Synthesis of quinolinyl-based pincer copper(ii) complexes: an efficient catalyst system for Kumada coupling of alkyl chlorides and bromides with alkyl Grignard reagents

Article abstract of DOI:10.1039/C8DT03210F

Quinolinamide-based pincer copper(ii) complexes, κ^N,κ^N,κ^N-{C₉H₆N-(μ-N)-C(O)CH₂NEt₂}CuX [(^QNNN^Et₂)CuX (X = Cl, 2; X = Br, 3; X = OAc, 4)], were synthesized by the reaction of ligand (^QNNN^Et₂)-H (1) with CuX₂ (X = Cl, Br or OAc) in the presence of Et₃N. The reaction of (^QNNN^Et₂)-H with CuX (X = Cl, Br or OAc) also afforded the Cu(ii) complexes 2, 3 and 4, respectively, instead of the expected Cu(i) pincer complexes. The formation of Cu(ii) complexes from Cu(i) precursors most likely occurred via the disproportionation reaction of Cu(i) into Cu(0) and Cu(ii). A cationic complex [(^QNNN^Et₂)Cu(CH₃CN)]OTf (5) was synthesized by the treatment of neutral complex 2 with AgOTf. On the other hand, the reaction of (^QNNN^Et₂)-H (1) with [Cu(MeCN)₄]ClO₄ produced cationic Cu(i) complex, [(^QNN(H)N^Et₂)Cu(CH₃CN)]ClO₄ (6), in good yield. All complexes 2-5 were characterized by elemental analysis and HRMS measurements. Furthermore, the molecular structures of 2, 3 and 4 were elucidated by X-ray crystallography. Complex 4 crystallizes in a dimeric and catemeric pattern. The cationic complex 5 was found to be an efficient catalyst for the Kumada coupling reaction of diverse nonactivated alkyl chlorides and bromides with alkyl magnesium chloride under mild reaction conditions.

Full text of DOI:10.1039/C8DT03210F

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Synthesis of quinolinyl-based pincer copper(II)
complexes: an efficient catalyst system for
Kumada coupling of alkyl chlorides and bromides
with alkyl Grignard reagents†
Cite this: Dalton Trans., 2018, 47,
16747
a,b
Hanumanprasad Pandiri,a,b Rajesh G. Gonnade c and Benudhar Punji
*
Quinolinamide-based pincer copper(II) complexes, κN,κN,κN-{C9H6N-(μ-N)-C(O)CH2NEt2}CuX [(QNNNEt
)
2
CuX (X = Cl, 2; X = Br, 3; X = OAc, 4)], were synthesized by the reaction of ligand (QNNNEt )-H (1) with
2
CuX2 (X = Cl, Br or OAc) in the presence of Et3N. The reaction of (QNNNEt )-H with CuX (X = Cl, Br or
2
OAc) also afforded the Cu(II) complexes 2, 3 and 4, respectively, instead of the expected Cu(I) pincer com-
plexes. The formation of Cu(II) complexes from Cu(I) precursors most likely occurred via the disproportio-
nation reaction of Cu(I) into Cu(0) and Cu(II). A cationic complex [(QNNNEt )Cu(CH3CN)]OTf (5) was syn-
2
thesized by the treatment of neutral complex 2 with AgOTf. On the other hand, the reaction of
(QNNNEt )-H (1) with [Cu(MeCN)4]ClO4 produced cationic Cu(I) complex, [(QNN(H)NEt )Cu(CH3CN)]ClO4
(6), in good yield. All complexes 2–5 were characterized by elemental analysis and HRMS measurements.
Furthermore, the molecular structures of 2, 3 and 4 were elucidated by X-ray crystallography. Complex 4
crystallizes in a dimeric and catemeric pattern. The cationic complex 5 was found to be an efficient cata-
lyst for the Kumada coupling reaction of diverse nonactivated alkyl chlorides and bromides with alkyl
magnesium chloride under mild reaction conditions.
2
2
Received 6th August 2018,
Accepted 19th October 2018
DOI: 10.1039/c8dt03210f
rsc.li/dalton
unique features of these transition metals.3 In that regard,
complexes of Mn,4 Fe,5 Co,6 Ni,3a,7 and Cu8 are developed and
Introduction
Pincer-ligated transition metal complexes have found extensive efficiently employed in numerous applications. Among them,
applications in catalysis as well as in materials chemistry.1 the pincer copper complexes are used in biological appli-
Particularly, the high thermal stability and rigid structure of cations such as in luminescence and medicine, and exhibit
pincer complexes make them extensively useful in diverse antioxidant and antibacterial activity.9 In addition, the pincer
organic transformations, because the tight tridentate coordi- copper complexes are explored in several catalytic cross-coup-
nation keeps the pincer and metal together in a catalytic cycle, lings as well as in direct C–H bond functionalizations.10
wherein the sterics and electronics on a ligand are effectively
The Kumada coupling is one of the most important cross-
transferred to the metal centre. Pincer complexes of noble coupling reactions, useful in achieving C–C coupled products
metals, such as Pd, Rh and Ir, are extensively studied and by the reaction of a Grignard reagent with an organic halide.11
employed as catalysts in important chemical reactions.2 Kumada coupling is well precedented by early transition-metal
Recently, pincer complexes based on naturally-abundant, less salts, such as Fe,12 Co,13 and Ni.14 Moreover, the pincer com-
expensive first-row transition metals have attracted special plexes of Mn,15 Fe,16 and Ni17 are also employed in the
attention because of the sustainability, economic viability and Kumada coupling reaction. However, copper complexes in the
Kumada coupling reaction are relatively less explored. For
example, Burns and Liu independently demonstrated the
Kumada coupling of alkyl tosylates and mesylates employing
aOrganometallic Synthesis and Catalysis Group, Chemical Engineering Division,
India. E-mail: b.punji@ncl.res.in
Cu-precursors with added ligands.18 Recently, Kirchner uti-
lized a well-defined phosphazine-based PNP-pincer copper
complex in the Kumada coupling of aryl triflates and bromides
with aryl Grignard reagents.10b Unfortunately, the Kumada
coupling of challenging alkyl chlorides employing a copper
catalyst has been rarely explored, with an extremely limited
scope.18b We hypothesized that a copper catalyst based on a
bAcademy of Scientific and Innovative Research (AcSIR), New Delhi 110 020, India
cCentre for Material Characterization. CSIR–National Chemical Laboratory
(CSIR–NCL), Dr. Homi Bhabha Road, Pune – 411 008, Maharashtra, India.
Fax: + 91-20-25902621; Tel: + 91-20-2590 2733
†Electronic supplementary information (ESI) available: Analytical data of com-
pounds and spectra. CCDC 1832852 for 2, 1832854 for 3, 1832845 for 4·(H2O)0.5
and 1832853 for (4)n. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c8dt03210f
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strong sigma-donating tridentate nitrogen-ligand would favour be due to the fact that Cu(I) salts in solution undergo dispro-
the reactivity of unactivated alkyl chlorides in the Kumada portionation reaction into Cu(II) and Cu(0) species. The result-
coupling reaction.19 Thus, as a part of our interest on the ing Cu(II) species would react with ligand 1 to produce the
development of pincer complexes of 3d metals,20 herein, we corresponding pincer copper(II) complexes in less than 50%
have synthesized a series of quinolinyl-based NNN-pincer yields (Scheme 2). Similarly, the disproportionation reaction of
copper complexes κN,κN,κN-{C9H6N-(μ-N)-C(O)CH2NEt2}CuX copper is well established by various groups.21 The molecular
[(QNNNEt )CuX] (2–5) and demonstrated them in the Kumada composition and the molecular structure of complexes 2–4,
2
coupling of diverse alkyl chlorides and bromides with alkyl synthesized via this approach, are verified that well correlates
magnesium chlorides.
with the complexes synthesized by employing Cu(II) precur-
sors. The XPS analysis of the crude reaction mixture of
complex 4 obtained via this method (Scheme 2) indicates the
existence of both Cu(II) and Cu(0) species (discussed vide
supra).
Results and discussion
Synthesis of (QNNNEt )-ligated copper(II) complexes
2
The reaction of complex (QNNNEt )CuCl (2) with AgOTf in
2
Recently, we have developed
a
C9H6N-NHC(O)CH2NEt2
acetonitrile at room temperature resulted in the formation of a
Q
Et2
[(QNNNEt )-H; 1] ligand and pincer nickel complexes [( NNN
)
2
cationic complex, [(QNNNEt )Cu(CH3CN)](OTf) (5), in 80% yield
2
NiX], wherein the pincer nickel complexes were found to be
efficient catalysts for the C–H bond alkylation, arylation and
benzylation of indoles.20a,c,d We further wanted to explore the
reactivity of ligand 1 with copper precursors, and demonstrate
the catalytic activity of the synthesized copper complexes for
the Kumada coupling of unactivated alkyl chlorides. Thus, the
(Scheme 3). Complex 5 was characterized by elemental analysis
as well as by MALDI-TOF-MS analysis. The MALDI-TOF ana-
lysis of complex 5 showed m/z values of 508.8325 and 318.9736
that correspond to [M]+ and [M − (CH3CN + OTf)]+, respect-
ively. The cationic complex 5 was found to be very robust com-
pared to complexes 2–4 as it decomposes above 230 °C.
metallation of ligand, (QNNNEt )-H (1), with CuCl2, CuBr2 and
2
On the other hand, treatment of ligand 1 with the copper(I)
precursor [Cu(CH3CN)4](ClO4) in acetonitrile under reflux con-
Cu(OAc)2 in the presence of Et3N in THF under reflux con-
ditions afforded copper(II) complexes κN,κN,κN-{C9H6N-(μ-N)-C
ditions afforded complex [(QNN(H)NEt )Cu(CH3CN)](ClO4) (6)
2
N
N
N
(O)CH2NEt2}CuCl [(QNNNEt )CuCl; 2], κ ,κ ,κ -{C9H6N-(μ-N)-C
2
in 77% isolated yield (Scheme 4). The 1H NMR spectrum of
complex 6 shows a peak at 10.21 ppm corresponding to co-
ordinated NH, which is 1.2 ppm shielded compared to that
observed for ligand 1. Furthermore, NMR analysis indicates
the coordination of the quinolinyl-N and NEt2 arm to the
copper(I) center. In addition, an acetonitrile moiety is ligated
to Cu(I) in complex 6 that appears as a singlet at 2.61 ppm.8f,22
The MALDI-TOF analysis of complex 6 showed m/z values of
(O)CH2NEt2}CuBr [(QNNNEt )CuBr; 3] and κ ,κ ,κ -{C9H6N-
N
N
N
2
(μ-N)-C(O)CH2NEt2Cu(OAc)} [(QNNNEt )Cu(OAc); 4], respect-
2
ively, in good yields (Scheme 1). The resulting complexes are
1
NMR inactive and hence could not be characterized by H and
13C NMR analyses. However, all three complexes were charac-
terized by elemental analysis and HRMS. The HRMS of 2
shows peaks at m/z 319.0734 and 355.0500 corresponding to
[2–Cl]+ and [2 + H]+, respectively. Similarly, complexes 3 and 4
show peaks at 319.0735, 396.9861, 319.0745 and 379.0307 for
[3–Br]+, [3–H]+, [4–OAc]+ and [4 + H]+, respectively. The mole-
cular structures of complexes 2, 3 and 4 were further con-
firmed by single crystal X-ray diffraction studies.
Furthermore, we planned to synthesize copper(I) complexes
[(QNNNEt )Cu(thf)] employing different Cu(I) precursors.
2
However, upon treatment of ligand 1 with CuCl, CuBr and
Cu(OAc) in the presence of Et3N in THF under reflux conditions,
copper(II) complexes 2, 3 and 4 were obtained in 49%, 50%
and 46% yields, respectively. Notably the yields of the com-
plexes 2, 3 and 4 by the reaction of ligand 1 with Cu(I) salts are
around 50% (see the ESI† for experimental details). This may
Scheme 2 Reaction of ligand (QNNNEt )-H with Cu(I)X to produce
2
(QNNNEt )Cu(II)X.
2
Q
Scheme 1 Synthesis of pincer complexes (QNNNEt )CuX.
Scheme 3 Synthesis of cationic complex [(Et NNN )Cu(MeCN)]OTf (5).
2
2
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Q
Scheme 4 Synthesis of cationic Cu(I) complex [(Et NN(H)N )Cu(MeCN)]
2
ClO4 (6).
460.12 and 320.07 that correspond to [M]+ and [M − (CH3CN +
Fig. 3 Thermal ellipsoid plot of (QNNNEt )Cu(OAc)·(H2O)0.5 [4·(H2O)0.5].
2
ClO4)]+, respectively.
All the hydrogens are omitted for clarity except on water molecules.
Crystal structure description of (QNNNEt )CuX complexes
2
CH2-NH2}Cu(OCOCH3),23 whereas the Cu–N(1) bond lengths
(2.0690(10) Å in 2 and 2.0691(12) Å in 3) and Cu–N(3) bond
lengths (2.0093(10) Å in 2 and 2.0128(12) Å in 3) are com-
parable to the respective bond lengths in {C9H6N-(μ-N)-C(O)
CH2-NH2}Cu(OCOCH3). The shorter Cu–N(2) bond length com-
pared to Cu–N(1) and Cu–N(3) bond lengths is consistent with
the amido sigma Cu–N(2) bond, whereas Cu–N(1) and Cu–N(3)
are coordinate bonds. The N(1)–Cu–N(3) bond angles in 2
(166.80(4)°) and 3 (166.84(5)°) are comparable to each other,
and significantly larger than that reported for {C9H6N-(μ-N)-
C(O)CH2NH2}Cu(OCOCH3) (159.7(2)°).23 The N(1)–Cu–N(2)
bond angles in 2 and 3 are around 84.5°, whereas the N(2)–
Cu–N(3) bond angles are slightly smaller (∼82.6°). The five-
membered ring containing Cu, N(2), and N(3) is almost planar
with the quinolinyl-moiety (Cu(1)–N(2)–C(3)–C(11) = −5.17(13)°
for 2 and 6.02(15)° for 3), whereas the other five-membered
Cu-containing ring is highly distorted (Cu(1)–N(1)–C(1)–C(2)
torsion angle 15.80(11)° for 2 and −15.99(13)° for 3). The
comparison of important bond parameters of complexes 2 and
The ORTEP diagrams of complexes 2, 3 and 4 are shown in
Fig. 1, 2 and 3, respectively. Selected bond lengths and bond
angles are given in Tables S1 and S2 in the ESI.† In all three
complexes, ligand 1 provides a tridentate coordination to the
copper through quinolinyl-N3, amido-N2 and amine-N1, and the
fourth site is occupied by an anionic ligand –Cl (2) or –Br (3) or
–OAc (4). The coordination geometry around copper is slightly
distorted from the expected square-planar in all three complexes
2, 3 and 4. The Cu–N(2) bond lengths in 2 (1.9086(10) Å) and
3 (1.9097(12) Å) are slightly shorter than the Cu–N bond length
(1.936(4) Å) in a similar amido-complex, {C9H6N-(μ-N)-C(O)
3
with that of complex {C9H6N-(μ-N)-C(O)CH2-NH2}Cu
(OCOCH3) suggests that the structural features of newly syn-
thesized complexes are in accordance with the reported
ones.23
Complex 4 crystallizes in two different patterns: (i) in one, a
molecule of H2O bridges between two copper complexes 4
(Fig. 3) and (ii) in the other, complex 4 exists as a catemeric
form (Fig. 4). Complex 4·(H2O)0.5 is most likely obtained due to
Fig. 1 Thermal ellipsoid plot of (QNNNEt )CuCl (2). All the hydrogen
atoms are omitted for clarity.
2
Fig. 2 Thermal ellipsoid plot of (QNNNEt )CuBr (3). All the hydrogen
Fig. 4 Complete model of [(QNNNEt )Cu(OAc)]n (4)n. All the hydrogen
2
2
atoms are omitted for clarity.
atoms are omitted for clarity.
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the presence of ubiquitous water in the solvent used for recrys- 2p3/2 and 2p1/2. To explain this asymmetric nature of the
tallization. Particularly, this complex was isolated from the peaks, each Cu 2p peak is deconvoluted into two peaks –
crystallization process of the reaction of ligand 1 with Cu binding energies at 933.5, 953.4 eV correspond to Cu2+ and
(OAc)2. However, the catemeric form of 4 was often recrystal- 932, 951.8 eV appear due to Cu0. Along with the main peaks, a
lized from the reaction of 1 with Cu(OAc), and upon complete low intense broad satellite peak (s) at 943.6 eV binding energy
drying of the mother liquor. In complex 4·(H2O)0.5, two mole- to the main peaks of Cu 2p3/2 was observed. However, due to
cules of (QNNNEt )Cu(OAc) are held together by hydrogen the presence of Cu0 species on the surface, the relative inten-
2
bonding between the bonded oxygen atom of acetate and sity of the satellite peak was less than that expected in the case
water in a [Cu]–AcO⋯H–O–H⋯OAc–[Cu] mode, wherein the of pure Cu2+. The obtained binding energies are in accordance
AcO–H distance is 2.055 Å and the AcO–H–O bond angle is with the earlier reports.24
177.56° (Fig. 3). In the catemeric structure of complex 4, the
Catalytic activity of (QNNNEt )CuX complexes for Kumada
coupling of alkyl halides
2
ligand carbonyl oxygen of (QNNNEt )Cu(OAc) forms a bond
2
with the Cu-centre of another (QNNNEt )Cu(OAc), resulting in
2
the formation of a catemeric structure (Fig. 4). The Cu–N(2)
bond lengths in 4·(H2O)0.5 (1.9109(14) Å) and (4)n (1.926(2) Å)
are slightly shorter than the similar Cu–N bond length (1.936
(4) Å) in the amido-complex, {C9H6N-(μ-N)-C(O)CH2-NH2}Cu
(OCOCH3),23 whereas the Cu–N(1) bond lengths in 4·(H2O)0.5
(2.0633(13) Å) and (4)n (2.083(2) Å) as well as the Cu–N(3) bond
lengths in 4·(H2O)0.5 (2.0107 Å) and (4)n (2.035(2) Å) are com-
parable. The Cu–O(1) bond length (1.9618(11) Å) in 4·(H2O)0.5
is slightly longer than the Cu–O(1) bond length (1.9302(19) Å)
in (4)n, which could be due to the involvement of the oxygen
atom O(1) of the former complex in hydrogen bonding with a
water molecule. The N(1)–Cu–N(3) bond angles in 4·(H2O)0.5
(162.42(6)°) and (4)n (162.18(9)°) are comparable to each other
and significantly larger than that reported for {C9H6N-(μ-N)-C
(O)CH2NH2}Cu(OCOCH3) (159.7(2)°).23 The N(1)–Cu–N(2) and
N(2)–Cu–N(3) bond angles in 4·(H2O)0.5 and (4)n are 83.99(6),
82.51(6) and 82.89(9), 81.50(9)°, respectively. The Cu(1)–O(3)–C
(2) bond angle in the catemeric structure of 4 is 134.47°.
The newly developed neutral pincer copper complexes
Q
Et2
(QNNNEt )CuX (2–4) along with cationic complexes, [( NNN
)
2
Cu(CH3CN)](OTf) (5) and [(QNN(H)NEt )Cu(CH3CN)](ClO4) (6),
were screened and employed for the Kumada coupling of alkyl
halides with alkyl magnesium chloride. Initially, complex 2
was screened for the Kumada coupling of (3-bromopropyl)
benzene (0.20 mmol) with cyclohexylmagnesium chloride (8a,
0.40 mmol) using a mild base LiOMe at 0 °C (Table 1), a reac-
tion condition previously reported by Liu for a similar reac-
tion.18b The desired coupled product 9aa was obtained in 15%
yield using catalyst 2 (entry 1). By employing catalysts 3 and 4,
the NMR yield of product 9aa was 20% and 18%, respectively
(entries 2 and 3). Notably, the use of cationic complex 5 as a
catalyst afforded 9aa in 57% isolated yield (entry 4). The better
activity of catalyst 5 might be due to the more electrophilic
nature of copper in complex 5 that would prefer the Grignard
nucleophiles. Furthermore, an easy access of a vacant coordi-
nation site around the copper center in complex 5 could be an
additional advantage. The use of complex 6 as a catalyst
afforded only 12% yield of product 9aa (entry 5), which might
be due to the less electrophilic character of 6. Next, we shifted
2
X-ray photoelectron spectroscopy studies of complexes
Compounds 4·(H2O)0.5 and (4)n were characterized by XPS to
determine the oxidation state of copper in both the complexes.
Although X-ray studies indicate that copper is in the +2 oxi-
dation state in both the complexes 4·(H2O)0.5 and (4)n, XPS
analysis has been performed to assign the oxidation state of
Cu in bulk compounds. XPS analysis has been carried out
using a Thermo Scientific instrument with an Al Kα mono-
chromator source and the obtained results are shown in
Fig. S1 in the ESI.† The Cu 2p spectrum of both the complexes
4·(H2O)0.5 and (4)n shows two main intense spin orbit splitting
peaks for 2p3/2 (933.5 eV) and 2p1/2 (953.4 eV). Along with the
main peaks, two broad satellite peaks (s) at 942.2 and 962.2 eV
were also observed. These satellite peaks normally appear for
Cu(II) complexes due to charge transfer from the ligand (AcO−)
to the Cu(II) centre. Hence, both the complexes have the
copper centre in the +2 oxidation state. The obtained binding
energies are in accordance with the earlier reports.24
Table 1 Optimization of reaction conditions for (QNNNEt )CuX cata-
2
lyzed Kumada couplinga
Entry
Cat. [Cu]
X
8a (equiv.)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
2
3
4
5
6
5
5
5
5
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
2
2
2
2
2
2
2
3
3
24
24
24
24
24
24
16
8
15
20
18
67 (57)c
12
58 (45)c
42
Furthermore, XPS analysis of the reaction mixture of 4
(crude mixture obtained via Scheme 2 before isolation) was
carried out to ascertain residual Cu(0) species that would be
obtained by the disproportionation of Cu(OAc) into Cu0 and
Cu2+. As shown in Fig. S2 in the ESI,† the Cu 2p spectrum
shows two main intense asymmetric spin orbit splitting peaks
70 (63)c
38
6
a Conditions: (3-Halopropyl)benzene (0.2 mmol), cyclohexyl-
magnesium chloride (0.3–0.6 mmol), LiOMe (0.2 mmol), THF
(1.0 mL). b Yield determined by 1H NMR. c Isolated yield is given in
parentheses.
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our attention to check the feasibility of the coupling of MeO and Cl(aryl) groups are well tolerated under the reaction
(3-chloropropyl)benzene, wherein 45% yield of product 9aa conditions. Notably, a C(sp3)–Cl bond is selectively reacted in
was isolated employing catalyst 5 (entry 6). Notably, the for- the presence of the C(sp2)–Cl bond (entry 5). This selectivity is
mation of the cyclohexyl-dimer as a by-product from CyMgCl very crucial as the aryl–Cl can be used for further functionali-
was severe when the reaction was continued for a longer time. zations. The dioxolane ring containing substrates 7f and 7g
Thus, on increasing the amount of CyMgCl to 3.0 equiv. and reacted with CyMgCl to give the alkylated products 9fa and 9ga
reducing the reaction time to 8 h, the coupled product 9aa was in 82% and 56% yields, respectively (entries 6 and 7).
isolated in 63% yield (entry 8). Furthermore, lowering the reac- Similarly, the linear halide, 1-octyl chloride reacted with
tion time to 6 h didn’t improve the reaction and resulted in CyMgCl and afforded product 9ha in 55% yield. In addition to
38% yield of 9aa (entry 9). The optimization data show that the coupling of CyMgCl, various acyclic alkyl Grignard coup-
performing the reaction for a longer time using a lower ling partners reacted smoothly with (3-bromopropyl)benzene
amount of Grignard reagent is not suitable for achieving a to deliver the alkylated products in excellent yields (entries
good yield of the desired product; however, a shorter reaction 9–12). The major by-products in all these reactions are the
time with more Grignard reagent was helpful.
alkyl-dimers resulting from the alkyl magnesium chlorides
With the optimized reaction conditions in hand, we (Grignard reagents). Although some reports exist for the
explored the scope for coupling of various alkyl and benzyl copper-catalyzed Kumada coupling of alkyl tosylates and mesy-
halides with different alkyl Grignard reagents (Table 2). Thus, lates with alkyl Grignard reagents,18 herein we have demon-
the aryl substituted alkyl chlorides and bromides bearing strated the Kumada coupling of unactivated and more challen-
β-hydrogen atoms efficiently reacted with cyclohexyl mag- ging alkyl chlorides in addition to alkyl bromides employing a
nesium chloride (CyMgCl) to give the desired alkylated pro- well-defined pincer copper catalyst. Considering the activity of
ducts 9aa–9ea in good yields (Table 2, entries 1–5). Both the the copper-catalyst for the coupling of challenging alkyl chlor-
ides demonstrated here, the present catalytic protocol is
unique from the literature precedented examples.18
Apart from the coupling of alkyl halides, diverse benzyl
halides could couple with CyMgCl to afford the desired benzy-
Table 2 Scope for the Kumada coupling of alkyl halides catalyzed by 5a
lated alkylation in moderate yields (Scheme 5). Though the
Grignard
reagent (8)
Yield
(%)b
yields of the coupling products were moderate, the functional
group tolerability for this coupling is significant. Important
functional groups such as –Br, –Cl, and –NO2 at different posi-
tions of the aryl ring are well tolerated. The benzyl halides
bearing both the electron-donating (–OMe) as well as electron-
withdrawing (–NO2) substituents were reacted with compara-
tive activity. The observed low yields for the coupling of benzyl
halides with CyMgCl are primarily due to the low reactivity of
benzyl halides under the optimized reaction conditions.
Entry Alkyl halide (7)
Product (9)
1
2
R = H, X = Cl (7a′)
R = OMe, X = Br (7b)
R = H, (9aa)
R = OMe, (9ba)
63
90
3
4
5
6
R = H, (7c)
R = OMe, (7d)
R = Cl, (7e)
R = H, (9ca)
R = OMe, (9da)
R = Cl, (9ea)
76
82
62c
82
7
8
56
55
n-Octyl-Cl (7h)
9
MeMgCl (8b)
EtMgCl (8c)
93
92
10
7a
7a
7a
11
12
iPrMgCl (8d)
tBuMgCl (8e)
90
90
a Conditions: Alkyl halide (0.2 mmol), alkyl magnesium chloride
(0.6 mmol), LiOMe (0.2 mmol), THF (1.0 mL), 0 °C, 8 h. b Yields of iso-
lated compounds. c Yield determined by 1H NMR using CH2Br2 as an
internal standard.
Scheme 5 Kumada coupling of benzyl halides with cyclohexyl mag-
nesium chloride. Yield determined by 1H NMR using CH2Br2 as an
internal standard.
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(dec). Anal. calcd for C15H18ON3ClCu: C, 50.70; H, 5.11; N,
Conclusions
11.83. Found: C, 50.35; H, 4.79; N, 11.38. HRMS (ESI): m/z
In summary, we have demonstrated the synthesis and catalytic calcd for C15H18ON3ClCu + H+ [M + H]+ 355.0507, found
application of (QNNNEt )CuX complexes for Kumada coupling 355.0500 and C15H18ON3ClCu–Cl+ [M − Cl]+ 319.0740, found
2
of unactivated alkyl chlorides and bromides. Both neutral and 319.0734.
cationic copper(II) complexes, (QNNNEt )CuX (2–5), were devel-
Note: On employing CuCl as a metal precursor instead of
2
oped from copper(II) as well as from copper(I) precursors. A cat- CuCl2, compound 2 was obtained in 49% yield (0.067 g).
ionic copper(I) complex, [(QNN(H)NEt )Cu(MeCN)](ClO4) (6),
Synthesis of (QNNNEt )CuBr (3). This compound was syn-
2
2
was synthesized using
a
[Cu(MeCN)4](ClO4) precursor. thesized following a procedure similar to the synthesis of
Complexes 2–5 were characterized by elemental analysis and complex 2, using (QNNNEt )-H (0.1 g, 0.389 mmol), CuBr2
HRMS, whereas complex 6 was characterized by NMR, MALDI (0.087 g, 0.389 mmol) and Et3N (0.506 mmol, 0.071 mL). The
and elemental analysis. Molecular structures of 2, 3 and 4 were crude product was recrystallized in toluene/Et2O to obtain a
elucidated by single crystal X-ray diffraction studies. Complex 4 crystalline compound 3. Yield: 0.115 g, 74%. M.p. = 124 °C
2
crystallizes in dimeric form with bridged H2O i.e. [4·(H2O)0.5
]
(dec). Anal. calcd for C15H18ON3BrCu: C, 45.07; H, 4.54; N,
as well as in catemeric form (4)n. XPS analysis of complexes 10.51. Found: C, 44.48; H, 4.28; N, 10.19. HRMS (ESI): m/z
[4·(H2O)0.5] and (4)n, obtained from both the Cu(II) and Cu(I) calcd for C15H18ON3BrCu–Br+ [M − Br]+ 319.0740, found
precursors, highlights that the copper species are present in 319.0735 and C15H18ON3BrCu–H+ [M − H]+ 396.9846, found
the +2 oxidation state. The cationic copper complex 5 is found 396.9861.
to be an active catalyst for the Kumada coupling of unactivated
Note: On employing CuBr as a metal precursor instead of
alkyl and benzyl chlorides and bromides with alkyl Grignard CuBr2, compound 3 was obtained in 50% yield (0.078 g).
Synthesis of (QNNNEt )Cu(OAc)·(H2O)0.5 [4·(H2O)0.5]. This
2
reagents.
compound was synthesized following the procedure similar to
the synthesis of complex 2, using (QNNNEt )-H (0.1 g,
2
0.389 mmol), Cu(OAc)2 (0.071 g, 0.389 mmol) and Et3N
(0.506 mmol, 0.071 mL). The crude product was recrystallized
Experimental section
General considerations
in toluene/Et2O to obtain a crystalline compound 4·(H2O)0.5
.
All manipulations were conducted under an argon atmosphere Yield: 0.103 g, 70%. M.p. = 131 °C (dec). Anal. calcd for
either in a glove box or using standard Schlenk techniques in C17H21O3N3Cu·(H2O)0.5: C, 52.64; H, 5.72; N, 10.83. Found: C,
pre-dried glassware. The catalytic reactions were performed in 52.57; H, 5.61; N, 10.96. HRMS (ESI): m/z calcd for
flame-dried Schlenk tubes. Solvents were dried over Na/benzo- C17H21O3N3Cu + H+ [M + H]+ 379.0952, found 379.0307 and
phenone or CaH2 and distilled prior to use. Liquid reagents C17H21O3N3Cu–OAc+ [M − OAc]+ 319.0740, found 319.0745.
were flushed with argon prior to use. The ligand 120a and
Note: On employing Cu(OAc) as a metal precursor instead of
25
[(CH3CN)4Cu]ClO4
were synthesized according to the pre- Cu(OAc)2, compound (4)n was obtained in 46% yield (0.068 g).
Synthesis of [(QNNNEt )Cu(MeCN)]OTf (5). To an oven dried
2
viously described procedures. All other chemicals were
obtained from commercial sources and were used without Schlenk flask, complex 2 (0.05 g, 0.141 mmol) and AgOTf
further purification. Yields refer to isolated compounds, esti- (0.036 g, 0.141 mmol) were introduced and acetonitrile
mated to be >95% pure. TLC: TLC Silica gel 60 F254. Detection (10 mL) was added into it. The reaction mixture was stirred at
under UV light at 254 nm. Chromatography: separations were room temperature for 3 h. The resultant mixture was filtered
carried out on Spectrochem silica gel (74–149 µm, through a cannula and the mother liquor was concentrated
100–200 mesh). M.p.: Büchi 540 capillary melting point appar- under vacuum and Et2O (3 mL) was added to obtain a pure
atus, values are uncorrected. NMR (1H and 13C) spectra were blue-green compound 5. Yield: 0.057 g, 79%. M.p. = 230 °C
recorded at 400 or 500 (1H), 100 or 125 {13C, DEPT}, respect- (dec). Anal. calcd for C18H21O4N4F3SCu: C, 42.39; H, 4.15; N,
ively on Bruker AV 400 and AV 500 spectrometers in CDCl3 10.99; S, 6.29. Found: C, 42.97; H, 4.22; N, 10.29; S, 6.29.
solutions, if not specified; chemical shifts (δ) are given in MALDI-TOF: m/z calcd for C18H21O4N4F3SCu [M+] 509.0532,
1
ppm. The H and 13C NMR spectra are referenced to residual found 508.8325 and [C18H21O4N4F3SCu–(CH3CN
+
OTf)]+
solvent signals (CDCl3: δH = 7.26 ppm, δC = 77.2 ppm).
[M − (CH3CN + OTf)]+ 319.0740, found 318.9736.
Synthesis of (QNNNEt )CuCl (2). To an oven dried Schlenk
Synthesis of [(QNN(H)NEt )Cu(CH3CN)]ClO4 (6). A Schlenk
2
2
flask were introduced ligand (QNNNEt )-H (0.1 g, 0.389 mmol) flask was charged with ligand 1 (0.1 g, 0.389 mmol) and
and CuCl2 (0.052 g, 0.387 mmol), and THF (10 mL) was added [(CH3CN)4Cu]ClO4 (0.127 g, 0.389 mmol), and acetonitrile
into it. To the resultant reaction mixture, Et3N (0.071 mL, (10 mL) was added into it. The reaction mixture was refluxed
0.506 mmol) was added and the reaction mixture was stirred at for 3 h and was cooled to ambient temperature. The resultant
70 °C for 3 h in a preheated oil bath. The reaction was cooled reaction mixture was filtered and the filtrate was evaporated
2
to room temperature and all the volatiles were evaporated. The under vacuum. Washing the solid residue with diethyl ether
product was then extracted with toluene (10 mL × 2), concen- (8 mL × 3) afforded the crystalline compound [(QNN(H)NEt
)
2
1
trated under vacuum and Et2O (3 mL) was added to obtain Cu(CH3CN)]ClO4 (6). Yield: 0.138 g, 77%. H NMR (400 MHz,
crystalline compound 2. Yield: 0.104 g, 75%. M.p. = 127 °C CD3CN): δ 10.22 (br s, 1H, NH), 8.89 (d, J = 3.8 Hz, 1H, Ar–H),
16752 | Dalton Trans., 2018, 47, 16747–16754
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8.64 (d, J = 7.6 Hz, 1H, Ar–H), 8.33 (d, J = 8.4 Hz, 1H, Ar–H),
7.71 (d, J = 8.4 Hz, 1H, Ar–H), 7.63–7.55 (m, 2H, Ar–H), 4.21 (s,
2H, CH2), 3.30 (q, J = 6.9 Hz, 4H, CH2), 2.61 (s, 3H, CH3CN),
1.31 (t, J = 7.2 Hz, 6H). 13C NMR (100 MHz, CD3CN): δ 164.1
(Cq), 150.0 (CH), 138.9 (Cq), 137.3 (CH), 134.2 (Cq), 128.9 (Cq),
127.7 (CH), 124.0 (CH), 123.1 (CH), 117.8 (CH), 55.5 (CH2),
51.0 (2C, CH2), 9.5 (2C, CH3), 1.8 (CH3CN). Anal. calcd for
C17H22ClO5N4Cu: C, 44.26; H, 4.81; N, 12.14. Found: C, 43.97;
H, 4.52; N, 11.94. MALDI-TOF: m/z calcd for C17H22ClO5N4Cu
[M+] 460.06, found 460.12 and [C17H22ClO5N4Cu–(CH3CN +
ClO4)]+ [M − (CH3CN + ClO4)]+ 320.08, found 320.07.
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Representative procedure for Kumada coupling
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Synthesis of (3-cyclohexylpropyl)benzene (9aa). To a flame-
dried Schlenk tube equipped with a magnetic stir bar was
introduced catalyst 5 (0.005 g, 0.01 mmol), LiOMe (0.008 g,
0.2 mmol) and (3-chloropropyl)benzene (0.031 g, 0.2 mmol),
and THF (1.0 mL) was added under an argon atmosphere. The
reaction mixture was cooled to 0 °C and CyMgCl (8a, 0.3 mL,
2.0 M in Et2O, 0.6 mmol) was added under argon. The resul-
tant reaction mixture was then stirred at 0 °C for 8 h. At
ambient temperature, saturated aqueous ammonium chloride
solution (10 mL) was added and the reaction mixture was
extracted with CH2Cl2 (20 mL × 3). The combined organic
layers were dried over Na2SO4 and the solvent was evaporated
in vacuo. The resulting residue was purified by column chrom-
atography on silica gel (petroleum ether) to yield 9aa
(0.0255 g, 63%) as a colorless liquid.
Conflicts of interest
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There are no conflicts to declare.
Acknowledgements
This work was financially supported by the SERB, New Delhi,
India (EMR/2016/000989). H. P. thanks the CSIR – New Delhi
for research fellowship. We are grateful to Dr (Mrs)
Shanthakumari for HRMS analyses, Dr S. P. Borikar for GC-MS
analyses and Mr Deo Shriniwas for XPS analysis.
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