Cobalt-catalyzed cross-coupling reactions of aryl bromides with alkyl grignard reagents

Article abstract of DOI:10.1246/cl.2008.1178

Aryl bromides react with primary alkyl Grignard reagents in the presence of N,N,N′,N′-tetramethyl-1,3-propanediamine and catalytic amounts of cobalt(II) chloride and an N-heterocyclic carbene to yield the corresponding cross-coupling products in high yields. Copyright

Full text of DOI:10.1246/cl.2008.1178

1178
Chemistry Letters Vol.37, No.11 (2008)
Cobalt-catalyzed Cross-coupling Reactions of Aryl Bromides with Alkyl Grignard Reagents
Hiroyuki Hamaguchi, Minoru Uemura, Hiroto Yasui,
Hideki Yorimitsu,ꢀ and Koichiro Oshimaꢀ
Department of Material Chemistry, Graduate School of Engineering, Kyoto University,
Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510
(Received August 22, 2008; CL-080804;
E-mail: yori@orgrxn.mbox.media.kyoto-u.ac.jp, oshima@orgrxn.mbox.media.kyoto-u.ac.jp)
Aryl bromides react with primary alkyl Grignard reagents in
the presence of N,N,N0,N0-tetramethyl-1,3-propanediamine and
catalytic amounts of cobalt(II) chloride and an N-heterocyclic
carbene to yield the corresponding cross-coupling products in
high yields.
Table 1. Cobalt-catalyzed reaction of p-bromoanisole (1a) with
octylmagnesium chloride
5mol% CoCl2, 6mol% ligand
1.5 equiv additive
Br
n-C8H17
1.5 equiv n-C8H17MgCl
Et2O, 25 °C, 1 h
MeO
MeO
2a
1a, 0.50 mmol
Palladium- and nickel-catalyzed cross-coupling reactions
are powerful tools for carbon–carbon bond formation. Recently,
cross-coupling reactions catalyzed by transition metals other
than palladium and nickel have attracted increasing attention.1
We have been interested in cobalt-catalyzed cross-coupling re-
actions.2 To expand the scope of cobalt-catalyzed cross-coupling
reactions, we report herein cobalt-catalyzed cross-coupling
reactions of aryl bromides with alkyl Grignard reagents.
Entry
Liganda
.
IMes HCl
None
Additive
TMPDA
2a/%
1a/%
1
2
3
4
5
6
7
8
9
10
11
12
13
91
0
0
0
0
75
0
0
72
0
0
0
85
55
57
77
0
81
50
0
83
69
79
61
TMPDA
TMPDA
TMPDA
TMPDA
TMPDA
TMPDA
None
TMEDA
H2N(CH2)3NH2
Et3Nb
2,20-Bipyridyl
TMPDAc
PPh3
P(c-C6H11
Pt-Bu3
)
3
.
IPr HCl
Mes HASPO
.
.
IMes HCl
Treatment of p-bromoanisole (1a) with octylmagnesium
chloride in the presence of catalytic amounts of cobalt(II)
chloride and a precursor of an N-heterocyclic carbene (NHC),
.
IMes HCl
.
IMes HCl
.
IMes HCl
3
IMes HCl, and 1.5 equiv of N,N,N0,N0-tetramethyl-1,3-pro-
.
.
IMes HCl
.
IMes HCl
0
0
panediamine (TMPDA) in diethyl ether afforded p-octylanisole
4
.
(2a) in 91% yield (Table 1, Entry 1). In this reaction, IMes HCl
and TMPDA played key roles. In the absence of either
a
i-Pr
i-Pr
.
IMes HCl or TMPDA, no 2a was obtained (Entries 2 and 8).
The use of phosphine ligands also failed to afford 2a (Entries
N
N
N
N
Cl–
5
3
.
3–5). In contrast, the bulkier NHC precursor, IPr HCl, served
as an effective ligand in the cross-coupling reaction (Entry 6).
Cl–
IMes•HCl
i-Pr
i-Pr
IPr•HCl
6
0
0
.
Mes HASPO did not work (Entry 7). The use of N,N,N ,N -
tetramethylethylenediamine (TMEDA) instead of TMPDA
provided 2a in good yield (Entry 9). No 2a was obtained when
1,3-propanediamine, 2,20-bipyridyl, and triethylamine were
employed as an additive (Entries 10–12). The stoichiometric
amount of TMPDA was essential: the reaction in the presence
of 10 mol % of TMPDA led to no formation of 2a (Entry 13).
This result suggests that TMPDA coordinates to magnesium to
promote the reaction. Diethyl ether was the best solvent. The re-
actions in THF, 1,4-dioxane, and 1,2-dimethoxyethane afforded
2a in 60%, 57%, and 68% yields, respectively.
N
N
P
O H
Mes•HASPO
b3.0 equiv. 10 mol %.
c
(Entries 10 and 13). However, the reaction of o-bromotrifluoro-
methylbenzene (1m) led to low yield, albeit with full conversion
(Entry 12). p-Iodoanisole, generally the more reactive than 1a,
was converted to 2a in only 28% yield, and a significant amount
of anisole was obtained. The reaction of p-chloroanisole suffered
from low conversion as well as formation of a trace amount of
2a. The effect of the leaving groups is not clear at this stage.
Other primary alkylmagnesium chlorides participated in the
cross-coupling reaction (Table 3). Hexyl- and butylmagnesium
chloride reacted with 1a to yield the corresponding p-alkylani-
soles in good yields (Entries 1 and 2). However, attempted ethyl-
ation suffered from low yield, possibly because of the slower
transmetalation (Entry 3). Octylmagnesium bromide was as re-
active as the corresponding chloride (Entry 4). Methyl and allyl
Grignard reagents did not react with 1a (Entries 5 and 6). Methyl
Grignard reagent might undergo transmetalation sluggishly. Al-
lyl Grignard reagent could be too reactive, and the carbene li-
gand can be decomposed. Silyl-substituted methylmagnesium
The scope of aryl bromides in the cobalt-catalyzed cross-
coupling reaction is summarized in Table 2. Acetals (Entries 2
and 4) and silyl ether (Entry 3) were compatible under the reac-
tion conditions. The coupling reaction occurred at the brominat-
ed carbon exclusively to yield 2f, leaving the chloro moiety
untouched (Entry 5). Dimethylamino-substituted aryl bromide
1g underwent the coupling reaction smoothly (Entry 6). The re-
action of 1h having an electron-withdrawing trifluoromethyl
group resulted in moderate yield (Entry 7). Not only p-bromo-
anisole (1a) but also m- and o-bromoanisole were efficiently
converted to the corresponding products (Entries 8 and 9). Steri-
cally demanding 1k and 1n were also octylated in good yields
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.11 (2008)
1179
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Table 2. Scope of aryl bromides
5mol% CoCl2, 6mol% IMes•HCl
1.5 equiv TMPDA
1.5 equiv n-C8H17MgCl
Br
n-C8H17
Et2O, 25 °C, 1 h
R
R
2
1, 0.50 mmol
Entry
1
R
2
Yield/%
´
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1
2
3
4
5
6
7
8
9
10
11
12
13
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
p-Me
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
84
89
93
93
69
88
56
83
85
72
68
24
64
p-CMe(OCH2CH2O)
p-OSitBuMe2
p-OTHP
p-Cl
p-Me2N
p-CF3
m-MeO
o-MeO
o-Me
H
o-CF3
(1-Naphthyl)
´
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Table 3. Scope of Grignard reagents
2
5mol% CoCl2, 6mol% IMes•HCl
1.5 equiv TMPDA, 1.5 equiv RMgX
Br
R
Et2O, 25 °C, 1 h
MeO
MeO
1a, 0.50 mmol
3
Entry
RMgX
3
Yield/%
1
2
3
4
5
6
7
8
9
n-C6H13MgCl
n-C4H9MgCl
EtMgCl
n-C8H17MgBr
MeMgI
CH2=CHCH2MgBr
Me3SiCH2MgCl
i-PrMgCl
c-C6H11MgCl
CH2=CH(CH2)9MgCl
PhMgBr
3a
3b
78
66
11
87
3c
3d (=2a)
3
4
A. J. Arduengo, III, R. Krafczyk, R. Schmutzler, H. A. Craig, J. R.
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3e
3f
3g
3h
3i
3j0
0a
0b
72c
9 (34d)
32
Typical experimental procedure: Anhydrous cobalt(II) chloride
.
(3.2 mg, 0.025 mmol) and IMes HCl (10 mg, 0.030 mmol) were
placed in a 20-mL reaction flask and were heated with a hair
dryer in vacuo for 2 min. After the color of the cobalt salt became
blue, substrate 1a (94 mg, 0.50 mmol) in anhydrous diethyl ether
(1.0 mL) and TMPDA (0.13 mL, 0.75 mmol) were added under
argon. Octylmagnesium chloride (0.75 mL, 1.0 M ethereal solution,
0.75 mmol) was then added at one push at 25 ꢁC under argon. The
resulting mixture was stirred for 1 h at 25 ꢁC. The reaction mixture
was poured into a saturated ammonium chloride solution. The prod-
uct was extracted with ethyl acetate (20 mL ꢂ 2). The combined
organic layer was passed through a pad of Florisil, dried over
Na2SO4, and concentrated to provide an oil. Purification by silica
gel column chromatography (hexane/ethyl acetate = 20:1) provid-
ed 2a (100 mg, 0.45 mmol) in 91% isolated yield. Product 2a
showed the identical spectra in the literature: T. Brenstrum, D. A.
Gerristma, G. M. Adjabeng, C. S. Frampton, J. Britten, A. J.
Robertson, J. McNulty, A. Capretta, J. Org. Chem. 2004, 69, 7635.
The only by-product was anisole, which would result from ꢀ-
hydride elimination from an alkyl(aryl)cobalt intermediate
followed by reductive elimination.
10
11
(75e)
3k
21
a90% of 1a was recovered. b82% of 1a was recovered. cPerformed
at reflux for 16 h. dYield of p-propylanisole (3h0). Yield of p-9-
e
undecenylanisole (3j0).
reagent reacted in boiling ether (Entry 7). The reaction of 1a
with isopropylmagnesium chloride was sluggish, providing p-
isopropylanisole (3h) in only 9% yield (Entry 8). As a byprod-
uct, aside from anisole, p-propylanisole (3h0) was obtained in
34% yield. It is probable that isomerization of isopropylcobalt
to n-propylcobalt would take place through ꢀ-hydride elimina-
tion followed by the anti-Markovnikov hydrocobaltation.7 The
reaction of cyclohexylmagnesium chloride thus afforded 3i in
32% yield as the sole alkylated product (Entry 9). Under the re-
action conditions, isomerization of terminal to internal olefin oc-
curred (Entry 10).8 Unfortunately, the reaction with phenylmag-
nesium bromide was sluggish, affording a large amount of ani-
sole (Entry 11).
5
6
7
L. Ackermann, Synthesis 2006, 1557; L. Ackermann, Synlett 2007,
507.
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Kodama, I. Nakajima, A. Minato, M. Kumada, Bull. Chem. Soc.
Jpn. 1976, 49, 1958; Y. Kiso, K. Tamao, M. Kumada, J. Organo-
met. Chem. 1973, 50, C12; K. Tamao, Y. Kiso, K. Sumitani, M.
Kumada, J. Am. Chem. Soc. 1972, 94, 9268; A. Iida, M. Yamashita,
Bull. Chem. Soc. Jpn. 1988, 61, 2365.
Dedicated to Professor Ryoji Noyori on the occasion of his
70th birthday.
References and Notes
1
Selected examples. Cu: B. H. Lipshutz, S. Sengupta, Org. React.
1992, 41, 149; J. Terao, H. Todo, S. A. Begum, H. Kuniyasu, N.
8
T. Kobayashi, H. Ohmiya, H. Yorimitsu, K. Oshima, J. Am. Chem.
Soc. 2008, 130, 11276.
Published on the web (Advance View) November 5, 2008; doi:10.1246/cl.2008.1178