Facile synthesis of bibenzyl by reductive homocoupling of benzyl halides in aqueous media

Article abstract of DOI:10.1080/00397910701483340

A facile synthesis of bibenzyl derivatives by reductive homocoupling reaction is described. Treatment of benzyl halides with metallic iron powder in the presence of a catalytic amount of cuprous salt in aqueous media gives bibenzyls in good yields. Copyright Taylor & Francis Group, LLC.

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Facile Synthesis of Bibenzyl by Reductive
Homocoupling of Benzyl Halides in Aqueous
Media
Jian Liu a & Bin Li a
a Department of Chemistry, Beijing Normal University, Beijing, China
Version of record first published: 05 Oct 2007.
To cite this article: Jian Liu & Bin Li (2007): Facile Synthesis of Bibenzyl by Reductive Homocoupling
of Benzyl Halides in Aqueous Media, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 37:19, 3273-3278
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Synthetic Communicationsw, 37: 3273–3278, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701483340
Facile Synthesis of Bibenzyl by Reductive
Homocoupling of Benzyl Halides in Aqueous
Media
Jian Liu and Bin Li
Department of Chemistry, Beijing Normal University, Beijing, China
Abstract: A facile synthesis of bibenzyl derivatives by reductive homocoupling
reaction is described. Treatment of benzyl halides with metallic iron powder in the
presence of a catalytic amount of cuprous salt in aqueous media gives bibenzyls in
good yields.
Keywords: aqueous media, bibenzyl, iron powder, reductive homocoupling reaction
INTRODUCTION
Reductive homocoupling of benzyl halides catalyzed by transition metals is
one of the most useful synthetic methods to prepare bibenzyls, which are
extensively used in synthesis of polymers, agrochemicals, and pharmaceutical
intermediates. Several metals and metal complexes, such as vanadium,[1,2]
titanium,[3] copper,[4,5] nickel,[6] chromium,[7] zinc,[8] iron,[9,10] and
lanthanum metal,[11] are employed in this type of reaction. These reactions
must be carried out in an inert gas atmosphere and in anhydrous organic
solvents because they are air and moisture sensitive. Sometimes, complicated
coordinating ligands are required to stabilize the active metal intermediates,
and some metals are not compatible with sensitive functional groups, such
as cyano and nitro, on the benzene ring, which are often reduced to corre-
sponding groups by metals or metal complexes.
Received in Japan September 15, 2006
Address correspondence to Jian Liu, Department of Chemistry Beijing Normal Uni-
versity, Beijing 100875, China. E-mail: liujianbnu@sina.com
3273

3274
J. Liu and B. Li
Scheme 1.
Water is an easily available, economical, and environmentally friendly
solvent, and carrying out organic reactions in aqueous medium is of great
interest. Recently, we found that reductive homocoupling of benzyl halides
can be performed in water in the presence of metallic iron powder and
cuprous salt. Herein, we report a facile synthesis of bibenzyls and discuss
the substituted effects on the reaction (Scheme 1).
RESULTS AND DISCUSSION
In air, treatment of benzyl halides with metallic iron powder in the presence of
a catalytic amount of cuprous salt in aqueous media gives bibenzyls in good
yields. The results are summarized in Table 1. Reaction of benzyl bromide
and iron powder at 808C gives bibenzyl in 81% yield (Table 1, entry 2),
whereas benzyl chloride gives 73% yield (Table 1, entry 1). The yield
varies with the structure of the substituted groups on benzyl; for example,
Table 1. Reductive coupling of different benzyl halides with iron powder and
CuCl in water
Entry
Ar
X
Y
Condition
Yielda (%)
1
-C6H4-
-C6H4-
-C6H4-
-C6H4-
-C6H4-
-C6H4-
-C6H4-
-C6H4-
-C6H4-
-C6H4-
1-naphthyl
2-naphthyl
Cl
Br
Cl
Cl
Cl
Cl
Cl
Br
Br
Br
Cl
Cl
H
H
908C/2 h
808C/1h
908C/1 h
808C/2 h
808C/2 h
708C/2 h
708C/2 h
808C/2 h
708C/2 h
708C/2 h
758C/1 h
758C/1 h
73
81
77
62
66
58
42
73
60
51
48
56
2
3
4-CH3
4-Cl
4-Br
4-CN
4-NO2
4-Br
4-CN
4-NO2
H
4
5
6
7
8
9
10
11
12
H
aIsolated yields based on benzyl halides.

Bibenzyl Synthesis
3275
introducing the substituted group into the 4-position of the benzyl ring leads to
a decrease in the isolated yields generally (Table 1, entries 4–10). Nitro- and
cyano-groups are compatible with iron under these reaction conditions and
give bi-(nitrobenzyl) and bi-(cyanobenzyl), respectively (Table 1, entries 6,
7, 9, and 10).
Under similar reaction conditions, the reaction of naphthalenyl halides and
iron in the presence of cuprous salt also gives the corresponding dimmers
(Table 1, entries 11 and 12). Reaction of 1-(chloromethyl)-naphthalene with
iron gives the product in 48% yield (Table 1, entry 11), and 2-(chloro-
methyl)-naphthalene gives the product in 56% yield (Table 1, entry 12).
To study the effects on the reaction, benzyl halide is used as substrate and
the experiments are carried out under the reaction conditions similar to those
described in the experimental section. The results are summarized in Table 2.
The mixture of iron and cuprous salt plays an essential role in the reaction. In
the absence of iron powder, treatment of benzyl bromide with cuprous salt
leads to no bibenzyl being isolated. In the absence of cuprous salt, reaction
of benzyl chloride and benzyl bromide with iron in water gives bibenzyl in
11% and 19% yields (Table 2, entries 1 and 2), respectively, and in the
presence of CuCl, bibenzyl is isolated in 73% and 81% yields (Table 2,
entries 3 and 6), respectively, indicating that cuprous salt CuCl can greatly
promote the coupling reaction.
Under similar reaction conditions, treatment of benzyl chloride with
various cuprous salts such as CuCl, CuBr, and CuI gives bibenzyl in good
yields (.70%) (Table 2, entries 3–5), and CuBr is the most efficient in this
type of reaction (Table 2, entry 4).
The mass is a heterogeneous mixture; vigorous agitating is critical for
these reactions. Increasing temperature facilitates the coupling reactions. By
keeping the maintenance temperature in the range from 808C to 958C, the
reaction can be completed in 2–4 h.
Table 2. Effect of different copper salts on reductive coupling of benzyl halides
Entry
X
Y
Cu salt
Condition
Yielda (%)
1
2
3
4
5
6
7
8
9
Cl
Br
Cl
Cl
Cl
Br
Br
Cl
Cl
H
H
H
H
H
H
H
—
—
908C/2 h
908C/2 h
908C/2 h
808C/2 h
908C/2 h
808C/2 h
808C/2 h
908C/2 h
908C/2 h
11
19
73
80
77
81
79
78
68
CuCl
CuBr
CuI
CuCI
CuI
CuI
4-CH3
4-Cl
CuI
aIsolated yield.

3276
J. Liu and B. Li
In conclusion, a facile synthesis of bibenzyls in aqueous media is
described. Given the ease of accessing bibenzyls (in isolated yield up to
80%) by the procedure outlined in this article, this should facilitate its use
not only as reported but also possibly in further applications. Further investi-
gation is under way.
EXPERIMENTAL
All chemicals were purchased from commercial sources and used without
further treatment except as specified. Melting points were determined on an
electrothermal digital apparatus and were uncorrected. IR spectra were
recorded on a Hitachi 260–50 infrared spectrometer as KBr pellets. 1H
NMR spectra were recorded on a 500-MHz UltraShield 500 spectrometer in
CDCl3 using TMS as an internal standard.
General Procedure
To a suspension of iron powder (0.60 g, 11.0 mmol) in H2O (15 mL), CuCl
(0.10 g, 1.0 mmol) was added. After this suspension was stirred at room temp-
erature for 20 min, the resulting mixture was heated to 908C. Benzyl chloride
(2.52 g, 20.0 mmol) was added dropwise to the reaction mixture within
30 min, and the mixture was stirred at 908C for 2 h. After the mixture was
cooled to 658C, the solid was filtered off and washed with hot water
(30 mL Â 2). The water solution was further cooled to 108C, and a crude
product was isolated by filtration, which was recrystallized from methanol
to give the pure bibenzyl in 85% yield (1.55 g).
Data
1
1,2-Diphenylethane. Mp 50–528C (lit.[12] mp 51–528C); H NMR(CDCl3,
500 MHz): d 2.9 (s, 4H), 7.2 (m, 10H).
1
1,2-Bis(4-chlorophenyl)ethane. Mp 98–1008C (lit.[6] mp 98.5–99.58C); H
NMR (CDC13, 500 MHz): d 2.82 (s, CH, 4H), 7.02 (m, Ar, 4H), 7.21
(m, Ar, 4H).
1,2-Bis(4-bromopheny1)ethane. Mp 113–1158C (lit.[6] mp 114–1158C);1H
NMR (CDCl3, 500 MHz): d 2.81 (s, CH2, 4H), 7.00 (d, Ar, 4H), 7.37
(d, Ar, 4H).
1,2-Bis(4-methylphenyI)ethane. Mp 83–848C (lit.[13] mp 82–838C); 1H
NMR (CDCl3, 500 MHz): d 2.27 (s, CH3, 6H), 2.83 (s, CH2, 4H), 7.03
(s, Ar, 8H).

Bibenzyl Synthesis
3277
1,2-Bis(4-cyanophenyl)ethane. Mp 199–2008C (lit.[14] mp 200–2038C); 1H
NMR (CDC13, 500 MHz): d 3.00 (s, CH2, 4H), 7.21 (d, Ar, 4H), 7.58 (d, Ar,
4H); IR (KBr) 2215 cm21 (CN).
1
1,2-Bis(4-nitrophenyl)ethane. Mp 181–1828C (lit.[15] mp 179–1808C); H
NMR (CDC13, 500 MHz): d 3.10 (s, CH2, 4H), 7.30 (d, J ¼ 8.6 Hz, Ar,
4H), 8.10 (d, Ar, J ¼ 8.6 Hz, 4H); IR (KBr) 1505 and 1335 cm21 (NO2).
1,2-Bis(1-naphthyl)ethane. Mp 159–1618C (lit.[16] mp 163–1658C); 1H
NMR (CDC13, 500 MHz): d 3.48 (s, CH2, 4H), 7.15–8.32 (m, Ar, 14H).
1,2-Bis(2-naphthyl)ethane. Mp 182–1848C (lit.[17] mp 183–1848C); 1H
NMR (CDC13, 500 MHz): d 3.17 (s, CH2, 4H), 7.25–7.94 (m, Ar, 14H).
ACKNOWLEDGMENT
We are grateful to Xuebin Wang for his guidance during this research and
Cong Zhang for his assistance on this manuscript.
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