Activation of aryl halides by Cu0/1,10-phenanthroline: Cu 0 as precursor of CuI catalyst in cross-coupling reactions

Article abstract of DOI:10.1039/b814364a

The activation of aryl iodides and bromides by Cu⁰/1,10- phenanthroline in acetonitrile (S) generates Cu^I(phenanthroline) S₂⁺ which can be a catalyst for cross-coupling reactions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b814364a

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Activation of aryl halides by Cu0/1,10-phenanthroline: Cu0 as precursor
of CuI catalyst in cross-coupling reactionsw
Mounir Mansour,a Roberto Giacovazzi,a Armelle Ouali,b Marc Tailleferb and
Anny Jutand*a
Received (in College Station, MA, USA) 19th August 2008, Accepted 29th September 2008
First published as an Advance Article on the web 22nd October 2008
DOI: 10.1039/b814364a
The activation of aryl iodides and bromides by Cu0/1,10-
phenanthroline in acetonitrile (S) generates CuI(phenanthroline)
+
S2 which can be a catalyst for cross-coupling reactions.
Copper-catalyzed arylations of N- or O-nucleophiles were
reported in the early 20th by Ullmann and Golberg, however
at high temperatures.1,2 In 2001, important improvements
were achieved by Taillefer et al.3a,b and Buchwald et al.3c with
the discovery of very efficient versatile new copper/ligand
systems allowing the use of catalytic amounts of CuII, CuI
or Cu0 under mild conditions.2,3 The mechanism of these
cross-coupling reactions is however not fully established.4
There is a general agreement for considering that CuI is the
real catalyst generated from either CuII or Cu0 precursors by
an in situ chemical reduction or oxidation respectively.4 In our
Fig. 1 Cyclic voltammetry performed in acetonitrile containing
search to get evidence of a catalytic cycle induced by the
nBu4NBF4 (0.3 M) at
a steady glassy carbon disk electrode
reaction of CuI species with aryl halides by means of electro-
chemical techniques, we have discovered that an electrogener-
ated Cu0 complex is at the origin of the formation of CuI
through its activation of aryl halides by electron transfer. This
reaction explains why Cu0 can be used as a precursor in
CuI-catalyzed cross-coupling reactions.2
(d = 3 mm), at the scan rate of 0.5 V sÀ1, at 20 1C. (a) ReductioÀn
of Cu(CH3CN)4+PF6À (2 mM); (b) Reduction of Cu(CH3CN)4+PF6
(2 mM) in the presence of 1,10-phenanthroline (1 equiv.); (c) (dashed
À
line) Reduction of Cu(CH3CN)4+PF6 (2 mM) in the presence of
phenanthroline (2 equiv.).
The ligands 1,10-phenanthroline or 4,7-substituted-1,10-
phenanthroline associated with CuI (Cu2O, CuBr, CuI, CuOTf,
Cu(CH3CN)4PF6) have been used to catalyze C–C, C–O or
C–N cross-couplingÀreactions.2,4g,k,5 The cyclic voltammetry of
CuI(CH3CN)4+PF6 (2 mM) was performed in CH3CN (con-
taining nBu4NBF4, 0.3 M). It exhibited an irreversible reduc-
tion peak at Epred = À0.681 V vs. SCE (Fig. 1(a)). The reduction
generated a naked ‘‘Cu0’’ which deposited at the electrode
surface and was characterized by an irreproducible broad
adsorption oxidation wave at ca. À0.2 V (Fig. 1(a)). After
In the presence of more phenanthroline (up to 5 equiv.), the
reduction peak current at R1 slightly increased as well as that
of R2 to reach a limit. The reduction peak current of R3 also
increased. R3 characterizes the first reduction step of phenan-
throline (comparison to an authentic sample).
According to literature, two successive equilibrium are
involved in acetonitrile (Scheme 1).6
From the value of K1 (Scheme 1), it emerges that 97% of
CuS4+ was converted to CuI(phen)S2+ when phen (2 mM) was
added to CuS4+ (2 mM). Consequently the reduction peak R0
observed as a shoulder characterizes CuS4+ (less easily reduced
than in the absence of ligand (phenanthroline) because of
equilibrium (1)7) while the reduction peak R1 characterizes
Cu(phen)S2+. R3 characterizes the reduction of the phenan-
throline in the equilibrium (1) in Scheme 1. The relative
reduction peak currents at R0, R1 and R3 did not necessarily
reflect the thermodynamic concentration of the three species in
equilibrium (1) because the equilibrium might be shifted due to
the consumption of the species by the reduction process in the
diffusion layer at low scan rate (CE mechanism).7 Indeed, R0
was no longer observed at higher scan rates (Fig. 2(b)). The
addition of 1,10-phenanthroline (phen,
1
equiv.), the
reduction peak of CuS4 (S = CH3CN) disappeared leading
to complex voltammagram, involving four successive
irreversible reduction peaks: R0 (ERp 0
+
a
=
À1.54 V),
R1 (EpR1 = À1.65 V), R2 (ERp 2 = À1.82 V, hardly detectable),
and R3 (EpR3 = À2.03 V) (Fig. 1(b)). Addition of a second equiv.
of phenanthroline led to the disappearance of R0, an increase of
the reduction peak currents at R1, R2 and R3 (Fig. 1(c)).
a Ecole Normale Supe´rieure (ENS), De´partement de Chimie, UMR
CNRS, 8640, 24 Rue Lhomond, F-75231 Paris Cedex 05, France.
E-mail: Anny.Jutand@ens.fr; Fax: 33144322402; Tel: 33144323872
b Institut Charles Gerhardt Montpellier, AM2N, ENSCM, 8 rue de
l’Ecole Normale, F-34296 Montpellier Cedex 5, France
+
reduction of Cu(phen)S2 at R1 was irreversible at low scan
rates and a badly defined oxidation peak was observed on the
reverse scan at Ox (Fig. 2(a)). The reduction peak R1 became
partly reversible at higher scan rates while the oxidation peak
w Electronic supplementary information (ESI) available: Experimental
procedure and discussion. See DOI: 10.1039/b814364a
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was fully dissociated to ionic Cu(CH3CN)4 and IÀ (conduc-
+
tivity: 110 mS cmÀ1 for CuI (2 mM)).8 Consequently, CuI
+
behaved as the cationic Cu(CH3CN)4 in acetonitrile.
The reactivity of complex CuI(phen)S2+À formed by addition
of phen (2 mM) to Cu(CH3CN)4+PF6 (2 mM) with aryl
halides was investigated in acetonitrile. If a fast reaction
occurred, the reduction peak current at R1 should decrease
since currents are proportional to the concentrations of
electroactive species.7 Surprisingly, the reverse situation was
observed: the reduction peak current at R1 increased as the
concentration of PhI was increased whereas the oxidation
peak Ox on the reverse scan assigned to the oxidation of an
electrogenerated {Cu0} moiety disappeared (Fig. 3).
Scheme 1
current of Ox decreased (Fig. 2(b)). The unsaturated complex
Cu0(phen) generated in the reduction of CuI(phen)S2+ was not
stable and decomposed to a badly defined {Cu0} species
oxidized at Ox. Cu0(phen) was however intercepted at
shorter times (higher scan rates) and was oxidized back to
CuI(phen)S2 at O1 (Fig. 2(b), Scheme 2 left).
+
Addition of a second equiv. of phenanthroline achieved to
+
convert all CuS4 into CuI/phen complexes (R0 no longer
Consequently, the reduction of CuI(phen)S2+ at R1 generated
Cu0(phen) which reacted with PhI to regenerate the initial
detected in Fig. 1(c)). The reduction peak current at R1 did not
decrease upon addition of more than two equiv. of phenanthro-
line. Consequently, R1 characterizes the reduction of the two
complexes CuI(phen)S2+ and CuI(phen)2+ involved in a fast equi-
librium (right part of Scheme 2). From the value of K2,6 it emerges
that Cu(phen)2+ was the main complex when phen (4 mM) was
+
CuI(phen)S2 and a catalytic reduction current was observed
at R1 which increased as the PhI concentration was increased
(Fig. 3(b)). It is worthwhile to note that the direct reduction of
PhI occurred at a more negative potential (Table 1).9 For a given
PhI concentration, the catalytic effect at R1, expressed by
the ratio ipR1(PhI)/iRp 1(0) (ipR1(PhI): reduction peak current at R1 in
the presence of PhI; iRp 1(0): reduction peak current at R1 in the
absence of PhI), was less important when the scan rate or
phenanthroline concentration was increased. At high scan rates
(shorter times), the concentration of Cu0(phen) remained high
because it had less time to decompose to {Cu0} but on the other
hand, it had less time to react with PhI. This competition seems
to be in favor of the latter effect. A catalytic current at R1 was
observed for 4-Z–C6H4–X (Z = CN, H, MeO, X = I, Br) but
not for 4-NC–C6H4–Cl. All ArX were reduced at potentials more
negative than that of R1 (Table 1). Therefore, the Cu0(phen)
added to CuS4 (2 mM) ([Cu(phen)2+]/[Cu(phen)S2+] = 89/11
+
+
at 20 1C). The mixture CuS4 (2 mM) and phenanthroline
(10 mM) was investigated at various scan rates (from 0.5 to
1000 V sÀ1). The ratio iRp 2/iRp 1 decreased when the scan rate
increased, while R1 became partly reversible. This indicates that
the species reduced at R2 was not directly generated by the
+
reduction of CuI(phen)2 to Cu0(phen)2 at R1 but characterized
a badly defined {Cu0{ species formed in the chemical evolution of
Cu0(phen)2 (Scheme 2). Such complex is not a dimer since the
ratio ipR2/iRp 1 was not affected by the initial CuI concentration.
The addition of phenanthroline to CuI used as precursor in
catalytic reactions,4 led to similar voltammograms since CuI
complex generated in the reduction of CuI(phen)S2 could
+
activate aryl iodides and bromides (even deactivated by EDG
groups) but could not activate aryl chlorides (even activated by
an EWG group as CN) during the potential scan. Exhaustive
electrolyses of solutions containing ArI or ArBr (0.5 mmol),
Cu(CH3CN)4PF6 (10 mol%) and phenanthroline (10 mol%) in
12 mL of acetonitrile, performed at the reduction potential of R1
À
2
Cyclic voltammetry of Cu(CH3CN)4+PF6 (2 mM) in
Fig.
the presence of 1,10-phenanthroline (2 mM), in acetonitrile containing
nBu4NBF4 (0.3 M) at
a steady glassy carbon disk electrode
(d = 1 mm) at the scan rate of: (a) 5 V sÀ1; (b) 100 V sÀ1 at 20 1C.
Fig. 3 (a) Cyclic voltammetry of Cu(CH3CN)4+PF6À (2 mM) in the
presence of 1,10-phenanthroline (2 mM), in acetonitrile containing
nBu4NBF4 (0.3 M) at a steady glassy carbon disk electrode (d = 3 mm)
at the scan rate of 0.5 V sÀ1, at 20 1C. (b) After successive additions of
PhI (the numbers indicate the cumulative number of equivalents
of added PhI). The current scale has been divided by 4 when going
from (a) to (b).
Scheme 2
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supported by DFT calculations by Liu, Guo et al.,4f and very
recently established by Hartwig et al.,4k for N-nucleophiles.
In conclusion, cross-coupling reactions of ArX (X = I, Br)
and nucleophiles can be performed by using a Cu0 precursor
which will be transformed in situ into a CuI species by its reaction
with ArX at the very beginning of the catalytic reactions.
Table 1 Reduction peak potential Epred of ArX (X = I, Br, Cl) in
acetonitrile. Results of the electrolyses performed in the presence of
catalytic Cu(CH3CN)4PF6 and 1,10-phenanthroline.
ArH ArX
Ce F/molf (%)g (%)h
b
c
electd
red
ArXa
Epred (E0red
)
E
4-CNC6H4I
C6H5I
4-CH3C6H4I
4-CH3OC6H4I
4-NCC6H4Br
À2.00 (À1.81) À1.5 106 2.0
À2.51 (À1.91) À1.5 112 2.11
78
—
0
0
0
2
0
i
À2.43
À2.26
À1.5 115 2.17
À1.5 106 2.0
12i
76
80
54
0
À2.11 (À1.94) À1.5 123 2.3
Moreover, such reaction can be used for preparative purpose
since the electrogenerated Cu0(phenanthroline) catalyses the
selective electrochemical reduction of ArX (X = I, Br) to ArH.
We thank ANR (ANR-07-CP2D-08-03), the Centre National
de la Recherche Scientifique and the Ministere de la Recherche
4-CH3OC6H4Br oÀ2.8
À1.5 119 2.24
0
100
4-NCC6H4Cl
À2.16 (À2.03) À1.5
26 0.49
a
ArX = 0.5 mmol in 12 mL of acetonitrile containing nBu4NBF4
b
(0.3 M). Peak potentials vs. SCE at a steady glassy carbon disk electrode
(d = 1 mm) at the scan rate of 0.5 V sÀ1, 20 1C. Standard reduction
c
potentials of ArX vs. SCE in DMF at 20 1C.9b d Cathodic electrolysis
potential. C: charge passed through the cell expressed in Coulomb
(Ecole Normale Superieure) for supporting this work.
´
e
(theoretical charge: 106 C). f Faraday per mol. Isolated yield.
g
Notes and references
h
i
Recovered. ArH was isolated in low yield due to low boiling point.
1 (a) F. Ullmann, Ber. Dtsch. Chem. Ges., 1903, 36, 2382; (b) I.
Goldberg, Ber. Dtsch. Chem. Ges., 1906, 39, 1691–1692.
(À1.5 V) delivered after consumption of 2 electrons per mole of
ArX the corresponding ArH after fast protonation of ArÀ by the
solvent (Table 1). When an electrolysis was performed from
4-CN–C6H4–Cl at À1.5 V, the reduction current dropped to zero
2 (a) S. V. Ley and A. W. Thomas, Angew. Chem., Int. Ed., 2003, 42,
5400; (b) I. P. Beletskaya and A. V. Cheprakov, Coord. Chem.
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2833947-WO 0353225 (Pr. Nb. Fr 2001 16547), 2001; (b) S. L.
Buchwald, A. Klapars, J. C. Antilla, G. E. Job, M. Wolter, F. Y.
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(Pr. Nb. US 2001 0286268), 2001; (c) M. Taillefer, H.-J. Cristau,
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03101966 (Pr. Nb. Fr 2002 06717), 2002.
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+
just after the reduction of the complex CuI(phen)S2 and no
PhCN was produced in agreement with the absence of catalytic
current observed on the cyclic voltammogram.
Therefore, the electrogenerated Cu0(phen) complex cata-
lyses the reduction of ArX (X = I, Br) to ArÀ at a potential
which is less negative than the reduction potentials of ArX
(Table 1). Such reactions generate a CuI complex after activa-
tion of the aryl halide by electron transfer in an inner sphere
mechanism (see ESIw).
This electrochemical reduction of ArX to ArH catalysed by
a Cu0 moiety is unprecedented. The reaction is selective since
no biaryl ArAr is formed. This contrasts with the reported
electrochemical reduction of ArX in the presence of Pd or Ni
catalysts which do not provide any arene ArH (except as by-
product) but the homocoupling product ArAr.10 This is a
consequence of the formation of Ar–PdII–X or Ar–NiII–X
complexes by oxidative addition of ArX to electrogenerated
Pd0 or Ni0 complexes, respectively.10
All our attempts to characterize Ar–CuIII complexes formed
by oxidative addition of ArX to the cationic CuI(phen)S2
+
failed. The reaction of 4-NC–C6H4–I with the neutral yellow
complex CuBr(PPh3)(phen) in reflux toluene provided an
elusive pale green complex whose ESI MS revealed the for-
mation of 4-NC–C6H4–CuIII–Br(phen)+ by an oxidative ad-
dition. But the reaction has never been reproduced. Cationic
CuI complexes must be a priori poor candidates for the
activation of Ar–X bond (X = I, Br, Cl) by oxidative addition
owning to their cationic character.11 However, DFT calcula-
tions by Liu, Guo et al.4f suggest that the oxidative addition
could take place from a cationic CuI. In the presence of a
nucleophile (NuÀ or NuH + base), neutral complexes
Nu–CuILn must be formed and be good candidates for the
oxidative addition of ArX, as proposed by Buchwald et al.,4g,j
7 A. J. Bard and L. R. Faulkner, Electrochemical Methods.
Fundamentals and Applications, Wiley, New York, 2001, 2nd edn.
8 (a) W. J. Geary, Coord. Chem. Rev., 1971, 7, 81; (b) A. Jutand, Eur.
J. Inorg. Chem., 2003, 2017.
9 (a) We are conscious that we are not comparing standard poten-
tials E0 but peak potentials Ep which slightly differ from standard
potentials7; (b) For comparison, the standard potentials E0 of ArX
determined in DMF are given in Table 1, see: R. J. Enemaerke,
T. B. Christensen, H. Jensen and K. Daasbjerg, J. Chem. Soc.,
Perkin Trans. 2, 2001, 1620.
10 For a recent review, see: A. Jutand, Chem. Rev., 2008, 108, 2300.
11 For some examples, see: (a) T. Osaka, K. D. Karlin and S. Itoh,
Inorg. Chem., 2005, 44, 410; (b) D. Maiti, A. A. N. Sarjeant, S. Itoh
and K. D. Karlin, J. Am. Chem. Soc., 2008, 130, 5644.
ꢀc
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Chem. Commun., 2008, 6051–6053 | 6053