Copper-catalyzed cross-coupling of alkyl and aryl grignard reagents with alkynyl halides

Article abstract of DOI:10.1002/anie.200905816

(Chemical Equation Presented) Good old copperl A new general procedure to couple aliphatic and aromatic Grignard reagents with alkynyl halides under copper catalysis is described (see scheme; NMP=N-methylpyrrolidinone). The reaction is chemoselective and allows preparation of a vast array of simple and functionalized internal alkynes in high yields.

Full text of DOI:10.1002/anie.200905816

Communications
DOI: 10.1002/anie.200905816
Homogeneous Catalysis
Copper-Catalyzed Cross-Coupling of Alkyl and Aryl Grignard
Reagents with Alkynyl Halides**
Gꢀrard Cahiez,* Olivier Gager, and Julien Buendia
Alkyl–alkynyl cross-coupling can be achieved through one of
two pathways. The first one consists in alkylating a metal
acetylide with a primary alkyl iodide or bromide. Sodium,
potassium, or lithium acetylides have been extensively used,
but the reaction suffers from some limitations.[1] As an
example, the substitution reaction does not tolerate the
presence of reactive functional groups such as esters or
nitriles.[1a] On the other hand, b-branched primary, secondary,
or tertiary alkyl halides mainly undergo an elimination
reaction and do not lead to the substitution product, or in
only poor yields.[1a] The coupling of tertiary alkyl halides with
alkynylalanes was reported.[2] However, the method is not
general, and only one of the three alkynyl groups is
transferred.
The alkyl–alkynyl cross-coupling can also be performed
according to a second pathway, which involves reacting an
alkylmetal with an alkynyl halide. The first attempts were
carried out by reacting iodo- or bromoalkynes with Grignard
reagents in the presence of cobalt salts,[3] or organocopper
derivatives.[4] However, poor to moderate yields are generally
obtained. Later, a few trialkylaluminium reagents were
coupled successfully with alkynyl bromides under nickel
catalysis,[5] however only one alkyl group was transferred in
moderate to good yields. An interesting general method was
described by Yeh and Knochel in 1989,[6] in which function-
alized organocuprate reagents, AlkFg–Cu(CN)ZnI (Fg =
functional group), react with simple bromo- or iodoalkynes
to afford functionalized alkynes in good yields. However, the
reaction conditions (12–16 h at À658C), and the use of a
stoichiometric amount of copper are not very convenient for
large-scale preparations.
In fact, it should be noted that as a rule, no general
procedure is currently available to couple aryl or alkyl
Grignard reagents with alkynyl halides. Herein we report the
first efficient copper-catalyzed alkynylation of alkyl and aryl
Grignard reagents.
Weedon and co-workers[4] reported that the copper-
catalyzed coupling of alkyl Grignard reagents with alkynyl
halides only gives poor yields of the substitution product. As
mentioned by Normant and co-workers,[9] the halogen/
magnesium exchange generally takes place predominantly.
Recently, in the light of our experience in copper-catalyzed
cross-coupling reactions with Grignard reagents,[10] we
decided to reinvestigate this reaction and we discovered
that, in fact, satisfactory yields can be obtained by slowly
introducing the Grignard reagent to the reaction mixture. As
an example, in the presence of 3 mol% CuCl2 in THF at 08C,
the addition of nBuMgCl (12 mmol) to heptynyl bromide
(10 mmol) over a 45 minute period gave 73% of 5-undecyne
(Table 1, entry 1). To improve this result, we have tested the
Table 1: Influence of various ligands on the copper-catalyzed cross-
coupling of nBuMgCl with heptynyl bromide.[a]
Entry
Ligand
Yield [%][b]
1
2
no ligand
73
79
3
4
5
6
7
8
9
dppe
<5
84
87
87
90
91
91
PPh3
TMEDA[c]
Me2S
The aryl–alkynyl coupling is generally performed by using
the well-known Sonogashira reaction between aromatic
halides and terminal alkynes in the presence of both copper
and palladium salts.[7] In contrast, the coupling of an arylmetal
with an alkynyl halide has been almost ignored until now. In
1972, Oliver and Walton[8] studied the reaction of arylcopper
reagents with iodo-trimethylsilylacetylene, but the reaction
has not been extended to other substrates.
OP(OEt)3
NMP
NMP (4 mol%)
[a] The reactions were performed on a 10 mmol scale. nBuMgCl
(12 mmol) was added by using a syringe pump over a 45 minute
period. [b] Yield determined by GC (pentadecane as internal standard).
[c] TMEDA=N,N,N’,N’-tetramethylethylenediamine.
[*] Dr. G. Cahiez, O. Gager, J. Buendia
Department of Chemistry (FRE 3043)
CNRS—Universitꢀ de Paris 13
influence of various ligands on the course of the reaction
(Table 1). 1-Phenylpropyne, successfully used as a ligand by
Kambe and co-workers for the copper-catalyzed coupling
between alkyl Grignard reagents and alkyl chlorides,[11] has
only a moderate effect on the reaction (Table 1, entry 2).
Surprisingly, the addition of 10 mol% dppe (diphenylphos-
phinoethane) has a clear detrimental effect whereas the
addition of triphenylphosphine is beneficial (Table 1,
74 Rue Marcel Cachin, 93017 Bobigny (France)
E-mail: gerard.cahiez@univ-paris13.fr
[**] The authors thank Fondation de France (ESCOM) for a grant to J.B.
as well as the Ministꢁre de l’Education Nationale et de la Recherche
and the CNRS for financial supports.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905816.
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Chemie
entries 3 and 4). As a rule, excellent yields are obtained with
s-donor ligands (10 mol%) like TMEDA (Table 1, entry 5),
dimethylsulfide (Table 1, entry 6), triethylphosphate (Table 1,
entry 7), or N-methylpyrrolidinone (NMP; Table 1, entry 8).
Notably, an excellent yield of the coupling product (91%) was
obtained by using only 4 mol% NMP and 3 mol% CuCl2
(Table 1, entry 9).
Scheme 1. Copper-catalyzed cross-coupling of nBuMgCl with 1-bromo-
2-phenylacetylene.
As illustrated in Table 2, various aliphatic internal alkynes
were prepared in good to excellent yields by using this
procedure. Satisfactory results were obtained with primary
Table 3: Copper-catalyzed cross-coupling of alkyl Grignard reagents with
1-chloro-2-phenylacetylene.[a]
Table 2: Copper-catalyzed cross-coupling of alkyl Grignard reagents with
alkynyl bromides.[a]
Entry
1
Product
Yield [%][b]
78
12
13
Entry AlkMgX
Product
Yield [%][b]
2
77
1
2
3
nBuMgCl
nOctMgCl
sBuMgCl
1
2
3
91[c]
89
90
3
4
5
14
15
16
92
93
91
4
5
6
7
4
5
6
7
78
92
94
65
tBuMgCl
[a] The reactions were performed on
(12 mmol) was added by using a syringe pump over a 15 minute
period. [b] Yield of isolated product.
a 10 mmol scale. AlkMgCl
nBuMgCl
8
9
nPentMgCl
8
9
88
92
nOctMgCl
(Table 3, entries 1 and 2). Yields are still more impressive with
secondary cyclic or acyclic, as well as with tertiary alkyl
Grignard reagents (Table 3, entries 3–5).
This procedure was used to prepare various functionalized
arylacetylenes from 2-bromobenzonitrile 17 (Scheme 2). The
arylacetylene derivative 18 was prepared from 17 in two steps.
After a classical Br/Mg exchange reaction,[13] the resulting 2-
cyanophenylmagnesium halide was coupled with trimethylsi-
lylethynylmagnesium chloride by action of oxygen in the
10
11
10
11
89
81
[a] The reactions were performed on
(12 mmol) was added by using a syringe pump over a 45 minute
period. [b] Yield of isolated product. [c] From nPentC CI, the reaction
gave a mixture of nPentC CH (55%) and nPentC C C CnPent (35%).
a 10 mmol scale. AlkMgCl
ꢀ
ꢀ
ꢀ À ꢀ
alkyl (Table 2, entries 1, 2, and 7–9), as well as with acyclic or
cyclic secondary alkyl Grignard reagents (Table 2, entries 3
and 4). Interestingly, tertiary alkyl Grignard reagents can also
be used successfully; as an example, 2,2-dimethyl-3-nonyne
was obtained in 92% yield from tert-butylmagnesium chloride
(Table 2, entry 5). Notably, the reaction is very chemoselec-
tive, and various functionalized Grignard reagents or func-
tionalized alkynyl halides have been employed successfully
(Table 2, entries 6–11). The procedure allows a rapid and
simple access to various functionalized alkynes in high yields.
The reaction failed with alkynyl bromides derived from
arylacetylenes since the Br/Mg exchange is mainly observed
(Scheme 1).
Fortunately, we discovered that the substitution product
can be prepared successfully by using the corresponding
alkynyl chlorides (Table 3); in this case the use of NMP was
not necessary.[12] In this way, good yields of 1-alkyl-2-phenyl-
acetylenes were obtained from n-alkyl Grignard reagents
Scheme 2. Copper-catalyzed cross-coupling of alkyl Grignard reagents
with chloroarylacetylene 19.
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Communications
presence of MnCl2 according to a procedure recently
To explain our results, a reasonable mechanism is
proposed in Scheme 3. The catalytic cycle starts by the
formation of the cuprate B from the Grignard reagent A. The
reported.[14] Compound 18 was obtained in 81% yield, and
subsequent desilylation[15] delivered 2-cyanophenylacetylene,
which was isolated and then chlorinated[16] to give 19 in 89%
yield. Chloroalkyne 19 was then coupled in the presence of
copper chloride with tert-butylmagnesium chloride or with the
secondary alkyl Grignard reagent 21 to give respectively the
arylacetylenes 20 and 22 in excellent yields. On the other
hand, the use of the functionalized Grignard reagent 23 led to
the highly functionalized arylacetylene 24 in 71% yield.
Encouraged by these results, we have tried to extend the
reaction to the arylation of alkynyl bromides. Indeed,
arylacetylenes derivatives are attractive, since they have
numerous applications as optical materials or organic con-
ductors.[17] Our first attempts were effective and aryl Grignard
reagents bearing electron-withdrawing (Table 4, entry 1) or
electron-donating groups (Table 4, entry 2), as well as various
functionalized alkynyl bromides (Table 4, entries 3–9) have
been coupled in satisfactory yields.
Scheme 3. Putative mechanism of the copper-catalyzed cross-coupling
between R’MgX and alkynyl halides.
The Br/Mg exchange is not observed, except in the case of
the reaction of methyl 3-bromopropiolate with 4-anisylmag-
nesium bromide which only gave 48% yield of the cross-
coupling product, since 4-bromoanisole is formed as a side
product in 50% yield (Table 4, entry 7). In this case, the use of
electron-deficient (Table 4, entry 8) or hindered (Table 4,
entry 9) aryl Grignard reagents precludes the Br/Mg
exchange reaction, and the expected methyl 3-arylpropiolates
were isolated in good yields.
haloalkyne C then reacts with B to give the vinylcopper
reagent E via the complex D/D’ (carbocupration). Formally,
E results from a reductive elimination from the metal-
lacyclopropene D’. The unstable vinylcopper E quickly
undergoes a b-halogen elimination to afford the substitution
product F and the organocopper G, which then reacts with A
to regenerate the cuprate B.
This mechanism is consistent with our observations and
with the results described in the literature. It is known that
Table 4: Copper-catalyzed cross-coupling of aryl Grignard reagents with
alkynyl bromides.[a]
ꢀ
organocopper RCu readily reacts with alkoxyacetylene R’C
COR’’ to give a vinyl copper species in which the R group is
connected to the carbon center bearing the oxygen atom.[18]
ꢀ
From chloroalkyne R’C CCl, the carbocupration also occurs,
but the vinylcopper bearing a copper and a chlorine atom in
Entry ArMgBr
Product
Yield
[%][b]
the b-position is very unstable and only the elimination
product R’C CR is obtained, albeit in moderate yield.[18] It is
1
2
3
25
26
27
84
78
52
ꢀ
reported that the carbocupration of compounds such as
RC CZ (Z = OEt, Cl) must be performed in THF,[18] whereas
ꢀ
ꢀ
the same reaction with simple terminal alkynes RC CH takes
place in diethyl ether.[19] In the case of RC CZ, the triple
ꢀ
bond is more p acceptor than in the case of a simple terminal
alkyne because of the presence of the electron-withdrawing
substituent Z. Therefore, the role of THF, a better s-donor
ligand than diethyl ether, would be to favor the complexation
of the copper(I) species to the triple bond by increasing the
electronic density of the copper atom. According to the
results described in the literature, the carbocupration is not a
very rapid reaction.[19d,20] Therefore, the addition to the triple
bond is very likely the slow step in the catalytic cycle
proposed in Scheme 3. Therefore, the improvement in the
yield observed in the presence of NMP, a good s-donor ligand,
could result from the strenghtening of the interaction
between the copper atom and the triple bond (D’ rather
than D), which favors the carbocupration process. A similar
effect was observed with various other s-donor ligands such
as TMEDA or triethyl phosphate (Table 1, entries 5 and 7). In
the light of these considerations, the slow addition of the
4
5
28
29
58
73
6
7
8
30
31
32
72
48
85
9
33
83
[a] The reactions were performed on
(12 mmol) was added by using a syringe pump over a 48 minute
period. [b] Yield of isolated product.
a 10 mmol scale. ArMgBr
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Chemie
[8] R. Oliver, D. R. M. Walton, Tetrahedron Lett. 1972, 13, 5209.
[9] A. Commerꢂon, J. F. Normant, J. Villiꢃras, Tetrahedron 1980, 36,
1215.
Grignard reagent is required to avoid the presence of a large
excess of Grignard reagent which directly reacts with the
halogenoalkyne C through a metal–halogen exchange reac-
tion.[9] Of course, the presence of NMP also disfavors this side
reaction since it accelerates the copper-catalyzed reaction.
In conclusion, we disclosed herein a very simple and
efficient method for the preparation of a vast array of simple
or functionalized internal alkynes. It is the first general
procedure to couple aryl as well as secondary or tertiary alkyl
Grignard reagents with alkynyl halides. This high yielding
procedure is very attractive for large-scale preparations since
it is performed under mild reaction conditions by using
Grignard reagents, which are the most common commercially
available organometallic reagents.
[10] a) G. Cahiez, C. Chaboche, M. Jꢃzꢃquel, Tetrahedron 2000, 56,
2733; For a similar Cu-catalyzed reaction with organomanganese
reagents, see: b) G. Cahiez, S. Marquais, Synlett 1993, 45; c) G.
Cahiez, S. Marquais, Pure Appl. Chem. 1996, 68, 53.
[11] J. Terao, H. Todo, S. A. Begum, H. Kuniyasu, N. Kambe, Angew.
Chem. 2007, 119, 2132; Angew. Chem. Int. Ed. 2007, 46, 2086.
[12] In the case of the copper-catalyzed alkylation or Grignard
reagents, we have previously observed that the presence of NMP
is not always beneficial. See referefences [10b,c].
[13] L. Boymond, M. Rottlꢄnder, G. Cahiez, P. Knochel, Angew.
Chem. 1998, 110, 1801; Angew. Chem. Int. Ed. 1998, 37, 1701.
[14] G. Cahiez, C. Duplais, J. Buendia, Angew. Chem. 2009, 121, 6859;
Angew. Chem. Int. Ed. 2009, 48, 6731.
[15] a) H. Gilman, A. G. Brook, L. S. Miller, J. Am. Chem. Soc. 1953,
75, 4531; b) C. Eaborn, A. R. Thompson, D. R. M. Walton, J.
Chem. Soc. 1967, 1364; c) B. W. Metcalf, K. Jund, Tetrahedron
Lett. 1977, 18, 3689.
Received: October 16, 2009
Published online: January 8, 2010
[16] The chlorination of 2-cyanophenylacetylene was performed
according to: R. Truchet, Ann. Chim. 1931, 16, 309. The
procedure has been slightly modified as described in the
Supporting Information.
[17] M. B. Nielsen, F. Diederich, Chem. Rev. 2005, 105, 1837.
[18] J. F. Normant, A. Alexakis, G. Cahiez, J. Villiꢃras, C. R. Acad.
Sci. Ser. C 1974, 279, 763.
[19] a) J. F. Normant, G. Cahiez, C. Chuit, A. Alexakis, J. Villiꢃras, J.
Organomet. Chem. 1972, 40, C49; b) J. F. Normant, G. Cahiez, C.
Chuit, J. Villiꢃras, Tetrahedron Lett. 1973, 14, 2407; c) J. F.
Normant, G. Cahiez, C. Chuit, J. Villiꢃras, J. Organomet. Chem.
1973, 54, C53; d) J. F. Normant, G. Cahiez, M. Bourgain, C.
Chuit, J. Villiꢃras, Bull. Soc. Chim. Fr. 1974, 1656; e) J. F.
Normant, G. Cahiez, C. Chuit, J. Villiꢃras, J. Organomet. Chem.
1974, 77, 269; f) J. F. Normant, G. Cahiez, C. Chuit, J. Villiꢃras, J.
Organomet. Chem. 1974, 77, 281. For a review see: g) A.
Alexakis, J. F. Normant, Synthesis 1981, 841.
Keywords: alkynes · copper · cross-coupling · Grignard reagents
.
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