Cobalt-Catalyzed Reductive Cross-Coupling Between Styryl and Benzyl Halides

Article abstract of DOI:10.1002/chem.201603832

A simple and efficient protocol for the direct reductive cross-coupling between alkenyl and benzyl halides using a Co/Mn system has been developed. This reaction proceeds smoothly in the presence of [CoBr₂(PPh₃)₂] as the catalyst, with NaI as an additive in acetonitrile with a broad scope of functionalized alkenyl and benzyl halides. Different functional groups are tolerated on both coupling partners, thus, significantly extending the general scope of transition-metal-catalyzed benzylation of alkenyl halides. Moderate to excellent yields were also obtained. From a mechanistic point of view, a radical chain mechanism was proposed. This reaction is stereospecific and some studies suggest the retention of the double-bond configuration.

Full text of DOI:10.1002/chem.201603832

A Journal of
Accepted Article
Title: Cobalt-Catalyzed Reductive Cross-Coupling Between Styryl and
Benzyl Halides
Authors: Yingxiao Cai, Andreas Benischke, Paul Knochel, and Corinne
Gosmini
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To be cited as: Chem. Eur. J. 10.1002/chem.201603832
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Cobalt-Catalyzed Reductive Cross-Coupling Between Styryl and
Benzyl Halides
Yingxiao Cai,[a] Andreas D. Benischke, [b] Paul Knochel [b]and Corinne Gosmini*[a]
Abstract: A simple and efficient protocol for the direct reductive
cross-coupling between alkenyl and benzyl halides using a Co/Mn
system has been developed. This reaction proceeds smoothly in the
presence of [CoBr2(PPh3)2] as catalyst, with NaI as additive in
acetonitrile with a broad scope of functionalized alkenyl and benzyl
halides. Different functional groups are tolerated on both coupling
partners, thus, significantly extending the general scope of transition-
metal-catalyzed benzylation of alkenyl halides. Moderate to excellent
yields were obtained. From a mechanistic point of view, a radical
chain mechanism was proposed. This reaction is stereospecific and
some studies suggest the retention of the double bond configuration.
well documented. H. Lipshutz’s group reported Pd-catalyzed
cross-coupling between alkenyl and benzylic halides without
organic solvent under mild reaction conditions. However, an
expensive catalyst was used and only few functional groups
were present.[12] More recently, Reisman’s group established an
elegant, efficient and highly enantioselective Ni-catalyzed
reductive cross-coupling between styrenyl bromides and
secondary benzyl chlorides.[13] However, Reisman’s work was
focused on the enantoselective cross-coupling of vinyl halides
and benzyl halides involving secondary benzyl systems to be
enantioselective. Moreover, Z-alkenyl bromide only afforded E-
product in low yield.
In our group, we have already disclosed some reductive
cross-coupling reactions using Co/Mn systems to form C-C
bonds between aryl-alkenyl, aryl-aryl, aryl-alkyl, allyl-alkyl
moities.[14] In such methods, a catalytic cobalt organometallic
reagents is generated under mild conditions. However, to the
best of our knowledge, Co-catalyzed cross-coupling between
alkenyl and benzyl compounds has not been studied with the
advantage to use a cheap with environmentally friendly catalyst.
Transition-metal-catalyzed Csp2-Csp3 bond formation has been
intensively developed during the last decades and has been one
of the most significant tools for organic synthesis.[1] Among these
couplings, the formation of 1,3-diarylpropenes was studied.
These compounds are important molecular structures or
intermediates in the synthesis of natural or biological products.[2]
A variety of strategies involving stoichiometric organometallic
species has been established to obtain such compounds via
Csp2-Csp3 bond formation. However, most of them involve aryl-
alkyl cross-couplings with allylic compounds as C sp3 reagent.[3]
Several alkenylation or benzylation reactions were also reported
but, these methods often required pre-generated stoichiometric
organometallic compounds which could limit the presence of
functional groups in some cases.[4-5] Oxidative alkenyl-benzyl
cross-couplings are another alternative but they always have
some drawbacks such as high temperature (> 99°C) and long
reaction time (>10h).[6-8] Another method involving a radical
alkylation of nitro-olefins by alkyl substituted Hantzsch esters
was also devised by Tang’s group in order to form 1,3-
diarylpropenes. [9] Although this method was highly efficient and
metal-free, it presented a low atom economy. Compared with
the above methods, versatile metal-catalyzed reductive cross-
coupling reactions have been developed in order to create
operationally simple C-C bond formation.[10] Very recently,
First, we have investigated the cross-coupling in MeCN as
solvent between β-bromostyrene and benzyl chloride, using
CoBr2 as catalyst, manganese as reductant, pyridine as
cosolvent and trifluoroacetic acid to activate manganese (Table
1, entry 1). An excess of the more reactive benzyl chloride was
necessary to obtain the cross-coupling product in good yield.
However, we observed the corresponding dimer in small amount.
In these conditions, the desired cross-coupling product was
obtained in only 20% GC yield. However, as previously
described with nickel, addition of sodium iodide had a beneficial
effect (Table 1, entry 2). Probably, this additive facilitates the
reduction of the Co catalyst[13,15] and/or generate a small amount
of the more reactive benzyl iodide in the medium.[16] A lower
yield was obtained without pyridine which was generally required
in our previous cross-coupling reaction using a Co/Mn system[14]
(Table 1, entry 2-3). Performing the reaction with zinc instead of
manganese as reducing metal decreased the yield (Table 1,
entry 4). Moreover, no improvement of the yield was observed
by adjoining 2 PPh3 as ligand for cobalt, in acetonitrile/pyridine
(Table 1, entry 5). However, replacing pyridine by 2 PPh3
increased the yield (Table 1, entry 6). The temperature is also
an important parameter using TFA for Mn activation and 50 °C
seemed to be the best one since either decreasing or increasing
the reaction temperature lowered the yield (Table 1, entries 6-8).
With these conditions in hand, we tried to cutback the catalytic
loading to 6% but the yield decreased (Table 1, entry 9). A
screening of ligands revealed that PPh3 was the best one (Table
1, entries 9-15). It is worth mentioning that the homemade
preformed [CoBr2(PPh3)2] complex,[17] instead of adding CoBr2
and PPh3 separately, slightly improved the yield (Table 1, entry
16) . With the preformed [CoBr2(PPh3)2] complex as catalyst,
Weix’s group reported
a nickel-catalyzed reductive cross-
coupling of vinyl halides with various functionalized alkyl halides
in mild conditions.[11] However, the Co-catalysed alkenyl-benzyl
reductive cross-coupling has not been investigated and more
generally, these couplings catalyzed by other metals were not
[a]
[b]
Y. Cai, Dr. C. Gosmini
LCM, CNRS, Ecole Polytechnique, Université Paris-Saclay, 91128
Palaiseau, France
E-mail: corinne.gosmini@polytechnique.edu
A. D. Benischke, P. Knochel
Department of Chemistry
Ludwig-Maximilians- universität, Butenandtstr. 5-13, 81377 Munich,
Germany
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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other additives were tested and sodium iodide was confirmed to
be the most effective (Table 1, entry 17-19). To activate
manganese, TMSCl[18] was used instead of TFA and to our
delight, we observed an excellent NMR yield of 84% (Table 1,
entry 20). Besides, the advantage to use TMSCl instead of TFA
is the possibility to carry out the reaction at room temperature
with the same yield (Table 1, entry 21-22). However, using
TMSCl required the use of the preforming [CoBr2(PPh3)2]
complex otherwise no cross-coupling product was observed
Encouraged by these results, we investigated the scope of
substrates. First, the reaction of various functionalized alkenyl
bromides with benzyl chloride was examined (Table 2). A wide
variety of functional group on the styrenyl bromides is tolerated
in this reaction. Styryl bromides bearing both electron-donating
and electron-withdrawing groups on the aromatic nucleus were
coupled with benzyl chloride and afforded moderate to excellent
yields. In general, electron-donating groups on the aromatic
nucleus led to higher yields than those bearing electron-
withdrawing groups. Moreover, a temperature of 50 °C was
required with a alkenyl bromide bearing an electron-withdrawing
group (Table 2, entry 9, 10), except with 4-(2-
bromovinyl)benzonitrile which only gave moderate yield despite
the temperature (Table 2, entry 7). In this case, the dimers of
each starting materials were obtained. However, several alkenyl
bromides bearing electron-donating groups were also coupled
with the benzyl chloride most efficiently at 50 °C (Table 2, entry
4, 8). An amino group was tolerated in this reaction but a
moderate yield was obtained (Table 2, entry 6). Moreover, it
should be pointed that the cross-coupling of an alkyl-substituted
alkenyl bromide instead of a styryl bromide with the benzyl
chloride did not work.
(Table 1, entry 23).
Table 1. Optimization of reaction conditions
Additive
(0.5
equiv.)
Temperature
entry
catalyst
ligand
Yield(%)[a]
(°C)
1[b]
2[b]
3
CoBr2
CoBr2
/
50
50
50
50
50
50
30
70
50
50
50
50
50
50
50
50
50
50
50
50
r.t.
/
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
/
20
60
/
CoBr2
/
37
4[c]
5[b]
6
CoBr2
/
23[b]
38[d]
70[d]
n.d.
60
CoBr2
2 PPh3
Table 2. Scope of alkenyl bromides
CoBr2
2 PPh3
[CoBr2(PPh3)2] (10 mol%)
Mn (3.8 equiv.)
7
CoBr2
2 PPh3
NaI (0.5 equiv.)
TMSCl (30 mol%)
Br
Cl
R
8
CoBr2
2 PPh3
R
+
MeCN
54[d]
temperature, 3-6 h
9
CoBr2 (6%)
CoBr2 (6%)
CoBr2 (6%)
CoBr2 (6%)
CoBr2 (6%)
CoBr2
2 PPh3
2.5 mmol
1.25 mmol
10
11
12
13
14
15
16
17
18
19
20[e]
21[e]
2 L1
14
2 L2
51
Entry
R
temperature
0 °C – r.t.
0 °C – r.t.
0 °C – r.t.
50 °C
product
3aa
3ba
3ca
3da
3ea
3fa
yield (%)
86[a]
80
2 L3
9
1
2
3
4
5
6
7
8
9
H (1a)
4-OMe (1b)
2-OMe (1c)
3,4,5-triOMe (1d)
4-F (1e)
2 L4
n.d.
10
2 L5
CoBr2
1 dppe
25
90
CoBr2(PPh3)2
CoBr2(PPh3)2
CoBr2(PPh3)2
CoBr2(PPh3)2
CoBr2(PPh3)2
CoBr2(PPh3)2
/
/
/
/
/
/
77[d]
53[d]
61[d]
61[d]
84[d]
84[d]
89
TMSCl
KBF4
NaI
NaI
0 °C – r.t.
0 °C – r.t.
50 °C
62[a]
47
4-NMe2 (1f)
4-CN (1g)
3ga
3ha
3ia
29
22[e]
23[e]
CoBr2(PPh3)2
/
0-r.t.
NaI
86[d]
4-OCOMe (1h)
4-CO2Me (1i)
50 °C
80
CoBr2
2 PPh3
50
NaI
0
50 °C
68
[a]: GC yield calculated with dodecane as an internal standard ; [b]: adding 0.5
mL of pyridine; [c]: using Zn instead of Mn; [d]: yield calculated by NMR with
mesitylene as internal standard; [e]: using TMSCl (30mol%) for activation
instead of TFA
[a] yield calculated by NMR with mesitylene as internal standard.
L1 : tri(p-methoxyphenyl)phosphine
L2 : tri(p-fluorophenyl)phosphine
L3 : tri(pentafluorophenyl)phosphine
L4 : tri(p-methylphenyl)phosphine
L5 : biisobutylphenylphosphine
The functional diversity on the benzyl chloride was next
investigated (Table 3). Electron-donating groups and electron-
withdrawing groups were well tolerated with good to excellent
yields. The reaction was successfully carried out at with benzyl
chloride bearing methoxy group in meta- and ortho- position
(Table 3, entry 1,2) while a methoxy and thiomethyl group in
para- position only provided moderate yields (Table 3, entry 3, 4).
Addition of an electron-donating group on the aromatic ring of
the styryl bromide did not change the reaction conditions (Table
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17
18
19
4-F (1e)
2-OMe (2b)
2-OMe (2b)
2-OMe (2c)
0 °C – r.t.
50 °C
3eb
3jb
80
3, entries 8-10). Moreover, the presence of an halogen
substituent on the benzyl moiety allowed the coupling product in
good yield whatever the halogen (Table 3, entry 11-13).
Interestingly, the presence of an alkenyl group did not hinder the
coupling reaction, no polymer was detected and 3bp was
isolated in 77% yield (Table 3, entry 15) and 2-
(chloromethyl)naphthalene could replace benzyl chloride (Table
3, entry 16) affording a good yield. 1,3-dienyl bromide and
secondary alkenyl bromide worked also well (Table 2, entry 19,
20). However, heating to 50 °C was necessary when either
benzyl chlorides or styrenyl bromides borne at least an electron-
withdrawing groups such as ester, cyano, trifluoromethyl and
methylsulfonyl group (Table 3, entry 5-7, 14, 18). In all cases,
very good yields were also obtained except with a nitro and
carboxy groups that were not tolerated. Moreover, the more
challenging cross-coupling of (E)-4-(2-bromovinyl)benzonitrile
with 3-trifluoromethyl-benzyl chloride provided the desired
product in moderate yield (Table 3, entry 21). Noteworthy, the
reaction did not work with secondary benzyl halides; with TMSCl
at room temperature, the benzyl chloride did not react and with
TFA, it homocoupled.
4-OCF3 (1j)
75
Ph(CH=CH)2-
0 °C – r.t.
3kc
70*
(1k)
20
21
2-indene (1l)
4-CN (1g)
2-OMe (2b)
3-CF3 (2o)
0 °C – r.t.
50 °C
3lb
77
39
3go
* Yield determined by 1H NMR spectroscopy
To investigate the outcome of the reaction concerning the
double bond configuration, several experiments were conducted
using Z/E-β-bromostyrene mixtures (scheme 1). The
corresponding products gave the same E/Z ratio as the starting
bromide. This suggests that the reaction occurs with retention of
the double bond configuration and opens the way to
stereocontrol the synthesis. In these reactions, the different
products were formed in good yield contrary to the similar nickel
catalyzed cross-coupling (scheme 1).
bromostyrene reacted faster than Z-isomer and afforded a
Furthermore, E-β-
higher yield.
Table 3. Scope of benzyl chlorides
R1
R2
T°C
product
yield
Entry
Scheme 1. Experiments of stereochemistry
(%)
80
77
37
40
65
73
90
73
73
79
61
62*
59*
77
77
71
1
H (1a)
H (1a)
2-OMe (2b)
3-OMe (2c)
4-OMe (2d)
4-SMe (2e)
4-CO2Me (2f)
4-CN (2g)
4-SO2Me (2h)
4-Me (2i)
0 °C - r.t.
0 °C - r.t.
0 °C - r.t.
0 °C - r.t.
50 °C
3ab
3ac
3ad
3ae
3af
Some experiments were also conducted to provide insight
into the mechanism. The addition of a free radical scavenger
such as 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) after the
activation of manganese inhibited the cross-coupling reaction.
2
3
H (1a)
4
H (1a)
Therefore, we assumed that
generated in the reaction.
a radical intermediate was
5
H (1a)
According to these results, we supposed that mechanism is
similar both to those proposed for Ni-catalyzed cross-coupling
involving alkyl halides[19] and those we proposed previously for
other Co-catalyzed cross-coupling involving alkyl halides. [14d,e] [20]
However, further studies to gain insight into its mechanism are
currently underway in our laboratory.
6
H (1a)
50 °C
3ag
3ah
3bi
7
H (1a)
50 °C
8
4-OMe (1b)
4-OMe (1b)
4-OMe (1b)
4-OMe (1b)
4-OMe (1b)
4-OMe (1b)
4-OMe (1b)
4-OMe (1b)
4-OMe (1b)
0 °C - r.t.
0 °C - r.t.
0 °C - r.t.
0 °C - r.t.
0 °C - r.t.
0 °C - r.t.
50 °C
9
3-Me (2j)
3bj
In conclusion, a novel and efficient method of alkenyl-benzyl
reductive cross-coupling using cobalt as a catalyst has been
developed in mild conditions using an environmentally friendly
catalyst. A broad scope of substrates can be employed under
very mild conditions affording moderate to excellent yields.
Furthermore, the stereochemical investigation demonstrated the
retention of the double bond configuration. As the reaction is
sensitive to the presence of TEMPO, a radical seems to be
involved. Further developments are underway in our laboratory.
10
11
12
13
14
15
16
2-Me (2k)
4-F (2l)
3bk
3bl
4-Cl (2m)
3bm
3bn
3bo
3bp
3bq
4-Br (2n)
3-CF3 (2o)
4-vinyl (2p)
0 °C - r.t.
0 °C - r.t.
2-naphthyl
(2q)
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To an oven-dried flask were added CoBr2(PPh3)2 (0.125 mmol), Mn (4.75
mmol), NaI (0.675 mmol) and MeCN (2 mL). Then TMSCl (30 mmol%)
was added to activate Mn. After the mixture was cooled down to room
temperature (around 5 min), alkenyl bromide (1.25 mmol) and benzyl
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10.1002/chem.201603832
Chemistry - A European Journal
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Yingxiao Cai, Andreas D. Benischke,
Paul Knochel and Corinne Gosmini*
Page No. – Page No.
Cobalt-Catalyzed Reductive Cross-
Coupling Between styryl and Benzyl
Halides
An efficient Co-catalyzed reductive cross-coupling of benzyl chlorides with alkenyl
bromides has been achieved. The complete retention of the double bond
configuration was kept and moderate to excellent yields were obtained in mild
conditions.
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