Iron nanoparticle-promoted Cu (I)-catalysed homocoupling of arylmethyl halides for the synthesis of 1,2-diarylethanes

Article abstract of DOI:10.3184/174751912X13252619521880

Carbon-carbon bond formation is important in organic synthesis for the preparation of natural products, organic materials and polymers and so on. An efficient and mild reductive homocoupling of arylmethyl halides is now described. Treatment of various benzylic halides in the presence of zero valent iron nanoparticles (nZVI) and a catalytic amount of CuBr in air and water at room temperature afforded 1,2-diarylethanes in excellent yields.

Full text of DOI:10.3184/174751912X13252619521880

JOURNAL OF CHEMICAL RESEARCH 2012
JANUARY, 29–30
RESEARCH PAPER 29
Iron nanoparticle-promoted Cu (I)-catalysed homocoupling of arylmethyl
halides for the synthesis of 1,2-diarylethanes
Marzieh Shekarriz *, Mehdi Adib , Tayebe Biabani and Sohrab Taghipoora
a
b
a,b
a
Research Institute of Petroleum Industry (RIPI), West Blvd., Near Azadi Sports Complex, Tehran, Iran
bSchool of Chemistry, University College of Science, University ofTehran, PO Box 14155-6455, Tehran, Iran
Carbon–carbon bond formation is important in organic synthesis for the preparation of natural products, organic
materials and polymers and so on. An efficient and mild reductive homocoupling of arylmethyl halides is now
described. Treatment of various benzylic halides in the presence of zero valent iron nanoparticles (nZVI) and a
catalytic amount of CuBr in air and water at room temperature afforded 1,2-diarylethanes in excellent yields.
Keywords: iron nanoparticles, benzylic halides, reductive homocoupling, C–C bond formation, Cu (I) catalysis, 1,2-diarylethanes,
reaction in water
The development of useful reagents and efficient catalysts
which enable carbon–carbon bond formation is key to
contemporary organic synthesis. Reductive metal-catalysed
homocoupling and cross-coupling reactions have played
crucial roles in these bond formations over recent decades.
These coupling reactions have been widely employed for the
preparation of natural products, building blocks for supramo-
lecular chemistry and self-assembly, organic materials and
polymers, and lead compounds in medicinal chemistry from
Experimental
All reagents and solvents were purchased from Merck Chemical
Company and used without further purification. All yields refer to
products isolated by column chromatography or crystallisation. IR
spectra were run on a Perkin-Elmer 781 spectrophotometer. NMR
spectra were recorded on a Bruker Avance DPX-250. The purity of
both starting materials as well as the reaction products were checked
by TLC on silica-gel polygram SILG/UV254 plates or by a Shimadzu
Gas Chromatograph GC-10A instrument with a flame-ionisation
detector using a 15% carbowax 20M chromosorb-w acid washed
60–80 mesh column. TEM images of nanoparticles were obtained
using a Philips model TM 200 FGE electron microscope.
1,2
simpler entities.
The reductive coupling of organic halides is an important
3
4
method for C–C bond formation, where Wurtz and Ullmann
Synthesis of nZVI
Iron nanoparticle was synthesised according to the reported method.
A water solution of 5 × 10 M of sodium luryl sulfate (SDS) was
added into a clear solution of ferric chloride (0.026 mol) dissolved in
reactions are classical methods for the preparation of bialkyl
and biaryl compounds. Transition metal catalysed coupling
reactions between organic electrophiles and organometallic
compounds constitute the most straightforward approach
2
2
−
2
7
2 mL of ethanol and 21.6 mL of deionised water. Then a solution of
5
for the formation of C–C bonds. Several metals and metal
potassium borohydride (0.08 mol) dissolved in 42 mL of ethanol and
10 mL of deionised water was added gradually and mixed with the
above solution under constant and vigorous stirring. Stirring was con-
tinued for a further hour after a black precipitate was first formed.
Then the products were separated and rinsed with ethanol and acetone,
respectively. Reactions were run under nitrogen atmosphere. The
6
7
8
complexes, such as titanium, vanadium, chromium, lantha-
9
10
11
12
13
num, copper, nickel, zinc and iron, have been employed
in this type of reaction.
The homocoupling of several organic halides has been
performed in good yields under inert gas atmosphere and
2
−1
average particle size and surface area were about 12 nm and 70 m g
respectively (Fig. 1).
1
4
anhydrous conditions, in some cases using activated metals,
1
5
and the interest for these reactions has increased. Advances
in metal-promoted Barbier-type additions to carbonyl com-
pounds in aqueous media led to the hope that a Wurtz-type
coupling could also be performed in water.
Preparation of 1,2-Diphenylethane (2a)
A suspension of iron nanoparticles (0.205 g, 2.2 mol) and CuBr
1
6
(
0.029 g, 0.2 mmol) in H O (5 mL) was stirred at room temperature
2
for 10 min. Then benzyl brimide (0.342 g, 2 mmol) was added to
the reaction mixture which was stirred for a further 25 min. Next, the
solid was filtered off and washed with hot water (2 × 10 mL). The
water solution stood at 5 °C for 2 h. Then the crude solid product
Water is a desirable solvent because it is safe, non-toxic,
environmentally friendly, readily available and cheap com-
1
7
pared to organic solvents. Since the pioneering studies on
1
8
Diels–Alder reactions by Breslow, there has been increasing
recognition that organic reactions can proceed well in aqueous
media and offer advantages over those occurring in organic
(
0.178 g, 98%) was isolated by filtration and recrystallised from
methanol to give pure 1,2-diphenylethane 2a (Table 1).
17
solvents.
In 2007, Liu et al. reported that reductive homocoupling of
benzylic halides with metallic iron powder in the presence of
a catalytic amount of a cuprous salt in aqueous media leading
to the corresponding bibenzyls. CuBr was the most efficient
1
9
cuprous halide that they used in this type of reaction. How-
ever, their procedure has drawbacks such as long reaction
times (1–2 h), in some cases, fairly low yields of the products
(
down to 48%) and relatively high reaction temperatures (up
to 90 °C).
Recently transition-metal nanoparticles have attracted atten-
tion; their preparation, structure determination, and applica-
2
0
tions are of interest. One of these applications is their use in
reactions generating C–C bonds.
2
1
We now report the use of zero valent iron nanoparticles
nZVI) with water as the solvent in air in the presence of CuCl
(
under mild reaction conditions.
Fig. 1 TEM images of zero valent iron nanoparticles (average
*
Correspondent. E-mail: shekarriz@ripi.ir
particle size = 12 nm).22

3
0
JOURNAL OF CHEMICAL RESEARCH 2012
Results and discussion
As part of our current studies on the development of efficient
and simple methods for the preparation of organic compounds
from readily available building blocks, we report here an
improvement of homocoupling of benzylic halides under
mild conditions. Thus a mixture of an arylmethyl halide 1,
22
iron nanoparticles and a catalytic amount of CuBr in air and
water undergoes a reductive homocoupling reaction at room
temperature to afford 1,2-diarylethanes 2 in 88–98% yields
(
Scheme 1, Table 1).
All the reactions went to completion within 25 min. H
1
NMR analysis of the reaction mixtures clearly indicated
the formation of the corresponding 1,2-diarylethanes 2 in
excellent yields.
A range of 1,2-diarylethanes 2 were synthesised by reduc-
tive homocoupling reactions of the corresponding arylmethyl
halides. The results are given in Table 1. The structures of
the isolated 1,2-diarylethanes were confirmed by comparison
Scheme 2 Proposed reaction mechanism of coupling of
benzylic halides in the presence of nZVI and CuBr.
1
of their spectral data ( H NMR spectra) and their melting
points with those of authentic samples (see Table 1).
A mechanistic rationalisation for this reaction is provided
in Scheme 2. On the basis of the chemistry of metal-catalysed
excellent yields of the products and a simple purification
process are the main advantages of this method.
23
coupling reactions, it is reasonable to assume that Cu(0)
could be the active catalytic species, which is generated in
the presence of iron nanoparticles by reduction of Cu(I).
This active catalytic species could undergo two successive
oxidative addition of the halide 1 to form intermediate 4 via 3.
Reductive elimination of 1,2-diarylethane 2 may form Cu(I)
species which would complete the catalytic cycle.
This research was supported by Research Institute of Petro-
leum Industry (RIPI) and the Research Council of University
of Tehran as a research project (6102036/1/03).
Received 29 October 2011; accepted 22 December 2011
Paper 1100961 doi: 10.3184/174751912X13252619521880
Published online: 31 January 2012
Conclusion
In conclusion, we have developed an efficient and mild Cu (I)
catalysed reductive homocoupling of arylmethyl halides
promoted by zero valent iron nanoparticles leading to 1,2-
diarylethanes. Use of water as a green medium, carrying out
the reactions in air without any need to inert gas atmosphere,
fairly fast reaction times, reduced reaction temperature,
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87
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88
90
96
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90
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12
6
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