Direct cross-coupling of C-H bonds with grignard reagents through cobalt catalysis

Article abstract of DOI:10.1002/anie.201005394

Go go Grignard! The first highly regioselective, cobalt-catalyzed C-H transformation of benzo[h]quinoline and phenylpyridine derivatives with Grignard reagents at room temperature has been achieved (see scheme). Both aryl and alkyl Grignard reagents showed significant reactivity.

Full text of DOI:10.1002/anie.201005394

DOI: 10.1002/anie.201005394
À
C H Activation
À
Direct Cross-Coupling of C H Bonds with Grignard Reagents through
Cobalt Catalysis**
Bin Li, Zhen-Hua Wu, Yi-Fan Gu, Chang-Liang Sun, Bai-Quan Wang,* and Zhang-Jie Shi*
À
Many methods have recently been developed to construct C
C bonds through direct C H transformations.[1] Among these
À
methods, various transition metals have been shown to be
effective in the last two decades.[2,3] The first-row transition-
metal catalysts, such as iron complexes,[4,5] have drawn much
attention recently as a result of their ready availability, low
Scheme 1. New catalytic pathway beyond the traditional nucleophilic
addition of RMgBr in the presence of a Co catalyst.
cost, relatively low toxicity, and unique catalytic abilities.
À
Cases where the catalytic direct C H transformation is
mediated by Co catalysis are very rare.[6] On the other hand,
various organometallic reagents have been successfully used
to facilitate such transformations.[7] However, much more
active Grignard reagents have never been successfully applied
the presence of a suitable transition-metal catalyst and
lowering the reaction temperature might be the good strategy
to decrease the reactivity of RMgBr toward the direct
addition to pyridine derivatives.
À
as an intermolecular coupling partner with C H bonds.
Herein, we demonstrated the first successful example of
À
cobalt-catalyzed direct C H transformation with both aryl
After many trials to achieve the goal, the desired cross-
coupling finally took place smoothly with Co(acac)3 as a
catalyst in the presence of TMEDA and DCB in THFat room
temperature (Table 1, entry 1).[11] Certainly, no desired prod-
uct 3aa was formed in the absence of catalyst, under
otherwise identical conditions with Grignard reagents.
Next, the reactivity of various Grignard reagents was
investigated (Table 1). Phenyl Grignard reagents bearing
different substituents provided the desired products in
moderate to excellent yields. The steric effect was important
in influencing the reaction outcome. Thus, increase of the
steric hindrance dramatically decreased the yield (compare
Table 1, entries 2, 3, and 4). Meanwhile, further increase of
steric hindrance completely inhibited the transformation
(Table 1, entry 14). Importantly, the functional groups MeO
(Table 1, entries 8 and 9), F (Table 1, entry 11), and Cl
(Table 1, entry 12) are compatible. These findings offer the
opportunity for the orthogonal coupling to afford more
complicated molecules.
À
and alkyl Grignard reagents as nucleophiles to facilitate C C
bond formation.
In other studies, the directing group orientation was
considered useful to control the regioselectivity in C H
À
activation.[8] Among different anchoring groups, nitrogen-
contained groups were frequently used owing to their good
compatibility with transition metals and synthetic applica-
tions.[5g,h,7h,9] Initially, we chose the benzo[h]quinoline (1a) as
the substrate to explore the reactivity of Grignard reagents in
the presence of transition-metal catalysts. However, in the
absence of any catalyst, the direct nucleophilic attack of
pyridine derivatives at the 2-position by Grignard reagents is
a big challenge (traditional pathway, Scheme 1).[10] For
instance, even much less active organozinc reagents promote
such a reaction through nickel catalysis.[7k] We propose that
[*] Prof. Dr. B. Li, Z.-H. Wu, Y.-F. Gu, C.-L. Sun, Prof. Dr. Z.-J. Shi
Beijing National Laboratory of Molecular Sciences (BNLMS)
and Key Laboratory of Bioorganic Chemistry and Molecular
Engineering of Ministry of Education, College of Chemistry
and Green Chemistry Center, Peking University
Beijing 100871 (China)
This direct cross-coupling can be extended to alkyl
2
3
À
Grignard reagents for the constructions of C(sp ) C(sp )
bonds. Interestingly, MeMgBr showed the best reactivity and
the desired methylated product 3aq was isolated in an
excellent yield (Table 1, entry 17). However, other alkyl
Grignard reagents gave much lower yields and only linear
products were obtained (Table 1, entries 18–21). Unfortu-
nately, under the same conditions, vinylMgBr completely
failed to facilitate this cross-coupling reaction (Table 1,
entry 22).
Fax: (+86)10-6276-0890
E-mail: zshi@pku.edu.cn
Homepage: http://www.shigroup.cn/
Prof. Dr. Z.-J. Shi
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
Prof. Dr. B. Li, Prof. Dr. B.-Q. Wang
Moreover, different benzo[h]quinoline compounds were
investigated (Scheme 2). We found that this transformation
was very sensitive to steric effects. When either a Ph or Me
group was introduced at the 2-position, the coupling was
completely inhibited and only the starting material was
recovered (3ba and 3ca). If the substituent was introduced
at the 9-position, the cross-coupling indeed took place while
State Key Laboratory of Elemento-Organic Chemistry
College of Chemistry, Nankai University, TianJin 300071 (China)
[**] Support of this work by the NSFC (Nos. 20672006, 20821062,
20832002, 20925207, GZ419) and the “973” Project from the MOST
of China (No. 2009CB825300) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005394.
Angew. Chem. Int. Ed. 2011, 50, 1109 –1113
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1109

Communications
Table 1: Direct functionalization of 1a with various Grignard reagents.[a]
Entry
R (2)
Product (3)
Yield [%][b]
1
2
3
4
5
6
7
8
Ph (2a)
2-tolyl (2b)
3-tolyl (2c)
4-tolyl (2d)
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
92
41
93
75
75
68
82
93
88
63
76
66
85
<5
71
70
83
54
40
19
42
0
4-tBuC6H4 (2e)
4-C6H5C6H4 (2 f)
4-vinylC6H4 (2g)
3-MeOC6H4 (2h)
4-MeOC6H4 (2i)
4-Me2NC6H4 (2j)
4-FC6H4 (2k)
4-ClC6H4 (2l)
3,5-Me2C6H3 (2m)
2,4,6-Me3C6H2 (2n)
1-naphthyl (2o)
2-naphthyl (2p)
Me (2q)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
3aj
3ak
3al
3am
3an
3ao
3ap
3aq
3ar
3as
3at
À
Scheme 2. Direct functionalization of C H bond with different sub-
strates. Reaction conditions: 0.28 mmol of substrate, 1.12 mmol of
PhMgBr, 0.028 mmol of Co(acac)3, 0.28 mmol of TMEDA, 0.42 mmol
of 2,3-dichlorobutane, RT, 48 h. Reported yield is of isolated product.
[a] Product was detected by GC analysis with n-dodecane as an internal
standard. [b] Carried out using 1.68 mmol of PhMgBr and 0.56 mmol
of 2,3-dichlorobutane. TES=triethylsilyl.
Et (2r)
n-hexyl (2s)
n-octyl (2t)
benzyl (2u)
3au
3av
vinyl (2v)
radical process was generally proposed.[6a–c] We first tested
our developed process in the presence of radical scavengers.
To our surprise, the reaction ran very smoothly even in the
presence of two equivalents of TEMPO (2,2,6,6-tetramethyl-
1-piperidinyloxy, free radical). When 1,1-diphenylethylene
was added to the reaction mixture, the radical addition
product was not observed. On the other hand, the dimeriza-
tion of starting materials (10,10’-bibenzo[h]quinoline) was not
detected. Thus, the radical process was tentatively ruled out.
Interestingly, with the use of iPrMgBr as a reagent, a mixture
of the products with both linear and branched alkylation was
isolated in a ratio of 1.6:1 [Eq. (1)]. Therefore, we hypothe-
[a] Reaction conditions: 0.28 mmol of 1a, 1.12 mmol of Grignard
reagent, 0.028 mmol of Co(acac)3, 0.28 mmol of TMEDA, 0.42 mmol of
2,3-dichlorobutane. [b] Yield of isolated product. acac=acetylacetonate,
DCB=2,3-dichlorobutane, THF=tetrahydrofuran, TMEDA=N,N,N’,N’-
tetramethylethylenediamine.
the yield was also low (3ea). To our satisfaction, the
substituents at other positions did not affect the yield and
the desired products were obtained in good to excellent yields
(3da, 3 fa–3na). Notably, various groups survived under the
reaction condition. A free hydroxy group and a silyl protected
ether survived well (3ia–3ka). Furthermore, a tertiary amino
group did not affect the yield (3la–3na),
while the secondary amine dramatically
lowered the yield (3oa).
2-Phenylpyridine derivatives were also
suitable for this transformation (Table 2).
When 2-phenylpyridine was the substrate,
the direct arylation took place and afforded
monoarylated and diarylated products as a
À
À
mixture in good yield (Table 2, entries 1, 3, and 5). To our
delight, diarylated products could be selectively produced by
simply lengthening the reaction time or increasing the amount
of Grignard reagents (Table 2, entries 2, 4, and 6). Meanwhile,
when one of the ortho positions or meta positions was
blocked, only a single product was obtained in moderate to
good yields (Table 2, entries 7–13, and 15). Moreover, the
mixture of mono- and di-arylated products was also obtained
when the para-substituted substrate was applied (Table 2,
entry 14).
size that both R Co and H Co species exist in the catalytic
system. Moreover, starting from 10-[D]benzo[h]quinoline,
the amount of 6-[D]2-methoxylnaphthylene is almost equal to
that of the desired coupling product [Eq. (2)]. Finally, the
intermolecular isotopic studies indicated that the cleavage of
À
the C H bond was not involved in the rate determining step
[Eq. (3); yield determined by 1H NMR spectroscopy].
Thus, we propose the catalytic cycle shown in Scheme 3.
In the presence of TMEDA and Grignard reagents, the
catalytic active species 5 is produced followed by reductive
elimination of the CoIII intermediate 4. Species 5 could further
undergo oxidative addition with benzo[h]quinoline (1a) to
Further investigations unveiled the mechanistic pathway
of this unprecedented transformation.[11] In previous reports a
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Table 2: Direct functionalization with different 2-phenylpyridinyl substrates.[a]
is produced. After subsequent
reductive elimination to generate
the desire coupling product 3, the
CoI species 5 is regenerated to
facilitate the catalytic cycle. If the
b-hydrogen atom is present and the
steric hindrance increases in the
Grignard reagents, for example
when using iPrMgBr, b-hydride
Entry
Substrate
Product(s)
Yield of monosubstituted,
disubstituted
products [%][b]
1
2
3
4
5
6
7
8
9
30, 35 (Ar=Ph)
–, 62[c] (Ar=Ph)
17, 49 (Ar=4-MeOC6H4)
–, 63[d] (Ar=4-MeOC6H4)
10, 55 (Ar=3-MeOC6H4)
–, 64[d] (Ar=3-MeOC6H4)
68, – (Ar=Ph)
À
elimination produces the Co H
species 10, which undergoes the
insertion in the reverse direction
to generate the isomerized inter-
mediate 5. By following the same
coupling catalytic pathway, the pro-
pylation was observed.
64, – (Ar=4-MeOC6H4)
59, – (Ar=3-MeOC6H4)
10
11
70, – (Ar=4-MeOC6H4)
74, – (Ar=3-MeOC6H4)
In summary, we have reported
the first highly regioselective
À
12
13
14
40, –
38, –
8, 52
cobalt-catalyzed C H transforma-
tion of benzo[h]quilonine and phe-
nylpyridine
derivatives
with
Grignard reagents at room temper-
ature. An unprecedented reactivity
of Grignard reagents with pyridine
derivatives was presented. Various
substituents on the heterocycles
were compatible under these mild
reaction conditions. In addition to
aryl Grignard reagents, alkyl
Grignard reagents also showed
good to excellent reactivity. To
date, this is the only successful
example that applies Grignard
reagents as the coupling partners
15
63, –
[a] Reaction conditions: 0.28 mmol of substrate, 1.68 mmol of Grignard reagent, 1.12 mmol of LiCl,
0.028 mmol of Co(acac)3, 0.28 mmol of TMEDA, 0.56 mmol of 2,3-dichlorobutane, RT, 48 h. [b] Yield of
isolated monoarylated product and (if applicable) the diarylated product. [c] Carried out with 8.0 equiv of
ArMgBr, 3.0 equiv of 2,3-dichlorobutane, 72 h. [d] Carried out with 6.0 equiv of ArMgBr, 2.0 equiv of 2,3-
dichlorobutane, 72 h.
À
in direct C H functionalization.
Based on our preliminary studies,
the radical process was excluded
and a new catalytic pathway via an
oxidative addition is now highly preferred.
Further studies to explore this new method
and clearly understand the reaction pathway
are underway.
Experimental Section
Typical procedure for cobalt-catalyzed arylation
of benzo[h]quinoline with an aryl Grignard
reagent (Table 1, entry 1): Phenylmagnesium bro-
mide (1.12 mL, 1.00m, 1.12 mmol) in a solution of
THF, TMEDA (33 mg, 0.28 mmol), and 2,3-
dichlorobutane (53 mg, 0.42 mmol) were added
to a Schlenk tube containing benzo[h]quinoline
(50 mg, 0.28 mmol) and Co(acac)3 (10 mg,
0.028 mmol) at RT. The reaction mixture was
generate a new CoIII species 6. After the reductive elimination
stirred at RT for 48 h and then quenched by the addition of saturated
aqueous solution of NaHCO3 (5 mL). After extraction with ethyl
acetate (10 ꢀ 3 mL), the organic layer was dried over MgSO4 and
concentrated under reduced pressure. The crude residue was purified
by column chromatography on silica gel (eluent: first n-hexane to
À
to generate the by-product R H with the sacrifice of
1 equivalent of RMgX, the CoI complex 7 that is produced
is oxidized by 2,3-butyldichloride to produce the CoIII species
8. With the double transmetalations, the new CoIII complex 9
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Scheme 3. Plausible catalytic pathway for direct C H functionalization
with RMgX through Co catalysis.
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Received: August 29, 2010
Published online: December 22, 2010
À
Keywords: C H activation · cobalt · cross-coupling ·
Grignard reagents · homogeneous catalysis
.
À
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