Cooperative copper(I) and primary amine catalyzed room-temperature carbocyclization of formyl alkynes

Article abstract of DOI:10.1002/ejoc.201100464

An efficient Cu^I/amine catalytic system is described for the carbocyclization of α-disubstituted formylalkynes at room temperature. Merging aminocatalysis to the copper(I)-catalyzed activation of alkynes led to clean carbocyclization of a wide range of functionalized substrates under mild conditions. This novel cooperative catalytic system allowed the formation of a variety of carbo- and heterocycles, including pyrrolidines, in good to excellent yields. Mechanistic aspects of this process are also discussed. Copyright

Full text of DOI:10.1002/ejoc.201100464

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201100464
Cooperative Copper(I) and Primary Amine Catalyzed Room-Temperature
Carbocyclization of Formyl Alkynes
Benjamin Montaignac,[a,b] Maxime R. Vitale,[a,b] Virginie Ratovelomanana-Vidal,*[a,b] and
Véronique Michelet*[a,b]
Keywords: Copper / Amines / Organocatalysis / Cyclization
An efficient CuI/amine catalytic system is described for the
carbocyclization of α-disubstituted formylalkynes at room
temperature. Merging aminocatalysis to the copper(I)-cata-
lyzed activation of alkynes led to clean carbocyclization of a
wide range of functionalized substrates under mild condi-
tions. This novel cooperative catalytic system allowed the for-
mation of a variety of carbo- and heterocycles, including pyr-
rolidines, in good to excellent yields. Mechanistic aspects of
this process are also discussed.
Introduction
Over the past few years, we have witnessed the tremen-
dous expansion of carbophilic Lewis acid catalysis. Transi-
tion metals, typically palladium, platinum, and gold, while
coordinating to C–C unsaturations, allow the addition of
various nucleophiles onto the π system.[1] Heteroatom nu-
cleophiles were shown to exhibit good reactivities,[2]
whereas carbon nucleophiles, aside from carbon–carbon
unsaturations,[3] usually required stabilized enol forms[4] or
surrogates.[5] Although less explored, the akin activation of
alkynes by copper(I) complexes was also proven to be effec-
tive.[1,5] In 1992, Balme et al. described for the first time the
intramolecular stoichiometric carbocupration of unacti-
vated alkynes bearing an α-sulfonylester moiety at room
temperature.[6a,7] This Conia–ene type reaction,[8] based on
the use of copper(I) iodide and tBuOK and proceeding
through an anti-carbometalation pathway, was later ex-
tended to a catalytic version applicable to a broader range
of stabilized nucleophiles [Scheme 1, Equation (1)];[6b] it
was later used in tandem “Michael addition/carbocupr-
ation” reactions.[9]
Scheme 1. Intramolecular carbocupration of unactivated alkynes.
We and others recently reported the carbocyclization of
α-disubstituted aldehydes onto nonactivated alkynes by
merging aminocatalysis to indium[11] or gold[12] catalysis.[13]
Under such cooperative “metallo–organocatalysis” condi-
tions,[14] the metal catalyst allows the electrophilic acti-
vation of the alkyne moiety, whereas the nucleophilicity of
the aldehyde is induced through the formation of an en-
amine. As a straight benefit of such catalytic systems, the
Conia–ene-type reaction of α-disubstituted aldehydes was
achieved without using stabilized enols or enol surrogates,
affording directly carbocyclized quaternary carbaldehydes.
One limitation of these metallo–organocatalytic systems be-
ing the rather elevated temperature (80 to 100 °C) required
for such carbocyclization reactions, we decided to investi-
gate the ability of copper(I)/amine combinations to activate
triple bonds at room temperature [Scheme 1, Equation (2)].
We wish to report herein our endeavors towards the discov-
ery of an efficient metallo–organocatalytic system based on
copper(I)/amine allowing the room-temperature carbocy-
clization of α-disubstituted formylalkynes.
Very recently, a copper(I)-catalyzed asymmetric Conia–
ene reaction of alkynyl-β-keto esters was reported by Dixon
and co-workers, whilst the cooperative use of a cinchona-
based thiourea organocatalyst, accountable for the enantio-
selectivity, led to syn-carbocupration.[10]
[a] E.N.S.C.P., Chimie-ParisTech, UMR 7223, Laboratoire Charles
Friedel (LCF),
75005 Paris, France
Fax: +33-1-44071062
E-mail: virginie-vidal@chimie-paristech.fr
veronique-michelet@chimie-paristech.fr
[b] CNRS UMR 7223,
75005 Paris, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100464.
Eur. J. Org. Chem. 2011, 3723–3727
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3723

B. Montaignac, M. R. Vitale, V. Ratovelomanana-Vidal, V. Michelet
SHORT COMMUNICATION
whereas twice the amount of triphenylphosphane strongly
decreased the reactivity of the catalytic system (Table 1, En-
tries 6 & 7). The influence of the counterion was also sur-
veyed by using perchlorate or trifluoromethanesulfonate
complexes, but no significant enhancement was observed
(Table 1, Entries 8 & 9). Switching from triphenylphos-
phane to electron-rich tricyclohexylphosphane did not
meaningfully change the catalytic behavior (Table 1, En-
try 10), whereas the use of electron-poor tris(p-trifluoro-
methylphenyl)phosphane merely ameliorated the conver-
sion (Table 1, Entry 11). Important progress arose with the
copper(I) trifluoromethanesulfonate benzene complex. In-
deed, complete conversion of 1a was obtained in 16 h, and
cyclopentane 2a was isolated in a promising 65% yield
(Table 1, Entry 12). Addition of an equimolar amount or
more of triphenylphosphane to this complex resulted in a
dramatic drop in reactivity (Table 1, Entries 13 & 14).
At this point, we decided to broaden our investigation
while in situ generating copper(I) complexes from phos-
phane-mediated reduction of copper(II) sources.[16] Starting
from 5 mol-% copper(II) trifluoromethanesulfonate and an
equimolar amount of triphenylphosphane, the reactivity
was still low (Table 1, Entry 15). However, with 4 equiv. of
phosphane the presumably in situ generated CuI(PPh3)3-
OTf[13b] promoted complete conversion in 22 h at room
temperature and yielded 2a in 86% (Table 1, Entry 16).
Interestingly, the use of Cu(OTf)2 without triphenylphos-
phane also resulted in complete consumption of 1a; how-
ever, 2a was only isolated in 60% yield (Table 1, En-
try 17).[17]
Results and Discussion
At the outset of our investigations, the carbocyclization
of model substrate 1a was realized at room temperature in
1,2-dichloroethane (DCE) in the presence of cyclohex-
ylamine (20 mol-%) and different copper(I) sources (5 mol-
%) with or without a phosphane additive (Table 1). Cyclo-
hexylamine was selected as the organocatalyst for this study
considering our recent results demonstrating that primary
amines are more efficient than secondary amines for the
generation of enamines from α-disubstituted aldehydes.[11b]
Preliminary experiments using copper(I) iodide led, under
these reaction conditions, to very sluggish reactivity, as only
a few percent of desired product 2a was detected after 16 h,
whereas only half conversion was observed after 2 weeks
(Table 1, Entries 1 & 2). The addition of triphenylphos-
phane (15 mol-%) to generate the in situ soluble (PPh3)3CuI
catalyst[9d] did not improve the carbocyclization reaction
(Table 1, Entry 3). We next examined the use of cationic
copper(I) complexes such as tetrakis acetonitrile copper(I)
tetrafluoroborate employed by Fehr et al. for the cycloisom-
erization reactions of 5-enyn-3-ols.[15] Under these reaction
conditions we observed an improvement in reactivity, al-
though the 1a/2a ratio only reached 88:12 after 16 h and
66:34 after 64 h (Table 1, Entries 4 & 5). Addition of an
equivalent amount of triphenylphosphane to this copper
source led to a significantly better conversion after 64 h,
Table 1. Optimization of the copper-based metallo–organocatalytic
systems.
Other copper(II) precatalysts such as CuBr2, Cu(OAc)2,
and CuSO4 were tested in the presence of triphenylphos-
phane but none displayed a better catalytic behavior than
Cu(OTf)2, thus emphasizing the determining influence of
the counterion (Table 1, Entries 18–20). A control experi-
ment without cyclohexylamine led to the recovery of start-
ing material 1a (Table 1, Entry 21).
Entry
Copper source
PR3
(mol-%)
Time
[h]
SM/P[a]
With this new catalytic system in hand, we next con-
fronted it with a broad range of α-disubstituted carbon-
tethered formyl alkynes starting with α-methyl-substituted
substrates (Table 2). The replacement of the gem-dimethyl
malonate moiety by the more sterically hindered gem-di-
isopropyl malonate did not affect the carbocyclization reac-
tion (Table 2, Entry 1). Notably, this novel metallo–organo-
catalytic system demonstrated good functional group toler-
ance, considering the efficient room-temperature carbocy-
clization reactions of diethers 1c and 1d, diacetate 1e, and
disilyl ether 1f (Table 2, Entries 2–5). Interestingly, the mild-
ness of this copper-based catalytic system allowed the for-
mation of cyclopentane 2g in 92% yield (Table 2, Entry 6),
whereas some problems of retro-Michael side reactivity
were previously observed, leading to 4,4-bis(phenylsulf-
onyl)-1-butyne starting from gem-disulfone 1g.[11b] The in-
fluence of the group in the α position relative to the alde-
hyde moiety was then examined. When the more encum-
bered α-ethyl-substituted substrate 1h was submitted to the
cyclization reaction, a decrease in reactivity occurred and
64 h was required to reach full conversion. Corresponding
1
2
3
4
5
6
7
8
CuI
CuI
CuI
–
–
16
2 w
16
16
64
64
64
64
64
48
48
16
16
16
16
22
24
16
16
16
22
Ͼ95:5
50:50
Ͼ95:5
88:12
66:34
30:70
85:15
30:70
70:30
50:50
PPh3 (15)
–
[Cu(CH3CN)4]BF4
[Cu(CH3CN)4]BF4
[Cu(CH3CN)4]BF4
[Cu(CH3CN)4]BF4
[Cu(CH3CN)4]ClO4
[Cu(CH3CN)4]OTf
[Cu(CH3CN)4]BF4
[Cu(CH3CN)4]BF4
Cu(OTf)·1/2benzene
Cu(OTf)·1/2benzene
Cu(OTf)·1/2benzene
Cu(OTf)2
–
PPh3 (5)
PPh3 (10)
PPh3 (5)
PPh3 (5)
PCy3 (5)
P(p-CF3C6H4)3 (5)
–
9
10
11
12
13
14
15
16
17
18
19
20
21
35:65
Ͻ5:95[b]
Ͼ95:5
87:13
PPh3 (5)
PPh3 (15)
PPh3 (5)
PPh3 (20)
–
PPh3 (20)
PPh3 (20)
PPh3 (20)
PPh3 (20)
76:24
Cu(OTf)2
Cu(OTf)2
CuBr2
Cu(OAc)2
CuSO4·5H2O
Cu(OTf)2
Ͻ5:95[c]
Ͻ5:95 [d]
90:10
85:15
Ͼ95:5
Ͼ95:5[e]
[a] Determined by 1H NMR spectroscopic analysis of the crude
reaction mixture. [b] 65% isolated yield. [c] 86% isolated yield.
[d] 60% isolated yield. [e] Without cyclohexylamine.
3724
www.eurjoc.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 3723–3727

Room-Temperature Carbocyclization of Formyl Alkynes
Table 2. Carbocyclization reactions of carbon-tethered substrates tions of the sterically hindered α-butyl- and α-phenyl-sub-
1b–j.
stituted substrates were performed. Indeed, 1i and 1j poorly
reacted under the same reaction conditions with 80 and
65% conversion, respectively, after 64 h at room tempera-
ture. Cyclopentane 2i was obtained in 48% isolated yield
(Table 2, Entry 8). When increasing the temperature to
50 °C, substituted substrate 1j was efficiently cyclized to cy-
clopentane 2j in 90% yield (Table 2, Entry 9), whereas such
modification did not improve the cyclization of 2i. Thus,
while considering carbon-tethered substrates, the group in
the α position relative to the aldehyde moiety displayed an
important influence on the efficiency of the carbocycliz-
ation reaction.
Taking into account the mildness of this copper-based
metallo–organocatalytic system, we finally investigated cy-
clization reactions leading to pyrrolidine heterocycles. In-
deed, in comparison to gem-disulfone 1g, the corresponding
precursors usually evolved according to a retro-Michael re-
arrangement in the presence of our previously described
CyNH2/InCl3 catalytic system.[11b] To this end, N-tosyl-pro-
tected α-disubstituted nitrogen tethered substrates 3a and
3b were prepared according to a straightforward four-step
synthesis[18] and submitted to the cyclization reaction
(Scheme 2).
Scheme 2. Carbocyclization reactions leading to pyrrolidines.
Gratifyingly, both α-methyl- and α-butyl-substituted sub-
strates 3a and 3b led to smooth and clean carbocyclization
reactions, and the corresponding pyrrolidines were isolated
in 86 and 91% yield, respectively. Notably, a significant in-
crease in the rate of the reaction was observed for the syn-
thesis of these heterocycle precursors compared to the anal-
ogous carbon-tethered substrates.
Considering that both cyclohexylamine and copper(I) are
compulsory for the reaction to take place, the metallo–or-
ganocatalytic process would involve the amine catalyst as
accountable for the formation of a nucleophilic enamine,
whereas the copper complex would enhance the electrophi-
licity of the alkynyl residue (Scheme 3). Both activations
would then trigger the cyclization process, and the catalysts
would be regenerated by hydrolysis and protodemetalation.
Some reactivity discrepancies of carbon-tethered substrates
were observed compared to nitrogen-tethered substrates. In-
deed, whereas malonate-linked formylalkynes reacted more
sluggishly when the steric hindrance in the α position of
[a] Isolated yield. [b] Incomplete conversion was observed: SM/P =
20:80. [c] Reaction performed at 50 °C.
the aldehyde moiety was higher, comparable N-tosyl-linked
substrates were very good substrates for this transforma-
tion. In this context, the rate-determining step of this reac-
cyclopentane 2h was isolated in 67% yield (Table 2, En- tion would be the cyclization step in which a chair-like tran-
try 7). A similar trend was encountered when the cycliza- sition state might be at stake (Scheme 3). The low reactivity
Eur. J. Org. Chem. 2011, 3723–3727
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3725

B. Montaignac, M. R. Vitale, V. Ratovelomanana-Vidal, V. Michelet
SHORT COMMUNICATION
of the α-hindered carbon-tethered substrates could be ra-
tionalized on the basis of 1,3-diaxial steric strains
Acknowledgments
(Scheme 3, TS-A). Nitrogen-tethered substrates, in which This work was supported by the Centre National de la Recherche
Scientifique (CNRS) and the Ministère de lЈEducation et de la Re-
cherche. B.M. is grateful to the Ministère de lЈEducation et de la
Recherche for a grant (2009–2012).
the nitrogen atom displays a trigonal or slightly pyramidal
geometry,[19] would be free from these deleterious strains
(Scheme 3, TS-B) and thus would reach more easily the
adequate conformation necessary for cyclization.
[1] a) Z. Li, C. Brouwer, C. He, Chem. Rev. 2008, 108, 3239–3265;
b) G. C. Lloyd-Jones, Org. Biomol. Chem. 2003, 1, 215–236; c)
A. R. Chianese, S. J. Lee, M. R. Gagné, Angew. Chem. Int. Ed.
2007, 46, 4042–4059; d) A. Fürstner, P. W. Davies, Angew.
Chem. Int. Ed. 2007, 46, 3410–3449; e) S. I. Lee, N. Chatani,
Chem. Commun. 2009, 371–384; f) P. Y. Toullec, V. Michelet,
Top. Curr. Chem. 2011, DOI: 10.1007/128_2010_116; g) G.
Balme, D. Bouyssi, T. Lomberget, N. Monteiro, Synthesis 2003,
2115–2134.
[2] a) H. C. Shen, Tetrahedron 2008, 64, 3885–3903; b) N. T. Patil,
Y. Yamamoto, Chem. Rev. 2008, 108, 3395–3442; c) N. Krause,
V. Belting, C. Deutsch, J. Erdsack, H.-T. Fan, B. Gockel, A.
Hoffmann-Röder, N. Morita, F. Volz, Pure Appl. Chem. 2008,
80, 1063–1069; d) I. Nakamura, Y. Yamamoto, Chem. Rev.
2004, 104, 2127–2198.
Scheme 3. Mechanism rationale.
[3] a) P. Belmont, E. Parker, Eur. J. Org. Chem. 2009, 35, 6075–
6089; b) E. Jimenez-Nunez, A. M. Echavarren, Chem. Rev.
2008, 108, 3326–3350; c) V. Michelet, P. Y. Toullec, J.-P. Genêt,
Angew. Chem. Int. Ed. 2008, 47, 4268–4315; d) L. Zhang, J.
Sun, S. A. Kozmin, Adv. Synth. Catal. 2006, 348, 2271–2296;
e) C. Nevado, A. M. Echavarren, Synthesis 2005, 167–182; f)
I. J. S. Fairlamb, Angew. Chem. Int. Ed. 2004, 43, 1048–1052;
g) O. Buisine, C. Aubert, M. Malacria, Chem. Rev. 2002, 102,
813–834; h) B. M. Trost, M. J. Krische, Synlett 1998, 1–16.
[4] a) S. T. Staben, J. J. Kennedy-Smith, F. D. Toste, Angew. Chem.
Int. Ed. 2004, 43, 5350–5352; b) J. J. Kennedy-Smith, S. T.
Staben, F. D. Toste, J. Am. Chem. Soc. 2004, 126, 4526–4527.
[5] a) C. Nevado, D. J. Cárdenas, A. M. Echavarren, Chem. Eur. J.
2003, 9, 2627–2635; b) S. T. Staben, J. J. Kennedy-Smith, D.
Huang, B. K. Corkey, R. L. LaLonde, F. D. Toste, Angew.
Chem. Int. Ed. 2006, 45, 5991–5994; c) J. W. Dankwardt, Tetra-
hedron Lett. 2001, 42, 5809–5812; d) K. Lee, P. H. Lee, Adv.
Synth. Catal. 2007, 349, 2092–2096; e) E. C. Minnihan, S. L.
Colletti, F. D. Toste, H. C. Shen, J. Org. Chem. 2007, 72, 6287–
6289; f) T. J. Harrison, B. O. Patrick, G. R. Dake, Org. Lett.
2007, 9, 367–370; g) P. Belmont, T. Belhadj, Org. Lett. 2005, 7,
1793–1795; h) T. J. Harrison, G. R. Dake, Org. Lett. 2004, 6,
5023–5026; i) G. Abbiati, A. Arcadi, G. Bianchi, S. Di Giu-
seppe, F. Marinelli, E. Rossi, J. Org. Chem. 2003, 68, 6959–
6966.
Conclusions
In summary, we have developed a new metallo–organo-
catalytic system based on the synergistic use of catalytic
quantities of cyclohexylamine and an in situ generated cop-
per(I) complex, allowing efficient room-temperature car-
bocyclization reactions of α-disubstituted aldehydes onto
unactivated alkynes. We demonstrated that this combina-
tion promotes the cyclization of a broad range of substrates
leading to a variety of carbo- and heterocycles in high yields
and a good functional group tolerance. The set of the exper-
imental results tends to support a chair-like transition state
for the cyclization process. Further investigations concern-
ing an enantioselective version of this reaction are currently
underway and will be reported in due course.
Experimental Section
General Procedure for the Carbocyclization Reaction: In a sealed
vial under an argon atmosphere was successively introduced tri-
phenylphosphane (0.08 mmol, 0.2 equiv.), copper(II) trifluorome-
thanesulfonate (0.02 mmol, 0.05 equiv.), and DCE (0.4 mL). The
resulting mixture was stirred for 20 min at room temperature before
the freshly purified formylalkyne (0.4 mmol, 1 equiv.) in a 0.2 m
solution of amine in DCE (0.4 mL, 0.08 mmol, 0.2 equiv.) was
added. After introduction of an additional amount DCE (0.2 mL),
the reaction mixture was stirred at room temperature until GC or
TLC analysis indicated complete conversion. The reaction mixture
was then treated with an aqueous solution of AcOH (1:1, 1 mL)
and then vigorously stirred for 15 min at room temperature before
extraction of the aqueous layer with CH2Cl2. The combined or-
ganic layers were dried with MgSO4, filtered, and concentrated un-
der reduced pressure, and the resulting crude material was purified
by silica gel flash chromatography to afford the desired carbocy-
clized aldehyde.
[6] a) N. Monteiro, G. Balme, G. Gore, Synlett 1992, 227–228; b)
D. Bouyssi, N. Monteiro, G. Balme, Tetrahedron Lett. 1999,
40, 1297–1300; c) S. Montel, D. Bouyssi, G. Balme, Adv. Synth.
Catal. 2010, 352, 2315–2320.
[7] For a review on the addition of metal–enolates onto alkynes,
see: F. Dénès, A. Pérez-Luna, F. Chemla, Chem. Rev. 2010, 110,
2366–2447.
[8] For a review, see: J. M. Conia, P. Le Perchec, Synthesis 1975,
1–19.
[9] a) D. Bouyssi, N. Monteiro, G. Balme, Tetrahedron Lett. 1999,
40, 1301–1304; b) N. Coia, D. Bouyssi, G. Balme, Eur. J. Org.
Chem. 2007, 3158–3165; c) M. Woods, N. Monteiro, G. Balme,
Eur. J. Org. Chem. 2000, 1711–1718; d) B. Clique, S. Vassiliou,
N. Monteiro, G. Balme, Eur. J. Org. Chem. 2002, 1493–1499;
e) M. Cavicchioli, X. Marat, N. Monteiro, B. Hartmann, G.
Balme, Tetrahedron Lett. 2002, 43, 2609–2611; f) B. Clique, C.
Anselme, D. Otto, N. Monteiro, G. Balme, Tetrahedron Lett.
2004, 45, 1195–1197.
[10] T. Yang, A. Ferrali, F. Sladojevich, L. Campbell, D. J. Dixon,
J. Am. Chem. Soc. 2009, 131, 9140–9141.
[11] a) B. Montaignac, M. R. Vitale, V. Michelet, V. Ratoveloman-
ana-Vidal, Org. Lett. 2010, 12, 2582–2585; b) B. Montaignac,
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, analytical descriptions, and copies
1
of the H and 13C NMR spectra of new compounds.
3726
www.eurjoc.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 3723–3727

Room-Temperature Carbocyclization of Formyl Alkynes
M. R. Vitale, V. Ratovelomanana-Vidal, V. Michelet, J. Org.
Chem. 2010, 75, 8322–8325.
[12] J. T. Binder, B. Crone, T. T. Haug, H. Menz, S. F. Kirsch, Org.
Lett. 2008, 10, 1025–1028.
2904; c) C. Fehr, I. Magpantay, J. Arpagaus, X. Marquet, M.
Vuagnoux, Angew. Chem. Int. Ed. 2009, 48, 7221–7223; d) C.
Fehr, B. Winter, I. Magpanty, Chem. Eur. J. 2009, 15, 9973–
9784.
[13] For other examples relying on the association π-Lewis acid
metal catalysis to aminocatalysis, see: a) Q. Ding, J. Wu, Org.
Lett. 2007, 9, 4959–4962; b) T. Yang, A. Ferrali, L. Campbell,
D. J. Dixon, Chem. Commun. 2008, 2923–2925; c) N. T. Patil,
V. S. Ra u t , J. Org. Chem. 2010, 75, 6961–6964; for asymmetric
variants, see: d) K. L. Jensen, P. T. Franke, C. Arróniz, S. Kob-
belgaard, K. A. Jørgensen, Chem. Eur. J. 2010, 16, 1750–1753;
e) G.-L. Zhao, F. Ullah, L. Deiana, S. Lin, Q. Zhang, J. Sun,
I. Ibrahem, P. Dziedzic, A. Córdova, Chem. Eur. J. 2010, 16,
1585–1591; f) S. Lin, G.-L. Zhao, L. Deiana, J. E. Sun, Q.
Zhang, H. Leijonmarck, A. Córdova, Chem. Eur. J. 2010, 16,
13930–13934; g) T. Zweifel, D. Hollmann, B. Prüger, M. Niel-
sen, K. A. Jørgensen, Tetrahedron: Asymmetry 2010, 21, 1624–
1629; h) C. Yu, Y. Zhang, S. Zhang, J. He, W. Wang, Tetrahe-
dron Lett. 2010, 51, 1742–1744.
[16]
[17]
F. H. Jardine, L. Rule, A. G. Vohra, J. Chem. Soc. A 1970, 238–
240.
This result seems to indicate that the reduction of copper(II)-
trifluromethanesulfonate may occur in the presence of the
amine similarly to PdII complexes, or that this copper salt is
sufficiently acidic to activate the alkynyl moiety. For amine-
induced reduction of CuII and PdII, see, respectively: a) Y. L.
Sheludyakov, V. A. Golodov, J. Appl. Chem. USSR 1986, 59,
1233–1235; b) S.-I. Murahashi, T. Hirano, T. Yano, J. Am.
Chem. Soc. 1978, 100, 348–350.
[18] See Supporting Information for details.
[19] a) I. Chataigner, C. Panel, H. Gérard, S. R. Piettre, Chem.
Commun. 2007, 3288–3290; b) P. V. Bharatam, Amita, A.
Gupta, D. Kaur, Tetrahedron 2002, 58, 1759–1764; c) T.
Ohwada, I. Okamoto, K. Shudo, K. Yamaguchi, Tetrahedron
Lett. 1998, 39, 7877–7880.
[14] For reviews, see: a) Z. Shao, H. Zhang, Chem. Soc. Rev. 2009,
38, 2745–2755; b) C. Zhong, X. Shi, Eur. J. Org. Chem. 2010,
2999–3025.
Received: April 1, 2011
Published Online: May 24, 2011
[15] a) C. Fehr, I. Farris, H. Sommer, Org. Lett. 2006, 8, 1839–1841;
b) C. Fehr, I. Farris, Angew. Chem. Int. Ed. 2006, 45, 2901–
Eur. J. Org. Chem. 2011, 3723–3727
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3727