Cross-coupling of nonactivated alkyl halides with alkynyl grignard reagents: A nickel pincer complex as the catalyst

Article abstract of DOI:10.1002/anie.201105964

In a pinch: The nickel pincer complex 1 catalyzes the cross-coupling of the title compounds with remarkable substrate scope and functional group tolerance. A nickel/alkynyl species was isolated and shown to be catalytically competent. THF=tetrahydrofuran, O-TMEDA=bis[2-(N,N-dimethylaminoethyl)] ether. Copyright

Full text of DOI:10.1002/anie.201105964

DOI: 10.1002/anie.201105964
Synthetic Methods
Cross-Coupling of Nonactivated Alkyl Halides with Alkynyl Grignard
Reagents: A Nickel Pincer Complex as the Catalyst**
Oleg Vechorkin, Aurꢀlien Godinat, Rosario Scopelliti, and Xile Hu*
Alkynes are an important class of organic molecules because
they are frequently used as synthetic intermediates and
precursors for natural products, biologically active molecules,
and organic materials.[1,2] Alkynes are also essential coupling
partners for the azide–alkyne Huisgen cycloaddition reac-
tion.[3,4] The streamlined synthesis of alkynes containing
various functional groups is therefore highly desirable.
Alkynes containing nonactivated alkyl groups, especially
those with b-hydrogen atoms, are difficult to synthesize.
Reactions of alkali metal acetylides with alkyl halides in
liquid ammonia, or with hexamethylphosphoramide (HMPA)
as the solvent or cosolvent at a low temperature (e.g.,
À788C), have long been used for the alkylation of alkynes.
These reactions suffer from the limited solubility of acetylides
in liquid ammonia, the carcinogenic effect of HMPA, and the
inconvenience of working at low temperatures.
The first three paths to an alkylated alkyne are: 1) Cou-
pling of alkyl halides with terminal alkynes under Sonoga-
shira-type conditions (path A, Scheme 1);[5–7] 2) Palladium-
catalyzed oxidative alkyl–alkynyl coupling (path B,
Scheme 1);[8–11] 3) Cross-coupling of alkynyl halides with
organometallic alkyl reagents (path C, Scheme 1).[12–15] Only
a small number of protocols have been developed based on
these reactions. Thus, the scope remains limited. Further-
more, most of these protocols have one or more of the
following drawbacks: 1) Copper-catalyzed or cocatalyzed
reactions have a low tolerance for functional groups that
have a high affinity for copper, such as sulfur-containing
groups; 2) Oxidative coupling so far requires two organome-
tallic reagents as coupling partners, or excess amounts of alkyl
zinc reagents; 3) Functionalized alkyl Grignard reagents are
scarce.
The limitations of uncatalyzed nucleophilic alkylation of
alkali metal acetylides motivate the search for alternative,
transition-metal-catalyzed alkyl–alkynyl coupling methods.
However, the alkyl–alkynyl cross-coupling is among the most
challenging coupling reactions for two reasons: 1) the metal
alkyl intermediates have a tendency to undergo unproductive
b-hydride elimination; 2) the metal alkynyl moieties are
weakly nucleophilic and are subject to oxidative dimerization.
As a result, only a few methods for alkyl–alkynyl coupling
have been reported. These methods can be classified into four
categories (Scheme 1).
A fourth method for constructing the alkyl–alkynyl bond
is the coupling of nonactivated alkyl halides with alkynyl
organometallic reagents (path D, Scheme 1). Despite recent
progress in the cross-coupling of nonactivated alkyl electro-
philes,[16–21] there are only two prior reports of successful
cross-coupling reactions of nonactivated alkyl halides with
alkynyl Grignard reagents.[22,23] Only a narrow range of
alkynyl Grignard reagents could be used, namely phenyl-
ethynyl- and trimethylsilylethynyl magnesium halide (Br, I)
for the palladium-based system, and trimethylsilylethynyl
magnesium bromide for the cobalt-based system.
We find that by using a well-defined nickel pincer
complex[24–26] as the catalyst, general and efficient cross-
coupling of nonactivated alkyl halides with alkynyl Grignard
reagents can be achieved. Herein, we describe the develop-
ment of this nickel-catalyzed coupling method, the explora-
tion of its scope, and the investigation of its mechanism.
The coupling of n-octyliodide with 1-propynyl magnesium
bromide was chosen as the test reaction (Table 1). Surpris-
ingly, the reported palladium- and cobalt-based methods
failed to effect this seemingly simple coupling reaction.[27] No
reaction occurred between the two substrates in the absence
of a catalyst or additive after 1 hour in THF at room
temperature (entry 1, Table 1). Amines are known to pro-
mote the reactivity of metal acetylides,[23] therefore upon
addition of a chelating amine, bis[2-(N,N-dimethylamino-
ethyl)]ether (O-TMEDA) there was an increase in the
conversion of n-octyliodide to 31%. However, the yield of
the cross-coupling product was only 3% (entry 2, Table 1).
Upon adding 5 mol% of [(MeN2N)NiCl] (1, Nickamine)[28–30]
as the catalyst, the conversion was 38%, and the yield of the
cross-coupling product was 20% (entry 3, Table 1). The
combination of the catalyst 1 and an amine additive
(TMEDA or O-TMEDA) significantly improved both the
Scheme 1. Four types of transition-metal-catalyzed cross-coupling
methods for the synthesis of alkyl-substituted alkynes. X=halide,
M=metal.
[*] Dr. O. Vechorkin, A. Godinat, Dr. R. Scopelliti, Prof. Dr. X. L. Hu
Institute of Chemical Sciences and Engineering
Ecole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
SB-ISIC-LSCI, BCH 3305, Lausanne, CH 1015 (Switzerland)
E-mail: xile.hu@epfl.ch
Homepage: http://lsci.epfl.ch
[**] This work is supported by the Swiss National Science Foundation
(project no. 200021_126498).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105964.
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Table 1: Optimization of the reaction conditions for the coupling of n-
octyliodide with 1-propynyl magnesium bromide.[a]
Entry Conditions
Conversion[b] [%] Yield[c] [%]
1
2
3
4
5
6
no catalyst, no additive[d]
2 equiv of O-TMEDA
0
31
38
68
85
0
3
20
44
68
93
5 mol% 1, no additive
5 mol% 1, 2 equiv of TMEDA
5 mol% 1, 2 equiv of O-TMEDA
5 mol% 1, 3 equiv of O-TMEDA[d]
100
[a] Reaction conditions unless otherwise specified: 1-propynyl magne-
sium bromide (0.6 mmol) was added over a 1 h period to a THF solution
of [(MeN2N)Ni-Cl] (0.025 mmol), O-TMEDA, and n-octyliodide
(0.5 mmol). [b] Conversion of n-octyliodide. [c] Yield as determined by
GC analysis of the crude reaction mixture using decane as an internal
standard. [d] Addition of Grignard reagent at once.
Scheme 2. Optimized reaction conditions for testing reactions of
catalytic alkyl–alkynyl cross-coupling. Reported yields were determined
by GC analysis of the reaction mixture using decane as an internal
standard.
conversion and yield (entries 4 and 5, Table 1). O-TMEDA is
a better additive than TMEDA (compare entries 4 and 5,
Table 1). Additional optimization showed that the best result
for this reaction was achieved using 5 mol% of 1 and
3 equivalents of O-TMEDA (entry 6, Table 1).[31] The
Grignard reagent could be added at once, which gave a
slightly better yield than adding it over a period of 1 hour.
Under these reaction conditions, the conversion was 100%
and the yield was 93%. Additional details on the influence of
various reaction parameters on the transformation can be
found in Table S1 in the Supporting Information.[31]
Several additional test reactions were carried out to
examine the generality of this catalytic system. With some
minor modifications, the optimized protocol used in Table 1
could be extended to other substrates (Scheme 2).[31] The
coupling of n-octyliodide with 2-phenylethynyl magnesium
bromide required 6 hours [Eq. (1), Scheme 2]. The slower
reaction rate is likely due to the reduced nucleophilicity of
aryl-substituted alkynyl anions. Interestingly, ethynyl magne-
sium bromide was coupled to give a terminal alkyne in a
modest yield [Eq. (2), Scheme 2]. This reaction obviates the
need of protecting groups for alkynes, and provides easy
access to alkyl-substituted terminal alkynes. The coupling of
n-octylbromide with 1-propynyl magnesium bromide was
complete after 3 hours [Eq. (3), Scheme 2]. For these three
reactions, the employment of an excess of the Grignard
reagent was slightly beneficial.
After establishing the nickel-catalyzed alkyl–alkynyl
Kumada coupling method, we explored its scope. As shown
in Table 2 and Table S2 in the Supporting Information, a wide
range of nonactivated alkyl iodides and bromides are suitable
substrates. Modest to high yields (45–91%) of isolated
products were obtained within 1–6 hours at room temper-
ature. Not only robust chloride and ether groups (entries 1–3,
Table S2),[31] but also sensitive amide, ester, and nitrile groups
(entries 1–5, Table 2; entries 4–6, Table S2) were tolerated.
Amine groups did not pose a problem, regardless of whether
they were Boc protected (entries 6 and 7, Table 2; entry 7,
Table S2), or present as a tertiary amine (entry 8, Table 2).
Acetal and olefinic groups did not interfere with the coupling
(entries 9 and 10, Table 2). Gratifyingly, substrates containing
important N and O heterocycles were successfully coupled
(entries 11–13, Table 2; entries 8 and 9, Table S2). An aro-
matic enone was also coupled (entry 14, Table 2). Sulfur-
containing thioether and thiophene groups could be tolerated
(entries 3, 15, and 16, Table 1). The scope of the alkynyl
coupling partner is impressive. Alkyl-, aryl-, vinyl-, and silyl-
substituted alkynyl Grignard reagents were all readily cou-
pled. The alkynyl coupling partner can contain various
sensitive functional groups. The results show that functional-
ized alkynyl Grignard reagents are easily prepared, stable,
and attractive reagents for cross-coupling reactions.
Commercially unavailable alkynyl Grignard reagents are
normally synthesized by reaction of a terminal alkyne with
EtMgBr. These in situ prepared alkynyl Grignard reagents
are also suitable coupling partners. For example, cross-
coupling of 1-octynyl magnesium bromide with n-octylhalide
(Br, I) proceeded with yields of approximately 74% [Eq. (4),
Scheme 2]. Unfortunately, n-octylchloride or secondary alkyl
iodides like cyclohexyliodide could not be coupled.
One major advantage of this nickel catalysis is its high
functional group tolerance. It is sometimes possible to find
conditions for alkylation of lithium acetylides in THF at room
or elevated temperature even without HMPA.[32] In our
hands, reaction of 1-hexynyllithium (prepared by deprotona-
tion of 1-hexyne with nBuLi) with octyliodide took place
slowly to give 5-tetradecyne in a yield of 84% after 24 hours
[Eq. (S1) in the Supporting Information].[31] However, this
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Table 2: Cross-coupling of nonactivated alkyl halides with alkynyl
type of alkylation method is not efficient using alkyl
bromides. Reaction of 1-octynyllithium with octylbromide
gave 7-hexadecyne in a yield of only 8% after 24 h
[Eq. (S2)].[31] The biggest drawback of this alkylation
method is its poor functional group tolerance. For example,
reaction of 1-octynyllithium with an ester-containing sub-
strate, ethyl 4-iodobutanoate, gave no coupling product
[Eq. (S3)].[31]
Grignard reagents.[a]
Entry X-Alkyl
1
Product
Yield[b] [%]
69
We reported earlier that complex 1 was an efficient
catalyst for nickel-catalyzed Sonogashira coupling of non-
activated alkyl halides with terminal alkynes.[7] The Sonoga-
shira method required a temperature of 1008C to 1408C
whereas the present Kumada coupling method can be
conducted at room temperature. As a consequence, the
Kumada coupling is significantly more tolerant. For example,
indoles containing an H atom at the 2-position could not be
used for the Sonogashira coupling, but could be used for the
Kumada coupling (entries 12 and 16, Table 2; entry 9,
Table S2). Furthermore, because no copper cocatalyst is
required for the Kumada coupling, sulfur-containing groups
are now tolerated (entries 3, 15, and 16, Table 2). The
improved group compatibility is important for applications
in medicinal and materials chemistry.
A number of experiments were conducted to give insight
into the mechanism of the nickel catalysis. The mercury test
suggests that the catalysis is homogeneous, that is, in the
presence of 100 equivalents of mercury (relative to the
catalyst), a coupling reaction gave a nearly identical yield of
product as the reaction conducted without mercury.[31]
Activation of alkyl halides in nickel-catalyzed cross-
coupling reactions often proceeds through a radical mecha-
nism.[16,20,21,33,34] To ascertain that this is the case for the
current nickel catalysis, coupling reactions with radical-
probe-type substrates were carried out (Scheme 3). The
2
3
78
73
4
5
45
56
6
7
73
90
8
70
77
83
91
9
10
11
12
13
65
85
14
15
52
88
Scheme 3. Nickel-catalyzed alkyl–alkynyl Kumada coupling of radical-
probe-type substrates.
16
54
reaction of cyclopropylmethylbromide with octynyl magne-
sium bromide gave both 2 and 3 as the cross-coupling
products, albeit with low yields [Eq. (5), Scheme 3]. The
reaction of 1-iodo-5-hexene with hexynyl magnesium bro-
mide gave 4, but not 5, as the coupling product [Eq. (6),
[a] See the Supporting Information for experimental details. [b] Yield of
isolated product. Boc=tert-butoxycarbonyl.
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Communications
Scheme 3]. These results suggest that alkyl halides are also
Based on the aforementioned results, a catalytic cycle can
activated through a radical mechanism. The recombination of
the alkyl radical with the catalyst has a rate that is comparable
to the ring-opening rearrangement of the cyclopropylmethyl
radical, which has a first-order rate constant of 108 sÀ1.[35] The
recombination is much faster than the ring-closing rearrange-
ment of 5-hexenyl radical, which has a first-order rate
constant of 105 sÀ1.[35]
be proposed (Scheme 5). Reaction of the complex 1 with an
To probe as to whether nickel/alkynyl species are
intermediates in the catalysis, [(MeN2N)Ni-C CCH3] (6) was
ꢀ
synthesized. The identity of 6 was confirmed by X-ray
crystallography, which revealed the square-planar structure
of 6 (Figure 1). Using 5 mol% of 6 as the catalyst, the
Scheme 5. Proposed catalytic cycles for the nickel-catalyzed alkyl–
alkynyl coupling reactions.
alkynyl Grignard reagent yields the nickel/alkynyl complex 7.
There are two possible reactions of 7 that would lead to the
key intermediate species 10. In the potential pathway 1, 7
reacts with an alkyl halide to form a [Ni(alkyl)(alkynyl)]
species (8), which is transmetalated by the alkynyl Grignard
reagent to form 10. In pathway 2, 7 reacts with a second
molecule of the Grignard reagent to form a nickel/bis-
(alkynyl) species (9). 9 has enhanced nucleophilicity com-
pared to 7, and it reacts with the alkyl halide to give the
intermediate 10. Reductive elimination from 10 produces the
alkyl–alkynyl coupling product and regenerates 7. According
to Equations (7) and (8), the transformation from 7 into 8 or 9
must be thermodynamically uphill, and thus cannot be
observed in stoichiometric reactions. The role of O-
TMEDA may be multifold. It may coordinate to Mg and
activate the Grignard reagents for transmetalation. In the
case of functionalized alkynyl Grignard reagents, it may
stabilize the reagents against decomposition. It may suppress
homocoupling, as found in nickel-catalyzed alkyl–aryl cou-
pling.[25]The details of the mechanism are the subject of a
future study.
Figure 1. Molecular structure of complex 6. The thermal ellipsoids are
displayed in 50% probability. Selected bond lengths [ꢁ] and angles
[deg.]: Ni1–N1 1.9703(17), Ni1–N2 1.853(3), Ni1–C9 1.890(4); C9–C10
1.193(6); N2-Ni1-C9 180.0, Ni1-C9-C10 180.0.[37]
coupling of n-octyliodide with 1-propynyl magnesium bro-
mide gave an 86% yield of 2-undecyne, a yield that is
comparable to the result obtained using 1 as the catalyst
(entry 6, Table 1). Therefore, a nickel/alkynyl species is likely
involved in the catalytic cycle.
Under catalytically relevant conditions (in the presence of
O-TMEDA, in THF at room temperature), the reaction of 6
with n-octyliodide did not give 2-undecyne [Eq. (7),
Scheme 4]. In fact, no reaction was observed that would be
kinetically relevant to the catalysis. And no reaction was
In summary, we have disclosed the first general nickel-
catalyzed cross-coupling of nonactivated alkyl halides with
alkynyl Grignard reagents. The wide scope and high func-
tional group tolerance makes the method attractive for the
streamlined preparation of alkynes containing nonactivated
alkyl groups. At this moment, only primary alkyl halides can
be coupled. Methods to couple secondary alkyl halides are
currently being developed in our lab.[36]
Scheme 4. Stoichiometric reactions of complex 6. All reactions were
conducted in the presence of 1 equiv of O-TMEDA.
observed by NMR analysis of a mixture of 6 and 1-propynyl
magnesium bromide [Eq. (8), Scheme 4]. However, when
equal amounts of 6, n-octyliodide, and 1-propynyl magnesium
bromide were mixed in THF, 2-undecyne was produced in a
73% yield [Eq. (9), Scheme 4].
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Keywords: alkylation · alkynes · cross-coupling ·
Grignard reaction · nickel
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