Direct method for carbon-carbon bond formation: The functional group tolerant cobalt-catalyzed alkylation of aryl halides

Article abstract of DOI:10.1002/chem.201000178

(Figure Presented). A new protocol for the direct cobaltcatalyzed alkylation of aryl halides has been developed that proceeds smoothly in the presence of phosphanes or bipyridines as ligands with a variety of alkyl halides, including challenging alkyl electrophiles bearing β hydrogen atoms (see scheme). Sensitive functional groups are tolerated on both coupling partners, thus, significantly extending the general scope of transition-metal-catalyzed alkylation of aryl halides.

Full text of DOI:10.1002/chem.201000178

DOI: 10.1002/chem.201000178
Direct Method for Carbon–Carbon Bond Formation: The Functional Group
Tolerant Cobalt-Catalyzed Alkylation of Aryl Halides
Muriel Amatore and Corinne Gosmini*[a]
Dedicated to the memory of Professor Pascal Le Floch; an inspiring scientist and wonderful individual
Modern transition-metal-catalyzed reactions have been
shown to be of indispensable value for organic synthesis.[1]
In particular, the cross-coupling of alkyl halides, including
problematic unactivated alkyl halides, have now emerged as
promising candidates to complement and extend the scope
of the traditional Friedel–Crafts alkylation.[2,3] The incorpo-
ration of a new alkyl chain on an aromatic moiety often pro-
ceeds by the metal-catalyzed coupling of stoichiometric or-
ganometallic derivatives (alkyl–M or aryl–M, M=Zn, B, Si,
Sn, Mg, Mn, In, Al) using a large variety of transition
metals, such as palladium, nickel, iron, cobalt, copper, rhodi-
um, manganese, vanadium, or even silver.[2,4] However, it is
intriguing to consider a direct approach for cross-coupling
that does not rely on the utilization of preformed organome-
tallic reagents, thus resulting in reduced step count and
waste. Two main strategies have been reported, the palladi-
functional groups such as esters or ketones that are prone to
react with organomagnesium reagents. We previously docu-
mented the development of straightforward procedures for
the direct cobalt-catalyzed functionalization of aryl halides
using CoBr2/ligand/metal systems.[8] Employing non-toxic
cobalt salts, we discovered highly reactive catalyst systems
suitable for inexpensive and operationally trivial carbon–
carbon bond-forming processes. Most importantly, these cat-
alyst systems also showed compatibility with otherwise sen-
sitive functionalized coupling partners. Herein, we report a
versatile method for the direct functional group tolerant al-
kylation of aryl halides, a transformation that employs
simple catalytic systems consisting of CoBr2/Ligand/Mn.
Our new cross-coupling protocol proceeds smoothly in the
presence of phosphanes or bipyridines as ligands with a vari-
ety of alkyl halides, including challenging alkyl electrophiles
bearing b-hydrogen atoms. Though the reaction requires
presently activated aryl halides, the synthetic scope is in our
view largely compensated by the use of a simple catalyst
formed in situ from ubiquitous reagents and by the fact that
sensitive functional groups such as esters, nitriles, or ketones
are tolerated on both coupling partners, thus significantly
extending the general scope of transition-metal-catalyzed al-
kylation of aryl halides (Scheme 1).
À
um-catalyzed C H activation for the domino ortho-alkyla-
tion/Heck coupling sequence,[5] and the direct cross-coupling
of the corresponding halides.[6] The latter process involves a
nickel-catalyzed activation of a-chlorocarbonyls for the syn-
thesis of functionalized a-arylcarbonyl compounds. More re-
cently, a magnesium-mediated direct cross-coupling using
catalytic amounts of iron or cobalt salts has been developed
by Jacobi von Wangelin and co-workers, thus documenting a
practical alternative to the use of electrosynthesis or toxic
nickel salts.[7] Despite the inherent practical advantages of
these methods, the mechanistic fundament is the in situ for-
mation of reactive Grignard reagents, which, in turn, nar-
rows the substrate scope. The latter is generally restricted to
electron-rich aryl and alkyl bromides and does not tolerate
Scheme 1. General concept for the cobalt-catalyzed direct alkylation of
aryl halides.
[a] Dr. M. Amatore, Dr. C. Gosmini
Laboratoire “Hꢀtꢀroꢀlꢀments et Coordination”
Ecole Polytechnique, CNRS (France)
Fax : (+33)1-6933-4440
Having successfully developed methods using a CoBr2/
PPh3/Mn system for the direct and selective synthesis of un-
symmetrical biaryls,[8c] we directed our efforts to exploit
such catalytic systems for the cobalt-promoted coupling of
aryl and alkyl halides. We envisaged the use of readily avail-
E-mail: corinne.gosmini@polytechnique.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000178.
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Chem. Eur. J. 2010, 16, 5848 – 5852

COMMUNICATION
able yet poorly reactive alkyl bromides (as opposed to the
corresponding iodides) bearing b-hydrogen atoms that
would be subject to slow oxidative addition and subsequent-
ly to rapid b-hydride elimination hampering the coupling
process.[2] Initial studies devoted to identifying suitable con-
ditions were conducted using ethyl 4-bromobenzoate and
ethyl 4-bromobutyrate as a model reaction (Table 1). The
proposed alkylation was first examined at 658C in presence
of 10 mol% CoBr2 and 10 mol% 2,2’-bipyridine (1) in a
mixture of DMF and pyridine using four equivalents of man-
ganese and two equivalents of the alkyl bromide. Gratifying-
ly, the desired coupling product was obtained in 76% yield
(as determined by GC) within 4 h (Table 1, entry 1). Subse-
quent screening of ligand motifs unveiled promising effects
of phosphane-type ligands on this reaction (Table 1, en-
tries 3–7). Although initially employed bidentate nitrogen-
based bipyridine 1 offered good activity, monodentate phos-
phane ligands, more precisely trialkyl phosphane 3 or dialk-
ly formed cross-coupling product. As highlighted in Table 1,
presence of ligand was necessary (Table 1, entry 8) and re-
lated phosphanes led to inferior results in comparison to
those obtained with ligand 4.[9] Among the ligands that we
have investigated, the mixed electronic properties of
iPr2PhP (Table 1, entry 7) compared favorably to Ph3P or
iPr3P (Table 1, entries 9 and 11). Whereas iPr3P was fairly
effective, the use of less (nBu3P, Table 1, entry 10) or more
(tBu3P, Cy3P, Table 1, entries 12 and 13) sterically hindered
trialkylphosphanes resulted in lower activities. Finally, Buch-
wald-type biaryl phosphane ligands 8[10] gave moderate re-
sults under these conditions (Table 1, entry 14). Having suc-
cessfully developed this protocol for the alkylation of aryl
halides, we decided to undertake scope studies. The cobalt-
catalyzed reactions of various aryl bromides with alkyl bro-
mides containing electron-withdrawing groups were investi-
gated (Table 2). Reactions of primary alkyl bromides gave
the coupling products in good to excellent yields, and a
range of sensitive functional groups were tolerated on both
partners (ester, ketone, nitrile, trifluoromethyl Table 2, en-
tries 1, 2, 4, 5, 8, and 11). Probably due to a cobalt chelation
effect, 4-bromobenzonitrile and 4-bromobutyronitrile re-
quired higher catalyst loadings to afford good levels of effi-
ciency in these reactions (Table 2, entries 4, 8, and 9). Elec-
tron-poor aryl bromides readily participated in a coupling
with non-functionalized alkyl bromides in excellent yields
(Table 2, entries 6, 7, 9, 10, and 12). Secondary alkyl bro-
ACHTUNGTRENNUNGylACHTUNGTRENNUNGaryl phosphane 4, showed interesting reaction efficiency
(Table 1, entries 5 and 6). We were pleased to find that the
use of iPr2PhP (4) afforded the corresponding product with
an excellent GC yield of 99% within 15 h (Table 1, entry 7).
Moreover, the reaction temperature could be decreased to
308C and only a slight excess of alkyl bromide (1.1 equiv)
was necessary to obtain a quantitative yield of the exclusive-
Table 1. Ligand effect on cross-coupling between ethyl 4-bromobenzoate
and ethyl 4-bromobutyrate.
Table 2. Cobalt-catalyzed direct alkylation of aryl bromides with alkyl
bromides: Substrate scope.
Entry
1
Product
Entry
7
Product
2
3
8[a]
Entry
CoBr2
[mol%]
Ligand
[mol%]
RBr
[equiv]
T
[8C]
GC yield
[%][a]
Time
[h]
9[a,b]
G
R
N
1
2
3
4
5
6
7
8
10
20
10
20
20
20
10
10
10
10
10
10
10
10
1 (10)
1 (20)
2 (10)
2 (20)
3 (20)
4 (20)
4 (10)
none
2 (10)
3 (10)
5 (10)
6 (10)
7 (10)
8 (10)
2.0
2.0
2.0
2.0
2.0
2.0
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
65
65
65
65
65
65
30
30
30
30
30
30
30
30
76
62
50
68
72
78
99
63
72
55
80
74
78
63
4
2
2
1
1
4[a]
10
11
1
15
24
15
15
15
36
36
72
5
9
10
11
12
13
14
6
12
[a] With 20 mol% CoBr2/iPr2PhP. [b] Reaction conducted with the corre-
sponding aryl chloride at 658C with 20 mol% CoBr2/iPr2PhP and two
equivalents alkyl bromide.
[a] Full conversion of ethyl 4-bromobenzoate was observed. Yields deter-
mined by GLC analysis using an internal standard.
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5849

C. Gosmini and M. Amatore
mides such as cyclohexyl bromide reacted efficiently with
ethyl 4-bromobenzoate to afford the corresponding coupling
product in 96% yield (Table 2, entry 6). Satisfyingly, ortho-
functionalized aryl bromide could also be employed to fur-
nish moderate yields under the same conditions (Table 2,
entry 10). Finally, preliminary results using aryl chlorides
such as 4-chlorobenzonitrile showed the feasibility of this
cobalt-catalyzed protocol to mediate a cross-coupling pro-
cess with alkyl bromides under slightly modified conditions
(Table 2, entry 9).
reaction conditions demonstrated that aryl chlorides could
be employed as well affording the coupling products in rea-
sonable to good yields (Table 3, entries 3 and 5).
Lastly, we investigated the viability of a one-step synthesis
of functionalized diarylmethanes that would constitute an
interesting alternative to the Barbier-type procedure previ-
ously described by our group using CoBr2 and zinc dust in-
volving aryl zinc derivatives.[4i] As a representative example,
the reaction of ethyl 4-bromobenzoate with benzyl chloride
proceeded smoothly in the presence of 4,4’-dimethyl-2,2’-bi-
pyridine to afford the desired diarylmethane in 85% yield
(Scheme 2). Under the same conditions, the use of the
Unexpectedly, the cross-coupling of ethyl 3-bromopropio-
nate remained unsuccessful under these conditions due to a
rapid consumption of the alkyl
partner by dimerization (as ob-
served by GC) and presumably
also by b-elimination and re-
duction (Table 2, entry 3).[11]
The catalytic system involving
iPr2PhP as a ligand remained
Scheme 2. Direct synthesis of diarylmethanes.
inefficient even at higher tem-
peratures (after 4 h with two
equivalents of RBr at 508C,
only a partial conversion of ArBr was observed at full con-
parent 2,2’-bipyridine allowed the formation of the corre-
sponding diarylmethane in 74% yield.[13]
version of alkyl bromide, 24% GC yield).[12]
In contrast, the use of 2,2’-bipyridine allowed the reaction
to go to completion and the desired coupling product was
isolated in 76% yield (Table 3, entry 1). Moreover, very low
levels of conversion to the desired compound were accom-
plished by using pyridine only or PPh3 (<5% GC yield
within a full conversion of ArBr).[12] Under these optimized
conditions, the reaction proceeded smoothly with electron-
poor aryl bromides. Moderate to good yields were obtained
with para-functionalized aryl halides (Table 3, entries 1–3, 5,
and 6). In this case, ortho-substitution appeared more prob-
lematic due to the high reactivity of the alkyl bromide cou-
pling partner (Table 3, entry 4). However, slightly modified
Our current mechanistic hypothesis closely resembles our
previous results describing the synthesis of biaryl com-
pounds using cobalt complexes (Scheme 3).[8c,14] Initial re-
duction of a cobalt(II) complex should furnish a catalytically
active low-valent cobalt species. Subsequent oxidative addi-
tion to the aryl halide gives an aryl cobalt intermediate that
is, again, subject to reduction by the stoichiometric reduc-
tant (manganese). The key alkyl aryl cobalt complex formed
upon a second, selective oxidative addition into the CACHTUNGTRENNUNG
(sp3)–
Br bond, is believed to display sufficient stability towards
unproductive b-hydride elimination. Thus, reductive elimi-
nation can occur to furnish the cross-coupling product along
with the regeneration of the catalytic active species. The
generation of alkyl or aryl manganese derivatives as active
Table 3. Cobalt-catalyzed direct cross-coupling of aryl bromides with
ethyl 3-bromopropionate: Substrate scope.
Entry
1
Product
Entry
4
Product
2
5[a]
3[a]
6
[a] Reaction conducted with the corresponding aryl chloride at 658C and
three equivalents of alkyl bromide.
Scheme 3. Postulated mechanism for the direct alkylation of aryl halides.
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Cobalt-Catalyzed Alkylation of Aryl Halides
COMMUNICATION
Cross-coupling of aryl halides with ethyl-3-bromopropionate or benzyl
chlorides—general procedure: To a solution of CoBr2, 2,2’-bipyridine or
4,4’-diMe-2,2’-bipyridine, and manganese powder (4.0 equiv, 10 mmol,
550 mg) in a mixture of DMF and pyridine were successively added at
room temperature the corresponding aryl bromide (2.5 mmol) and alkyl
halide (2.0 equiv, 5.0 mmol). Manganese powder was activated by traces
of trifluoroacetic acid (50 mL) and the medium was then stirred at 50 or
658C until aryl bromide was consumed (2 h). The amount of the corre-
sponding coupling product was measured by GC using an internal stan-
dard (dodecane, 50 mL). The medium was filtered through a Celite pad
and washed with ethyl acetate or Et2O. The mixture was then poured
into a solution of 2n HCl or NH4Cl (50 mL). The mixture was stirred vig-
orously until the layers turned clear.
species is unlikely since the reaction occurred with similar
yields in the presence of air[15] or acetic anhydride.[16] How-
ever, a radical pathway based on single-electron transfer
(SET) cannot be ruled out.[17,7b] As a preliminary result,
under standard conditions, we found that reaction of ethyl
4-bromobenzoate with bromomethylcyclopropane afforded
the corresponding product expected from a radical-based
mechanism as the major compound with ArH (Scheme 4).
Further studies are underway to further elucidate this mech-
anism.
The solution was extracted with
EtOAc or Et2O (3ꢂ50 mL), washed
with brine (1ꢂ100 mL), dried over
MgSO4, filtered, and concentrated in
vacuo. Purification of the resulting oil
by flash chromatography over silica
with petroleum ether/EtOAc or petro-
leum ether/Et2O mixtures afforded the
Scheme 4. Reaction of ethyl 4-bromobenzoate with bromomethylcyclopropane.
title compounds. See the Supporting
Information for more details.
In conclusion, we have demonstrated that a cobalt-based
catalytic system is suited for the direct alkylation of func-
tionalized aryl bromides and chlorides with unactivated or
activated alkyl halides. CoBr2/iPr2PhP/Mn has been shown
to catalyze efficiently the direct Csp2–Csp3 coupling of vari-
ous aryl bromides and even chlorides with relatively unreac-
tive alkyl bromides bearing b-hydrogen atoms under mild
conditions. A wide range of functionalized substrates con-
taining ester, ketone, or nitrile moieties, as well primary or
secondary alkyl bromides could be coupled in high yields.
To the best of our knowledge, this method represents the
first direct cross-coupling of aryl halides and alkyl bromides
that is not limited to electron-rich substrates.[18] In the case
of ethyl 3-bromopropionate or activated benzylic chlorides,
a different choice of ligands facilitated comparably success-
ful results. Further studies to extend the scope of this meth-
odology and to gain detailed mechanistic insight are current-
ly underway in our laboratory.
Acknowledgements
The authors would like to thank Loren Berthomieu for a late-stage ex-
perimental contribution. This work was supported by CNRS and Ecole
Polytechnique.
Keywords: alkylation
·
cobalt
·
cross-coupling
·
homogeneous catalysis · synthetic methods
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Cross-coupling of aryl halides with alkyl bromides—general procedure:
To a solution of CoBr2 (10 mol%, 0.25 mmol, 55 mg) and manganese
powder (4.0 equiv, 10 mmol, 550 mg) in a mixture of DMF (3 mL) and
pyridine (0.5 mL) were successively added at room temperature the cor-
responding aryl bromide (2.5 mmol) and alkyl bromide (1.1 equiv,
2.75 mmol). Manganese powder was activated by traces of trifluoroacetic
acid (50 mL) and the medium was then stirred at room temperature for
5 min until the smoke had disappeared. Then, iPr2PhP (10 mol%,
0.25 mmol, 50 mL) was rapidly added and the medium was then stirred at
308C until aryl bromide was consumed (15 h). The amount of the corre-
sponding coupling product was measured by GC using an internal stan-
dard (dodecane, 50 mL). The medium was filtered through a Celite pad
and washed with ethyl acetate. The mixture was then poured into a solu-
tion of 2n HCl or NH4Cl (50 mL). The mixture was stirred vigorously
until the layers turned clear. The solution was extracted with EtOAc (3ꢂ
50 mL), washed with brine (1ꢂ100 mL), dried over MgSO4, filtered, and
concentrated in vacuo. Purification of the resulting oil by flash chroma-
tography over silica with petroleum ether/EtOAc mixtures afforded the
title compounds.
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C. Gosmini and M. Amatore
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Received: January 22, 2010
Published online: April 8, 2010
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Chem. Eur. J. 2010, 16, 5848 – 5852