Functionalized organocuprates: Structures of lithium and magnesium grignard 2-methoxyphenylcuprates

Article abstract of DOI:10.1021/om300488j

Lithium and magnesium Grignard diorganocuprates incorporating the functionalized aryl group 2-methoxyphenyl have been prepared and structurally characterized in the solid state. [Cu₄Li₂(C ₆H₄OMe-2)₆(T

Full text of DOI:10.1021/om300488j

Article
pubs.acs.org/Organometallics
Functionalized Organocuprates: Structures of Lithium and
Magnesium Grignard 2-Methoxyphenylcuprates
Roberta Bomparola, Robert P. Davies,* Steven Lal, and Andrew J. P. White
Department of Chemistry, Imperial College London, South Kensington, London, U.K. SW7 2AZ.
*
S Supporting Information
ABSTRACT: Lithium and magnesium Grignard diorgano-
cuprates incorporating the functionalized aryl group 2-
methoxyphenyl have been prepared and structurally charac-
terized in the solid state. [Cu Li (C H OMe-2) (THF) ] (2)
4
2
6
4
6
2
and [Cu(C H OCH -2) Mg(THF) X] (3-X; X = Cl, Br) all
6
4
3
2
2
exhibit coordination of the s-block metal center by the
methoxy oxygen, resulting in the formation of novel aggregates
and favoring contact ion pair structures. In contrast, separate
ion pair structures had previously been observed under similar conditions for nonfunctionalized arylcuprates. The magnesium
organocuprates 3-Cl and 3-Br are of particular interest, being rare examples of structurally characterized Grignard-derived
organocuprates and the first examples of functionalized Grignard organocuprates. All reported organocuprates undergo oxidative
aryl coupling in the presence of O or PhNO to give 2,2′-dimethoxybiphenyl.
2
2
INTRODUCTION
Grignard organocuprates, revealing [Ph Cu (Mg(OEt )I) ] to
4 2 2 2
adopt a dimeric CIP structure in diethyl ether which is
■
Organocuprates are excellent reagents for the formation of
carbon−carbon bonds and have been extensively employed in
synthetic methodology since the 1960s for a number of
important reactions, including conjugate addition, substitution
reactions, coupling reactions, and carbocupration. However,
it is only relatively recently that significant insights have been
obtained into the mechanism of operation of these reagents and
analogous to that of lithium diorganocuprate CIPs (I) but with
7
MgI formally replacing Li in the ring. Larger “inverse crown”
8
type aggregates were observed when employing the bulkier
1
,2
mesityl (C H Me -2,4,6) aryl group, and similar to the case for
6
2
3
lithium organocuprates, solvent-separated species were dis-
cerned when more strongly coordinating THF solvent
7
3
−5
molecules were present.
the origin of their unique reactivity,
including recent
Most structural studies on organocuprates to date have
evidence of the important role played by Cu(III) intermedi-
9
−14
6
employed simple nonfunctionalized aryls (such as phenyl
or mesityl
ates.
15−17
) as the organo R group. However, many
One of the first key steps in building the current
understanding of organocuprates was identification of the
structural forms adopted by these complex species. Thus, the
organocuprate synthetic protocols employ aryl or alkyl groups
with additional functionalization such as amine, ether, alkenyl,
3
18
and alkynyl groups and it is still unclear how closely the
structures and behavior of these species match those of their
unfunctionalized cousins. A small number of lithium organo-
cuprates containing dimethylamino-functionalized aryl groups
have been studied, including the lithium homocuprate
resting state and also reactive form of lithium organocuprates
(
“R CuLi”, R = organo group) in a nonpolar or weakly
2
coordinating solvent such as diethyl ether is now commonly
accepted to be the dimeric contact ion pair (CIP) species I
Figure 1). However, in a more strongly coordinating solvent,
(
[
Cu Li (C H CH NMe -2) ] (III), which was shown to
2 2 6 4 2 2 4
adopt a CIP structure with dimethylamino substituents
coordinating to the lithium atoms via their nitrogen lone
19
pairs (Figure 2). An additional advantage to studying such
species is the demonstrated ability of intramolecular Lewis
donor groups to stabilize reagents or aggregates which are
otherwise too reactive or unstable to be isolated. Hence, the
aforementioned dimethylamino-substituted aryl was also
employed by van Koten and co-workers to prepare
Figure 1. CIP (I) and SSIP (II) structures of lithium homocuprates.
such as THF, the less reactive solvent-separated ion pair (SSIP)
species II is predominant. Studies on the structures of
analogous magnesium Grignard derived organocuprates
[
(C H (CH NMe )-2) CuLi (CN)(THF) ] (IV), which
6 4 2 2 2 2 4 ∞
(
“R CuMgX”, X = halide) are far less developed, despite the
2
Special Issue: Copper Organometallic Chemistry
fact that the heritage and synthetic usage of these reagents rival
that of their lithium-based analogues. We have recently
reported the first structural characterizations of magnesium
1
Received: June 1, 2012
Published: July 13, 2012
©
2012 American Chemical Society
7877
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Organometallics
Article
4
9.26; H, 4.13. Found: C, 49.15; H, 4.19 (note that the THF solvent
of crystallization was lost during the isolation procedure required to
prepare the sample for analysis).
Synthesis of [Cu Li (C H OMe-2) (THF) ] (2). A solution of
4
2
6
4
6
2
nBuLi in hexane/cyclohexane (2.5 M, 1.06 mmol, 0.43 mL) was added
dropwise to a solution of 2-bromoanisole (1.06 mmol, 0.13 mL) in
hexane (5 mL) at 0 °C. The reaction mixture was warmed to room
temperature and stirred for 1 h. A solution of 2-methoxyphenylcopper
(
1; 0.182 g, 1.06 mmol) in toluene (4.6 mL) was added to give a
yellow precipitate, which was dissolved with the addition of THF (0.5
mL) and then filtered over Celite. Storage at room temperature for 5
days yielded yellow blocks of 2 suitable for X-ray crystallography (85
1
mg, 24% yield based on 2-bromoanisole). H NMR (400 MHz, 25 °C,
C D ): δ 8.02 (m, 6H, H6), 7.23 (m, 6H, H4), 7.05 (m, 6H, H5), 6.69
6
6
(
1
1
m, 6H, H3), 3.68 (m, 8 H, CH O-THF), 3.40 (s, 18H, −OCH ),
2
3
7
.49 (m, 8 H, CH -THF). Li NMR (155.6 MHz, 25 °C, C D ) δ
2
6
6
.46.
Exposure of 2 to atmospheric oxygen over several days or treatment
Figure 2. Organocuprates containing amine-functionalized aryl groups.
with PhNO at room temperature produced 2,2′-dimethoxybiphenyl.
2
1
H NMR (400 MHz, 25 °C, C D ): δ 7.49 (d, 2H, J = 7.4 Hz, H6),
6
6
proved key in understanding the chemistry of cyano Gilman
7
.28 (m, 2H, H4), 7.06 (m, 2H, H5), 6.78 (d, 2H, J = 8 Hz, H3), 3.39
(
Lipshultz) cuprates at a time when there was much
(s, 6H, −OCH ).
20
3
controversy regarding the nature of these species. In addition,
a diamine-functionalized aryl was thought to play a large role in
contributing to the thermodynamic stability of the lithium
diorganocuprate−lithium halide complex [Cu(C H (CH N-
Synthesis of [Cu(C H OCH -2) Mg(THF) Br] (3-Br). A solution
6
4
3
2
2
of 2-methoxyphenylcopper (1; 182 mg, 1.10 mmol) in toluene (2.6
mL) was added to a solution of (2-methoxyphenyl)magnesium
bromide in THF (1 M, 1.10 mL, 1.10 mmol), previously prepared
from the addition of 2-bromoanisole (20 mmol, 2.5 mL) to a
suspension of magnesium metal (22 mmol, 0.48 g) in THF (20 mL).
In order to dissolve the yellow precipitate that was formed, additional
toluene (2 mL) and THF (2 mL) were added; the solution was stirred
at room temperature for 15 min before filtration through Celite.
Storage at −35 °C for 19 days yielded colorless crystals suitable for X-
6
4
2
21
(
Et)CH CH NEt )-2) Li Br] (V). Using similar principles,
2 2 2 2 2
Ribas et al. employed a triaza macrocyclic ligand to yield the
22
first example of an isolable Cu(III) monoaryl species.
In this work we investigate the role of ether-functionalized
aryl groups in lithium and Grignard organocuprates. There are
currently no structurally characterized examples of Grignard-
derived organocuprates with functionalized aryls of any sort,
and as discussed above, studies on functionalized lithium
diarylcuprates are currently limited to just those with one or
more amine donor groups. The 2-methoxyphenyl group has
been selected as the organo group, since organocuprates
containing this ligand are readily accessible and have previously
been employed in a number of synthetic protocols, including
1
ray analysis (142 mg, 25%); mp 282 °C dec. H NMR (400 MHz,
C D , 25 °C): δ 7.48 (d, 2H, J = 8.0 Hz, H6), 7.22 (m, 2H, H4), 7.05
6
6
(
m, 2H, H5), 6.93 (m, 2H, H3), 3.82 (m, 8H, CH O-THF), 3.39 (s,
2
6H, −OCH ), 1.45 (m, 8H, CH -THF).
3
2
Synthesis of [Cu(C
H OCH -2) Mg(THF) Cl] (3-Cl). A solution
6 4 3 2 2
of 2-methoxyphenylcopper (1; 182 mg, 1.10 mmol) in toluene (2.6
mL) was added to a solution of (2-methoxyphenyl)magnesium
chloride in THF (1 M, 1.10 mL, 1.10 mmol), previously prepared
from the addition of 2-chloroanisole (20 mmol, 2.5 mL) to a
suspension of magnesium metal (22 mmol, 0.48 g) in THF (20 mL).
Toluene (2 mL) and THF (2 mL) were added to dissolve the solid,
and the solution was stirred at room temperature for 15 min before
filtration through Celite. Storage at room temperature for 14 days
23
the syntheses of dihydromultifidene, hallucinogenic amphet-
2
4
amine derivatives, and the anticancer natural product
2
5
yomogin.
EXPERIMENTAL SECTION
1
yielded colorless crystals suitable for X-ray analysis (98 mg, 19%). H
■
General Considerations. All experimental work was carried out
under an inert atmosphere of nitrogen using standard Schlenk double-
manifold and glovebox techniques. Purification and drying of the
solvents was carried out following standard methods or using an
Innovative Technologies PureSolv solvent purification system with
purification-grade solvents. NMR spectra were recorded on a Bruker
DPX400 spectrometer with internal standards. Melting points were
measured in capillaries sealed under nitrogen, and microanalytical data
were obtained from the Science Technical Support Unit, London
Metropolitan University.
NMR (400 MHz, C
(m, 2H, H4), 7.05 (m, 2H, H5), 6.93 (m, 2H, H3), 3.69 (m, 8H,
CH O-THF), 3.39 (s, 6H, −OCH ), 1.52 (m, 8H, CH -THF).
Synthesis of [Cu(C OCH -2) Mg(THF) I] (3-I). A solution of
2-methoxyphenylcopper (182 mg, 1.10 mmol) in toluene (2.6 mL)
was added to a solution of (2-methoxyphenyl)magnesium iodide in
THF (0.5 M, 2.20 mL, 1.10 mmol), previously prepared from the
addition of 2-iodoanisole (20 mmol, 2.6 mL) to a suspension of
magnesium metal (22 mmol, 0.48 g) in THF (20 mL). Toluene (2
mL) and THF (2 mL) were added, and the mixture was heated to 50
°C before filtration through Celite. Storage of the filtrate at room
temperature for 12 h yielded a yellow solid, which was dried in vacuo
(95 mg, yield 16%). H NMR (400 MHz, C
J = 7.6 Hz, H6), 7.22 (m, 2H, H4), 7.05 (m, 2H, H5), 6.93 (m, 2H,
H4), 3.71 (m, 8H, CH O-THF), 3.39 (s, 6H, −OCH ), 1.48 (m, 8H,
CH -THF).
X-ray Structure Determinations of 1, 2, 3-Cl, and 3-Br. The
crystals were all taken directly from the mother liquor, covered with a
perfluorinated ether, and mounted on the top of a glass capillary under
a flow of cold gaseous nitrogen. The data were collected using a
Oxford Diffraction Xcalibur PX Ultra diffractometer fitted with an
Oxford Cryostream low-temperature device, and the structures were
refined on the basis of F2 using the SHELXTL and SHELX-97
program systems. Table 1 provides a summary of the crystallographic
D , 25 °C): δ 7.48 (d, 2H, J = 7.2 Hz, H6), 7.22
6 6
2
3
2
H
6
4
3
2
2
Synthesis of [Cu(C H OMe-2)] ·8THF (1). A 40 mmol portion of
6
4
8
2
-bromoanisole (5.0 mL) was added dropwise to a suspension of
1
magnesium metal (44 mmol, 1.07 g) in THF (40 mL) at 0 °C. After
complete addition the reaction mixture was brought to room
temperature and stirred for 1 h. The resultant Grignard reagent was
then added to a suspension of copper(I) chloride (40 mmol, 3.94 g) in
THF (40 mL) at 0 °C, and the mixture was stirred overnight. A 20 mL
portion of 1,4-dioxane was added to aid the precipitation of MgBrCl,
which was removed by filtration, and the filtrate volume was
concentrated under vacuum. Storage at −35 °C for 3 days yielded
6
D
6
, 25 °C): δ 7.48 (d, 2H,
2
3
2
orange crystals (2.58 g, 38% yield relative to 2-bromoanisole); mp 128
1
°
C dec. H NMR (400 MHz, 25 °C, C D ): δ 9.2−6.1 (m, 32 H, Ar-
6
6
H), 3.70 (m, 32 H, CH O-THF), 3.5−2.1 (s, 24 H, −OCH ), 1.52 (m,
2 H, CH -THF). Anal. Calcd for C H Cu O (FW = 1365.3): C,
2
3
3
2 56 56 8 8
7
878
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Organometallics
Article
Table 1. Summary of Crystal Structure Data for 1, 2, 3-Br, and 3-Cl
1
2
3-Br
3-Cl
empirical formula
fw
C H Cu O ·8C H O
1942.16
173
C H Cu Li O
C H BrCuMgO4
C H ClCuMgO
4
5
6
56
8
8
4
8
50 58
4
2
8
22 30
22 30
1055.00
526.22
481.76
173
temp (K)
wavelength (λ, Å)
cryst syst
space group
a (Å)
173
173
Cu Kα (1.541 84)
tetragonal
Mo Kα (0.710 73)
orthorhombic
Pbca (No. 61)
17.96778(17)
17.60950(18)
30.1255(4)
9531.82(18)
8
Cu Kα (1.541 84)
orthorhombic
P2 2 2 (No. 19)
Cu Kα (1.541 84)
orthorhombic
P4/n (No. 85)
18.15205(9)
P2 2 2 (No. 19)
1 1 1
1
1
1
8.18331(6)
8.43100(6)
33.1182(2)
2284.94(3)
4
7.9461(15)
8.5104(7)
33.255(3)
2248.9(5)
4
b (Å)
c (Å)
13.48120(9)
4442.01(4)
2
3
V (Å )
Z
ρcalcd (g cm−3)
θ range (deg)
μ (mm−1)
1.452
1.470
1.530
1.423
3.44−72.46
2.549
3.72−32.46
1.813
2.67−71.32
3.868
2.66−72.30
2.941
no. of rflns collected
no. of unique rflns (Rint)
no. of params
40 837
119 452
31 950
5720
4398 (0.0345)
165
15 909 (0.0494)
617
4480 (0.0217)
282
3779 (0.0391)
264
2
goodness of fit on F
1.148
1.131
1.101
1.179
R1 (F > 4σ(F))
0.0577
0.0563
0.0163
0.0610
wR2
0.2240
0.1410
0.0434
0.1467
data for all compounds. Full details of the X-ray structure solutions,
including the handling of any disorder present in the structures, is
given in the Supporting Information. The absolute structure of 3-Br
methoxyphenyl)cuprates discussed below, a new structural
analysis of 1 is briefly reported herein. Note that the crystals of
1
obtained in this work were from a THF solution and contain
+
−
was determined by a combination of R-factor tests (R1 = 0.0163, R1
noncoordinating THF within the crystal lattice, whereas the
1971 structural analysis by Camus contained toluene molecules
with the lattice, thus giving rise to very different crystallographic
+
−
=
1
0
0.0294) and by use of the Flack parameter (χ = 0.000(10), χ =
+
−
+
.018(10)). Similarly for 3-Cl R1 = 0.0610, R1 = 0.0687 and χ =
−
.00(4), χ = 1.00(4). The crystal structure data have been deposited
parameters. However, in both cases octomeric [Cu(C H OMe-
with the Cambridge Crystallographic Data Center under deposition
6
4
numbers CCDC 882792 (1), 882793 (2), 882794 (3-Br) and 882795
2)] aggregates are observed, as shown in Figure 3 for 1.
8
(3-Cl). This material can be obtained free of charge via www.ccdc.cam.
The structure of 1 is best considered as consisting of two
tetrameric rings which lie on top of one another in a staggered
conformation, so that the aryl group in one ring lies above a
copper atom in the ring below. The rings are held together via
coordination from anisyl methoxy oxygens in one ring to the
copper centers in the adjacent ring (Cu(1)−O(7), 2.350(3) Å;
Cu(2)−O(17), 2.387(3) Å), and there is no evidence of any
ac.uk/data_request/cif, by emailing data_request@ccdc.cam.ac.uk, or
by contacting the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB2 1EZ, U.K.
RESULTS AND DISCUSSION
■(
2-Methoxyphenyl)copper(I). The synthesis of (2-
intra-ring Cu−O interactions. Cu−C bond distances range
methoxyphenyl)copper was first reported by Camus and
Marsich in 1968, where it was observed that in comparison
to phenylcopper and tolylcopper complexes it was less air and
ipso
from 2.013(4) to 2.051(4) Å and are therefore comparable to
analogous Cu−C distances in tetrameric arylcuprates such as
ipso
26
[
Å)
Cu Mes ] (Mes = C H Me -2,4,6; 1.986(10)−1.999(9)
temperature sensitive and more soluble in organic solvents.
The reported synthesis of this complex was from the reaction of
2-methoxyphenyl)lithium with copper(I) bromide; however,
4
4
6
2
3
2
3
9,30
and [Cu (C H CH NMe -2) ] (1.994(2)−2.102(2)
4
6
4
2
2
4
1
Å) and are indicative of Cu−C−Cu two-electron−three-
center (2e−3c) bonding. The Cu−C−Cu angles within the
rings are acute (Cu(1)−C(12)−Cu(1A), 74.66(12)°; Cu(2)−
C(2)−Cu(2A), 75.00(13)°) and are also typical for 2e−3c
(
the reproducibility of the reaction was poor, due in part to
contamination from lithium (most likely in the form of
cocomplexed (2-methoxyphenyl)lithium). We therefore adop-
ted an alternative synthesis route via reaction of the Grignard
29−31
bonding of an aryl group to two copper atoms.
There is no
(
C H OMe-2)MgCl with copper(I) chloride in THF to give a
evidence for Cu−C interactions between the tetrameric rings in
1, the shortest Cu···C inter-ring distance being 3.130 Å. The
shortest inter-ring Cu···Cu distance of 2.690 Å is shorter than
the combined van der Waals radii of two copper atoms but is
not thought to represent any significant bonding interaction:
6
4
reproducible and high-purity yield of the orange crystalline
product [Cu(C H OMe-2)] ·8THF (1; see the Experimental
Section). Though it is somewhat complicated due to the highly
aggregated nature of the complex, the H NMR spectrum of 1
is fully consistent with Camus’ previously reported spectro-
scopic analysis of this compound.
A solid-state structure of (2-methoxyphenyl)copper crystal-
lized from toluene was previously reported in 1971; however,
the data were reported to be of poor quality due to
6
4
8
1
1
0
computational studies have shown that solid-state Cu(d )−
27
10
Cu(d ) interactions are likely to be very weak in nature with
Cu(I)···Cu(I) distances primarily governed by the presence of
32
other supramolecular interactions within the crystal lattice.
Lithium 2-Methoxyphenylcuprate 2. Reaction of (2-
methoxyphenyl)copper (1) with (2-methoxyphenyl)lithium in
toluene/THF gave a yellow solution from which crystals of
[Cu Li (C H OMe-2) (THF) ] (2) were grown (Scheme 1).
“decomposition of the crystal” during the data collection and
are consequently not included in the Cambridge Structural
Database. Hence, in order to allow comparisons to be drawn
with the novel lithium and Grignard derived (2-
2
8
4
2
6
4
6
2
The solid-state structure for complex 2, as determined using
7
879
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Organometallics
Article
Figure 4. (a) Molecular structure of [Cu Li (C H OMe-2) (THF) ]
4
2
6
4
6
2
(
2), with hydrogen atoms and disorder in the THF molecules omitted
for clarity. (b) Depiction of the core connectivity in 2, omitting all
other atoms. Thermal ellipsoids are displayed at the 40% probability
level.
Table 2. Selected Bond Lengths (Å) and Angles (deg) in
[
Cu Li (C H OMe-2) (THF) ] (2)
Figure 3. (a) Molecular structure of [Cu(C H OMe-2)] ·8THF (1).
4
2
6
4
6
2
6
4
8
Hydrogen atoms and solvent THF molecules are omitted for clarity.
b) Simplified view of 1 showing the aryl ipso carbon atoms and just
Cu(1)−C(11)
Cu(1)−C(1)
Cu(2)−C(1)
Cu(2)−C(41)
Cu(3)−C(21)
Cu(3)−C(31)
Cu(4)−C(31)
Cu(4)−C(51)
1.924(3)
1.992(3)
1.998(3)
1.922(3)
1.925(3)
1.980(3)
1.991(3)
1.911(3)
Li(1)−O(7)
Li(1)−O(17)
Li(1)−O(27)
Li(1)−O(60)
Li(2)−O(37)
Li(2)−O(47)
Li(2)−O(57)
Li(2)−O(70)
1.970(6)
1.953(6)
1.921(7)
1.964(6)
1.951(7)
1.940(7)
1.924(7)
1.959(7)
139.57(14)
(
one of the methoxyphenyl groups. Thermal ellipsoids are displayed at
the 30% probability level. Symmetry transformations used to generate
3
3
equivalent atoms: (A) y, −x + / , z; (B) −y + / , x, z.
2
2
single-crystal X-ray diffraction, is shown in Figure 4, with key
bond lengths and angles given in Table 2.
C(1)−Cu(1)−C(11)
141.39(13) C(31)−Cu(4)−
Scheme 1. Synthesis of Lithium Diarylcuprate 2
C(51)
C(1)−Cu(2)−C(41)
144.30(13) Cu(1)−C(1)−Cu(2)
76.71(12)
77.26(12)
C(21)−Cu(3)−
145.05(13) Cu(3)−C(31)−
C(31)
Cu(4)
−
2
c−2e bonds. A similar but ion-separated [Cu Mes ] anion
2
3
Lithium organocuprate 2 possesses a copper to lithium ratio
of 2/1, despite the fact that it was formed from an equimolar
mixture of its organocopper and organolithium precursors.
Similar copper-rich lithium homocuprates with Cu/Li ratios
greater than 1 are well-known in the literature and include the
has previously been reported that adopts an equivalent motif, in
which the two terminal mesityl groups form 2c−2e bonds with
the Cu atoms (Cu−C range 1.923(8)−1.925(8) Å) and the
central bridging mesityl group forms a longer 3c−2e bond
7
(Cu−C range 2.003(8)−2.020(7) Å). However, unique to 2,
−
12
−
13
−
anionic clusters [Cu LiPh ]
and [Cu Li Ph ]
and the
the two [Cu R ] cuprate units are held together via
4
6
3
2
6
2 3
15
neutral complex [Cu LiMes ]. These species have been
coordination of lithium cations by the methoxy oxygen atoms
with Li−O(Me) distances in the range 1.921(7)−1.970(6) Å
(mean 1.943 Å). The coordination sphere of the lithium is
completed by a single THF molecule (Li−O, 1.959(7) and
1.964(6) Å).
3
4
speculated to arise due to interaggregate exchange between
3
3,34
Cu Li R and Cu R molecules in solution.
2
2
4
4 4
Closer analysis of the structure of 2 reveals the aggregate to
−
be constructed from two [Cu (C H OMe-2) ] anionic
2
6
4
3
organocuprate units joined together by two THF-solvated
In contrast to previously reported amino-substituted
−
lithium cations. Within each of the [Cu R ] units there are
homocuprates such as III (Figure 2), there are no Li−C
2
3
ipso
two different types of aryl groups: the first bridges the two
copper(I) atoms symmetrically to give a 3c−2e bond with Cu−
C distances in the range 1.980(3)−1.998(3) Å (mean 1.990 Å).
The second type of aryl group is terminal to just one Cu(I)
atom, resulting in the formation of shorter Cu−C distances in
the range 1.911(3)−1.925(3) Å (mean 1.920 Å) indicative of
interactions evident within the solid-state structure of 2. The
bonding behavior of the methoxyphenyl groups therefore also
differs significantly from that observed for the parent
homometallic (2-methoxyphenyl)lithium complex, which
adopts a dimeric structure in THF solution exhibiting both
35
Li−Cipso and Li−OMe coordination. In the case of 2 this can
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Organometallics
Article
be rationalized by the strong carbophilicity of copper(I) in
comparison to that of lithium. However, despite the absence of
Scheme 3. Synthesis of Magnesium Grignard Diarylcuprates
3-X (X = Cl, Br, I)
any Li−C bonding complex 2 still forms a CIP structure as
ipso
opposed to a SSIP structure (Figure 1), which is rare for a
1
7,36
lithium organocuprate in the presence of THF
and can be
attributed to the strong propensity of the anisyl oxygen atoms
toward lithium cation coordination.
At first glance the four copper atoms in 2 appear to from a
central tetrahedron with Cu···Cu distances within the range
2
.476(5)−2.824(6) Å and C−Cu−C bond angles distorted
from linearity and pointing toward the center of the
tetrahedron (mean C−Cu−C, 142.74°). However, as discussed
32
above for 1 and on the basis of previous bonding studies, this
should not be automatically interpreted to support the
existence of any significant copper(I)−copper(I) bonding.
The preparation of 3-Br and 3-Cl gave crystalline products
which were shown by X-ray diffraction to exist as isostructural
complexes with similar crystallographic parameters (see Table
1). The molecular structure of 3-Br is shown in Figure 5, and
selected bond lengths and angles for 3-Br and 3-Cl are
compared in Table 3.
Nevertheless, the Cu cluster does differ from previously
4
2
−
37
reported Cu(I) clustersfor example [Cu I ]
,
4
4 6
2−
38
2− 39
[
Cu (SPh) ] , and [Cu (SePh) ]
in that in 2 only
4
6
4
6
two of the six anionic ligands directly bridge copper atoms
−
(
these being the central aryl groups in the [Cu R ] units), and
2 3
instead it is the peripheral Li−O(anisyl) bonding which must
therefore play a large part in the supramolecular assembly of
the structure.
On exposure to atmospheric oxygen, the lithium organo-
cuprate 2 was observed to decompose via oxidative coupling of
the aryl groups to give small quantities of 2,2′-dimethox-
ybiphenyl (Scheme 2). By using the controlled introduction of
Scheme 2. Oxidative Homocoupling of 2-Methoxylphenyl
Figure 5. Molecular structure of [Cu(C H OCH -2) Mg(THF) Br]
6
4
3
2
2
(
3-Br). Hydrogen atoms are omitted for clarity, and thermal ellipsoids
are displayed at the 40% probability level.
Table 3. Selected Bond Lengths (Å) and Angles (deg) in 3-X
(
X = Cl, Br)
nitrobenzene as the oxidizing agent in place of atmospheric
dioxygen, we have been able to improve the yield of this
coupling reaction to close to quantitative (>95%). In addition
and as noted in Introduction, the application of lithium (2-
methoxyphenyl)cuprate 2 in conjugate addition reactions is
X = Br
X = Cl
Cu−C(1)
Cu−C(11)
Mg−O(7)
Mg−O(17)
Mg−O(20)
Mg−O(25)
Mg−X
C(1)−Cu−C(11)
O(7)−Mg--O(17)
O(7)−Mg−X
1.9176(15)
1.9190(16)
2.0756(13)
2.0973(12)
2.0771(12)
2.0559(13)
2.5183(6)
172.57(7)
155.37(6)
101.90(4)
102.54(4)
1.923(6)
1.919(6)
2.075(5)
2.091(5)
2.071(5)
2.061(5)
2.352(3)
172.6(3)
154.1(2)
101.94(16)
103.77(16)
23−25
already well established in the organic chemistry literature.
Magnesium Grignard (2-Methoxyphenyl)cuprates 3-X
(
X = Cl, Br, I). Despite their widespread application in
1
synthesis, studies on the structures of Grignard organocuprates
(
2
commonly prepared from the reaction of a copper(I) salt with
equiv of Grignard reagent) are sparse in the literature, even in
comparison to lithium organocuprates. We recently reported
upon the first structurally characterized examples of Grignard
organocuprates: the iodo Grignard CIP organocuprate
O(17)−Mg−X
[
Ph Cu (Mg(OEt )I) ] and bromo Grignard CIP and SSIP
4 2 2 2
−
organocuprates [Cu Mg Mes Br ] and [Cu Mes ] [MgBr-
Both 3-Br and 3-Cl are monomeric but dinuclear R CuMgX
2
4
2
6
2
2
3
+
7
(
THF) ] , respectively. Two related structures derived from
complexes, with a 1/1 stoichiometric ratio of copper to
magnesium halide. This ratio is therefore as expected from the
original reaction stoichiometry but differs from that observed in
the homologous copper-rich organocuprate 2, with Cu/Li = 2/
1 (vide supra). In addition, the structure of 3-X contrasts with
those of previously reported Grignard organocuprates, being
the first example of a monomeric CIP complex. Although the
crystals of 3-Br and 3-Cl were obtained from THF solutions,
the organocuprates do not form THF-solvated SSIP structures,
as might be initially expected (Figure 1), but rather give CIP
structures, albeit with no apparent Cipso−Mg bonding. Similar
to the case for lithium organocuprate 2, the formation of a CIP
5
diorganomagnesium reagents have also been reported:
1
2
[
=
Cu MgPh (OEt )] and [Cu Mes ][μ-SAr] [MgSAr] (SAr
4
6
2
4
4
2
2
4
0,41
SC H CH(Me)NMe -2).
In an expansion of these
6
4
2
previous studies, employment of the 2-methoxyphenyl ligand
has led to the first structural characterization of a functionalized
Grignard organocuprate. Thus, the reaction of (2-
methoxyphenyl)copper with the corresponding aryl Grignard
(
itself prepared from the reaction of the aryl halide with
magnesium metal) in THF solution gave the magnesium
Grignard organocuprate complexes 3-X (X = Cl, Br, I), as
shown in Scheme 3.
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Organometallics
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structure can be attributed to the strong coordination of the s-
block metal by the anisole methoxy group.
Closer inspection of the makeup of 3-X reveals two key
structural components: a RCuR anionic fragment and a
(CIP) structures rather than THF-solvated separated ion pairs
(SSIPs) as observed for nonfunctionalized arylcuprates.
Previous studies have shown how a knowledge of the
organocuprate structure can be key to understanding the
3
MgX(THF) cationic fragment. The anionic RCuR organo-
reactivity and mechanism of reaction of these species, with the
2
cuprate unit is close to linear at the copper center (C−Cu−C,
balance between CIP and SSIP species being particularly
36
1
72.57(7)° (3-Br) and 172.6(3)° (3-Cl)) with mean Cu−C
important in this respect. In the past such structural studies
have predominately concentrated upon the role of the solvent
or the steric size of the organo group. However, from this work
it is apparent how the presence of additional functional groups
on the organo group can also play a key role in determining the
overall structure and aggregation of these reagents. It is through
such studies that we are able to build a more thorough
understanding of these important and widely used reagents.
distances of 1.918 Å (3-Br) and 1.921 Å (3-Cl). These copper
angles and distances are consistent with the presence of 2c−2e
copper(I)−carbon bonds and are directly comparable in length
to similar 2c−2e Cu−C bonds in 2 (mean 1.920 Å) as well as
being significantly shorter than the 3c−2e bond lengths
observed in 1 (mean 2.147 Å) and 2 (mean 1.990 Å). The
diarylcuprate unit is connected to the Mg center via
coordination of the two methoxy groups, with mean Mg−O
distances of 2.086 Å (3-Br) and 2.083 Å (3-Cl). The
magnesium metal centers are additionally complexed by two
THF oxygens and a halide anion. The geometry at the
magnesium is close to a square-based pyramid with the halide
in the apical position: the sum of cis O−Mg−O bond angles is
ASSOCIATED CONTENT
■
*
S
Supporting Information
Text, figures, and CIF files giving details of the crystal
structures of 1, 2, 3-Br, and 3-Cl. This material is available
free of charge via the Internet at http://pubs.acs.org.
3
58.57° in 3-Br and 358.4° in 3-Cl. The position trans to the
halide is effectively shielded by the copper atom, thus
AUTHOR INFORMATION
preventing any additional donor coordination at this site
■
Corresponding Author
*E-mail: r.davies@imperial.ac.uk. Tel: +44 207 5945754.
(
3
Cu···Mg distances are 2.7409(6) Å in 3-Br and 2.765(2) Å in
-Cl). The positioning of the methyl groups as well as twisting
in the THF molecules results in a chiral configuration for the
molecule in the solid state, although the bulk product is
racemic.
Notes
The authors declare no competing financial interest.
Although it was not possible to prepare crystals of the iodo
Grignard derivative 3-I suitable for X-ray diffraction analysis,
the NMR data of this complex are almost identical with those
observed for both 3-Br and 3-Cl, suggesting a similar Grignard
diarylcuprate structure (see the Experimental Section). In
addition, all 3-X complexes exhibit aryl−aryl oxidative coupling,
either slowly over several days in the presence of atmospheric
oxygen or quantitatively over 1 h in the presence of
nitrobenzene, to give 2,2′-dimethoxybiphenyl in a reaction
analogous to that reported for 2 (Scheme 2).
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(
(
(
4
(
(
2
(
6
(
3
(
(
2
(
(
5
(
1
(
(
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