An easy stereoselective copper(I)-catalyzed synthesis of (E)-3-trifluoromethylbut-2-en-3-ynoate via palladium-free stille coupling reaction

Article abstract of DOI:10.1055/s-0031-1290596

A general and efficient Cu(I)-catalyzed cross-coupling reaction of alkynyl bromides and β-tributylstannyl-α,β-unsaturated ester bearing a trifluoromethyl group in -position was developed under very mild conditions. This method provides easy access to a variety of 2,3-enynoate bearing a trifluoromethyl group from good to excellent yields with excellent stereoselectivity. This procedure does not require the use of any expensive supplementary additives and is palladium-free. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290596

LETTER
755
An Easy Stereoselective Copper(I)-Catalyzed Synthesis of
(E)-3-Trifluoromethylbut-2-en-3-ynoate via Palladium-Free
Stille Coupling Reaction
F
luorinated Enyno
hates alid Zine, Julien Petrignet, Jérôme Thibonnet, Mohamed Abarbri*
Laboratoire de Physicochimie des Matériaux et Biomolécules, EA 4244, Université François Rabelais, Faculté des Sciences,
Parc de Grandmont, 37200 Tours, France
Fax +33(2)47367073; E-mail: mohamed.abarbri@univ-tours.fr
Received 7 December 2011
F3C
Bu3SnH
Abstract: A general and efficient Cu(I)-catalyzed cross-coupling
reaction of alkynyl bromides and b-tributylstannyl-a,b-unsaturated
ester bearing a trifluoromethyl group in b-position was developed
under very mild conditions. This method provides easy access to a
variety of 2,3-enynoate bearing a trifluoromethyl group from good
to excellent yields with excellent stereoselectivity. This procedure
does not require the use of any expensive supplementary additives
and is palladium-free.
CO2Et
F3C
CO2Et
hexane, r.t.
95%
SnBu3
(Z)-1b
1
Scheme 1 Preparation of b-stannyl-4,4,4-trifluorobutenoate
However, to the best of our knowledge, no palladium-free
Stille coupling of b-stannyltrifluoromethylbutenoate (Z)-
1b using alkynyl bromides has yet been described. We re-
cently reported the preparation of the first highly regio-
selective metal-free hydrostannylation of ethyl 4,4,4-
trifluorobutynoate (1)15 without any additive (Scheme 1).
These new reagents reacted with a wide range of allyl and
propargyl bromides, leading to polyfunctional products
bearing a fluoroalkyl group.16
Key words: b-tributylstannyl-a,b-unsaturated ester, palladium-
free, copper-catalyst, trifluoromethylbut-2-en-3-ynoate
Organic compounds containing one or more fluorine at-
oms have received increasing attention in medicinal, agri-
cultural, and material sciences.1 They often confer
significant changes in their chemical and physical proper-
ties. The introduction of a fluorine or perfluoroalkyl group
into organic compounds often dramatically changes their
structure, stability, lipophilicity, reactivity, and biological
activity.2 The development of a simple method to obtain
perfluoroalkylated building blocks for their subsequent
utilization in the synthesis of fluoroalkyl-containing com-
pounds is therefore essential to organofluorine chemis-
try.3,4 Perfluoroalkylated vinyl metals constitute an
important class of these building blocks.
As a continuation of our previous research on the method
for preparing various derivatives bearing the trifluorome-
thyl group, we report here the first stereoselective copper-
catalyzed coupling reaction of b-stannyl-4,4,4-trifluo-
robutenoate (Z)-1b with alkynyl bromides without any ad-
ditive. More systematic investigations of Pd-catalyzed
alkynylation with alkynylstannanes were performed in the
early 1980’s by Bumagin, Beletskaya, and their
colleages17 and later by Stille.18 On the other hand, few re-
ports of the Pd-catalyzed alkynylation with alkenylstan-
nanes have been described.11
Perfluoroalkylated vinyl metals have been demonstrated
in which lithium,5 magnesium,6 zinc,7 silver,8 and
palladium9 species were prepared and alkylated with elec-
trophiles specific to the carbon attached to the metal. Vi-
nylstannanes bearing a perfluoroalkyl group also opened
the way for the preparation of these types of compounds.10
Vinylstannanes are the reagents of choice for the synthesis
of enyne structures by Stille coupling reaction.11
Although Stille reported that Pd(PPh3)4 alone catalyzed
the coupling reaction of vinylstannanes and alkynyl bro-
mides,11 no coupling reaction between (Z)-1b and 1-bro-
mo-2-phenylacetylene was observed when Pd(PPh3)4 was
used alone. Under Liebeskind conditions19 [5 mol% of
Pd(PPh3)4 and 10 mol% of CuI], the desired product was
formed with moderate yield (60%). In contrast, on the ba-
sis of transmetalation from the organotin compound to the
organocopper intermediate,20 we investigated a palladi-
um-free cross-coupling of vinylstannane (Z)-1b with
alkyne bromides in the presence of a catalytic amount of
CuI (10 mol%). A very clean reaction was observed, and
good yields of the desired product were obtained. It was
discovered that the reaction was complete within five
hours using 10 mol% of CuI at 20 °C (Scheme 2).
Enyne systems have attracted much attention from syn-
thetic organic chemists as enynes show interesting chem-
ical and biological activities.12 Conjugated enynes are
important synthetic intermediates since the conjugated
1,3-enyne moiety can be readily converted in a stereospe-
cific manner into the corresponding diene system.13 Many
methods can be used for the stereocontrolled synthesis of
conjugated enynes.10d,14
CF3
F3C
R
Br
CO2Et
SYNLETT 2012, 23, 755–759
Advanced online publication: 28.02.2012
x
x.
x
x.
2
0
1
2
SnBu3
CuI (10 mol%), DMF
r.t., 5 h
R
CO2Et
2a–m
DOI: 10.1055/s-0031-1290596; Art ID: B75211ST
© Georg Thieme Verlag Stuttgart · New York
(Z)-1b
Scheme 2 Preparation of (E)-3-trifluoromethylbut-2-en-3-ynoate

756
K. Zine et al.
LETTER
Table 1 CuI-Catalyzed Coupling of (Z)-1b with Alkynyl Bromides
The use of CuBr instead of CuI provided similar results.
To investigate the scope of the copper-alkylynated reac-
tion, several alkynyl bromides were tested to react with
(Z)-1b, and the reaction provided stereospecifically the
isomerically pure E-conjugated enynes.21 The results are
summarized in Table 1.
(continued)
Entry R
Product
Yield
(%)a
CF3
Table 1 CuI-Catalyzed Coupling of (Z)-1b with Alkynyl Bromides
8
9
3-thiophenyl
41
73
CO2Et
S
Entry R
Product
Yield
(%)a
2h
CF3
CF3
CO2Et
O
CO2Et
1
2
n-hexyl
83
77
OEt
CO2Et
5
2i
2a
CF3
CF3
HO
CO2Et
Ph
10
11
C6H4CH(OH)
79
78
CO2Et
2b
2c
2j
CF3
CF3
3
4-tolyl
4-anisyl
81
78
CO2Et
CH(OEt)2
EtO
CO2Et
OEt
2k
CF3
CF3
OEt
4b
12
13
CH2CH(OEt)2
89
0
CO2Et
EtO
CO2Et
MeO
2l
2d
CF3
CF3
TMS
5c
4-t-BuSiMe2OC6H4
50
Me3Si
CO2Et
CO2Et
2m
TBSO
a Yield of isolated product.
b E/Z = 70:30 (ratio determined on crude 1H and 19F NMR spectrum).
c E/Z = 60:40 (ratio determined on crude 1H and 19F NMR spectrum).
2e
CF3
As shown in Table 1, compounds (Z)-1b underwent reac-
tions with a variety of alkynyl bromides to provide the 3-
alkynylated products 2a–l with good yields and with a
clean E configuration of the double bond in the most of the
cases, demonstrating that the copper(I) catalyst coupling
reaction occurred with retention of configuration
(Scheme 2 and Table 1). Both aromatic and aliphatic
alkynyl bromides worked well. The coupling reactions
were not affected by the presence of either electron-donat-
ing group on the aromatic ring (Table 1, entries 3–5).
However, the presence of an electron-withdrawing group
on the aromatic ring provided moderate yields of the de-
sired product (51%; Table 1, entry 6). The reaction pro-
ceeded cleanly with retention of configuration of the
starting vinylstannanes (Z)-1b, except in the case of the ar-
omatic ring substituted by electron-donating groups
CO2Et
6
7
4-O2NC6H4
51
64
NO2
2f
CF3
2-pyridinyl
CO2Et
N
2g
Synlett 2012, 23, 755–759
© Thieme Stuttgart · New York

LETTER
Fluorinated Enynoates
757
(Table 1, entries 4 and 5) where partial isomerization of estannylation of (Z)-1b using a stoichiometric amount of
the double bond was observed (E/Z = ca. 60:40). The par- CuI and one equivalent of iodine in diethyl ether, which
tial E/Z isomerization of activated 1,3-enynes has already provided good yields of the known iodinated product 5
been reported by Gevorgyan.14e,22 In our case, we sup- (72%).27 However, no iododestannylation of (Z)-1b oc-
posed that this isomerization may occur under these reac- curred in the absence of a CuI catalyst, and the starting
tion conditions. It may be due to an equilibrium involving material was recovered. The stability of the cis copper in-
the formation of an enolate stabilized by the electron- termediate 3 was presumably attributable to the
donating group. Nevertheless, both isomers were separat- coordination28 between copper and oxygen of the ester
ed by column chromatography.
group (Scheme 3).
The presence of a trimethylsilyl group as substituent in the Interestingly, the use of vinyltin compound (Z)-6 under
terminal alkyne was not successful in this cross-coupling the same reaction conditions in the presence of bromo-
reaction (Table 1, entry 13), only the starting material and ethynylbenzene did not provide the expected conjugated
product 4 of dimerization were recovered.
enyne. Indeed, only starting material and a mixture of
dienes 7 resulted from the homocoupling reaction were re-
covered after 16 hours of stirring (Scheme 4).26 This result
shows the importance of the presence of the trifluorome-
thyl group to facilitate the Bu3Sn/Cu transmetalation reac-
tion and, more particularly, the cross-coupling reaction
with bromoalkyne.
Finally, this new coupling method tolerated the presence
of a variety of functional groups (including hydroxy,
alkoxy, phenyl, ester, etc.) present in the acetylenic com-
pounds. Indeed, the ester, alcohol, and ketal functions
were highly tolerated in this cross-coupling reaction
(Table 1, entries 9–12).
One-pot hydrostannylation reaction of 4,4,4-trifluorobut-
2-ynoate (1) followed by cross-coupling of vinyltin
formed (Z)-1b with 1-bromophenyl acetylene were at-
tempted, but, unfortunately, only a low yield of the de-
sired product 2b was isolated (27%).
The retention of the double-bond configuration in most of
the cases indicated that the presumed intermediate copper
reagent 310a,16b,23 formed after catalytic transmetalation of
vinyltin reagents with copper iodide was configurational-
ly stable and reacted stereoselectively with alkynyl bro-
mide substrates. No coupling reaction was observed in the A plausible mechanism can be proposed for the copper
absence of Cu(I) catalyst, and the starting material was en- (I)-catalyzed coupling reaction that would proceed as fol-
tirely recovered. Similarly, no homocoupling product lows: copper(I) iodide would transmetalate the vinyltin
(diynes) was observed under the conditions employed.24 reagent (Z)-1b,19 leading to a vinyl copper species, which
In addition, no cyclization reaction producing butenolides then would react with alkynebromide to generate a cop-
or a-pyrones was observed.25
per(III) intermediate. Finally, the expected enyne 2 would
be obtained via a reductive elimination step giving the ac-
tive Cu(I) species.
The Bu3Sn/CuI transmetalation step was confirmed by the
experiment in which the vinyl stannane (Z)-1b was treated
with 10 mol% of CuI at room temperature in DMF, which In summary, we investigated the transition-metal-free
yielded 80% of the homocoupling product
4
cross-coupling reaction of b-stannylated alkenoate bear-
(Scheme 3).16b,26 Spectroscopic analyses (1H NMR and ing a trifluoromethyl group with alkynyl bromides. The
13C NMR) of product 4 were in perfect agreement with copper(I)-catalyzed coupling reaction offered a versatile
those previously described by our group for the same useful procedure for the preparation of substituted func-
product. These findings were also confirmed by iodod- tionalized enynes and provided good yields of the corre-
sponding acrylate esters with
a
clean olefinic
configuration. This method provided a new and efficient
means of entry to this important class of compounds.
Many extensions of this work can be envisaged. Further-
more, the structurally novel enyne products would appear
to be worthy intermediates for future investigations in or-
ganic synthesis. We are actively pursuing a number of
possibilities.
F3C
Cu
CuI
F3C
EtO2C
(1equiv)
CO2Et
(Z)-1b
Bu3SnI+
OEt
DMF, r.t.
CF3
4 (80%)
O
3
I2
F3C
I
OEt
Acknowledgment
O
5 (72%)
We thank the ‘Département d’analyses Chimiques et Médicales’
(Tours, France) for chemical analyses.
Scheme 3
CO2Et
H3C
Ph
Br
CO2Et
CO2Et
(Z)-6 (50%)
+
+
EtO2C
CuI (10 mol%)
DMF, r.t., 16 h
SnBu3
CO2Et
(Z,Z)-7 (25%)
(Z)-6
(E,E)-7 (25%)
Scheme 4 Tentative of cross-coupling reaction between (Z)-6 and bromoalkyne
© Thieme Stuttgart · New York
Synlett 2012, 23, 755–759

758
K. Zine et al.
LETTER
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Compound 1b (200 mg, 0.437 mmol), 1-bromooct-1-yne (83
mg,0.437 mmol), and dry DMF (5 mL) were introduced into
a dry Schlenk flask under argon. The mixture was degassed
under agitation (10 min), then CuI (8 mg, 0.04 mmol) was
introduced under argon flux. The mixture was brought to r.t.
and left for 5 h under stirring. The reaction mixture was
diluted with Et2O, washed with aq KF solution (1 M, 10 mL)
and the ether layer was separated, dried over MgSO4,
concentrated, and separated on a silica gel column (pentane–
Et2O = 95:5) to provide 100 mg (83%) of enyne 2a as a
colorless liquid. 1H NMR (300 MHz, CDCl3): d = 0.91 (t,
J = 7.2 Hz, 3 H), 1.22–1.66 (m, 11 H), 2.48 (d, J = 7.1 Hz, 2
H), 4.26 (q, J = 7.2 Hz, 2 H), 6.54 (s, 1 H). 13C NMR (75
MHz, CDCl3): d = 13.6, 14.1, 20.0, 22.4, 27.9, 28.4, 31.2,
61.1, 72.3, 106.8, 120.9 (q, JC–F = 273.4 Hz), 126.4 (q, JC–
F = 34.6 Hz), 127.1 (q, JC–F = 4.4 Hz), 163.5. 19F NMR (282
MHz, CDCl3): d = –67.9. IR (ATR): n = 2955, 2926, 2856,
2219 1734, 1634 cm–1.
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Synlett 2012, 23, 755–759
© Thieme Stuttgart · New York

LETTER
Fluorinated Enynoates
759
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© Thieme Stuttgart · New York
Synlett 2012, 23, 755–759

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