Nickel-catalyzed intermolecular [2 + 2] cycloaddition of conjugated enynes with alkenes

Article abstract of DOI:10.1021/ja3074775

A nickel-catalyzed intermolecular [2 + 2] cycloaddition of conjugated enynes with alkenes has been developed. A variety of electron-deficient alkenes as well as electronically neutral norbornene and 1-decene were applicable to this reaction. The use of conjugated enynes circumvented possible side rections, such as oligomerizations and cyclotrimerizations. The isolation of reaction intermediate complexes revealed that the ν³-butadienyl coordination is the key for the selective formation of cyclobutene.

Full text of DOI:10.1021/ja3074775

Communication
pubs.acs.org/JACS
Nickel-Catalyzed Intermolecular [2 + 2] Cycloaddition of Conjugated
Enynes with Alkenes
Akira Nishimura, Masato Ohashi, and Sensuke Ogoshi*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
S
* Supporting Information
co-workers showed that vinylacetylene favors [2 + 2]
ABSTRACT: A nickel-catalyzed intermolecular [2 + 2]
cycloaddition of conjugated enynes with alkenes has been
developed. A variety of electron-deficient alkenes as well as
electronically neutral norbornene and 1-decene were
applicable to this reaction. The use of conjugated enynes
circumvented possible side rections, such as oligomeriza-
tions and cyclotrimerizations. The isolation of reaction
intermediate complexes revealed that the η3-butadienyl
coordination is the key for the selective formation of
cyclobutene.
cycloaddition with norbornadiene over the competing homo-
Diels−Alder reaction.9 Hilt et al.6b reported that (1-cyclohexen-
1-ylethynyl)benzene showed complete selectivity for [2 + 2]
cycloaddition with cyclopentene while other internal alkynes
gave a mixture of [2 + 2] cycloadducts and Alder−ene
products. Here we report a nickel-catalyzed intermolecular [2 +
2] cycloaddition of conjugated enynes with alkenes. The
isolation of the key reaction intermediate and the role of the
alkenyl group of the conjugated enyne in the catalytic reaction
are also discussed.
Our initial study began with the reaction of 2-cyclo-
pentenone (1a) because cyclic enones did not afford
cyclohexene derivatives at all in the reaction with alkynes
using the Ni(cod)2/PCyp3 catalyst system. In the presence of
Ni(cod)2 and PCyp3, the reaction of 1a with 1-decen-3-yne
(2a) in toluene at 80 °C for 2 h afforded the corresponding
cyclobutene 3aa in 20% yield, although the major product was a
mixture of [2 + 2 + 2] cycloadducts of 1a with 2 equiv of 2a
(Table 1, run 1).10 Other monodentate phosphines, such as
eveloping new strategies for the synthesis of molecules
Dthat are not easy to access by conventional methods is a
challenging issue in modern organic chemistry. Among such
molecules, cyclobutene has an attractive structure because of its
high reactivity for versatile transformations originating from the
ring strain.1 In addition, there are a number of natural products
and biologically active compounds that also contain four-
membered carbon rings.2 The [2 + 2] cycloaddition of alkynes
with alkenes is a straightforward synthetic route for the
preparation of cyclobutenes that is thermally forbidden but
photochemically allowed according to the Woodward−
Hoffmann rules. Brønsted or Lewis acid-catalyzed3 and
transition-metal-catalyzed reactions4,5 are the alternatives
under thermal conditions. In the former case, a combination
of electron-rich and electron-deficient substrates is often
required, so the resultant product should bear at least one
heteroatom substituent on the cyclobutene ring. In the latter
case, a variety of transition-metal catalysts have been developed
for the [2 + 2] cycloaddition of alkynes with alkenes to date.
However, most of the intermolecular reactions require alkenes
containing a highly strained norbornene skeleton.4 Therefore,
only a few examples have been known to utilize less-strained
alkenes.6
We previously reported the [2 + 2 + 2] cycloaddition of an
alkyne with two acyclic enones catalyzed by Ni(cod)2/PCyp3
(cod = 1,5-cyclooctadiene; Cyp = cyclopentyl) to give
cyclohexene derivatives.7 During the course of the study, we
found the formation of a small amount of a cyclobutene along
with the major [2 + 2 + 2] cycloaddition product in the
reaction of (E)-1-phenyl-2-buten-1-one with 2-methyl-1-hexen-
3-yne.8 This result suggested that conjugated enynes must have
a distinct reactivity toward the [2 + 2] cycloaddition with
alkenes. In fact, two examples showing that conjugated enynes
can facilitate [2 + 2] cycloaddition with alkenes in cobalt-
catalyzed reactions can be found in the literature. Tolstikov and
Table 1. Optimization of the Reaction Conditions
a
run
ligand
x/y
solvent
time (h)
yield (%)
b
1
PCyp3
PCy3
PPh3
IPr
10/20
10/20
10/20
10/10
5/5
toluene
toluene
toluene
toluene
toluene
THF
2
2
2
2
5
5
20
12
b
2
b
c
3
n.d.
4
5
6
7
80
76
74
80
IPr
IPr
5/5
IPr
5/5
dioxane
5
a
b
GC yields using C14H30 as an internal standard. A mixture of [2 + 2
c
+ 2] cycloadducts was observed by GC. Not detected.
PCy3 (Cy = cyclohexyl) and PPh3, were less effective (runs 2
and 3). When the reaction was conducted in the presence of
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), the for-
mation of undesired [2 + 2 + 2] cycloadducts was completely
suppressed, and 3aa was obtained in 80% yield (run 4). When
the catalyst loading was reduced to 5 mol %, 3aa was obtained
Received: July 30, 2012
Published: September 11, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
in a slightly lower yield (76%; run 5). Various solvents were
examined under the same reaction conditions, and 1,4-dioxane
gave the best results, affording 3aa in 80% yield (runs 5−7).11
We next investigated the scope of electron-deficient alkenes
and conjugated enynes (Scheme 1). Under the optimized
reaction conditions, the reaction of 1a with 2a gave 3aa in 76%
isolated yield. The reaction of 1a with 2-methyl-1-hexen-3-yne
(2b) afforded cyclobutene 3ab in 72% yield as a mixture of
regioisomers in a 93:7 ratio. The reaction of phenyl-
vinylacetylene (2c) gave cyclobutene 3bc in 64% yield (97:3
regioisomeric ratio) upon slow addition of 2c to suppress the
oligomerization of 2c. Cyclobutenes 3ab and 3ac were the only
cases where the apparent formation of regioisomers was
observed. Although the yield was low, the reaction of
trimethyl(3-methylbut-3-en-1-ynyl)silane (2d) bearing a bulky
substituent also took place, giving 3ad. When cis- and trans-5-
phenyl-2-penten-4-yne [(Z)-2e or (E)-2e] were used, the
corresponding cyclobutenes (Z)-3ae or (E)-3ae were obtained,
and no E/Z isomerization was observed. However, (Z)-3ae
gradually isomerized into (E)-3ae under air at room temper-
ature after purification. 2-Cyclohexenone (1b) also reacted with
enynes 2a and 2f to give cyclobutenes 3ba and 3bf in good
yields. Acyclic enone 1c reacted with 2a to give cyclobutene
3ca as the major product in 60% yield, while cyclohexene
derivatives were also obtained in 28% yield.8 The reactions of
(E)-chalcone (1d) and (E)-benzalacetone (1e) gave good
yields of 3db and 3ea, respectively, and no cyclohexene
products were observed. In the case of 1d, PCyp3 was used as
the ligand because the reaction proceeded faster than with IPr.
Remarkably, ethyl acrylate (1f) was also applicable to this [2 +
2] cycloaddition, whereas 1f reacts with internal alkynes to give
a 2:1 cotrimerized product under similar reaction conditions.12
In addition, it has been reported that acrylates undergo [2 + 2 +
2] cycloaddition, codimerization, and 1:2 cotrimerization with
alkynes in the presence of a nickel catalyst.10b,12,13 However,
such undesired byproducts were not observed in this reaction,
and 3fa was obtained in 91% yield. The reactions of (E)-ethyl
crotonate (1g), γ-crotonolactone (1h), N,N-dimethylacryla-
mide (1i), and diethyl vinylphosphonate (1j) with 2a took
place to give the corresponding cyclobutenes 3ga, 3ha, 3ia, and
3ja. N-tert-Butylmaleimide (1k) underwent [2 + 2] cyclo-
addition with 2a to give 3ka in 69% yield. However, attempts to
obtain a cyclobutene from maleic anhydride or N-methyl- or N-
phenylmaleimide were unsuccessful, probably because of
insertion of a nickel complex into the carbonyl carbon−oxygen
or carbon−nitrogen bond.14 The reaction of dimethyl fumarate
[(E)-1l] with 2a gave anti-3la selectively. In contrast, a mixture
of anti and syn isomers of 3la was obtained in the reaction of
dimethyl maleate [(Z)-1l] because of the competitive E/Z
isomerization of (Z)-1l in the presence of Ni(cod)2 and IPr.8
Other electron-deficient alkenes, such as methyl vinyl ketone,
acrolein, and acrylonitrile, failed to give the corresponding [2 +
2] cycloadducts because of their rapid oligomerization under
the catalytic conditions. Neither methyl methacrylate nor
methyl β,β-dimethylacrylate reacted with 2a; instead, cyclo-
trimerization of 2a was observed.
Scheme 1. Nickel-Catalyzed [2 + 2] Cycloaddition of
a
Electron-Deficient Alkenes with Conjugated Enynes
We also examined whether electronically neutral alkenes
could be applied to the present catalytic system. The reaction of
norbornene (1m) with 2a occurred even at room temperature,
giving 3ma in 97% yield as the exo isomer selectively (eq 1). To
a
General reaction conditions: alkene 1 (0.6 mmol), conjugated enyne
2 (0.72 mmol), Ni(cod)2 (0.03 mmol), IPr (0.03 mmol), and dioxane
b
(2 mL). Isolated yields are shown. The ratio of regioisomers is given
c
d
in parentheses. With slow addition of 2. In toluene with a 10 mol %
e
f
catalyst loading. 2 equiv of 2a was used. The reaction was performed
at 60 °C using 10 mol % Ni(cod)2 and 20 mol % PCyp3 as the catalyst.
In THF with a 10 mol % catalyst loading. The reaction was
g
h
performed with 1.2 mmol (2 equiv) of 1g and 0.6 mmol (1 equiv) of
i
2a. The yield was based on 2a. The anti/syn ratio is given in
parentheses.
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our surprise, the reaction of 1-decene (1n) with 2f gave
cyclobutenes 3nf and 3′nf in high yields, although a 16-fold
excess of 1n was used as a solvent and 2f was added slowly via
syringe drive (eq 2).15
To clarify the reaction mechanism of this nickel-catalyzed [2
+ 2] cycloaddition of conjugated enynes with alkenes, the
isolation of a possible reaction intermediate was attempted.
Treatment of 1b with an equimolar amount of 2f in the
presence of Ni(cod)2 and IPr gave nickelacycle 4bf in 45%
isolated yield (eq 3). The ORTEP drawing of complex 4bf is
first example of the isolation of a cyclic (η3-butadienyl)nickel
complex, although the computational study reported by Houk
and Jamison suggested its formation as the most stable
intermediate in the oxidative cyclization of a conjugated
enyne with an aldehyde on nickel(0).16,17 The reaction of 1d
and 2b with Ni(cod)2 and IPr also gave an η3-butadienyl
nickelacycle, 4′db, in 48% isolated yield (eq 4). However, a
nickel O-enolate rather than a nickel C-enolate was formed in
4′db (Figure 1 bottom). The structural difference between 4bf
and 4′db might be due to the difference between the flexibilities
of cyclic enone 1b and acyclic enone 1d. Heating complex 4bf
at 80 °C in the presence of an excess amount of ethyl acrylate
afforded cyclobutene 3bf in 95% yield (eq 3). Complex 4′db
was also converted into the corresponding cyclobutene 3db in
the presence of ethyl acrylate at 60 °C, although 4′db has an O-
enolate structure (eq 4). This result indicates that the O-enolate
complex 4′db isomerized into the C-enolate intermediate prior
to reductive elimination. During these reactions, no insertion of
ethyl acrylate into either complex 4bf or 4′db was observed,
indicating that this was prevented by the η3-butadienyl
coordination. Therefore, these observations clearly show that
the η3-butadienyl nickelacycle mediates the [2 + 2] cyclo-
addition of conjugated enynes with alkenes.
A proposed reaction mechanism is depicted in Scheme 2.
Alkene 1 and conjugated enyne 2 simultaneously coordinate to
Scheme 2. Proposed Mechanism
shown at the top of Figure 1. In complex 4bf, the alkenyl group
and the adjacent alkynyl carbon of 2f are bound to the nickel
center in η3-butadienyl fashion. A carbon−carbon bond is
formed between the alkynyl carbon distal to the alkenyl group
of 2f and the β-carbon of 1c, and the α-carbon of 1c forms a
nickel−carbon bond. To the best of our knowledge, this is the
the nickel(0) center, and then oxidative cyclization to give η3-
butadienyl nickelacycle intermediate A occurs. In this step, the
regioselective incorporation of an alkyne moiety takes place as a
result of the formation of the thermodynamically favorable η3-
butadienyl structure.16 Subsequent reductive elimination from
A affords cyclobutene 3 and regenerates the nickel(0) species.
The η3-butadienyl coordination might suppress the possible β-
H elimination and insertion of another π component because
the four coordination sites of intermediate A are fully occupied.
Furthermore, the reductive elimination step might be facilitated
by the η3-butadienyl structure.18 In the case of acyclic enones,
intermediate A would be in equilibrium with an O-enolate
species as the resting state. However, the O-enolate complex is
also an intermediate in the [2 + 2 + 2] cycloaddition of two
enones with an alkyne.7 Thus, the reaction of enone 1c with 2a
gave a [2 + 2 + 2] cyloaddduct as a byproduct. However, the
reaction of 1d gave cyclobutene 3db selectively because the [2
+ 2 + 2] reaction of 1d is much slower than that of 1c.
In conclusion, we have demonstrated the nickel-catalyzed
intermolecular [2 + 2] cycloaddition of conjugated enynes with
alkenes to provide cyclobutene derivatives with high chemo-
Figure 1. Molecular structures of (top) 4bf and (bottom) 4′db with
thermal ellipsoids set at the 30% probability level. H atoms, iPr groups,
and solvated molecules have been omitted for clarity.
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1162. Ir: (k) Fan, B.-M.; Li, X.-J.; Peng, F.-Z.; Zhang, H.-B.; Chan, A.
S. C.; Shao, Z.-H. Org. Lett. 2010, 12, 304.
(5) For intramolecular reactions, see: Pd: (a) Trost, B. M.; Tanoury,
and regioselectivities. Various types of electron-deficient
alkenes as well as electronically neutral alkenes such as
norbornene and 1-decene were applicable to this reaction.
The isolation of the key reaction intermediate revealed that the
η3-butadienyl coordination derived from the conjugated enyne
plays an important role in the selective formation of
cyclobutenes. Further investigation of the reaction conditions
for which more versatile alkenes can be applied is currently
underway in our laboratory.
G. J. J. Am. Chem. Soc. 1988, 110, 1636. (b) Trost, B. M.; Yanai, M.;
Hoogsteen, K. J. Am. Chem. Soc. 1993, 115, 5294. Pt: (c) Furstner, A.;
̈
Stelzer, F.; Szillat, H. J. Am. Chem. Soc. 2001, 123, 11863. (d) Furstner,
̈
A.; Davies, P. W.; Gress, T. J. Am. Chem. Soc. 2005, 127, 8244.
(e) Bajracharya, G. B.; Nakamura, I.; Yamamoto, Y. J. Org. Chem.
2005, 70, 892. Au: (f) Nieto-Oberhuber, C.; Lop
Cardenas, D. J.; Bunuel, E.; Nevado, C.; Echavarren, A. M. Angew.
Chem., Int. Ed. 2005, 44, 6146. (g) Nieto-Oberhuber, C.; Lop
Echavarren, A. M. J. Am. Chem. Soc. 2005, 127, 6178. (h) Odabachian,
́
ez, S.; Munoz, M. P.;
̃
́
̃
́
ez, S.;
ASSOCIATED CONTENT
■
Y.; Gagosz, F. Adv. Synth. Catal. 2009, 351, 379. Ru: (i) Furstner, A.;
̈
S
* Supporting Information
Schlecker, A.; Lehmann, C. W. Chem. Commun. 2007, 4277.
(6) (a) Rosenblum, M.; Scheck, D. Organometallics 1982, 1, 397.
(b) Hilt, G.; Paul, A.; Treutwein, J. Org. Lett. 2010, 12, 1536.
Detailed experimental procedures, analytical and spectral data,
and crystallographic data for 3db, 4bf and 4′db (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
́
(c) Lopez-Carrillo, V.; Echavarren, A. M. J. Am. Chem. Soc. 2010, 132,
9292. (d) Motokura, K.; Nakayama, K.; Miyaji, A.; Baba, T.
ChemCatChem 2011, 3, 1419.
(7) Ogoshi, S.; Nishimura, A.; Ohashi, M. Org. Lett. 2010, 12, 3450.
(8) See the Supporting Information.
(9) Dzhemilev, U. M.; Khusnutdinov, R. I.; Muslimov, Z. S.;
Tolstikov, G. A. Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.)
1987, 36, 977.
(10) (a) Mori, N.; Ikeda, S.; Sato, Y. J. Am. Chem. Soc. 1999, 121,
2722. (b) Sambaiah, T.; Li, L.-P.; Huang, D.-J.; Lin, C.-H.;
Rayabarapu, D. K.; Cheng, C.-H. J. Org. Chem. 1999, 64, 3663.
(11) The reaction of 1a with a conjugated diyne (2,4-hexadiyne)
instead of a conjugated enyne was also attempted. However, it gave a
complicated mixture, and a trace amount of a cyclobutene derivative
was detected by GC−MS.
(12) Horie, H.; Kurahashi, T.; Matsubara, S. Chem. Commun. 2010,
46, 7229.
(13) Horie, H.; Koyama, I.; Kurahashi, T.; Matsubara, S. Chem.
AUTHOR INFORMATION
■
Corresponding Author
ogoshi@chem.eng.osaka-u.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid for Scientific
Research (21245028) and Scientific Research on Innovative
Area “Molecular Activation Directed toward Straightforward
Synthesis” (23105546) from MEXT and by the Asahi Glass
Foundation. A.N. expresses his special thanks for the Research
Fellowship for Young Scientists from JSPS.
Commun. 2011, 47, 2658.
(14) Insertion of a nickel complex into the carbonyl carbon−oxygen
bond of cyclic anhydrides has been reported previously. For example,
see: Uhlig, V. E.; Fehske, G.; Nestler, B. Z. Anorg. Allg. Chem. 1980,
465, 141.
(15) In the reaction of styrene under the reaction conditions outlined
in eq 2, [2 + 2] cycloadducts were also obtained, and the
regioselectivity of the products was almost 50:50. However, a
considerable amount of unidentified and inseparable byproducts that
might be 4π-ring-opened products of cyclobutenes and/or cyclo-
trimerized enynes was also observed.
REFERENCES
■
(1) Namyslo, J. C.; Kaufmann, D. E. Chem. Rev. 2003, 103, 1485.
(2) Dembitsky, V. M. J. Nat. Med. 2008, 62, 1.
(3) (a) Clark, R. D.; Untch, K. G. J. Org. Chem. 1979, 44, 248.
(b) Clark, R. D.; Untch, K. G. J. Org. Chem. 1979, 44, 253.
(c) Fienemann, H.; Hoffmann, H. M. R. J. Org. Chem. 1979, 44, 2802.
(d) Snider, B. B.; Rodini, D. J.; Conn, R. S. E.; Sealfon, S. J. Am. Chem.
Soc. 1979, 101, 5283. (e) Snider, B. B.; Roush, D. M.; Rodini, D. J.;
Gonzalez, D.; Spindell, D. J. Org. Chem. 1980, 45, 2773. (f) Faron, K.
L.; Wulff, W. D. J. Am. Chem. Soc. 1988, 110, 8727. (g) Quendo, A.;
Rousseau, G. Tetrahedron Lett. 1988, 29, 6443. (h) Narasaka, K.;
Hayashi, Y.; Shimadzu, H.; Niihata, S. J. Am. Chem. Soc. 1992, 114,
8869. (i) Sweis, R. F.; Schramm, M. P.; Kozmin, S. A. J. Am. Chem. Soc.
2004, 126, 7442. (j) Inanaga, K.; Takasu, K.; Ihara, M. J. Am. Chem.
Soc. 2005, 127, 3668. (k) Takenaka, Y.; Ito, H.; Hasegawa, M.; Iguchi,
K. Tetrahedron 2006, 62, 3380. (l) Ishihara, K.; Fushimi, M. J. Am.
Chem. Soc. 2008, 130, 7532. (m) Commandeur, M.; Commandeur, C.;
Paolis, M. D.; Edmunds, A. J. F.; Maienfisch, P.; Ghosez, L.
Tetrahedron Lett. 2009, 50, 3359. (n) Li, H.; Hsung, R. P.;
DeKorver, K. A.; Wei, Y. Org. Lett. 2010, 12, 3780. (o) Schotes, C.;
Mezzetti, A. Angew. Chem., Int. Ed. 2011, 50, 3072.
(4) For intermolecular reactions, see: Ni: (a) Schrauzer, G. N.;
Glockner, P. Chem. Ber. 1964, 97, 2451. (b) Huang, D.-J.; Rayabarapu,
D. K.; Li, L.-P.; Sambaiah, T.; Cheng, C.-H. Chem.Eur. J. 2000, 6,
3706. Ru: (c) Mitsudo, T.; Kokuryo, K.; Takegami, Y. J. Chem. Soc.,
Chem. Commun. 1976, 722. (d) Mitsudo, T.; Naruse, H.; Kondo, T.;
Ozaki, Y.; Watanabe, Y. Angew. Chem., Int. Ed. Engl. 1994, 33, 580.
(e) Yi, C. S.; Lee, D. W.; Chen, Y. Organometallics 1999, 18, 2043.
(f) Jordan, R. W.; Tam, W. Org. Lett. 2000, 2, 3031. Co: (g) Chao, K.
C.; Rayabarapu, D. K.; Wang, C.-C.; Cheng, C.-H. J. Org. Chem. 2001,
66, 8804. (h) Treutwein, J.; Hilt, G. Angew. Chem., Int. Ed. 2008, 47,
6811. Rh: (i) Shibata, T.; Takami, K.; Kawachi, A. Org. Lett. 2006, 8,
1343. Re: (j) Kuninobu, Y.; Yu, P.; Takai, K. Chem. Lett. 2007, 36,
(16) (a) Liu, P.; McCarren, P.; Cheong, P. H.-Y.; Jamison, T. F.;
Houk, K. N. J. Am. Chem. Soc. 2010, 132, 2050. (b) Miller, K. M.;
Luanphaisarnnont, T.; Molinaro, C.; Jamison, T. F. J. Am. Chem. Soc.
2004, 126, 4130.
(17) η3-Butadienyl complexes of other transition metals have been
reported. For examples with group 10 metals, see: (a) Ogasawara, M.;
Okada, A.; Watanabe, S.; Fan, L.; Uetake, K.; Nakajima, K.; Takahashi,
T. Organometallics 2007, 26, 5025. (b) Benyunes, S. A.; Brandt, L.;
Fries, A.; Green, M.; Mahon, M. F.; Papworth, T. M. T. J. Chem. Soc.,
Dalton Trans. 1993, 3785. (c) Benyunes, S. A.; Brandt, L.; Green, M.;
Parkins, A. W. Organometallics 1991, 10, 57.
(18) (a) Kurosawa, H.; Ohnishi, H.; Emoto, M.; Kawasaki, Y.; Murai,
S. J. Am. Chem. Soc. 1988, 110, 6272. (b) Kurosawa, H.; Ohnishi, H.;
Emoto, M.; Chatani, N.; Kawasaki, Y.; Murai, S.; Ikeda, I.
Organometallics 1990, 9, 3038. (c) Gulías, M.; Collado, A.; Trillo,
B.; Lop
Chem. Soc. 2011, 133, 7660.
́
ez, F.; Onate, E.; Esteruelas, M. A.; Mascarenas, J. L. J. Am.
̃
̃
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