Facile reductive dehalogenation of organic halides with nickel boride at ambient temperature

Article abstract of DOI:10.1139/V08-156

The hydrodehalogenation of a series of aryl, alkyl, allyl, and benzyl chlorides, bromides, and iodides has been carried out efficiently using nickel boride in methanol at ambient temperature, leading to the corresponding products resulting from hydrogen/halogen exchange.

Full text of DOI:10.1139/V08-156

1
052
Facile reductive dehalogenation of organic halides
with nickel boride at ambient temperature
Jitender M. Khurana, Sanjay Kumar, and Bhaskara Nand
Abstract: The hydrodehalogenation of a series of aryl, alkyl, allyl, and benzyl chlorides, bromides, and iodides has
been carried out efficiently using nickel boride in methanol at ambient temperature, leading to the corresponding prod-
ucts resulting from hydrogen/halogen exchange.
Key words: hydrodehalogenation, reduction, organic halides, sodium borohydride, nickel boride.
Résumé : Faisant appel à l’action du borure de nickel à la température ambiante et dans le méthanol, on a effectué
d’une façon efficace la hydrodéshalogénation d’une série d’iodures, de bromures et de chlorures d’aryle, d’alkyle, d’allyle
et de benzyle conduisant à la formation des produits correspondants résultant d’un échange hydrogène/halogène.
Mots-clés : hydrodéshalogénation, réduction, halogénures organiques, borohydrure de sodium, borure de nickel.
[
Traduit par la Rédaction] Khurana et al. 1054
using nickel boride generated in situ from nickel(II) chloride
and sodium borohydride. Benzylic halides were almost in-
stantaneously reduced. Allylic halides underwent hydro-
dehalogenation and reduction of double bond concomitantly.
Aryl halides, which are generally resistant to hydrodehalo-
genation, could also be easily dehalogenated to the corre-
sponding arenes.
Introduction
The development of effective procedures for the
dehalogenation of organic halides is an important task, as it
helps in the reduction of pollution by halogen-containing in-
dustrial wastes as well as for the synthesis of fine chemicals.
Unlike oxidation methods, reductive dehalogenation pro-
cesses are advantageous, as no toxic side products are
formed. A wide variety of hydrodehalogenating systems
have been used over the years and have been reviewed in de-
tail (1–3). Thus, catalytic hydrogenation (4), dehalogenation
by metals or low-valent metal compounds (5), metal hy-
drides, or complex metal hydrides (6–10) are some general
reagents and methods able to accomplish the above-
mentioned transformation. Nickel boride (11), first reported
by Schlesinger and Brown, has been used as heterogeneous
hydrogenation catalyst over the years (12). However, it is
now known as a reducing agent by itself because of the ad-
sorbed hydrogen (13). In recent years, we have used nickel
boride as a novel reducing agent in multifarious transforma-
tions (14a–14m) (e.g., reductions, deoxygenations, desulfuri-
sations, and so forth). In continuation to our studies on
dehalogenation of various types of organic halides (15), we
now wish to report the dehalogenation of benzylic, allylic,
aryl, and alkyl halides with nickel boride in methanol at am-
bient temperature.
9
-Bromofluorene was chosen as the model substrate to in-
vestigate and optimize the conditions for hydrodebromina-
tion of benzylic halides. The reactions of 9-bromofluorene
were carried out with nickel boride in different solvents and
in different molar ratios of substrate to nickel boride to
achieve the desired transformation. Reactions carried out in
THF, ethanol, and DMF were sluggish and gave a mixture of
products, while no reaction was observed in acetonitrile and
dichloromethane. Reaction of 9-bromofluorene in methanol
yielded 96% fluorene in 15 min at ambient temperature. Fur-
ther, it has been shown that the hydrodehalogenation is pro-
ceeding due to in situ formation of nickel boride, as no
reaction of 9-bromofluorene was observed with nickel(II)
chloride or sodium borohydride independently.
The hydrodehalogenation of a variety of benzylic halides
was subsequently investigated under similar experimental
conditions. Benzylic chlorides, bromides, and iodides under-
went rapid hydrodehalogenation in different molar ratios of
substrate:NiCl ·6H O:NaBH (Scheme 1) (Table 1). The
2
2
4
ease of dehalogenation BnI > BnBr > BnCl follows the order
of bond energies C–Cl > C–Br > C–I, which is evident from
molar ratios and the time required for the dehalogenation
(Table 1, entries 1–21). Further, it can also be inferred from
Table 1 that primary halides (Table 1, entries 1–4, 11, 12,
Results and discussion
In this paper, we report hydrodehalogenation of aryl,
alkyl, and allyl halides to the corresponding hydrocarbons
Received 6 August 2008. Accepted 15 September 2008. Published on the NRC Research Press Web site at canjchem.nrc.ca on
2
4 October 2008.
1
J.M. Khurana, S. Kumar, and B. Nand. Department of Chemistry, University of Delhi, Delhi 110007, India.
1Corresponding author (e-mail: jmkhurana1@yahoo.co.in).
Can. J. Chem. 86: 1052–1054 (2008)
doi:10.1139/V08-156
© 2008 NRC Canada

Khurana et al.
1053
Table 1. Reductive dehalogenation of organic halides with nickel boride in methanol at room temperature.
Molar ratio
Time
(min)
Yield
(16) (%)
Entry
Substrate (S)
S/NiCl ·6H O/NaBH
Product
2
2
4
1
2
3
4
5
6
7
8
9
1-(Bromomethyl)naphthalene
2-(Bromomethyl)naphthalene
4-Bromobenzyl bromide
4-chlorobenzyl bromide
9-Bromofluorene
Bromodiphenylmethane
(1-Bromoethyl)benzene
(1-Bromopropyl)benzene
1-(1-Bromoethyl)naphthalene
Bromotriphenylmethane
1-(Chloromethyl)naphthalene
2-(Chloromethyl)naphthalene
9-Chlorofluorene
Chlorodiphenylmethane
(1-Chloroethyl)benzene
(1-Chloropropyl)benzene
1-(1-Chloroethyl)naphthalene
Chlorotriphenylmethane
1-(Iodomethyl)naphthalene
(1-Iodopropyl)benzene
Iododiphenylmethane
Cinnamyl iodide
Cinnamyl bromide
Cinnamyl chloride
1-Iodonaphthalene
1-Bromonaphthalene
1-Chloronaphthalene
9-Bromoanthracene
9-Chloroanthracene
9-Bromophenanthrene
4-Bromobiphenyl
1:2:6
1:2:6
1:2:6
1:2:6
1:2:6
1:2:6
1:2:6
1:2:6
1:2:6
1:2:6
1:3:9
1:3:9
1:3:9
1:3:9
1:3:9
1:3:9
1:3:9
1:3:9
1:1:3
1:1:3
1:1:3
1:1:3
1:1:3
1:2:6
1:1:3
1:2:6
1:3:9
1:2:6
1:3:9
1:2:6
15
15
15
15
15
20
20
30
20
40
20
20
25
30
30
30
25
45
5
10
10
10
15
10
15
15
25
30
30
30
120
60
90
45
60
20
20
35
30
30
1-Methylnaphthalene
2-Methylnaphthalene
4-Bromotoluene
4-Chlorotoluene
Fluorene
92
86
85
81
96
90
83
81
82
81
94
92
91
75
81
84
74
86
85
85
82
81
91
92
87
95
93
42 + 48
40 + 47
86
54
72
Diphenylmethane
Ethylbenzene
n-Propylbenzene
1-Ethylnaphthalene
Triphenylmethane
1-Methylnaphthalene
2-Methylnaphthalene
Fluorene
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Diphenylmethane
Ethylbenzene
n-Propylbenzene
1-Ethylnaphthalene
Triphenylmethane
1-Methylnaphthalene
n-Propylbenzene
Diphenylmethane
n-Propylbenzene
n-Propylbenzene
n-Propylbenzene
Naphthalene
Naphthalene
Naphthalene
Anthracene + 9,10-Dihydroanthracene
Anthracene + 9,10-Dihydroanthracene
Phenanthrene
Biphenyl
Phenol
Phenol
Methyl benzoate
Methyl benzoate
n-Dodecane
n-Hexadecane
Adamantane
1:10:30
1:10:30
1:10:30
1:10:30
1:10:30
1:5:15
1:5:15
1:5:15
1:5:15
1:5:15
4-Bromophenol
4-Chlorophenol
Methyl 4-bromobenzoate
Methyl 4-chlorobenzoate
1-Bromododecane
1-Bromohexadecane
1-Bromoadamantane
1-Bromo-3-phenylpropane
Methyl 4-bromobutanoate
75
84
82
85
84
82
87
78
n-Propylbenzene
Methyl butanoate
Scheme 1.
Aryl halides and alkyl halides, which are generally
resistant to dehalogenation, could also be dehalogenated suc-
cessfully with nickel boride (Scheme 1). Thus, 1-iodonaph-
thalene, 1-bromonaphthalene, and 1-chloronaphthalene
underwent rapid dehalogenation (Table 1, entries 25–27)
giving naphthalene in excellent yields. The order of reactiv-
ity was the same as observed earlier, i.e., I > Br > Cl.
Reaction of 9-bromoanthracene and 9-chloroanthracene with
nickel boride (Table 1, entries 28 and 29) resulted in a mix-
ture of anthracene and 9,10-dihydroanthracene. This is be-
cause anthracene itself was observed to undergo reduction
with nickel boride to give 9,10-dihydroanthracene in an in-
dependent reaction.
and 19) undergo faster reduction than secondary halides (Ta-
ble 1, entries 5–9, 13–17, 20, and 21), and tertiary halides
(
Table 1, entries 10 and 18) exhibit even slower reactivity.
Hydrodehalogenation of allylic halides (Table 1, entries
2–24), on the other hand, proceeds with the concomitant re-
2
duction of double bond to give the completely reduced prod-
uct. No selective dehalogenation could be achieved even at
lower temperatures.
Similarly, other aryl halides 9-bromophenanthrene, 4-
bromophenol, 4-chlorophenol, methyl 4-bromobenzoate,
©
2008 NRC Canada

1
054
Can. J. Chem. Vol. 86, 2008
methyl 4-chlorobenzoate, and 4-bromobiphenyl were easily
dehalogenated in quantitative yields. Dehalogenation of
halobenzenes, however, required much higher molar ratios
compared with halonaphthalenes and benzylic halides. This
fact has been utilized to achieve regioselective dehalo-
genation of benzylic C–X over Ph–X bond. Thus, 4-
bromobenzyl bromide and 4-chlorobenzyl bromide could be
selectively dehalogenated to give 4-bromotoluene and 4-
chlorotoluene, respectively (Table 1, entries 3 and 4).
Aliphatic alkyl halides, 1-bromohexadecane, 1-bromo-
dodecane, 1-bromoadamantane, 1-bromo-3-phenylpropane,
and methyl 4-bromobutanoate also underwent successful
dehalogenation but required higher molar ratio of 1:5:15
combined extract was dried over anhyd. MgSO , decanted
4
through a cotton pad, and concentrated on a rotary evapora-
tor. The product was recrystallized from ethanol to give
1
0.13 g of fluorene as characterized by mp and H NMR
spectra.
Acknowledgement
Financial assistance for the project by University Grants
Commission [Project no. F-32–203/2006 (SR)] and Junior/
Senior Research Fellowship to SK by Council of Scientific
and Industrial Research is gratefully acknowledged.
(
Table 1, entries 36–40). Selective debromination of
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2
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©
2008 NRC Canada