A class of effective decarboxylative perfluoroalkylating reagents: [(phen)2Cu](O2CRF)

Article abstract of DOI:10.1039/c6dt00277c

This article describes the invention of a class of effective reagents [(phen)₂Cu](O₂CR_F) (1) for the decarboxylative perfluoroalkylation of aryl and heteroaryl halides. Treatment of copper tert-butyloxide with phenanthroline ligands, with subsequent addition of perfluorocarboxylic acids afforded air-stable copper(i) perfluorocarboxylato complexes 1. These complexes reacted with a variety of aryl and heteroaryl halides to form perfluoroalkyl(hetero)arenes in moderate to high yields. Computational studies suggested that the coordination of the second phen ligand may reduce the energy barrier for the decarboxylation of perfluorocarboxylate to facilitate perfluoroalkylation.

Full text of DOI:10.1039/c6dt00277c

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A class of effective decarboxylative
perfluoroalkylating reagents: [(phen)2Cu](O2CRF)†
Cite this: DOI: 10.1039/c6dt00277c
Yangjie Huang,a Manjaly J. Ajitha,b Kuo-Wei Huang,*b Zhongxing Zhangc and
Zhiqiang Weng*a
This article describes the invention of a class of effective reagents [(phen)2Cu](O2CRF) (1) for the decarb-
oxylative perfluoroalkylation of aryl and heteroaryl halides. Treatment of copper tert-butyloxide with
phenanthroline ligands, with subsequent addition of perfluorocarboxylic acids afforded air-stable
copper(I) perfluorocarboxylato complexes 1. These complexes reacted with a variety of aryl and heteroaryl
halides to form perfluoroalkyl(hetero)arenes in moderate to high yields. Computational studies suggested
that the coordination of the second phen ligand may reduce the energy barrier for the decarboxylation of
perfluorocarboxylate to facilitate perfluoroalkylation.
Received 20th January 2016,
Accepted 13th April 2016
DOI: 10.1039/c6dt00277c
www.rsc.org/dalton
Introduction
The development of efficient methods for organofluorine com-
pound synthesis is a highly active research area in organic
chemistry.1–3,4–7 The incorporation of perfluoroalkyl groups
into bioactive molecules often dramatically alters their physi-
cal and chemical properties, and biological activities which
has led to the widespread use of fluorine-containing com-
pounds in the fields of medicinal chemistry8–10,11,12 and
materials science.13 For instance, pentafluoroethyl groups are
present in many bioactive compounds and drugs, such as the
angiotensin II receptor antagonist (DuP 532),14 the antihyper-
tensive K+ channel opener (KC-515),15 perfluoroalkyl substi-
tuted aniline insecticides16 and nucleobases17 (Scheme 1). The
exploration of efficient perfluoroalkylation reactions is there-
fore of great urgency.
Scheme 1 Bioactive compounds containing perfluoroalkyl groups.
perfluoroalkyl moieties, such as the C2F5 group, into organic
molecules (Scheme 2).36–38 For instance, Hartwig and co-
workers successfully employed stable (phen)Cu(C2F5), pre-
pared from the reaction of CuOt-Bu with C2F5SiMe3, for
pentafluoroethylation of arylboronate esters and heteroaryl
bromides.39–41 Grushin’s group reported the ligandless
Cu(C2F5) in situ prepared by the direct cupration of economical
C2F5H42 and Mikami’s group later reported the same reagent
synthesized from ethyl pentafluoropropionate.43 The CuC2F5
reagent was demonstrated as an exceedingly versatile
pentafluoroethylating reagent for a variety of substrates,
including arylboronic acids and aryl bromides. Despite the
popularity of these methods, some of them suffered from the
use of expensive perfluoroalkylsilane sources such as
C2F5SiMe3 or gaseous C2F5H (boiling point: −48.5 °C), or the
requirement of the addition of Et3N·3HF to enhance the
reaction efficiency. An alternative approach to CuC2F5 is the
decarboxylation of low-cost and easy-to-handle alkali metal
Transition metal-mediated or -catalyzed trifluoromethyl-
ation has emerged as an attractive new strategy for C–CF3
bond formation from arenes, aryl halides, arenediazonium
salts, and the corresponding boronic acids.18–25,26–35 There
are, however, few examples of the introduction of longer-chain
aState Key Laboratory of Photocatalysis on Energy and Environment, College of
Chemistry, Fuzhou University, Fuzhou, 350108, China. E-mail: zweng@fzu.edu.cn
bKAUST Catalysis Center and Division of Physical Sciences & Engineering,
King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900,
Saudi Arabia
cInstitute of Materials Research and Engineering, A*STAR (Agency for Science,
Technology and Research), 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634,
Singapore
†Electronic supplementary information (ESI) available: Experimental procedures
and spectral data for products and mechanistic study experiments. X-ray crystal-
lography details. CCDC 1427126 and 1427127. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c6dt00277c
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Scheme 3 Synthesis of copper(I) perfluoroacetato complexes 1a–d.
Scheme 2 Copper reagent for pentafluoroethylation of aryl halides and
Crystallization of 1a by vapor diffusion of diethyl ether into
a solution of CH3CN afforded dark red crystals suitable for
X-ray analysis. Crystals of complex 1b·C6H6 were obtained by
slow evaporation of a CH3CN/benzene solution at room temp-
erature. The molecular structures of 1a and 1b are shown in
Fig. 1 and 2, respectively. Each of these complexes contains a
cationic tetrahedral CuI center ligated by two phen ligands and
a free, unligated anionic perfluorocarboxylato group.
boronic acids.
pentafluoropropionates or methyl pentafluoropropionate.44–47
Unfortunately, these reactions have a number of limitations,
such as the requirement of high temperatures (150–180 °C),
lack of practicability, and poor substrate scope, despite their
economical nature.48
To evaluate the potential of these perfluorocarboxylato com-
plexes to be reactive reagents in decarboxylative perfluoroalkylation,
Besides the C2F5 group, the incorporation of other higher
fluorinated functional groups onto an aromatic core structure
has also gained exceptional attention.49–54 Transition metal-
catalyzed perfluoroalkylation of arenes and heteroarenes
has proven to be a useful and versatile method for their
synthesis.55–64 However, there still remain a number of limit-
ations. In particular, some of the approaches reported are of
limited reaction scope with respect to the even carbon number
CnF2n+1I (n = 2, 4, 6, 8, 10) species, and expensive reagents are
typically needed. Therefore, the development of general and
economical reagents for perfluoroalkylation is highly desirable.
In this context, we recently reported a copper(I) trifluoro-
acetate complex (phen)Cu(O2CCF3) for the decarboxylative
trifluoromethylation of (hetero)aryl halides,65 and the related
copper reagents for the synthesis of fluorinated organic
compounds.66–69 Herein, we report the synthesis of complexes
[(phen)2Cu](O2CRF), and demonstrate their applications in the
perfluoroalkylation of (hetero)aryl halides to furnish perfluoro-
alkylarenes in good yields.
Fig. 1 ORTEP structure of complex 1a with thermal ellipsoids set at
40% probability. Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (°): Cu(1)–N(1) 2.071(2), Cu(1)–N(2) 2.007(7),
Cu(1)–N(3) 2.012(4), Cu(1)–N(4) 2.062(2), C(1)–O(1) 1.221(4), C(1)–O(2)
1.209(4), C(1)–C(2) 1.599(6), N(1)–Cu(1)–N(2) 82.24(7), N(1)–Cu(1)–N(3)
109.23(7), N(2)–Cu(1)–N(4) 110.62(7), N(3)–Cu(1)–N(4) 82.20(7), O(1)–
C(1)–O(2) 130.9(3), O(1)–C(1)–C(2) 104.2(3), O(2)–C(1)–C(2) 123.8(3).
Results and discussion
We started our studies by the synthesis of copper(I)–O2CRF
complexes. The copper species 1a was generated by the reac-
tion of CuOt-Bu (prepared in situ from the reaction of CuCl
with NaOt-Bu) with two equiv. of phen ligand, with subsequent
addition of perfluoropropionic acid (1.0 equiv.) in THF at
room temperature. Complex 1a was isolated as a reddish
brown solid in good yield (79%). Its higher fluorinated ana-
logues 1b–d were also synthesized via the reactions with the
respective perfluorocarboxylic acids (Scheme 3). These com-
plexes containing longer chains of fluorinated groups were iso-
lated as reddish brown crystalline solids in slightly lower yields
(50–60%), probably due to their good solubility in solvents. All
of these complexes can be prepared on a multi-gram scale
(4–5 g) in the laboratory. All of them are air- and moisture
stable in the solid state and readily soluble in polar organic
solvents such as DMF, DMSO, CH3CN, and CH2Cl2.
Fig. 2 ORTEP structure of complex 1b with thermal ellipsoids set at
40% probability. Hydrogen atoms are omitted for clarity.
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we examined their reactions with aryl iodides in dioxane at catalyzed cross-coupling reactions. Furthermore, the use of
130 °C for 16 h (see the ESI†). The reagent proved to be 3-iodo-N-phenylcarbazole gave the corresponding pentafluoro-
broadly applicable and the desired products were generated in ethylated product 3o in good yield (83%). It should be empha-
moderate to high yields. 4-tert-Butyl-, 3,5-dimethyl-, or sized that heteroaromatic substrates, such as 2-iodopyridines,
4-phenyl-substituted phenyl iodides reacted readily with 1a to 2-iodoquinoline, and 2-iodoquinoxaline, were also suitable
give the desired pentafluoroethylated products 3a–c in 82%, reaction partners, affording the corresponding products 3p–t
74%, and 92% yields, respectively (Scheme 4). The sterically in 51–80% yields. Furthermore, an estradiol derivative reacted
hindered 2-iodobiphenyl provided the product 3d in 39% to form the pentafluoroethylated product 3u in modest yield.
yield. 1-Iodonaphthalene and 2-iodo-9H-fluorene underwent
To expand the scope of the transformation, we next
the reaction to furnish 3e and 3f in good yields. Reactions with embarked on an exploration of the incorporation of longer
electron-withdrawing aryl iodides also gave the desired pro- chain perfluoroalkyl groups into complexes 1b–d. A wide range
ducts 3g and 3h, in 39% and 62% yields, respectively. of aryl iodides exhibited excellent reactivity, including those
Additionally, aryl iodides containing an ester group were also with electron-neutral, electron-donating, electron-withdrawing
tolerated in this reaction to give the desired products (3i and and para, meta, or ortho substituents (Scheme 4). For sub-
3j, respectively) in good yields, and no transesterified by- strates with electron donating substituents, a higher reaction
products were detected. The electron-rich aryl iodides reacted temperature (140 °C) was required to reach full conversion. In
smoothly to give the desired products 3k–m in 61–87% fact, the decarboxylative perfluoropropylation, perfluorobutyl-
yields. 1-Chloro-3-iodobenzene underwent pentafluoroethylation ation, and perfluoropentylation provided the corresponding
smoothly to give the corresponding product 3n in 67% yield arene products 4a–4o, 5a–5o, and 6a–6o in similar yields to
with the chloro substituent intact. The ability to incorporate a those of pentafluoroethylation. Heteroarenes proved to be suit-
chloro substituent into the products provides opportunities for able substrates as well, delivering the synthetically useful pro-
further functional manipulations through transition-metal- ducts 4p–4t, 5p–5t, and 6p–6t.
The synthetic utility of the developed methodology was also
demonstrated by the synthesis of perfluoroalkyl substituted
aniline insecticides. The perfluoroalkylation of 4-iodo-2-
methylaniline 2v with 1a and 1b provided 3v and 4v in 53%
and 61% yields, respectively (Scheme 5). Compounds 3–4v
could readily be transformed into phthalic acid diamide
derivatives following the reported procedures.70
To gain a better understanding of the reaction mechanism,
density functional theory (DFT) was employed using the Gaus-
sian 09 suite of quantum chemistry packages.71 All the geome-
tries were optimized at the standard B3LYP level of density
functional theory with the 6-31G(d) basis set for C, H, O, N
and F atoms and the LANL2DZ (Los Alamos National Labora-
tory 2 double ζ) basis set for Cu and I atoms.72,73 The energies
were further refined with the 6-311+G(d,p) basis set for all
atoms except Cu and I which were treated with the LANL2DZ
basis set. In order to account for missing London dispersion
effects and basis set superposition error, dispersion (D3) cor-
rections including the Becke–Johnson (BJ) damping function
and geometrical counterpoise corrections (gCP) were also
added.74,75 The effect of dioxane solvent was considered using
the solvation model based on density (SMD).76 The free energy
values obtained at the B3LYP-SMD-gCP-D3(BJ)/6-311+G(d,p)//
B3LYP/6-31G(d) level of theory were used to discuss the ener-
getics in the reaction profile.
Scheme 4 Perfluoroalkylation of aryl iodides with 1a–d. Conditions:
1a–d (0.75 mmol), 2 (0.50 mmol), dioxane (5.0 mL), 16 h, N2, 130 °C.
Yields of isolated products are shown. a 19F NMR yield. b Conducted in a
2.7 mmol scale reaction.
Scheme 5 Formal synthesis of perfluoroalkyl substituted aniline
insecticides.
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Fig. 3 Free energy profile for the decarboxylation pentafluoroethylation of 4-iodobenzonitrile (ArI) with [phen2Cu][O2CC2F5]. Bond distances are
given in Å.
The mechanism can be considered to begin with the copper(I) materials. A series of copper(I) perfluorocarboxylato complexes
perfluorocarboxylate complex as the starting complex (a) supported by phenanthroline ligands have been prepared by
(Fig. 3). The previous DFT studies mainly focused on distorted treating CuOt-Bu and the phen ligand with perfluorocarboxylic
tetrahedral transition states with one phen ligand for the Cu- acids. The molecular structures of these complexes have been
catalyzed decarboxylation reaction,77–80 but we observed that an determined by X-ray diffraction. These air-stable complexes
octahedral transition state with two phen ligands is much more can serve as an efficient reagent for the decarboxylative per-
favored. The activation barrier for the decarboxylation (extru- fluoroalkylation of aryl and heteroaryl halides to form perfluoro-
sion of CO2) was found to be 27.6 kcal mol−1 with two coordi- alkyl (hetero)arenes of high value. DFT methods indicate that
nating phen ligands (via TS1). In contrast, the dissociation of the coordination of the second phen ligand may reduce the
one of the ligands from the Cu(I) complex resulted in a signifi- energy barrier for the decarboxylation of perfluorocarboxylate
cant increase (ΔΔG‡ = 7.7 kcal mol−1) in the activation barrier to facilitate perfluoroalkylation.
for the decarboxylation to 35.3 kcal mol−1 (via TS1′). The metal
center of TS1 appears to be more electron rich (Mulliken charge
= 0.142) compared to that of TS1′ (Mulliken charge = 0.246).
The initial ArI bond breaking before the decarboxylation was
Experimental
also modeled but a much higher activation barrier was com- General information
puted (46.8 kcal mol−1). In the case of ethyl carboxylate, the
1H NMR, 19F NMR and 13C NMR spectra were recorded using a
activation barrier was 34.8 kcal mol−1, in agreement with the
Bruker AVIII 400 spectrometer. 1H NMR and 13C NMR
less reactivity of Cu(I) ethylacetate. After the decarboxylation,
chemical shifts were reported in parts per million (ppm)
the dissociation of one of the phen ligands resulted in a
downfield from tetramethylsilane and 19F NMR chemical
more stable tricoordinate intermediate complex c (ΔGrel
=
shifts were determined relative to CFCl3 as the external
standard (0.00 ppm). Coupling constants (J) are reported in
Hertz (Hz). The residual solvent peak was used as an internal
−6.2 kcal mol−1) which underwent the oxidative addition of the
Ar–I bond to Cu with an overall activation barrier of 33.5 kcal mol−1
via TS2 (rate determining step). A less stable pentacoordinate
intermediate e was formed followed by an almost barrierless
C–C bond coupling (via TS3) to reductively release the product
1
reference: H NMR (chloroform δ 7.26) and 13C NMR (chloro-
form δ 77.0). The following abbreviations were used to explain
the multiplicities: s = singlet, d = doublet, t = triplet, q =
quartet, m = multiplet, br = broad. HRMS were obtained on a
Waters GCT-TOF at the Shanghai Institute of Organic
Chemistry. 2-Iodo-3,17-dimethoxy-β-estra-1,3,5(10)-triene was
prepared according to the published procedures.81 Other
f. Overall, the reaction is exergonic by 38.2 kcal mol−1
.
Conclusions
In summary, we have invented an effective reagent [(phen)2Cu] reagents were received from commercial sources. Solvents were
(O2CRF) from readily available and inexpensive starting freshly dried and degassed according to the published
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procedures prior to use. Column chromatography purifications C28H16CuF7N4O2·C6H6: C 57.11, H 3.10, N 7.83. Found:
were performed by flash chromatography using Merck silica C 57.32, H 3.19, N 8.06.
gel 60.
Synthesis of [(phen)2Cu][O2CC2F5] (1a).
A solution of
NaOt-Bu (345 mg, 3.6 mmol) in 10.0 mL of THF was added to
a suspension of CuCl (297 mg, 3.0 mmol) in 60 mL of THF,
and the resulting mixture was stirred at room temperature for
120 min. The resulting light yellow mixture was filtered
through a layer of Celite. To this filtrate was added a solution
of 1,10-phenanthroline (1080 mg, 6.0 mmol) in 10 mL of
THF. The resulting solution turned reddish brown immedi-
ately and was stirred at room temperature for an additional
5 min. A THF solution (1 mL) of C2F5CO2H (492 mg,
3.0 mmol) was added dropwise and the mixture was further
stirred at room temperature for 20 min. The solution was
filtered, and the filtrate was dried under vacuum to yield a
dark red solid. The resulting solid was washed with 2 × 2 mL
of diethyl ether and dried under vacuum to obtain 1.38 g
(79%) of 1a. 1H NMR (400 MHz, DMSO-d6) δ 9.02 (d, J = 3.7 Hz,
4H, H1), 8.82 (dd, J = 8.1, 1.1 Hz, 4H, H3), 8.27 (s, 4H, H4), 7.99
(dd, J = 8.1, 4.7 Hz, 4H, H2). 19F NMR (376 MHz, DMSO-d6)
δ −81.5 (t, J = 1.4 Hz, 3F, CF3), −118.1 (d, J = 1.4 Hz, 2F, CF2).
13C NMR (101 MHz, DMSO-d6) δ 158.3 (t, J = 21.9 Hz),
149.8 (s), 143.5 (s), 137.7 (s), 129.4 (s), 127.5 (s), 126.2 (s),
120.0 (qt, JCF = 286.4, 36.4 Hz, CF3), 107.6 (tq, JCF = 266.6,
35.4 Hz, CF2). Elemental Analysis (%) calculated for
C27H16CuF5N4O2: C 55.25, H 2.75, N 9.54. Found: C 55.47,
H 2.89, N 9.77.
General procedure for perfluoroalkylation of aryl iodides
In a dry box, [phen2Cu][O2CRF] (0.75 mmol), aryl or heteroaryl
halides (0.50 mmol), and 5 mL dioxane were added to a oven
dried 25 mL test tube with a Teflon screw cap. The tube was
sealed and the solution was stirred at 130 °C for 16 h. Then
the reaction mixture was filtered through a layer of Celite,
eluted with diethyl ether. The resulting mixture was extracted
with ethyl ether (20 mL × 3), and the combined organic layers
were washed with water (60 mL × 3), and then dried over mag-
nesium sulfate. The solvent was removed by rotary evaporation
and the resulting product was purified by column chromato-
graphy on silica gel with n-pentane/Et2O.
Acknowledgements
Financial support from the National Natural Science Foun-
dation of China (NSFC) (grant number 21372044), the
Research Fund for the Doctoral Program of Higher Education
of China (grant number 20123514110003), and Fuzhou Univer-
sity (grant number 022494) is gratefully acknowledged.
Notes and references
Synthesis of [(phen)2Cu][n-C3F7CO2] (1b). A solution of
NaOt-Bu (345 mg, 3.6 mmol) in 10 mL of THF was added to a
suspension of CuCl (297 mg, 3.0 mmol) in 60 mL of THF, and
the resulting mixture was stirred at room temperature for
120 min. The resulting light yellow mixture was filtered
through a layer of Celite. To this filtrate was added a solution
of 1,10-phenanthroline (1080 mg, 6.0 mmol) in 10 mL of THF.
The resulting solution turned reddish brown immediately and
was stirred at room temperature for an additional 5 min. A
THF solution (1 mL) of n-C3F7CO2H (642 mg, 3.0 mmol) was
added dropwise and the mixture was further stirred at room
temperature for 20 min. The solution was filtered, and the fil-
trate was dried under vacuum to yield a dark red solid. The
resulting solid was washed with 2 × 2 mL of diethyl ether and
dried under vacuum to obtain 1.14 g (60%) of 1b. 1H NMR
(400 MHz, DMSO-d6) δ 9.01 (d, J = 3.7 Hz, 4H, H1), 8.82 (dd, J =
8.1, 1.0 Hz, 4H, H3), 8.27 (s, 4H, H4), 7.99 (dd, J = 8.1, 4.7 Hz,
4H, H2). 19F NMR (376 MHz, DMSO-d6) δ −80.2 (t, J = 8.5 Hz,
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