 158.03, 143.73, 137.68, 137.32, 134.43 (q, J = 33.1 Hz), 130.66, 127.76, 127.70, 127.67, 123.12 (q, J = 273.50 Hz), 113.80, 55.40, 51.22, 42.06, 40.65, 39.93, 23.44. ¹⁹F NMR (376 MHz, CDCl₃): δ = –65.74. HRMS (ESI-TOF): m/z calcd for C₂₅H₂₅F₃NO₂S [M – H]^–: 476.1607; found: 476.1506.
• 16 20 mol% COD (1 equiv to [Ni]) was added into a trial run under conditions B in order to reproduce the overall [COD] under conditions A. This experiment resulted in 65% NMR yield, which is within 5% of the result without additional COD.