Copper(I) Phenoxide complexes in the etherification of aryl halides

Article abstract of DOI:10.1002/anie.200902245

"Chemical Equation Presented" No copping out! Copper(I) phenoxide complexes containing chelating ligands (see picture), proposed intermediates in copper-catalyzed etherification of aryl halides, have been synthesized and fully characterized. The kinetic and chemical competence of the isolated complexes are demonstrated for the synthesis of aryl phenyl ethers, and experiments provide evidence against mechanistic pathways involving the formation of either free or caged radicals.

Full text of DOI:10.1002/anie.200902245

Angewandte
Chemie
DOI: 10.1002/anie.200902245
Cross-Coupling
Copper(I) Phenoxide Complexes in the Etherification of Aryl
Halides**
Jesse W. Tye, Zhiqiang Weng, Ramesh Giri, and John F. Hartwig*
Copper-catalyzed Ullmann ether synthesis has been studied
for many years because aryl ethers comprise important classes
of medicinally active compounds and agrochemicals.[1,2] The
traditional Ullmann-type reactions required high temper-
atures, use of polar and high-boiling solvents, and stoichio-
metric quantities of the copper salt. Recently, by combining
the copper with a variety of different ligands, milder catalytic
Ullmann reactions have been developed [Eq. (1)]. Examples
several distinct mechanisms for reactions of copper alkoxides
and aryloxides with aryl halides have been proposed. The
haloarene has been proposed to react with either anionic,
two-coordinate cuprates, such as [Cu(OR)2]À,[14] or neutral
copper alkoxides, such as CuOR.[15] The C X bond-cleavage
À
step has been proposed to occur either by oxidative addition
III
À
of the C X bond to yield a Cu intermediate or through a
one-electron transfer from the copper center to the haloarene
À
to yield a haloarene radical anion that undergoes C X
cleavage. Moreover, because phenoxy radicals are particu-
larly stable, reactions of copper aryloxide complexes could
occur through radical pathways that would be less accessible
to complexes containing other types of anionic ligands.
Herein we report the synthesis and structural identifica-
tion of copper phenoxide complexes containing ancillary
nitrogen-donor ligands, and the reactions of these species with
haloarenes, including haloarenes containing or serving them-
selves as radical probes.[10] These studies reveal unexpected
structures, demonstrate the competence of the isolated
complexes to be intermediates in the catalytic process,
provide arguments against the intermediacy of aryl radicals
formed by electron transfer to the aryl halides, reveal
quantitatively the effect of the electronic properties of the
aryloxide ligand on the reactivity of these species with
haloarenes, and reveal the relative rates for reactions of
haloarenes with copper phenoxide, amidate, and imidate
complexes.
of such ligands include phenanthrolines,[3,4] N,N-dimethyl
glycine,[5] various pyridine derivatives,[6] b-diketones,[7] and
1,1,1-tris(hydroxymethyl)ethane.[8]
Despite the progress toward improving the scope and
developing milder reaction conditions for the coupling of aryl
halides with phenoxides, a mechanistic basis for the relative
À
reactivities of different catalysts toward various C O cou-
pling processes has not been established. Over 35 years ago, it
was shown that the addition of the dative ligand pyridine
improved the yield of the reaction of copper(I) phenoxide
with phenyl bromide to produce Ph2O, but the species formed
from the coordination of pyridine was not isolated, and little
additional information has been gained on the reactivity of
copper phenoxide complexes containing dative ligands.[9]
Only recently have any isolated copper complexes been
evaluated as intermediates in copper-catalyzed coupling
reactions.[10,11] In one recent study, copper(I) imidates and
amidates were isolated in pure form, structurally character-
ized, and shown to be intermediates in related copper-
catalyzed Goldberg reactions,[10] and in another study kinetic
data were obtained on copper amidates generated in situ.[12,13]
The relationship between intermediates in copper-catalyzed
The synthesis of the copper aryloxide complexes in this
study containing 1,10-phenanthroline (phen), 2,9-dimethyl-
1,10-phenanthroline (Me2phen), and trans-N,N’-dimethyl-1,2-
cyclohexanediamine (dmcyda) is summarized in Scheme 1.
À
coupling reactions that form C N bonds and potential
À
intermediates in couplings that form C O bonds is unknown.
Scheme 1.
In the absence of clear information on the composition
and structure of intermediates in the Ullmann ether synthesis,
Treating CuCl with 1 equivalent of NaOPh and subsequent
addition of the dative ligand, led to the formation of
complexes 1a–1c containing phen, Me2phen, and dmcyda as
the ancillary dative ligand, and phenoxide as the anionic
ligand. These phenoxide complexes were isolated in 87–98%
yield.
[*] Dr. J. W. Tye, Dr. Z. Weng, Dr. R. Giri, Prof. Dr. J. F. Hartwig
Department of Chemistry, University of Illinois Urbana-Champaign
Urbana, IL 61801 (USA)
Fax: (+1)217-244-8024
E-mail: jhartwig@uiuc.edu
All CuI aryloxide complexes were characterized by
elemental analysis and NMR spectroscopy. The 1H NMR
spectrum of each complex revealed a 1:1 ratio of the dative
ligand to the phenoxide ligand, and all analytical data were
[**] We thank the NIH NIGMS (GM-55382) for support of this work.
Z.W. thanks the National University of Singapore for support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902245.
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consistent with this 1:1 ratio. However, the solid-state
After isolation and full characterization of the phenoxide
complexes 1a–c, we evaluated the potential of these com-
plexes to serve as intermediates in the copper-catalyzed
etherification of aryl halides. The reactions of 1a–c with
iodoarenes are summarized in [Eq. (2)]. Reaction of the
structures of these complexes differed from a simple neutral
species containing a 1:1:1 ratio of dative ligand, phenoxide,
and copper.
The solid-state structure of 1b was determined by X-ray
diffraction. These data show that 1b (Figure 1) consists of a
double salt containing one cationic tetrahedral copper center
phen-ligated complex 1a with 5 equivalents of p-iodotoluene
in DMSO formed the coupled product in 91% yield after
75 minutes at 1108C, and reaction of the Me2phen-ligated
complex 1b with 5 equivalents of p-iodotoluene in DMSO
formed the coupled product in 90% yield after 19 hours at
1108C. The dmcyda-ligated complex 1c reacted much faster.
The reaction of 1c with 5 equivalents of p-iodotoluene in
DMSO formed the coupled product in 95% yield after
15 minutes at 1108C in DMSO. These data imply that the
complexes containing the more electron-donating dative
ligand (1c) form ether products faster and in higher yields
than those containing the less electron-donating dative
ligands, 1a and 1b. This result parallels related observations
of the reactions of copper imidate and amidate complexes
with haloarenes. As expected, the increased steric effect of
the ortho,orthoꢀ-disubstituted Me2phen ligand in 1b caused
this complex to react more slowly than complex 1a containing
the unsubstituted phen ligand.
Figure 1. ORTEP diagram of 1b at 30% ellipsoids. Selected bond
lengths [ꢀ] and angles[8]: Cu(1)–N(1) 2.027(3), Cu(1)–N(2) 2.048(3),
Cu(1)–N(3) 2.050(3), Cu(1)–N(4) 2.028(3), Cu(2)–O(1) 1.816(4),
Cu(2)–O(2) 1.787(4); O(1)-Cu(2)-O(2) 177.81(19).[36]
ligated by two of the dative Me2phen ligands and one
relatively open, anionic, two-coordinate linear copper center
ligated by just phenoxide ligands. A related structure
containing a bis(imine) ligand was observed as part of a
structural study of copper phenoxide complexes,[16] and
certain CuI–imidate complexes, although more hindered,
were shown to exist as related ionic structures. Such ionic
structures of ligated copper alkoxides and aryloxides have
rarely, if ever, been considered in mechanistic proposals, and
they have not been structurally characterized with ligands
that create catalysts for Ullman etherifications.
To address the ambiguity in past studies about the relative
reactivity of neutral and anionic alkoxides and aryloxides, we
compared the reactions of the isolated anionic species
[Bu4N+][Cu(OPh)2 ] (2) with the neutral complexes. Com-
À
plex 2 was synthesized and isolated from the reaction of
[Cu(OtBu)]4, [NBu4 ][OPhÀ] and PhOH [Eq. (3)] and was
+
Conductivity was used to determine if these complexes
were present in solution in either the ionic form as seen in the
solid state or in the neutral form containing one dative ligand
and one phenoxide. The molar conductivity of 1.0 mm
solutions containing complexes 1a–c in dimethylsulfoxide
(DMSO) was high (37.1, 27.0, 31.9 WÀ1 cm2 molÀ1, respec-
tively), with respect to ferrocene (0.3 WÀ1 cm2 molÀ1) as a
neutral standard and [NBu4][BPh4] (23.5 WÀ1 cm2 molÀ1) as an
ionic standard. These data imply that each of the phenoxide
complexes exists predominantly in the ionic form in polar
solvents. The conductivity of a 65.5 mm tetrahydrofuran
(THF) solution of 1c was 6.8 mWcmÀ1 (0.104 WÀ1 cm2 molÀ1).
For comparison, the conductivity of a 65.5 mm THF solution
of [(n-octyl)4N][Br] was 65.1 mWcmÀ1 (0.99 WÀ1 cm2 molÀ1),
and the value for a 65.5 mm solution of ferrocene was
0.0 mWcmÀ1 (0.0 WÀ1 cm2 molÀ1). These data indicate that the
double salt and the neutral species depicted in [Eq. (1)] exist
in equilibrium and that more of the neutral form is present in
the less polar THF solvent than in the more polar DMSO
solvent.
characterized by 1H and 13C NMR spectroscopy and elemen-
tal analysis. Reaction of 2 with 5 equivalents of p-iodotoluene
for 16 hours at 1108C in DMSO proceeded with low
conversion and formed the biaryl ether product in only
about 10% yield (Scheme 2).[17] In contrast, the reaction of a
mixture of 2, 1 equivalent of the phen ligand, and 5 equiv-
alents of p-iodotoluene for 135 minutes at 1108C in DMSO
gave the ether product in 81% yield based on the number of
phenoxide ligands (Scheme 2). These results imply that the
ligated complexes produce aryl ethers more efficiently than
do the anionic species.
The faster reaction of the ligated species is consistent with
the effect of added ligand on the reactions of phen-ligated CuI
phenoxide complex 1a with aryl halides. If the unligated
species reacted with the aryl halide, the added ligand would
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selectivity of the metal for the two haloarenes. The reaction of
a 1:1 ratio of the two iodoarenes catalyzed by CuI alone gave
a different 68:32 ratio of the two ethers [Eq. (5)]. Thus, the
similarity in the ratios of the products formed from the single-
turnover and catalytic reactions indicates that complex 1a is
competent to be an intermediate in the reactions of aryl
iodides with NaOPh catalyzed by the combination of CuI and
phenanthroline.
To probe for the potential intermediacy of free aryl radical
intermediates and the potential for initial electron transfer, a
series of experiments with specific aryl halides were con-
ducted. To probe the potential of an electron-transfer
mechanism, we studied the reactions of phenoxide complex
1a with 4-chlorobenzonitrile and 1-bromonaphthalene. The
reduction potential of the chloroarene is more positive than
that of the bromoarene, and the rate of chloride dissociation
from the radical anion of 4-chlorobenzonitrile is known to be
similar to that for bromide dissociation from the radical anion
of 1-bromonaphthalene.[18,19] Therefore, reaction by an outer-
sphere electron-transfer mechanism to generate a free or
caged aryl radical should be faster with 4-chlorobenzonitrile
than with 1-bromonaphthalene. However, reaction by a
concerted oxidative addition to form an arylcopper(III)
intermediate with the bromoarene should occur faster than
that with the chloroarene.
Scheme 2.
be expected to retard reactions initiated with ligated complex
1a. However, reactions conducted with no added phenan-
throline, 1 equivalent of added ligand, and 10 equivalents of
added ligand occurred with rate constants of 4.1 ꢀ 10À4 sÀ1,
3.9 ꢀ 10À4 sÀ1, and 2.9 ꢀ 10À4 sÀ1, respectively. Although there
was some variation in the rate constants, the roughly 25%
change in the rate constant with a 10-fold increase in the
amount of free ligand is consistent with a mechanism in which
the ligated species reacts with the haloarene.
To evaluate further the potential intermediacy of the
phenoxide compounds in copper-catalyzed etherification, we
conducted competition reactions to measure the selectivity of
two iodoarenes toward 1a in the stoichiometric reaction of
the phenoxide complex with the iodoarenes [Eq. (4)], and in
The reactions of 1c in DMSO at 1108C after 3 hours with
4-chlorobenzonitrile and 1-bromonaphthalene produced the
corresponding ether products in 13% (3) and 43% (4) yield,
respectively. Consistent with this observation, the reaction of
1c with a mixture of 4-chlorobenzonitrile and 1-bromonaph-
thalene in DMSO at 1108C after 3 hours afforded the
corresponding ether products 3 and 4 in 5% and 36% yield,
respectively [Eq. (6)]. The low yield, presumably, results from
catalytic reactions of NaOPh with the two iodoarenes in the
presence of 10 mol% CuI with and without 20 mol% phen
[Eq. (5)], ]. We used the sensitivity of the reaction to steric
effects as a probe for the potential intermediacy of the
the steric hindrance of the pseudo-ortho substituent in the
naphthalene substrate. The higher reactivity of the bro-
moarene among this pair of haloarenes, nevertheless,
argues against an outer-sphere electron-transfer pathway
to form aryl radical anions that dissociate a halide to form
aryl radicals, either free or in a solvent cage.
Reactions of o-(allyloxy)iodobenzene were studied to
provide an additional probe for the formation of free aryl
radicals. The corresponding aryl radical is known to undergo
rapid cyclization to form a [3-(2,3-dihydrobenzofuran)]-
methyl radical with a rate constant of 9.6 ꢀ 109 sÀ1 in DMSO
with subsequent formation of 2-methyldihydrobenzofuran.[20]
Therefore, the formation of coupled product without forma-
tion of accompanying cyclization products would indicate that
the recombination of the radical with the copper aryloxide
isolated copper complexes. Consistent with the intermediacy
of these complexes in the catalytic process, the reaction of
complex 1a with a 1:1 mixture of of p-iodotoluene and o-
iodotoluene catalyzed by CuI and phen produced the two
ether products in an 86:14 ratio [Eq. (4)], whereas the
reaction of sodium phenoxide with a 1:1 mixture of the two
iodoarenes formed the two aryl ethers in a nearly equal 80:20
ratio [Eq. (5)]. The presence of the phen ligand affects the
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must occur with a rate constant greater than 1012 to 1013 sÀ1 if
between 0.1% and 1% of the cyclized product can be
detected.
The reaction of o-(allyloxy)iodobenzene with the p-
cresolate analogue of 1a (5) [Eq. (7)] and reaction of o-
(allyloxy)iodobenzene with p-CH3C6H4ONa catalyzed by CuI
amounts of the hydrodeiodination products 10 and 11 (7 and
8% yields, respectively), as determined by GC/MS analysis of
the reaction mixture and comparison to independently
synthesized compounds 8–11 [Eq. (9)].
and phen [Eq. (8)] produced a mixture of the p-tolyl aryl
ether 6 and phenyl allyl ether 7 as the sole products. The
Finally, the ability to isolate pure aryloxide complexes
which react with aryl halides allows a quantitative assessment
of the effect of the electronic properties of the aryloxide
ligand on the rate of the reaction with iodoarenes to form
biaryl ethers. The rates of reaction of 4-fluoroiodobenzene
with complexes containing para-hydrogen, para-methyl-,
para-trifluoromethyl-, para-fluoro-, and para-methoxy sub-
stituents (1a, 5, 12, 13, and 14, respectively) were determined
by 19F NMR spectroscopy. A plot of kobs versus s was linear
(R2 = 0.98) with a 1 value of À1.52 (see the Supporting
Information). These data indicate that the reaction is faster
when the reactive ligand is more electron rich, most likely
because it helps make the metal more electron rich and
thereby accelerates oxidative addition of the aryl halide.
These data allow one to compare the rates of reaction of
intermediates in Ullmann etherifications containing arylox-
ides with intermediates in Goldberg reactions containing
amides. The phen-ligated phenoxide complex reacts faster
than the phen-ligated phthalimidate (phth) complex [Cu-
(phen)2][Cu(phth)2] and more slowly than the phen-ligated
pyrrolidinonate (pyrr) complex [Cu(phen)2][Cu(pyrr)2] in the
same DMSO solvent. These relative rates fall in line with the
basicity of the anionic ligand, as judged by acidities of imides,
amides, and phenoxides in DMSO.[23]
In conclusion, we have isolated copper phenoxide com-
plexes which are kinetically and chemically competent to be
intermediates in the copper-catalyzed etherification of aryl
halides. All these complexes appear to exist in an ionic form
in the solid state and in the polar solvents that are often used
for copper-catalyzed coupling of phenoxides.[24–30] This ionic
form contains two CuI sites, one bound by two chelating,
dative ligands, and one by two phenoxides. In contrast to prior
proposals of the reaction of anionic phenoxide complexes of
copper, the isolated anion containing an ammonium counter-
ion did not react with iodoarenes in high yields. Experiments
that probe for redox processes and the generation of free or
caged aryl radicals provide strong evidence against these
process to form such radical intermediates and argue in favor
of an oxidative addition to form an aryl–CuIII phenoxide
complex that undergoes reductive elimination of ether.
Although CuIII species are not common, alkylcopper(III)[31–34]
and arylcopper(III)[11,35] species have been identified recently.
modest yield of coupled product, again, results from the
presence of an ortho-alkoxy group.[21] The phenyl allyl ether
formed from catalytic reactions in [D6]DMSO contained 57%
deuterium, as determined by comparison of the GC/MS
spectra of the mixture to that of nondeuterated phenyl allyl
ether. The phenyl allyl ether formed from reactions of
complex 5 in [D6]DMSO contained 30–66% deuterium,
depending on the particular experiment. Therefore, the
origin of all of the reducing equivalents is not certain, but
the DMSO solvent is one source.
Although we cannot provide a definitive mechanism for
formation of this hydrodehalogenation product, previous
work showed that a hydrogen atom transfer to the free aryl
radical does not compete significantly with cyclization.[20]
Thus, the absence of products from cyclization and the
higher reactivity of the copper phenoxide with 1-bromonap-
thalene than with 4-chlorobenzonitrile imply that free aryl
radicals formed by initial electron transfer are unlikely to be
intermediates in the stoichiometric or catalytic reactions of
aryl halides with phenanthroline-ligated copper phenoxides.
To probe the formation of aryl radicals that could be
loosely bound to copper in a dynamic equilibrium, we
conducted the reaction of phen-ligated phenoxide complex
1a with 1,4-diiodo-2,6-dimethylbenzene. Dissociation of the
more hindered iodide has been shown to occur after electron
transfer from enolates.[22] Consistent with the absence of free
or loosely bound aryl radical anions leading to free aryl
radicals, the reaction of the phen-ligated phenoxide complex
1a with 1,4-diiodo-2,6-dimethylbenzene at 1108C in DMSO
yielded the aryl phenyl ether product 8 from cleavage of the
À
less hindered C I bond in 86% yield, along with minor
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Moreover, we have conducted computational studies by DFT
on the energy of a phen-ligated arylcopper(III) halide
phenoxide complex, and the DG for formation of this species
from the [Cu(phen)(OPh)] and PhI is computed to be
22.1 kcalmolÀ1 [Eq. (10)]. This energy is consistent with the
barrier corresponding to the conditions of the experiments in
Eq. (2). Additional structural and mechanistic studies of
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Experimental Section
[17] As requested by a reviewer, we also conducted the reaction of
cationic CuI species, [phen2Cu][PF6], with p-iodotoluene under
the conditions of Scheme 2, and no reaction was observed.
[18] R. J. Enemærke, T. B. Christensen, H. Jensen, K. Daasbjerg, J.
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[21] As a control, we conducted the reaction of o-(n-propoxy)iodo-
benzene with 1a in DMSO at 1108C for 12 hours. Consistent
with an ortho-alkoxy group leading to low yields from reaction
with copper phenoxides, this reaction produced the biaryl ether
product in 68% yield and phenyl n-propyl ether from hydro-
deiodination in 31% yield. Reaction with 5 equivalents of the
unhindered p-tert-butyl iodobenzene in DMSO at 1108C for
2 hours yielded the ether product in 80% yield and less than 2%
yield of tert-butyl benzene derived from hydrodeiodination.
[22] B. Branchi, C. Galli, P. Gentili, M. Marinelli, P. Mencarelli, Eur.
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Preparation of [bis(1,10-phenanthroline)Cu][bis(phenoxide)Cu]
(1a): A solution of NaOPh (46.3 mg, 0.399 mmol) in 1 mL of THF
was added to a suspension of CuCl (39.5 mg, 0.399 mmol) in 5 mL of
THF, and the resulting mixture was stirred at room temperature for
30 min. The resulting light yellow mixture was filtered through a layer
of Celite. To this filtrate was added a solution of 1,10-phenanthroline
(72 mg, 0.40 mmol) in 1.5 mL of THF. The resulting solution turned
reddish brown immediately and was additionally stirred at room
temperature for 40 min. n-Pentane (12 mL) was added to precipitate
the product. The product was separated from the supernatant by
filtration through a fine fritted funnel and washed with 2 mL of
pentane to afford 132 mg (98%) of 1a. 1H NMR (500 MHz,
[D6]DMSO): d = 6.25 (t, J = 7.0 Hz, 2H), 6.48 (d, J = 8.0 Hz, 4H),
6.87 (t, J = 7.0 Hz, 4H), 8.02–7.99 (m, 4H), 8.28 (s, 4H), 8.82 (d, J =
8.0 Hz, 4H), 9.01 ppm (d, J = 4 Hz, 4H); 13C{1H} NMR (126 MHz,
[D6]DMSO): d = 113.4, 119.4, 126.4, 127.8, 129.4, 129.7, 138.0, 144.0,
150.2, 169.0 ppm; Anal. Calcd for C36H26Cu2N4O2: C, 64.18; H, 3.89;
N, 8.32. Found: C, 64.45; H, 4.03; N, 7.94; Conductivity (258C,
1.00 mm in DMSO): 37.1 Wcm2 molÀ1
.
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Received: April 27, 2009
Revised: January 12, 2010
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Keywords: copper · cross-coupling · etherification · phenoxides
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