Isolation and characterization of copper(III) trifluoromethyl complexes and reactivity studies of aerobic trifluoromethylation of arylboronic acids

Abstract

The isolation, characterization and reactivity of transition metal trifluoromethyl complexes are fundamental and challenging topics in trifluoromethylation chemistry. We report herein the synthesis and isolation of two new complexes [(phen)Cu^I(PPh₃)₂]⁺[Cu^III(CF₃)₄]⁻ (2) and (phen)Cu^III(CF₃)₃ (3) as well as a known complex (bpy)Cu^III(CF₃)₃ (4) at room temperature. 2 and 3 have been fully characterized using ¹H, ¹⁹F, ³¹P NMR, elemental analyses and X-ray crystallography. Reactivity studies indicate that 2 is unreactive toward arylboronic acids. In contrast, 3 and 4 can react with various aryl and heteroaryl boronic acids to deliver trifluoromethylated arenes in good to quantitative yields under mild conditions. The presence of a fluoride additive in DMF under aerobic conditions is crucial to these reactions. This study provides fundamental information about the structure and reactivity of elusive Cu(III) trifluoromethyl complexes that have been proposed as relevant reactive intermediates in many trifluoromethylation reactions.

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Copper-mediated trifluoromethylation reactions have been greatly developed and constitute one important branch of organofluorine chemistry.^1–3 Trifluoromethylation of aryl halides,^4,5 boronic acids,⁶ arenes⁷ and others^8,9 with various CF₃ sources promoted by copper salts have been intensively studied during the past few decades. Despite the significant progress achieved in developing efficient reaction methodologies, the isolation and reactivity properties of possible copper trifluoromethyl intermediates are surprisingly scarcely described but are however essential to the mechanistic understanding of these reactions.¹⁰ Very recently, the characterization and reactivity properties of several Cu^I–CF₃ complexes have been reported that typically contains N-heterocyclic carbene (NHC) or phenanthroline (phen) ligand.¹¹

In contrast, extremely rare studies have been reported on the isolation, characterization and reactivity properties of high-valent Cu^III CF₃ complexes which have been frequently proposed as reactive intermediates for oxidative or electrophilic trifluoromethylation reactions.^12,13 In 1989, Burton et al. reported a pioneering study on the synthesis and X-ray crystal structure of a square planar (CF₃)₂Cu^III(dithiocarbamate) complex by oxidation of [Cu^I(CF₃)₂]⁻ using thiuramdisulphide.^13a This Cu(III) trifluoromethyl complex could react with aryl iodides to give trifluoromethylated arenes at 90–100 °C. As far as we know, this is the first example of the isolation and reactivity of well-characterized Cu(III) trifluoromethyl complex. Later, Naumann et al. reported anionic square planar [Cu^III(CF₃)₄]⁻ complexes with Ph₄P⁺ or Bu₄N⁺ counterions via oxidation of in situ Cu(I) trifluoromethyl by halogen.^13b Similar ion–pair complexes were obtained by Grushin by reaction of CuCl or in situ formed “Cu^I–CF₃” with CF₃SiMe₃ using air as the oxidant.^13c Heating the ion–pair complexes with bipyridine (bpy) in acetic acid gave neutral (bpy)Cu^III(CF₃)₃.

Herein, we report the preparation and isolation of Cu^III trifluoromethyl complexes 2–4 by reaction of Cu^I precursors with CF₃SiMe₃ in the presence of AgF (Scheme 1). Reactivity studies show that these Cu(III) trifluoromethyl complexes are able to efficiently trifluoromethylate arylboronic acids in up to quantitative yields even at room temperature. Considering the predominant role of phen ligand in copper trifluoromethylation chemistry,³ the isolation and reactivity properties of the phen-containing Cu^III trifluoromethyl complexes should be fundamental to this important area.

Scheme 1 Syntheses and reactivity of Cu^III–CF₃ complexes 2–4.

Reaction of (phen)Cu^I(PPh₃) (Br) (1)¹⁴ with 3 equivalents of CF₃SiMe₃ in the presence of AgF in CH₂Cl₂ at room temperature allows the concurrent access to 2 and 3 as pure yellow solids in 30% and 31% yields respectively (Scheme 1a, see ESI for details).† Alternatively, complex 3 can be synthesized and isolated by a different method of reaction of CuI/phen with CF₃SiMe₃ using AgF as oxidant in a higher yield of 63% (Scheme 1b, see ESI for details).† In both reactions, AgF is crucial, which should probably accelerate the transmetalation of CF₃SiMe₃ and act as a one-electron oxidant to convert the Cu(I) to higher oxidation states. When bpy was used instead of phen in Scheme 1b, a neutral (bpy)Cu^III(CF₃)₃ (4) was obtained in an isolated high yield of 71%.

¹⁹F NMR spectrum of complex 2 shows a resonance at −34.6 ppm with singlet splitting, consistent with previous reported [Cu^III(CF₃)₄]⁻ species.^{13b,c 19}F NMR spectrum of complex 3 shows two signals at −24.4 (septet) and −37.4 (quartet) ppm, with a ratio of ca. 1 : 2 with the signal at −37.4 ppm being the major one. ¹H NMR of complex 2 clearly shows a ratio of 1 : 2 of phen/PPh₃ and the typical splitting of the phen ligand in Cu(I) complexes (see ESI for more details).† ¹H NMR spectrum of complex 3 shows only resonances of phen ligand which are greatly low-field shifted compared to those in complex 2. ³¹P NMR spectrum of 2 shows a weak signal at 3.0 ppm, confirming the existence of coordinated PPh₃ ligand. Complex 4 shows a similar structure to 3, with two fluorine signals at −24.0 (septet) and −36.1 (quartet) ppm in ¹⁹F NMR and protons of bpy in ¹H NMR, which is consistent with previously reported data.^13c

The structures of 2 and 3 were further confirmed by X-ray crystallographic diffraction analyses (Fig. 1 and 2).¹⁵ Complex 2 is composed of a square planar [Cu^III(CF₃)₄]⁻ anion and a tetrahedral [(phen)Cu^I(PPh₃)₂]⁺ countercation (Fig. 1). The Cu–CF₃ bond lengths in [Cu^III(CF₃)₄]⁻ are averaged to about 1.936(7) Å. Complex 3 adopts an approximately trigonal bipyramidal geometry with phen, Cu and one CF₃ ligand lying in the equatorial plane and the other two CF₃s occupying the apical positions (Fig. 2). The equatorial Cu–CF₃ bond is shorter than apical Cu–CF₃ bonds (1.938(5) Å for equatorial vs. 1.953(5), 1.961(5) Å for apical Cu–CF₃ bonds). These bond lengths are shorter than Cu–CF₃ bonds in related Cu(I) trifluoromethyl complexes reported. For example, Cu–CF₃ bonds in (PPh₃)₃Cu^I–CF₃, phenCu^I(PPh₃)CF₃, Cu^I(CF₃)₂⁻, and (NHC)Cu^I–CF₃ are reported to be 2.025(7), 1.985(1), 1.970(6) and 2.022(4) Å.^11a,b,d This should partly be attributed to the different coordination geometries and the more electrophilic nature of the Cu(III) center in 3. The trigonal bipyramidal geometry of 3 is unusual considering that transition metal complexes with d⁸ electronic configuration, such as Pd(II), Ni(II) and Cu(III), often adopt square planar geometry.

Fig. 1 ORTEP drawing of complex 2 with thermal ellipsoids at 50% probability. All the hydrogen atoms and one solvent molecule are omitted for clarity. Disorders of fluorine atoms are observed.

Fig. 2 ORTEP drawing of complex 3 with thermal ellipsoids at 30% probability. All the hydrogen atoms are omitted for clarity.

As to the mechanism for the concurrent formation of 2 and 3 in Scheme 1a, it is proposed that a Cu(II) precursor A is initially formed.¹⁶ A disproportionates into 3 + B + PPh₃ which is in equilibrium with 2 + phen by ligand exchange (Scheme 2). Evidence supporting this mechanistic scheme include the observation of Cu(II) intermediates and Ag mirror during workup, ¹⁹F NMR detection of FSiMe₃ at −157.9 ppm and the access to X-ray crystal structure of PPh₃-ligated dimeric AgBr (5) (for more details, refer to ESI†).¹⁷ Additional evidence supporting the equilibrium between 2 and 3 comes from the observation that addition of phen to acetic acid solution of 2 led to the efficient formation of 3 in 45% isolated yield (see ESI for details).† Possibly, the addition of excess phen to the solution of 2 drives the equilibrium to the side of 3 (Scheme 2).

Scheme 2 Plausible mechanism for the concurrent formation of 2 and 3.

Reactivity properties of complexes 2–4 were further studied to evaluate their reaction with arylboronic acids.¹⁸ After initial screening of the reaction conditions (Table 1, entries 1–4), it was found that reaction of complex 3 with 2 equivalents of para-methoxyphenyl boronic acid (6a) gave the trifluoromethylated product 7a in 65% yield in the presence of 2 equivalents of AgF in DMF at 80 °C for 18 hours (entry 4).¹⁹ A breakthrough was achieved with the use of KF additive, which led to the quantitative trifluoromethylation of 5a even at room temperature in 3 hours (entries 5–10). These results should be the first examples of room-temperature quantitative trifluoromethylation of arylboronic acids by isolated Cu^III trifluoromethyl complexes. Reducing the reaction time to 1 hour still led to a 90% yield while the reaction can reach a 85% yield in only 0.5 hour (entries 11 & 12).

Table 1 Reactivity studies of complex 3 with arylboronic acid^a

Entry	Additive	T (°C)	Time (h)	Yield^b (%)
a Reaction conditions: 3 (0.1 mmol), 6a (0.2 mmol), additive (0.2 mmol), 4,4′-difluorobiphenyl (0.2 mmol, internal standard), DMF (1 mL), under dry O2 atmosphere.b Yields determined by ^19F NMR spectroscopy relative to 6a.
1	—	80	18	Trace
2	KI	80	18	23
3	Cs2CO3	80	18	12
4	AgF	80	18	65
5	KF	80	18	97
6	KF	80	10	99
7	KF	50	18	99
8	KF	RT	18	99
9	KF	RT	10	99
10	KF	RT	3	99
11	KF	RT	1	90
12	KF	RT	0.5	85

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With the optimized reaction conditions identified, we next examined reaction of 3 with a variety of substituted aryl boronic acids with diverse electronic properties (Table 2). The reactions show a complicated effect of electronic properties and substitution position of the substituents. Generally, electron-rich substituents are beneficial to the reactions. For substrates with strongly electron-withdrawing substituents, using AgF at a little higher 50 °C (in replace of KF at RT) is needed to achieve better yields. Additionally, para-substituted boronic acids are generally more reactive than meta- and ortho-substituted analogs (please see 7a vs. 7l, 7m; 7b vs. 7k). Finally, heteroaryl boronic acids such as 2-benzothiophenyl, 2-benzofuryl, and 3-pyridyl boronic acids can also be trifluoromethylated to produce the desired trifluoromethylated heteroarenes 7o–q in moderate to good yields. These results imply that 3 may be potential reactive intermediate in some copper/phen-mediated oxidative and electrophilic trifluoromethylation reactions.

Table 2 Substrate scope for reaction of 3 with various arylboronic acids^a

a Reaction yields were determined by ¹⁹F NMR spectroscopy using 4,4′-difluorobiphenyl (−117.0 ppm) as internal standard.b Values in parentheses are isolated yields using column chromatography.c AgF at 50 °C in the presence of 4 Å molecular sieves.

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Under similar conditions, complex 4 also shows good reactivity with various aryl boronic acids, as summarized in Table 3. In sharp contrast to 3 and 4, the ion–pair complex 2 shows little reactivity (Table S1 in ESI).† The reasons for this reactivity difference are currently not well understood.

Table 3 Substrate scope for reaction of 4 with various arylboronic acids^a

a Reaction yields were determined by ¹⁹F NMR spectroscopy using 4,4′-difluorobiphenyl (−117.0 ppm) as internal standard.b Values in parentheses are isolated yields using column chromatography.

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Although at the current stage, the detailed reaction pathway remains still elusive for the aerobic trifluoromethylation using these Cu(III) CF₃ complexes, a plausible mechanism is suggested as in Scheme 3 based on the experimental observations. A key Cu(III) intermediate i is proposed to account for the formation of Ar–CF₃ via reductive elimination, which should result from ligand exchange of initial complex 3 with Ar ligand from ArB(OH)₂ in the presence of F salt and water impurity. Accordingly, intermediate ii (or iii by comproportionation of ii with 3) is formed which is then oxidized by oxygen in the presence of water to generate a new Cu(III) intermediate iv. Ligand exchange of Ar with iv should produce intermediate i′ that can give a second Ar–CF₃ after reductive elimination. This mechanism proposal can rationalize the indispensable role of F salt and oxygen for this reaction, and the optimal reaction stoichiometry of 1 : 2 of 3 with ArB(OH)₂. In addition, the observation of better reactivity of electron-rich arylboronic acids and para-substituted arylboronic acids (please refer to Table 2) can also be explained by the more facile transmetalation of Ar of ArB(OH)₂ to copper center in critical intermediate i and i′ for electronically more rich and sterically less demanding Ar group. Further studies are underway on the elucidation of the detailed mechanism and the preparation and reactivity of Cu^III CF₃ complexes with other ancillary ligands.

Scheme 3 Proposed mechanism for the aerobic trifluoromethylation of arylboronic acids by Cu^III CF₃ complex 3.

In summary, Cu^III trifluoromethyl complexes 2–4 with representative phen or bpy ligand were prepared and isolated at room temperature via oxidation of Cu^I precursors by AgF in the presence of CF₃SiMe₃ and have been fully characterized in both solid state and solution phase. Reactivity studies show that 3 and 4 are highly reactive with aryl and heteroaryl boronic acids under mild conditions. Up to quantitative yields can be achieved at room temperature for trifluoromethylation of boronic acids using 3. In contrast, 2 shows little reactivity. The isolation and reactivity of these Cu^III CF₃ complexes provides valuable information for copper trifluoromethylation chemistry.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 21472068, 21202062) and the Natural Science Foundation of Jiangsu (No. BK2012108).

Notes and references

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15. CCDC 1402421 (3), 1402422 (2) and 1402423 (5) contain the supplementary crystallographic data for this manuscript. Please see also ESI for some details on crystallographic studies.†.
16. During the work-up process, a small amount of green impurity was observed which might imply the involvement of Cu(II) intermediates in the reaction process.
17. For the formation of intermediate A and crystal structure of PPh₃-ligated AgBr (5), please refer to ESI.†.
18. For detailed reaction procedure and ¹⁹F NMR determination of reaction yields, please refer to ESI.†.
19. Reaction of 1 eq. or 3 eq. of 6a with complex 3 led to lower yields.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, spectroscopic characterization data, X-ray crystallographic study, ¹⁹F NMR monitoring of reaction course and general procedures of reaction of 2–4 with arylboronic acids. CCDC 1402421–1402423. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra10302b

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