Nickel-catalyzed hydrodehalogenation of aryl halides

Article abstract of DOI:10.1016/j.jorganchem.2013.01.014

In the present study, the nickel-catalyzed dehalogenation of aryl and alkyl halides with iso-propyl zinc bromide or tert-butylmagnesium chloride has been examined in detail. With a straightforward nickel complex as pre-catalyst good to excellent yields and chemoselectivities were feasible for a variety of aryl and alkyl halides.

Full text of DOI:10.1016/j.jorganchem.2013.01.014

Journal of Organometallic Chemistry 729 (2013) 53e59
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Journal of Organometallic Chemistry
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Nickel-catalyzed hydrodehalogenation of aryl halides
,
Maik Weidauer a, Elisabeth Irran b, Chika I. Someya a, Michael Haberberger a, Stephan Enthaler a
*
a Technische Universität Berlin, Department of Chemistry, Cluster of Excellence “Unifying Concepts in Catalysis”, Straße des 17. Juni 115/C2, 10623 Berlin, Germany
b Institute of Chemistry, Metalorganics and Inorganic Materials, Technische Universität Berlin, Straße des 17, Juni 135/C2, D-10623 Berlin, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present study, the nickel-catalyzed dehalogenation of aryl and alkyl halides with iso-propyl zinc
bromide or tert-butylmagnesium chloride has been examined in detail. With a straightforward nickel
complex as pre-catalyst good to excellent yields and chemoselectivities were feasible for a variety of aryl
and alkyl halides.
Received 30 November 2012
Received in revised form
10 January 2013
Accepted 16 January 2013
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Catalysis
Nickel complexes
Hydrodehalogenation
Aryl halides
Alkyl halides
1. Introduction
pre-catalysts the well-defined nickel complexes 1 and 2 have been
found to be active in supporting this transformation. Interestingly,
The transformation of carbonehalogen to carbonehydrogen
bonds is of importance for organic synthesis as well as industry
[1]. Especially, the conversion of chlorinated arenes is a central
matter, due to the negative impact of such chlorinated compounds
(e.g., persistent organic pollutants) on the environment and human
health [2]. On the other hand halides have been applied as protec-
ting and directing groups in organic synthesis (Fig.1) [3]. Commonly,
after the halide functionalities have fulfilled their obligations
a removal is needed to create the final product [2a,4]. Based on this
issues efficient and straightforward hydrodehalogenation reactions
are highly requested. Until now numerous methodologies have been
reported for the hydrodehalogenation of aryl and alkyl halides [5,6].
In this regard, excellent performances have been exhibited by the
application of metal-based catalysts. For instance various homoge-
neous and heterogeneous nickel catalysts in combination with
hydrogen sources (e.g., borohydride, alkoxides, NaH) have been
demonstrated for an efficient removal of halide functions under mild
reaction conditions [6,7]. Especially, nickel complexes modified by
N-heterocyclic carbenes (NHCs) or phosphanes have been proven to
catalyze such transformations [6,7].
in case of iso-propyl zinc bromide as reagent to some extent
dehalogenation processes have been observed. Based on that,
we report herein our studies on the nickel-catalyzed dehalogena-
tion of organic halides applying organometallic zinc and Grignard
reagents.
2. Results and discussion
The 1-benzoyl-5-hydroxypyrazoline ligand 5 was synthesized in
accordance to the procedure reported in the literature (Scheme 1)
[8a,b,9]. Benzohydrazide (4) was reacted with equimolar amounts of
1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione (3) and refluxed for
24 h in ethanol. After work-up the desired ligand 5 was obtained
as colorless crystalline compound. Interestingly, the condensation
reaction was highly selective, while the isomer with the tri-
fluoromethyl function next to the hydroxyl group was not isolated.
With this ligand in hand we examined the coordination to nickel
salts (Scheme 1). In agreement to our previously established pro-
tocol a methanol solution of the ligand and an excess of triphenyl-
phosphane (2.0 equiv.) was added to a solution of Ni(OAc)2$4H2O in
methanol at room temperature [8b]. After stirring overnight, all
volatiles were removed to obtain a brown powder, which was
extracted with ethanol and purified by crystallization to obtain
brown crystals. Crystals suitable for X-ray measurements were
grown from ethanol by slow evaporation of the solvent at room
temperature. The solid-state structure of complex 6 has been
Recently, we have studied the nickel-catalyzed cross coupling
of aryl halides with organometallic zinc reagents (Fig. 2) [8]. As
* Corresponding author. Fax: þ49 3031429732.
E-mail address: stephan.enthaler@tu-berlin.de (S. Enthaler).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.01.014

54
M. Weidauer et al. / Journal of Organometallic Chemistry 729 (2013) 53e59
Fig. 1. Reasons for dehalogenation reactions.
center is formed. Moreover, the benefit of the triphenylphosphane
ligand in 2a and 6 in comparison to the DMAP ligand can be the
easier dissociation to create a more active complex as shown in
earlier NMR studies [8]. Besides, the loading of the pre-catalyst was
decreased to 2.5 mol%, resulting in a diminish of product 7a (68%)
(Table 1, entry 5). Additionally, the influence of the reaction tem-
perature was studied. Here, the amount of 7a is decreased at lower
temperature (40 ꢀC: yield 66%) (Table 1, entry 6).
Once the optimized reaction conditions were established, the
scope and limitations of the nickel-catalyzed dehalogenation of aryl
halides applying iPrZnBr as reagent were investigated. A number of
aryl halides with various functional groups were converted
(Table 2). In order to study the influence of the leaving halide
function 4-bromo- (8) and 4-chloroanisole (9) were reacted with
iPrZnBr demonstrating excellent yields of >99% after 24 h at 70 ꢀC
(Table 2, entries 1e3). In case of the bromobenzonitrile (11) the
desired product was obtained in low yields along with benzene as
side product, which is formed by removing the nitrile function
under described conditions (Table 2, entry 5). In contrast, better
results were realized for substrates containing ester, amino, and
thioether functions (Table 2, entries 6e8). Noteworthy, thioanisole
(14a) was obtained in 87%, while as minor product benzene was
monitored. Furthermore, 1-bromo-4-chlorobenzene (16) was
dehalogenated to the corresponding chlorobenzene (Table 2, entry
Fig. 2. Application of complexes 1 and 2 in cross coupling reactions (DMAP ¼ 4-
dimethylamino pyridine).
characterized by single crystal X-ray diffraction analysis. Thermal
ellipsoid plots selected bond lengths and angles are shown in Fig. 3.
The tridentate ligand is coordinated in a O,N,O0-mode creating a five-
membered as well as a six-membered ring system and therefore
shielding one side of the metal. The triphenylphosphane ligand is
cis-positioned to the oxygen donors, while the nitrogen of the 5-
hydroxypyrazoline ligand is connected to the nickel centre in the
trans-position. A similar square planar motif was observed by Joshi
and co-workers and more recently by us when applying ammonia
instead of triphenylphosphane as additional ligand, while with
DMAP an octahedral structure was achieved [8b,10,11]. The NieN1,
NieO1, and NieO2 bond lengths are found in a similar range of
those observed in nickel (II) complexes having the same coordinat-
ing atoms.
i
10). However, due to the excess of PrZnBr the produced chlor-
obenzene was to some extend converted to benzene.
More recently the group of Glorius studied the nickel-catalyzed
cross coupling of aryl bromides with tertiary Grignard reagents [7f].
Along with the desired coupling products to some extend dehalo-
genation was observed. In addition, in their recent work Jacobi von
Wangelin and co-workers showed the efficient iron-catalyzed
dehalogenation of aryl and alkyl halides applying Grignard re-
agents [5a]. Based on that, we studied the potential of tert-butyl-
Initially, the dehalogenation of 4-iodoanisole (7) with iPrZnBr in
THF was studied as a model reaction to explore appropriate con-
ditions and to examine the influence of various reaction parameters
(Table 1). As expected when performing the reaction in the absence
of nickel complexes no product formation was monitored (Table 1,
entry 1). However, when applying 5.0 mol% of 6 full conversion was
realized after 24 h at 70 ꢀC (Table 1, entry 2). The dehalogenated
product 7a was obtained in 87% yield, while as side product the
homocoupling product 4,40-dimethoxy biphenyl was detected.
i
magnesium chloride as dehalogenation reagent instead of PrZnBr
under same conditions (Table 3). Interestingly, in the absence of
complex 6 aryl iodides were dehalogenated in good to excellent
yields, probably via an iodine magnesium exchange and subse-
quent hydrolysis under work-up conditions (Table 3, entries 1
and 4). In contrast, for the dehalogenation of 4-bromo (8) and
4-chloroanisole (9) the presence of 6 was required to realize the
product formation (Table 3, entries 2e3). Noteworthy, in various
cases the corresponding homocoupling and cross coupling prod-
Noteworthy, the coupling of
7
with iPrZnBr to form 4-
isopropylanisole was not noticed. In addition, the complexes 1a
and 2a were tested as dehalogenation pre-catalysts (Table 1, entries
3 and 4). Only in case of 2a product formation (40%) was observed
along with 7% heterocoupling. Probably, the electron-withdrawing
effects of two CF3 groups reduce the catalytic abilities of the com-
i
ucts were detected [12]. In comparison to PrZnBr higher yields
were obtained. For instance the substrates containing nitrile or
ester functions were converted in good yields at room temperature
t
plex, while with the Bu group in complex 6 a more basic metal
Scheme 1. Synthesis of the nickel complex 6.

M. Weidauer et al. / Journal of Organometallic Chemistry 729 (2013) 53e59
55
Fig. 3. Molecular structure of 6. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [ꢀ]: Ni1eN1:
1.8513 (19), Ni2eN3: 1.8487(18), Ni1eO1: 1.8453(15), Ni2eO4 1.8375(16), Ni1eO2: 1.8229 (15), Ni2eO3: 1.8297(15), Ni1eP1: 2.2419(7), Ni2eP2: 2.2304(6), O1eNi1eO2: 176.61(7),
O3eNi2eO4: 176.80(7), N1eNi1eP1: 174.74(6), N3eNi2eP2: 174.74(6).
(Table 3, entries 5 and 6). Importantly, for the substrates contai-
ning nitrile and thioether functions to some extend the hydro-
decyanation or hydrodesulfurization was observed as side reaction
(Table 3, entries 5 and 8). Probably the C-CN or C-S bond is activated
by coordination to the nickel center [13]. Interestingly, the appli-
cation of bromobenzene-d5 (19) as substrate showed an excellent
selectivity of the catalyst for the carbon halogen bond, while no
scrambling of the deuterium atoms was noticed (Table 3, entry 9).
Moreover, to some extent defluorination was performed with
the described system (Table 3, entries 16 and 17). Furthermore,
alkyl based halides were dehalogenated (Table 3, entries 18 and 20).
For studying the influence of the leaving group a mixture of
4-iodoanisole (7), 4-bromoanisole (8), 4-chloroanisole (9), and
4-fluoroanisole (25) was reacted with stepwise increasing amounts
of tert-butylmagnesium chloride in the presence of pre-catalyst 2a
(Fig. 4). The obtained results are in accordance to the bond dis-
sociation energies of the leaving groups: CeI (w270 kJ/mol) < CeBr
(w340 kJ/mol) < CeCl (w400 kJ/mol) < CeF (w490 kJ/mol) [14].
Moreover, the potential of the complexes was evaluated in the
hydrodechlorination of polychlorinated organic pollutant e.g.,
hexachlorobenzenze (29, Fig. 5). In more detail, catalytic amounts
of complex 2a were reacted with 29 with stepwise increasing
amounts of tert-butylmagnesium chloride. The addition of 1 equiv.
of tert-butylmagnesium chloride resulted in the formation of sig-
nificant amounts of pentachlorobenzene, while increasing the
amount showed a broad variety of products, including all possible
compounds on the way to benzene. Noteworthy, for products 31, 32
and 33 different isomers were detected. Moreover, to some extend
the cross coupling of the chlorinated benzenes was observed,
resulting after dechlorination in biphenyl 35. Finally, after addition
of 6 equiv. of tert-butylmagnesium chloride benzene 24a was
observed as major product.
For the understanding of the origin of the hydrogen atom of the
created carbonehydrogen bond the dehalogenation of 8 was per-
formed in THF-d8, however, no incorporation of deuterium was
detected. Moreover, the reaction was performed as described in
Table 3 entry 2 and afterwards it was quenched with methanol-d4.
Noteworthy, no incorporation of deuterium was monitored, exclu-
ding probably a transfer of the magnesium to the aryl halide. On the
other hand, the reaction of 8 with EtMgBr-d5 under standard con-
ditions gave deuteroanisole in 51% yield, while as side reaction the
cross coupling was observed (49%). Based on that, we assume the
source of hydrogen should be the tert-butyl group of the Grignard
Table 1
Nickel-catalyzed dehalogenation of 4-iodoanisole (7).a
reagent by b-hydride elimination as figured out for other catalytic
system [5a,15]. On the other hand, for various nickel-catalyzed cross
couplings radical species has beenproposed as intermediates [16]. In
order to probe the existence of radical species substrate 36 was
applied (Scheme 2) [17]. In earlier studies it has been shown that the
corresponding radical of 36 forms 1-methylindane (37) due to 5-
exo-trig cyclization with a rate constant of 108 sꢁ1 [18]. Indeed,
significant amounts of 37 were observed by GCeMS along with the
desired dehalogenated product (38) and various undefined side
products, indicating a radical based reaction mechanism. Moreover,
the addition of 1 equivalent of TEMPO gives hint to a radical based
process, because a diminished yield (17%) was noticed.
Entry
Ni-source (mol%)
T [ꢀC]
Yield [%]b
1
2
3
4
5
6
e
70
70
70
70
70
40
<1
87
<1
40
68
66
6 (5.0)
1a (5.0)
2a (5.0)
6 (2.5)
6 (5.0)
a
Reaction conditions:
7
(1.0 mmol), pre-catalyst (2.5e5.0 mol%), iPrZnBr
(1.5 equiv., 0.5 M in THF), 40e70 ꢀC, 24 h.
b
Determined by GCeMS and 1H NMR.

56
M. Weidauer et al. / Journal of Organometallic Chemistry 729 (2013) 53e59
Table 2
Table 3
Nickel-catalyzed dehalogenation with iPrZnBr e scope and limitations.a
Nickel-catalyzed dehalogenation with tBuMgCl e scope and limitations.a
Entry
1
Substrate
Yield [%]b
Entry
1
Substrate
T [ꢀC]
Conv. [%]b
Yield [%]b
7a: 87
r.t.
>99
7a: >99c
7
2
3
7a: >99
7a: >99
2
3
4
5
70
70
r.t.
r.t.
>99
>99
52
7a: 72
7a: 60
10a: 52c
11a: 86d
4
5
10a: 78
>99
11a: 10c
6
7
r.t.
r.t.
>99
>99
12a: 73
6
7
12a: 35
13a: 61e
13a: >99
8
9
70
70
90
14a: 58f
19a: 62
8
14a: 87d
9
15a: <1
16a: 75e
>99
10
10
11
r.t.
70
>99
>99
16a: 36g
17a: 92
11
17a: 49
12
18a: <1
12
70
>99
20a: 67
a
Reaction conditions: substrate (1.0 mmol), pre-catalyst
(1.5 equiv., 0.5 M in THF), 70 ꢀC, 24 h.
6
(5 mol%), iPrZnBr
13
14
r.t.
70
>99
>99
21a: 65
22a: 81
b
Determined by GCeMS and 1H NMR.
c
As side product benzene (7%) was observed.
As side product benzene (13%) was observed.
As side product benzene (25%) was observed.
d
e
Based on the preliminary results we propose a reaction mech-
anism as presented in Scheme 3. First the triphenylphosphane
ligand dissociates from complexes 2 or 6 to create open coordina-
tion sites. In an earlier NMR study the dissociation was monitored
15
70
>99
23a: 57

M. Weidauer et al. / Journal of Organometallic Chemistry 729 (2013) 53e59
57
Table 3 (continued )
Entry
Substrate
T [ꢀC]
Conv. [%]b
59
Yield [%]b
16
70
24a: 49
17
18
r.t.
r.t.
34
7a: 34
>99
26a: 70
19
r.t.
>99
27a: 13
20
70
70
47
94
28a: 40
7a: 64
21h
22i
70
91
7a: 76
a
Reaction conditions: substrate (1.0 mmol), pre-catalyst 6 (5 mol%), tBuMgCl
(1.5 equiv., 2.0 M in diethyl ether), THF (2.0 mL), r.t ꢁ70 ꢀC, 24 h.
b
Determined by GCeMS and 1H NMR.
Without catalyst.
As side product benzene (6%) was observed.
As side product benzene (6%) was observed.
As side product benzene (7%).
As side product benzene (23%) was observed.
Toluene was used as solvent.
1.5 equiv. EtMgBr (3.0 M in n-hexane).
c
d
Fig. 5. Hydrodechlorination of hexachlorobenzene 29 (reaction conditions: substrate
29 (1.0 mmol), pre-catalyst 2a (5 mol%), tBuMgCl (1.0 equiv., 0.5 M in THF, every 24 h),
70 ꢀC. Determined by GCeMS. For 31, 32, and 33 the sum of isomers is shown).
e
f
g
h
i
mediates a single electron transfer (SET) to oxidize the complex and
produce a radical [16]. The one-electron oxidized complex can on
the one hand oxidized at the metal center or on the other hand the
electron can be stored in the ligand (non-inocent ligand). Subse-
quently, MgX2 is eliminated and the radical recombines with the
nickel species to form the species E. Finally, an elimination of AreH
occurs to regenerate complex A.
by 31P NMR and the free coordinations sites were occupied by the
solvent THF to form the complex A [8]. The complex A reacts with
the Grignard reagents to create complex B, which contains a 4-
membered ring system [19]. After b-hydride elimination ethylene is
formed and the nickel-magnesium hydride species C. The organic
halide approaches the coordination sphere of the nickel and
3. Conclusion
In summary, we have set up a protocol for the nickel-catalyzed
dehalogenation of organic halides with isopropyl zinc bromide or
tert-butylmagenesium chloride under mild reaction conditions.
With the well-defined nickel complex 6 a variety of aryl and alkyl
halides were converted. Future investigations will be dedicated to
a deeper understanding of the reaction mechanism.
4. Experimental section
4.1. General
All compounds were used as received without further purifi-
cation. THF and toluene were dried applying standard procedures.
1H, 19F and 13C NMR spectra were recorded on a Bruker AFM 200
spectrometer (1H: 200.13 MHz; 13C: 50.32 MHz; 19F: 188.31 MHz)
using the proton signals of the deuterated solvents as reference.
Single crystal X-Ray diffraction measurements were recorded on an
Oxford Diffraction Xcalibur S Sapphire spectrometer. GCeMS
measurements were carried out on a Shimadzu GC-2010 gas
chromatograph (30 m Rxi-5ms column) linked with a Shimadzu
GCMA-QP 2010 Plus mass spectrometer. IR spectra were recorded
on a Perkin Elmer Spectrum 100 FT-IR.
Fig. 4. Reactivity study of different anisole halides and pre-catalyst 2a (reaction con-
ditions: substrates 7, 8, 9, 25 (each 1.0 mmol), pre-catalyst 2a (1.25 mol%), tBuMgCl
(1.2 equiv., 0.5 M in THF, every 24 h), 70 ꢀC. Determined by GCeMS applying
n-dodecane as internal standard).

58
M. Weidauer et al. / Journal of Organometallic Chemistry 729 (2013) 53e59
Scheme 2. Investigation of the product composition of the nickel-catalyzed dehydrohalogenation of the radical probe 36 (yield determined by GCeMS methods).
4.2. Synthesis of 5
The residue was crystallized from ethanol. Crystals suitable for Xe
ray diffraction analysis were obtained from ethanol solution at
room temperature. Yield ¼ 1.52 g (75%, brown crystals). Mp: 154e
A mixture of 3 (33.6 mmol) and 4 (33.6 mmol) in ethanol (50 mL)
was stirred for 24 h under refluxing conditions. Afterwards the re-
action mixture was filtered and the ethanol was removed in vacuum.
A yellow solid was obtained by recrystallization from n-hexane.
Yield ¼ 2.97 g (28%, colorless crystals). Mp: 185e187 ꢀC. 1H NMR
156 ꢀC. 1H NMR (200 MHz, C6D6, 25 ꢀC)
d
¼ 8.07e8.12 (m, 2H, Are
H), 7.69e7.79 (m, 6H, AreH), 6.98e7.11 (m, 1H, AreH), 6.10 (s, 1H,
19
CeH), 0.69 (s, 9H, C(CH3)3) ppm. F NMR (188 MHz, C6D6, 25 ꢀC)
31
d
¼ ꢁ64.2 ppm. P NMR (81 MHz, CDCl3, 25 ꢀC)
d
¼ 18.1 ppm. IR
(200 MHz, CDCl3, 25 ꢀC)
d
¼ 7.75e7.78 (m, 2H, AreH), 7.36e7.56 (m,
(KBr):
n
¼ 3436 (br), 3057 (w), 2959 (w),1589 (m),1558 (m),1531 (s),
3H, AreH), 3.02e3.37 (m, 2H, CH2), 3.73 (s, 1H, OH), 1.06
(s, 9H, C(CH3)3) ppm.13C{1H} NMR (100 MHz, CDCl3, 25 ꢀC)
¼ 172.8,
133.4, 132.1, 129.9, 127.9, 125,8, 103.7, 41.9, 41.5, 27.0, 25.8 ppm. 19F
NMR (188 MHz, CDCl3, 25 ꢀC)
3016 (m), 2979 (m), 1716 (s), 1665 (s), 1603 (w), 1581 (w), 1556 (m),
1478 (w),1447 (w),1425 (w),1368 (m),1303 (m),1280 (m),1209 (m),
1184 (m), 1143 (s), 1118 (s), 1076 (w), 1057 (m), 1037 (m), 1002 (m),
949 (m), 735 (w), 698 (m) cmꢁ1. HRMS calc. for C15H17F3N2O2 þ H:
315.13149, found: 315.13116.
1515 (s), 1494 (m), 1481 (m), 1437 (m), 1413 (s), 1363 (m), 1325 (m),
1252 (m), 1223 (m), 1197 (m), 1174 (s), 1162 (s), 1144 (m), 1100 (m),
1068 (m), 1027 (m), 783 (m), 745 (m), 693 (s), 530 (m), 511 (m), 492
(m) cmꢁ1. HRMS calc. for C30H21F6N2O2NiP þ H: 633.14232, found
633.14202.
d
d
¼ ꢁ68.0 ppm. IR (KBr): ¼ 3165 (m),
n
4.4. Single-crystal X-ray structure determination [20]
Crystals were each mounted on a glass capillary in per-
fluorinated oil and measured in a cold N2 flow. The data were
collected using an Oxford Diffraction Xcalibur S Sapphire at 150(2)
4.3. Synthesis of 6
To a methanol solution (10 mL) of Ni(OAc)2$4H2O (3.18 mmol)
was added the methanol solution (10 mL) of 5 (3.18 mmol) and tri-
phenylphosphane (6.36 mmol). The solution was stirred over night.
After removing all volatiles in vacuum a brown solid was obtained.
K (MoK radiation,
l
¼ 0.71073 Å). The structures were solved by
a
direct methods and refined on F2 with the SHELX-97 software
package. The positions of the hydrogen atoms were calculated and
considered isotropically according to a riding model.
Scheme 3. Proposed catalytic cycle for the nickel-catalyzed dehydrohalogenation.

M. Weidauer et al. / Journal of Organometallic Chemistry 729 (2013) 53e59
59
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A Schlenk flask was charged with an appropriate amount of
complex 6 (0.05 mmol, 5.0 mol%) and the corresponding bromo,
chloro, fluoro or iodo arene (1.0 mmol). The flask was cycled with
nitrogen and vacuum. Afterwards THF (2.0 mL) and a diethylether
solution of tert-butyl-magnesiumchloride (0.75 mL, 1.5 mmol,
2.0 M in diethylether) were added. The flask was sealed and heated
at 70 ꢀC for 24 h or stirred at room temperature. After that time, the
mixture was cooled and a NH4Cl solution was added. The mixture
was extracted with dichloromethane and the organic layer was
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by GCeMS and NMR analysis using n-dodecane as internal stan-
dard. The analytical properties of the products are in agreement
with literature data.
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