Single-step oxidative homocoupling of aryl Grignard reagents via Co(II), Ni(II) and Cu(II) Complexes under air

Article abstract of DOI:10.1002/aoc.3130

A simple catalytic system of direct synthesis for the symmetrical biaryls using catalytic amounts of Co(II), Ni(II) and Cu(II) complexes has been developed. The reaction system involves in situ synthesis of Grignard reagents. The complexes, containing bidentate Schiff base and dmit (2-thioxo-1,3-dithiole- 4,5-dithiolate) ligands, were compatible with diverse functionalities and afford a high yield of biaryls in a single step, proving to be promising catalysts in homocoupling reactions. Atmospheric oxygen is used as an oxidant which renders a green, simple and economical catalytic route. Copyright

Full text of DOI:10.1002/aoc.3130

Full Paper
Received: 13 December 2013
Revised: 1 February 2014
Accepted: 1 February 2014
Published online in Wiley Online Library: 22 April 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3130
Single-step oxidative homocoupling of aryl
Grignard reagents via Co(II), Ni(II) and Cu(II)
Complexes under air
Aparna P. I. Bhat and Badekai Ramachandra Bhat*
A simple catalytic system of direct synthesis for the symmetrical biaryls using catalytic amounts of Co(II), Ni(II) and Cu(II) com-
plexes has been developed. The reaction system involves in situ synthesis of Grignard reagents. The complexes, containing
bidentate Schiff base and dmit (2-thioxo-1,3-dithiole-4,5-dithiolate) ligands, were compatible with diverse functionalities
and afford a high yield of biaryls in a single step, proving to be promising catalysts in homocoupling reactions. Atmospheric
oxygen is used as an oxidant which renders a green, simple and economical catalytic route. Copyright © 2014 John Wiley &
Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: Schiff base; homocoupling; Grignard reagents; biaryl; atmospheric oxygen
Herein, we wish to report the synthesis of symmetrical biaryls
directly from the combination of metallic magnesium and aryl
Introduction
The synthesis of biaryls is of particular interest to the industry, which
is easily shown by their broad applications in multidisciplinary
fields.[1,2] Transition-metal catalyzed homocoupling of Grignard
reagents is one of the most efficient synthetic methods for the con-
struction of a symmetrical biaryl backbone.[3] To date, many reports
have been published for coupling reactions using varieties of metal-
lic reagents such as NiCl2, CuCl2, TiCl4, Ce(OTf)4, FeCl3, MnCl2 and Pd
(OAc)2.[4–10] Demand for stoichiometric amounts of an organic oxi-
dant or an extra ligand, however, limits their use for industrial-scale
applications. In recent years, the necessity for homogeneous cata-
lysts with inexpensive green oxidants, such as molecular oxygen
for such transformations, has increased. Iron-,[11] manganese-[11]
and palladium-based[12] catalysts are well reported for such
homocoupling reactions, but the problem encountered with these
later methods is their two-step synthetic route, by which organo-
metallic compounds were prepared first, and then converted into
biaryl product in a separate reaction.
Transition metals in combination with different Schiff base li-
gands show high catalytic activity in reactions of industrial impor-
tance. The activity of Schiff base complexes showed significant
improvements, especially when a functional group like ―OH or
―SH was present close to the azomethine group so as to form a
five- or six-membered ring with the metal ion.[13] The easy elec-
tron-donating ability of Schiff base ligands play an important role
in C―C bond-forming reactions at low temperature. Schiff bases
of aminonaphthalene sulfonic acid (ANSA) and its complexes are
gaining importance in various reactions of organic transforma-
tions, especially in biological studies, because their ease of synthe-
sis, different coordination possibilities because of their flexibility
concerning the C¼N group and consequently their complexing
behavior with various metal ions.[14] Though these sulfur-containing
Schiff bases have been receiving increasing interest owing to the
aforementioned advantages, very little work has been reported on
the development of catalysts for the homocoupling reaction.
halides using catalytic amounts of Co(II), Ni(II) and Cu(II)
complexes. 2-Thioxo-1,3-dithiole-4,5-dithiolate (dmit) and 2-
aminonaphthalenesulfonic acid (ANSA) derivatized Schiff base
were used as ligands for complexation. The present catalytic
methodology involves in situ formation of the Grignard reagent
and atmospheric oxygen as an oxidant.
Experimental
Methods
All chemicals used during the experiments were of analytical
grade. The C, H, N and S contents of the ligand and complexes
were determined using a Thermoflash EA1112 series elemental
analyzer. Magnetic susceptibility measurements were recorded
on a Sherwood Scientific instrument (UK). Thermal analysis was
carried out (EXSTAR-6000) from room temperature to 700°C at a
heating rate of 10°C minꢀ1. The electronic spectrum of the com-
plex was measured on a GBC Cintra 101 UV–visible double-beam
spectrophotometer using DMF in the 200–800 nm range. FT-IR
spectra were recorded on a Thermo Nicolet Avatar FT-IR spec-
trometer in the frequency range 400–4000 cmꢀ1. Mass spectra
1
were recorded on a JEOL SX 102/Da-600 spectrometer. H NMR
spectra were recorded in a Bruker AV 400 instrument using
tetramethylsilane as an internal standard. Coupling reactions
were monitored by gas chromatography (Shimadzu 2014).
*
Correspondence to: Badekai Ramachandra Bhat, Catalysis and Materials Lab-
oratory, Department of Chemistry, National Institute of Technology Karnataka,
Surathkal, Srinivasanagar-575025, India. E-mail: chandpoorna@yahoo.com
Catalysis and Materials Laboratory, Department of Chemistry, National Insti-
tute of Technology Karnataka, Surathkal, Srinivasanagar-575025, India
Appl. Organometal. Chem. 2014, 28, 383–388
Copyright © 2014 John Wiley & Sons, Ltd.

A. P. I. Bhat and B. R. Bhat
Synthesis
calculated for C20H13CuNNaO6S6 [M+]: 642.2; found: 643.4; UV–visible
λ
max (nm): 275, 345, 420, 482, 562. μeff: 1.65.
Synthesis of ligand L1
General Procedure for Homocoupling Reaction
The ligand was synthesized by a similar method to that described
in the literature.[15,16] A 1:1 equimolar ethanolic solution of 2-
aminonaphthalene sulfonic acid (1 mmol) and 2,4-dihydroxy-
benzaldehyde (1 mmol) were mixed and heated for 2 h with
constant stirring. The characteristic yellow precipitate obtained
was filtered, washed, recrystallized from ethanol and dried in
vacuo. The purity of the prepared ligand was checked by thin-
layer chromatography and elemental analysis.
In this procedure, magnesium turnings (13 mmol, 0.320 g atom) were
placed in a two-necked round-bottom flask with a calcium chloride
guard tube and a crystal of iodine was added. Aryl halide (2 mmol
of 10 mmol in total) in 5 ml anhydrous diethyl ether was added with
constant stirring at room temperature. Initially, an increase in the tem-
perature of the reaction mixture was observed and the appearance of
turbidity after few minutes indicated the initiation of the reaction. The
remaining aryl halide (8 mmol) in 5 ml ether was added dropwise and
the reaction mixture was stirred for 40 min. The catalyst (0.13 mol%)
was added to the reaction mixture. Stirring was continued and after
5 h the reaction mixture was cooled and hydrolyzed with a saturated
solution of 10% aqueous ammonium chloride. The product was
extracted with ether; the combined organic layers were dried over an-
hydrous magnesium sulfate. The product obtained compared well
with the authentic samples using gas chromatography.
Synthesis of complexes (C1, C2, C3)
The complexes C1, C2 and C3 were synthesized strictly under an-
hydrous conditions (Fig. 1). To a methanolic (20 ml) solution
containing CS2 (0.2 ml, 3 mmol) and sodium metal, L1 (1 mmol)
and metal acetate [M(CH3COO)2] (M = Co, Ni and Cu) (1 mmol)
was added with constant stirring. The mixture was refluxed for
4 h. The colored complexes obtained were filtered, washed with
methanol followed by petroleum ether (60–80°C) and dried in
vacuo. Compounds were soluble in DMSO and DMF.
Results and Discussion
C1:, Yield 0.400 g, 61%; m.p. 320–323°C. Elemental analysis calcu-
lated for C20H15CoNNaO7S6 (%): C, 36.64; H, 2.31; N, 2.14; S, 29.34;
found: C, 36.56; H, 2.28; N, 2.13; S, 29.25; FT-IR (KBr, cmꢀ1): 3381
ν(OH), 1605 ν(CH¼N), 1220 ν(C―O), 1454, 1247 ν(C¼C), 1056,
1018 ν(C¼S), 994 ν(C―S), 617 ν(SC¼CS), 847 δr(H2O), 526 δw(H2O);
MS (ESI) (m/z) calculated for C20H15CoNNaO7S6 [M+]: 655.6; found:
656.3; UV–visible λmax (nm) 280, 353, 419, 456; μeff 4.26.
C2:, Yield 0.439 g, 67%; m.p. >350°C. Elemental analysis calculated
for C20H15NNaNiO7S6 (%): C, 36.65; H, 2.31; N, 2.14; S, 29.35. found:
C, 36.54; H, 2.28; N, 2.13; S, 29.23; FT-IR (KBr, cmꢀ1): 3299 ν(OH),
1609 ν(CH¼N), 1220 ν(C―O), 1427 ν(C¼C), 1047 ν(C¼S), 982, 931
ν(C―S), 615 ν(SC¼CS), 856 δr(H2O), 541 δw(H2O); MS (ESI) (m/z) cal-
culated for C20H15NNaNiO7S6 [M+]: 655.4; found: 656.7; UV–visible
Synthesis
Elemental analyses (C, H, N, S) were in good agreement with the
molecular formula proposed for L1 and complexes C1–C3. The
electronic absorption spectra of ligand and complexes were
made in DMSO. The spectra of the ligand displayed three main
bands. The bands in the region of 280 nm were assigned to the
π–π* transitions of the aromatic rings. The band at 312–359 nm
involved n–π* transition of the S¼O and C¼N groups. The band
in the region 404 nm can be attributed to a charge transfer tran-
sition within the ligand (CT). In the spectra of all the complexes,
the bands in the region 280–357 nm were assigned to intra-li-
gand charge transfer transition (ILCT). The spectra of C1 showed
ligand-to-metal charge transfer transition (LMCT) in the region
419–456 nm and the spectra of C2 showed d–d transition in the
range 481–515 nm, which suggests an octahedral geometry for
the complexes. The magnetic moment values for C1 and C2 were
4.26 and 3.58 BM, respectively, supporting octahedral geome-
try.[17,18] The spectrum of C3 showed d–d transition in the range
λmax (nm): 286, 357, 422, 481, 515; μeff: 3.58.
C3:, Yield 0.405 g, 63%; m.p. >350°C. Elemental analysis calculated
for C20H13CuNNaO6S6 (%): C, 37.40; H, 2.04; N, 2.18; S, 29.96; found:
C,37.37; H, 2.02; N, 2.17; S, 29.84; FT-IR (KBr, cmꢀ1): 3313 ν(OH), 1606
ν(CH¼N), 1200 ν(C―O), 1428 ν(C¼C), 1051 ν(C¼S), 1002, 954
ν(C―S), 615 ν(SC¼CS), 859 δr(H2O), 550 δw(H2O); MS (ESI) (m/z)
Figure 1. Synthesis of complexes C1, C2 and C3.
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Appl. Organometal. Chem. 2014, 28, 383–388

Single-step oxidative homocoupling of Grignard reagents under air
of 482–562 nm and its magnetic moment value 1.65 BM con-
firmed the trigonal bipyramidal structure.[19] The FT-IR spectrum of
the ligand exhibited intense bands at 1617 cmꢀ1, corresponding to
an azomethine ν(CH¼N). The band in the region 1241 cmꢀ1 was
assigned to phenolic ν(C―O). Thus the ligand is bidentate in nature
with N,O coordination sites. In the spectra of the complexes, ν(CH¼N)
shifted to lower frequencies, i.e. 1609–1605 cmꢀ1, which confirmed
imine N coordination to the metal ions. The ν(C―O) was shifted to
lower frequencies in all the complexes, suggesting the coordination
of phenolic oxygen to the metal ion. Thus the ligand binds the metal
ion in a bidentate fashion through the phenolic oxygen of the phenyl
ring and nitrogen of the azomethine group. The spectra of com-
plexes exhibited a broad band around 3381–3299 cmꢀ1, which was
assigned to water molecules, ν(OH), associated with the complexes.
In addition to these modes, coordinated water exhibited δr(H2O)
rocking near 859–847 cmꢀ1 and δw(H2O) wagging near
of in situ generated Grignard reagents was shown to be a more
convenient and clear-cut approach than the self-coupling of
organometallic derivatives in the presence of an oxidant. All
synthetic difficulties, related to the preparation of the Grignard
reagent when functional groups were present, were thus
avoided. Iodine was used as a surface activator.[24] To avoid cou-
pling side products, care was taken while adding the aryl halide
at the initial stage of preparation of Grignard reagents. On fast
addition of bromobenzene, we obtained phenol as a main side
product. The coupling reaction was successfully carried out at
room temperature, making the reaction system energy efficient.
The products formed were quantified by gas chromatography.
Control experiments
Various control experiments were carried out to optimize the
reaction conditions with respect to different parameters: solvent,
time, catalyst concentration and atmospheric oxygen.
Phenylmagnesium bromide (1e) was used as standard substrate.
The activities of complexes C1–C3 were examined in various sol-
vents such as tetrahydrofuran, diethyl ether and benzene, under
atmospheric oxygen (Table 1). The best conversion was observed
in diethyl ether as ether molecules actually coordinate with
Grignard reagent and thus help in stabilizing the intermediate
complex.[25] The lowest conversion was observed in benzene.
This could be attributed to the formation of an insoluble deposit
at the metallic surface in non-polar solvents, which could gradu-
ally inhibit the formation of the Grignard reagent. The reaction
mixture was analyzed at regular intervals of time under similar
reaction conditions. As shown in Fig. 2, the yield increased with
reaction time and maximum yield was observed in 6 h. Among
the complexes C1–C3, complex C2 gave the maximum yield.
The substrate:catalyst ratio varied from a total catalyst amount
of 0.04 to 0.21 mol% and the effect of catalyst concentration on
the coupling reaction was examined (Table 2). The coupling
product increased with increase in catalyst concentration and
all the complexes were capable of catalyzing the reaction to high
yield using 0.13 mol% catalyst loading (entry 4). In the present
study, environmentally friendly atmospheric oxygen was used
as an oxidant. Importantly, no extra organic oxidants, ligands or
additives were utilized, which makes the reaction system green
and cost effective. In an inert atmosphere, a lower yield of cou-
pling product was observed (entry 7). This can be attributed to
550–526 cmꢀ1 [20]
The FT-IR spectra of complexes exhibited strong
.
bands in the region 1454–1247 cmꢀ1, which was assigned to C¼C
stretching vibration of dmit.[21] The bands in the regions 1018–1056
and 931–1002 cmꢀ1 could be attributed to C¼S and C―S stretching
vibrations, respectively.[22,23] Bands in the region 615–617 cmꢀ1 were
assigned to SC¼CS twisting mode. From TG analysis, the weight loss
observed for C1 was in the temperature ranges of 150–193, 193–440
and 440–580°C, corresponding to 5.2% (calcd 5.4%) mass loss of two
water molecules, 29.1% (calcd 29.9%) mass loss of C3S5 moiety and
55.1% (calcd 55.5%) mass loss of one molecule of the ligand, respec-
tively. Weight loss observed for C2 was in the temperature ranges of
130–161, 161–532.9 and 532.9–784.5°C, corresponding to 5.4% (calcd
5.4%) mass loss of two water molecules, 29.4% (calcd 29.8%) mass
loss of C3S5 moiety and 55.2% (calcd 55.4%) mass loss of one mole-
cule of ligand, respectively. The weight loss observed for C3 was in
the temperature ranges of 150–189, 189–556 and 556–959.7°C, corre-
sponding to 2.6% (calcd 2.8%) mass loss of one water molecule,
56.5% (calcd 56.7%) mass loss of one molecule of ligand and 30.1%
(calcd 30.5%) mass loss of C3S5 moiety, respectively. In all cases,
respective metal oxides were obtained as the decomposition residue.
This further supports the assumed structure of the complexes.
Homocoupling Reaction
The catalytic methodology involves in situ preparation of
Grignard reagent. The present method of direct homocoupling
Table 1. Homocoupling of phenylmagnesium bromide (1e) in different solvents under aira
Entry
Solvent
Temp. (°C)
Time (h)
Yield (%)b
C1
C2
C3
1
2
3
THF
Et2O
60
r.t.c
40
8
6
41
57
20
60
75
39
23
30
14
Benzene
10
Bold entries is to emphasize the highest coupling yield in Et2O.
aReaction conditions: Mg turnings (13 mmol), aryl halide (10 mmol), catalyst (0.13 mol%), solvent (10 ml).
bIsolated yield.
cRoom temperature.
Appl. Organometal. Chem. 2014, 28, 383–388
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A. P. I. Bhat and B. R. Bhat
in the absence of complexes (entry 1), only traces of coupling
products were observed, underlining the vital role of complexes
as an efficient catalysts. For comparison purposes, a series of
blank experiments were carried out using cobalt acetate, nickel
acetate and copper acetate as catalysts without the addition of
either dmit or ligand L1 (entry 9). A reduced yield of coupling
product was observed. This confirms the effectiveness of synthe-
sized complexes as potential catalysts for the present catalytic
system. The yields are not very sensitive to the catalyst loading
at higher concentration. This could be reasoned by the fact that
the reactions conducted at high catalyst loading might experi-
ence rate-limiting mass transfer of oxygen gas into solution.[26]
Indeed, the solubility of oxygen in most of solvents is poor and
its rate of dissolution is very low. Hence, at high catalyst loading,
fixation of oxygen might be taking place. The mass transfer
effects are especially significant for aerobic reactions because of
the susceptibility of the reduced catalyst to decompose into inac-
tive metal. Thus slow transfer of oxygen gas into solution not only
affects the reaction rate but also contributes to catalyst decom-
position. Catalyst decomposition might be associated with cata-
lyst aggregation. If the catalyst decomposes during the course
of the reaction, the concentration of active Ni catalyst will
decrease and the rate will decrease proportionately. It should
be noted that mass transfer limitation effects include the invari-
ance of the rate on catalyst above a threshold concentration.[26]
Figure 2. Effect of reaction time on the coupling yield.
the lack of formation of peroxo-metal(III) active species, hence
proving the significance of atmospheric oxygen as an oxidant
in the present system. Prominently, the present coupling system
can be readily scaled up to the reaction of 50.0 mmol aryl halide
using 0.6 mol% catalyst, which afforded a satisfactory amount of
biaryl with a negligible amount of byproducts (entry 8). Notably,
Proposed mechanism
Table 2. Yield dependence on catalyst loading and aira
A proposed mechanism is depicted in Fig. 3 (top) for the C1–
C3-catalyzed reaction. A low-valent metal species is generated
by the Grignard reagent, which is a strong reducing agent[27] in-
volved in the catalytic cycle. The oxidative addition of molecular
oxygen to a low-valent metal complex (a) forms a peroxo-metal
(III) intermediate (b), which is the key step for this catalytic cy-
cle.[28] It is then reacted with two equivalents of RMgX to give
biaryl-metal(III) intermediate (c) and XMgOOMgX,[11,29] which un-
dergoes a rapid reductive elimination and eventually yields the
homocoupling product Ar―Ar (d) and (a). It is very reasonable
to think that the formation of the unstable metal(III) species (c)
is required to achieve a very quick reductive elimination that
gives (d). The best way to favor reductive elimination is to in-
crease the oxidation state of the metal.[11]
The formation of a peroxo-nickel(III) intermediate was con-
firmed by UV–visible analysis (Fig. 3 bottom). The absorption
peak of the catalytic reaction mixture that appeared at 420 nm
can be attributed to a peroxo-nickel(III) intermediate[28] and its
absence in the reaction mixture carried out under inert condition
further confirms the necessity for atmospheric oxygen in the
present catalytic system. Various iron-,[30] copper-[31] and palla-
dium-catalyzed[32]-catalyzed reactions are well established
with such a peroxo complex as catalytic intermediate.
Entry
Catalyst (mol%)
Time (h)
Yield (%)b
1
No catalyst
C1 (0.04)
C2 (0.04)
C3 (0.04)
C1 (0.08)
C2 (0.08)
C3 (0.08)
10
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
01
17
29
11
41
54
19
57
75
30
55
73
27
54
73
27
2
2
3
4
C
C
C
1 (0.13)
2 (0.13)
3 (0.13)
5
C1 (0.17)
C2 (0.17)
C3 (0.17)
C1 (0.21)
C2 (0.21)
C3 (0.21)
C1 (0.13)
6
7c
8d
9
C
C
2 (0.13)
3 (0.13)
4
1
C1 (0.6)
C2 (0.6)
55
72
29
18
29
9
C3 (0.6)
Scope of the reaction
Co(acetate) (0.13)
Ni(acetate) (0.13)
Cu(acetate) (0.13)
This promising result encouraged us to extend the scope of the
reaction to various aryl Grignard reagents (Table 3). Thus simple
and functionalized biaryl compounds were synthesized in good
yields (entries 1–10). The effect of the Grignard reagent with
various substituents on the coupling reaction was studied. The
para-substituted Grignard reagents resulted in higher coupling
yield (entries 1 and 3). Introduction of a methyl group at the ortho
position of the aryl halide resulted in somewhat lower yield un-
der similar conditions (entries 2 and 4). It is noteworthy that the
aReactions were carried out with 10 mmol bromobenzene in the
presence of atmospheric molecular oxygen.
bIsolated yield.
cInert atmosphere (argon).
dWith 50.0 mmol bromobenzene in the presence of 0.6 mol%
catalyst for10 h.
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Appl. Organometal. Chem. 2014, 28, 383–388

Single-step oxidative homocoupling of Grignard reagents under air
present reaction system is tolerant to aryl chloride functionality
(entry 6). Also, it successfully allows the coupling of heteroaryl
Grignard reagent 1g (entry 7). Although the sterically demanding
substrate 1h required a higher reaction temperature and longer
reaction time, it gave the corresponding biaryl in appreciable
yield (entry 8). The reaction is chemoselective; thus nitrile or nitro
groups are tolerated (entries 9 and 10). The catalytic activity
results reveals that an electron-donating group on the Grignard
reagent enhances the yield of the coupling product compared
to that of an electron-withdrawing group. This could be
accounted for by two factors: yield of the Grignard reagent and
the reductive elimination step. In the present system of
homocoupling of the in situ generated Grignard reagent, the
yield of biaryls mainly depend on the amount of Grignard re-
agent generated. The yield of Grignard reagent is enhanced with
the aryl halide-containing electron-donating substituent, com-
pared with that of an electron-withdrawing group. An enhanced
yield of the Grignard reagent results in an enhanced yield of cou-
pling product.[33] Another factor is the faster reductive elimina-
tion from the complex containing aryl halide substituted with
an electron-donating group, as compared with the complex
containing aryl halide attached to an electron-withdrawing
substituent. The electron-donating group on the aryl halide
decreases the strength of the M―C bond and makes it less ionic;
the decrease in ionic character decreases the thermodynamic
bond strength.[34] Thus the groups whose electronic properties
decrease the strength of the M―C bond tend to increase the
thermodynamic driving force for the reductive elimination.
From the catalytic activity studies, it was observed that com-
plex C2 showed the highest catalytic activity, which can be attrib-
uted to the faster oxidative addition reaction in the catalytic
cycle.[35] This can be credited to its electron-rich metal center
compared to that of C1. The lesser catalytic activity of complex
C3 can be attributed to the difficulty in the two-electron
oxidation of Cu(I) in association with the greater stability due to
its completely filled d orbitals.
Figure 3. (top) Proposed mechanism for the C1–C3-catalyzed
homocoupling reaction. (bottom) UV–visible spectral evidence for the
formation of peroxo-Ni(III) active species.
Table 3. C1–C3-catalyzed homocoupling of aryl Grignard reagents
Conclusions
under aira
We have developed a Co(II)-, Ni(II)- and Cu(II)-catalyzed procedure
for the homocoupling of Grignard reagents to afford symmetrical
biaryls in a single step. The reactions were carried out under mild
conditions using atmospheric oxygen as an oxidant. In situ prep-
aration of the Grignard reagent makes the reaction system simple
and economical. It should be noted that the reaction system is
tolerant to chloro-, nitro-, cyano- and hetero-aryl functionalities
and afford good to high yields of symmetrical biaryls with a
minimum amount of catalyst. This simple, economical and envi-
ronmentally friendly catalytic route constitutes an interesting
contribution to the large-scale synthesis of symmetrical biaryls.
Entry
R―
Yield (%)b
C1
C2
C3
1
p-MeC6H4― (1a)
o-MeC6H4― (1b)
p-MeOC6H4― (1c)
o-MeOC6H4― (1d)
C6H5― (1e)
63
54
70
55
57
53
48
44
40
43
81
71
87
70
75
69
65
64
59
62
37
28
41
29
30
27
23
16
11
14
2
3
4
5
Acknowledgments
6
4-ClC6H4― (1f)
4-Pyridyl (1g)
Mrs Aparna P. I. Bhat thanks the National Institute of Technology
Karnataka, Surathkal, for a research fellowship. The authors thank
the Indian Institute of Science, Bangalore, for NMR analysis.
7
8c
1-Naphthyl (1h)
4-CNC6H4― (1i)
2-NO2C6H4― (1j)
9
10
aReaction conditions: Mg turnings (0.320 g), aryl halide (10 mmol),
catalyst (0.13 mol %), Et2O (10 ml).
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cIn THF, 60°C, 11 h.
Appl. Organometal. Chem. 2014, 28, 383–388
Copyright © 2014 John Wiley & Sons, Ltd.
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A. P. I. Bhat and B. R. Bhat
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Appl. Organometal. Chem. 2014, 28, 383–388