Facile reductive coupling of benzylic halides with ferrous oxalate dihydrate

Article abstract of DOI:10.1039/b211792d

Facile reductive coupling of benzylic halides is reported with ferrous oxalate dihydrate in DMF or HMPA under nitrogen atmosphere at 155-160°C. The coupling is proposed to proceed by two successive oxidative additions of benzylic halides to ferrous oxalate to give an intermediate organoiron complex which undergoes concerted dimerization to give the corresponding reductively coupled dimers in high yields.

Full text of DOI:10.1039/b211792d

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Facile reductive coupling of benzylic halides with ferrous oxalate
dihydrate
Jitender M. Khurana,* Sushma Chauhan and Golak C. Maikap
Department of Chemistry, University of Delhi, Delhi-110007, India.
E-mail: jmkhurana@chemistry.du.ac.in; Fax: 91 11 27666605
Received 2nd December 2002, Accepted 18th March 2003
First published as an Advance Article on the web 14th April 2003
Facile reductive coupling of benzylic halides is reported with ferrous oxalate dihydrate in DMF or HMPA under
nitrogen atmosphere at 155–160 ЊC. The coupling is proposed to proceed by two successive oxidative additions of
benzylic halides to ferrous oxalate to give an intermediate organoiron complex which undergoes concerted
dimerization to give the corresponding reductively coupled dimers in high yields.
Introduction
Reductive coupling of organic halides by metals, metal ions
and metal complexes is of fundamental importance for the
formation of carbon–carbon single bonds in organic synthesis.
There are reports on reductive dimerization of benzylic halides
with different metals and metal salts or complexes.1 The reduc-
(1)
coupled products were obtained in high yields using a 1 : 1
molar ratio of substrate to ferrous oxalate in the case of
secondary benzylic halides. Primary benzylic halides required a
1 : 2 molar ratio of substrate to ferrous oxalate to give high
yields of dimers. The reactions of primary benzylic halides were
also complete using a 1 : 1 molar ratio of substrate to ferrous
oxalate, but gave lower yields of the reductively coupled dimers
as other undesired products were also obtained. Secondary
benzylic bromides and chlorides were observed to react at
nearly the same rate (runs 1 and 14, 6 and 10, 7 and 11), while
the primary benzylic chlorides reacted more slowly than the
corresponding bromides (runs 22 and 25, 23 and 26, 29 and 30).
These results are summarized in Table 1.
9-Bromofluorene (1a) and chlorodiphenylmethane (1c) did
not undergo any reaction with iron() oxalate in acetonitrile,
THF or dioxane at reflux temperatures under nitrogen atmos-
phere. 1a and 1c underwent only methanolysis in methanol at
reflux temperature and no coupled products were isolated. The
reaction of 1a and 1c with iron() oxalate at 100 ЊC was slug-
gish and did not proceed to completion even after 3 h both in
DMF and HMPA. Also, the starting halides were recovered
unchanged quantitatively when stirred with iron() oxalate
in DMF or HMPA at room temperature. The reactions of
9-bromofluorene (1a) with iron() chloride and iron() sulfate
using a 1 : 0.5 molar ratio gave 32% and 51% of the dimer while
reactions of chlorodiphenylmethane (1c) with iron() chloride
and iron() sulfate under identical conditions yielded 20% and
57% of the coupled products only unlike reactions of 1a and 1c
with iron() oxalate which gave nearly quantitative yields of
dimers. This could be due to the insolubility of iron() chloride
and iron() sulfate in DMF and HMPA even at elevated
temperature.
The formation of dimers was also accompanied by the form-
ation of alcohols and small amounts of carbonyl compounds in
the case of primary benzylic halides and also with secondary
benzylic halides when a lower molar ratio of substrate to Fe()
was used. The formation of oxygenated products proceeds by
competitive reactions of benzylic halides with DMF or HMPA,
as confirmed by independent reactions of 9-bromofluorene
(1a), bromodiphenylmethane (1b), chlorodiphenylmethane
(1c), 1-bromomethylnaphthalene (1k) and 1-chloromethyl-
naphthalene (1l) with DMF or HMPA in the absence of ferrous
oxalate which yielded corresponding alcohols and carbonyl
compounds in varying ratios. 9-Bromofluorene (1a) showed the
predominant formation of 9-(N,N-dimethylamino)fluorene
tion and/or coupling of halides by metals and metal ions/
complexes can take place by atom transfer, electron transfer or
oxidative addition.1 Little is known about the role of the
organometallic intermediates that are formed by oxidative addi-
tion of benzylic/aralkyl halides to low-valent transition metal
compounds. The yields of coupled products depend on the
specific halide, metal, ligands and reaction conditions used.
Some of the reagents for reductive coupling of halides include
NiBr2(PPh3)2–Zn,2 Pb–n-Bu4NϩBrϪ–DMF and PbBr2–Al–
DMF,3 metallic nickel,4 Cp2TiCH2ؒZnI2,5 Ni(CO)4,6 W(CO)6,7
and Co2(CO)8 8 etc. Among the iron metal salts and complexes,
Fe3(CO)12–Pyr-N-oxide9 is reported for the coupling of
p-substituted benzyl chlorides to give 1,2-diphenylethanes in
∼ 50% yield. Iron–water,10 lithium chloroferrate,11 Na[Fe(CO)2-
(C5H5)]12 and Fe3(CO)12 13 have also been found to be useful low
valent coupling agents for benzylic halides.
While iron–water, Fe3(CO)12 and lithium chloroferrate could
be used in different solvents such as water, acetonitrile, ethanol
and tetrahydrofuran, the reactions were very slow and resulted
in a mixture of oxygenated products besides the coupled prod-
uct. Some of the reagents for reductive dimerization of benzylic
halides (primary or secondary) are expensive, require hazard-
ous conditions and give poor yields of the coupled products.
Another major shortcoming of the methods discussed is
insolubility of the reagent in the solvent. To overcome these
disadvantages in the reactions of metal salts with organic
halides, we decided to develop a simple and efficient method
for reductive dimerization of benzylic halides using inexpensive
ferrous oxalate dihydrate. One of its major advantages is its
solubility in dimethylformamide (DMF) and hexamethyl-
phosphoramide (HMPA) at elevated temperatures. This
property of solubility of ferrous oxalate in DMF at elevated
temperatures has already been utilized for reductive coupling of
geminal dihalides to give olefins in high yields.14
Results and discussion
We report herein a novel method for the reductive coupling
of benzylic halides with ferrous oxalate dihydrate in dry
dimethylformamide (DMF) or hexamethylphosphoramide
(HMPA) at 155–160 ЊC under nitrogen atmosphere to give the
corresponding reductively coupled dimers in high yields. The
reactions were complete in reasonable time (< 80 min) with
both secondary and primary benzylic halides [eqn. (1)]. The
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 7 3 7 – 1 7 4 0
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Table 1 Reactions of benzylic halides with iron() oxalate dihydrate in dry DMF or HMPAa at 155–160 ЊC under nitrogen atmosphere
Yield (%)
Dimer
Substrate
1a–p
Molar ratio
1a–p : Fe()
Mp/ЊC of dimer (2)
Obs. (lit.)
Run no.
Solvent
Time/min
Alcohol
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1b
1b
1b
1b
1c
1c
1c
1c
1d
1d
1e
1f
1 : 1
1 : 1
1 : 0.5
1 : 0.5
1 : 0.25
1 : 1
HMPA
DMF
HMPA
DMF
HMPA
HMPA
DMF
HMPA
HMPA
HMPA
DMF
HMPA
HMPA
HMPA
HMPA
HMPA
HMPA
HMPA
HMPA
HMPA
HMPA
HMPA
DMF
HMPA
HMPA
DMF
HMPA
HMPA
HMPA
HMPA
HMPA
HMPA
HMPA
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
30
30
30
30
20
60
10
60
120
50
40
35
80
10
10
10
87
96
80
84
57
91
95
89
50
90
92
93
58
90
70
70
95
54
43
37
42d
42
31
65
70
42
85
39
55
66
27
37
40
—
—
246 (246–24815
245 (246–24815
246 (246–24815
246 (246–24815
244 (246–24815
210 (210–21216
210 (210–21216
210 (210–21216
210 (210–21216
210 (210–21216
210 (210–21216
210 (210–21216
210 (210–21216
244 (246–24815
245 (246–24815
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
3b, c
6b, c
7b, c
—
1 : 1
—
1 : 0.5
1 : 0.25
1 : 1
—
42c
—
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
1 : 1
—
1 : 0.5
1 : 0.25
1 : 1
1 : 0.5
1 : 3
1 : 1
1 : 2
1 : 3
1 : 2
1 : 2
1 : 1
1 : 1
1 : 2
1 : 1
1 : 1
1 : 2
1 : 1
1 : 2
1 : 2
—
26c
—
b, c
—
—
—
c
270 (27217
)
)
>325 (37018
e
1g
1h
1i
10c, d
23c, d
26c, d
—
124 (126.719
)
156–160 (160–16120
)
e
146 (147–14821
)
22 e
e
1j
92 (92
)
1k
1k
1k
1l
48c
52c
30c
15c
41c
10c
46c
39c
19c
50c
46c
34f
162–164 (162–16323
162–164 (162–16323
)
)
160 (162–16323
160 (162–16323
160 (162–16323
160 (162–16323
)
)
)
)
1l
1l
1m
1m
1n
1o
1o
1p
180 (18323
181 (18323
180 (18323
)
)
)
1 : 2
1 : 2
1 : 2
175 (176–17724
176 (176–17724
)
)
112 (11825
)
a ∼ 50 mL of solvent/10 mmol of substrate. b 9,9Ј-Bifluorenylidene (10–13%) was also isolated. c Small amount of carbonyl compounds (2–8%) was
also isolated. d Some other unidentified products were also obtained. e The products isolated were meso compounds. f 14% of corresponding carbonyl
compound was also isolated.
Table 2
Substrate
Ar
R
X
Substrate
Ar
R
X
1a
Br
1i
4-ClC6H4
Me
Br
1b
1c
Ph
Ph
Ph
Ph
Br
Cl
1j
1k
Ph
1-naphthyl
Et
H
Br
Br
1d
Cl
1l
1-naphthyl
2-naphthyl
H
Cl
1e
Br
1m
H
Br
1f
1g
1h
4-ClC6H4
Ph
4-BrC6H4
4-ClC6H4
Me
Me
Br
Br
Br
1n
1o
1p
2-naphthyl
2-Me-1-naphthyl
Mes
H
H
H
Cl
Br
Br
besides 9-fluorenol and 9-fluorenone in blank reactions. The
formation of oxygenated products and 9-(N,N-dimethylamino)-
fluorene was greatly suppressed when reactions were carried out
in the presence of ferrous oxalate. It is thus obvious that form-
ation of alcohols and carbonyl compounds is due to competing
reactions of the halides with DMF or HMPA even in the pres-
ence of ferrous oxalate. It is also inferred by a comparison of
the results of blank reactions with reactions carried out in the
presence of ferrous oxalate that formation of alcohols, carbonyl
compounds and N,N-dimethylamino-substituted products is
greatly suppressed or is completely eliminated when reactions
are carried out with a higher molar ratio of ferrous oxalate. The
primary benzylic halides invariably gave higher yields of oxy-
genated products. Among the primary benzylic halides, lower
amounts of oxygenated products are obtained from chlorides
compared to bromides due to slower nucleophilic substitution
of chlorides by the oxygen end of DMF or HMPA. The
amount of solvent also plays an important role since doubling
the amount of solvent reduced the yield of coupled product
and gave a higher yield of the oxygenated products.
The reaction of 1a with sodium oxalate in HMPA at 155–
160 ЊC did not show the formation of any dimeric product and
only products corresponding to blank reactions were observed.
Thence the formation of coupled products in the presence of
iron() oxalate dihydrate must be due to the involvement
of iron() rather than oxalate anion. The formation of the
dimers can be conceived to proceed via radical coupling and/or
nucleophilic attack of benzylic carbanion on halides (Scheme 1)
which is initiated by electron transfer (ET) from iron(). This
possibility has, however, been eliminated, for the following
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 7 3 7 – 1 7 4 0
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Scheme 1
Scheme 2
reasons. (i) Firstly, if the above route was applicable to our
reactions, a 1 : 1 molar ratio of the halide to iron() would be
required for the completion of reactions. But we found that
coupling was complete even in 1 : 0.5 molar ratio (runs 3, 4, 8,
12 and 15) of substrate to iron() oxalate and more than 50% of
coupling has been observed with a 1 : 0.25 molar ratio (runs 5, 9
and 13). (ii) Secondly, the reaction of 1c with iron() oxalate in
the presence of 10 molar excess of cumene did not show the
presence of any diphenylmethane. (iii) Thirdly, the radical cage,
if formed at all, is not stable at such a high temperature and
should escape to give reduced product (e.g. diphenylmethane).
The absence of carbanions in our reactions was confirmed by
the absence of diphenylmethane, when aliquots removed from
the reaction mixture after 15, 30 and 45 min were quenched
with acidified water.
All the reactions showed a distinctive change in the colour of
the reaction mixture from yellow to greenish black or reddish
orange within the first 5 min, which indicates formation of
some sort of an organoiron complex. Since the reaction was
complete even with half the molar equivalent of iron() and
more than 50% of the reaction was complete with one-fourth
molar equivalent of iron complex (runs 3, 4, 5, 8, 9, 12, 13 and
15), it suggests that the organoiron complex involves at least
two halide molecules for each iron() species. The higher
amounts of the coupled products obtained than expected from
a 1 : 0.25 molar ratio could be due to various redox reactions
which regenerate iron() for further coupling.
In view of the above arguments, it is likely that the proposed
organoiron complex could arise by a non-radical oxidative
addition by two successive three-centre, concerted, frontside
nucleophilic displacements (Scheme 2).1 It is noteworthy in this
connection that in a recent study on the reductive coupling of
benzylic chlorides in the presence of Fe3(CO)12 in benzene, a
broad signal at δ 1.22 ppm assignable to the methylene group of
the unstable system PhCH2Fe was recorded.9
The dimeric products could arise from the organoiron com-
plex 3 by a homolytic process in the solvent cage but a cage
reaction, if occurring, at the temperature of our reactions (i.e.
155–160 ЊC) would have allowed a part of the radicals to diffuse
out of the cage and as a consequence some diphenylmethane
would have been obtained. The absence of diphenylmethane
and the absence of cross coupled product suggest that the
homolytic cleavage of the organoiron complex 3 does not take
place. In our opinion, the proposed organoiron complex 3
(2)
undergoes a facile, concerted process [eqn. (2)] to give the
dimeric products as is known for two alkyl ligands attached to a
single metal centre.26
The iron species in higher oxidation states could be converted
to lower oxidation states by various permissible redox reactions.
The distinct colour change, stoichiometry of the reaction,
absence of radical-derived products and absence of carbanions
in these reactions support the proposed pathway for reductive
coupling of benzylic halides with ferrous oxalate in dry DMF
or HMPA at 155–160 ЊC under nitrogen atmosphere. The form-
ation of carbocations is inconceivable as this would have led to
the formation of oxygenated products in high yields due to
competitive coupling with solvent molecules. Also higher
amounts of oxygenated products would have been obtained
from secondary benzylic halides compared to primary benzylic
halides which is contrary to our observations. We conclude that
benzylic halides can be reductively coupled with inexpensive
ferrous oxalate dihydrate in DMF or HMPA at 155–160 ЊC to
give high yields of the dimers.
Experimental section
Melting points were recorded on a Tropical labequip apparatus
and are uncorrected. IR spectra were recorded on a Perkin-
Elmer FT-IR SPECTRUM-2000 instrument. NMR spectra
were recorded on an FT-NMR model R-600 Hitachi (60 MHz)
instrument with TMS as the internal standard. HMPA and
DMF27 were dried by allowing them to stand over calcium
oxide for 24 h followed by filtration and distillation under
vacuum. (Caution: DMF vapours are harmful, irritating to
skin, eyes, and mucous membranes. DMF is suspected to be a
carcinogen and must be used in a fume hood. HMPA is
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 7 3 7 – 1 7 4 0
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an industrial substance suspected of having carcinogenic
potential for man. Adequate precautions must be taken
to avoid all forms of exposure to HMPA). The solvents
were deaerated by bubbling nitrogen into the solvent. Ferrous
oxalate dihydrate was used as such. 9-Bromofluorene,28 bromo-
diphenylmethane,28 2,9-dibromofluorene,29 1-bromomethyl-
naphthalene,30 2-bromomethylnaphthalene,31 α-bromoethyl-
benzene, p,α-dibromoethylbenzene, p-chloro-α-bromoethyl-
benzene and α-bromopropylbenzene were prepared by allylic
bromination with N-bromosuccinimide. Bis(p-chlorophenyl)-
bromomethane32 was prepared by reaction of HBr with p,pЈ-
(12.5 mL) was carried out as above in an oil bath maintained at
155–160 ЊC under nitrogen atmosphere. The starting material
disappeared completely after 10 min, as observed by TLC using
petroleum ether–ethyl acetate (95 : 5, v/v) as eluent. The reac-
tion mixture was quenched with 0.5 M H2SO4 (100 mL) after
60 min and work up as above. Close examination of the reac-
tion mixture showed a complete absence of diphenylmethane.
1,1,2,2-Tetraphenylethane (91%) was isolated as confirmed by
mp and mixed mp.
dichlorobenzhydrol.
1-Bromomethyl-2-methylnaphthalene33
Acknowledgements
and bromomethylmesitylene34 were prepared by bromomethyl-
ation of 2-methylnaphthalene and mesitylene, respectively.
9-Chlorofluorene35 was prepared by treatment of 9-fluorenol
with HCl. 1-Chloromethylnaphthalene36 was prepared by
chloromethylation of naphthalene and 2-chloromethylnaph-
thalene37 was prepared by refluxing 2-hydroxymethylnaph-
thalene with thionyl chloride in toluene.
A grant of a Senior Research Fellowship to SC by CSIR,
New Delhi, India is gratefully acknowledged.
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General procedure
In a dried, N2-filled three-necked round bottomed flask,
mounted over a magnetic stirrer and fitted to a reflux condenser
and mercury trap, was placed a mixture of halide (2.5 mmol),
iron() oxalate dihydrate (according to Table 1) and dry deaer-
ated DMF or HMPA (12.5 mL). The system was deaerated by
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was allowed to cool to room temperature and poured in 0.5 M
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in runs 1, 2, 6, 7, 8, 10, 11, 12 and 14. The solid was filtered at
the pump and dried under vacuum. The product was pure as
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oxalate (2.5 mmol) and cumene (25 mmol) in dry HMPA
35 A. Kliegl, Chem. Ber., 1910, 43, 2488.
36 A. I. Vogel, Practical Organic Chemistry, 3rd Edn., 1956, p. 540.
37 C. R. Hauser, D. N. Van Eenam and P. L. Bayless, J. Org. Chem.,
1958, 23, 354.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 7 3 7 – 1 7 4 0
1740