Synthesis of cyclobutenes and allenes by cobalt-catalyzed cross-dimerization of simple alkenes with 1,3-enynes

Article abstract of DOI:10.1002/chem.201402218

Cobalt-catalyzed cross-dimerization of simple alkenes with 1,3-enynes is reported. A [2+2] cycloaddition reaction occurred, with alkenes bearing no allylic hydrogen, by reductive elimination of a η³-butadienyl cobaltacycle. On the other hand, a

Full text of DOI:10.1002/chem.201402218

DOI: 10.1002/chem.201402218
Communication
&
Synthetic Methods
Synthesis of Cyclobutenes and Allenes by Cobalt-Catalyzed
Cross-Dimerization of Simple Alkenes with 1,3-Enynes
Akira Nishimura,[a] Eri Tamai,[a] Masato Ohashi,[a] and Sensuke Ogoshi*[a, b]
Abstract: Cobalt-catalyzed cross-dimerization of simple al-
kenes with 1,3-enynes is reported. A [2+2] cycloaddition
reaction occurred, with alkenes bearing no allylic hydro-
gen, by reductive elimination of a h3-butadienyl cobaltacy-
cle. On the other hand, aliphatic alkenes underwent 1,4-
hydroallylation by means of exo-cyclic b-H elimination.
These reactions can provide cyclobutenes and allenes that
were previously difficult to access, from simple substrates
in a highly chemo- and regioselective manner.
Scheme 1. Cobalt-catalyzed cross-dimerization of alkenes with alkynes.
strained cyclopentene has an interim reactivity between
simple alkenes and norbornenes, affording both cyclobutene
and 1,4-diene.[8] However, the chemoselectivity mainly depends
on the structural nature of the alkenes. Therefore, it is a chal-
lenge to develop a transformation that can provide previously
inaccessible products beyond conventional reactivity. Herein,
we report the cobalt-catalyzed cross-dimerization of simple al-
kenes with 1,3-enynes to afford cyclobutenes or tetrasubstitut-
ed allenes. 1,3-enynes display totally different chemoselectivity
compared to that of internal alkynes.
Oxidative cyclization of two p components at a transition-
metal center to form a metallacycle is a fundamental reaction
in the field of organometallic chemistry. This process is accept-
ed as the key step for a variety of synthetic methods, such as
cycloaddition, linear oligomerization, reductive coupling, and
multicomponent reactions.[1–3] Catalytic variants of these trans-
formations are frequently achieved with late transition metals,
of which cobalt is attractive because it can efficiently catalyze
the dimerization of electronically unbiased unsaturated hydro-
carbons.[4,5] The groups of Hilt and Cheng have demonstrated
the cross-dimerization of simple alkenes with internal alkynes,
in the presence of a cobalt(I)/1,3-bis(diphenylphosphino)pro-
pane (dppp) catalyst, to furnish linear dienes (Scheme 1).[6]
These cross-dimerization reactions are proposed to proceed
via a cobaltacyclopentene intermediate, which undergoes b-H
elimination, followed by reductive elimination. The difference
in chemoselectivity between the formation of 1,3-dienes by
means of endo-cyclic b-H elimination and 1,4-dienes, by means
of exo-cyclic b-H elimination occurs in the absence and in the
presence of allylic hydrogen atoms on the alkene substrates.
On the other hand, [2+2] cycloaddition occurs in the reaction
of highly strained bicyclic alkenes, such as norbornene, with al-
kynes by means of direct reductive elimination from the corre-
sponding cobaltacycle.[7] Hilt et al. also reported that less-
We have previously developed a nickel-catalyzed [2+2] cy-
cloaddition of alkenes with 1,3-enynes (Scheme 2).[9] On the
Scheme 2. Nickel-catalyzed [2+2] cycloaddition of electron-deficient alkenes
with 1,3-enynes. EWG=electron-withdrawing group
basis of a stoichiometric study, the key step of this [2+2] cy-
cloaddition was the formation of an h3-butadienyl nickelacycle,
which might prevent undesired pathways, such as b-H elimina-
tion and the insertion of another p substrate. On comparison
with electron-deficient alkenes, unactivated simple alkenes are
less reactive in the nickel-catalyzed reaction, and the resultant
product was a mixture of isomers. Thus, we next focused on
the cobalt catalyst, which has both good reactivity and selec-
tivity in the cross-dimerization of unactivated alkenes with al-
kynes. We assumed that, as with nickel catalysis, if the oxida-
tive cyclization of alkenes with 1,3-enynes at the cobalt center
can occur to form a h3-butadienyl intermediate, then b-H elimi-
nation could be avoided, and cyclobutenes could be provided
by means of reductive elimination. In fact, two examples can
[a] A. Nishimura, E. Tamai, Dr. M. Ohashi, Prof. Dr. S. Ogoshi
Department of Applied Chemistry
Faculty of Engineering, Osaka University
Suita, Osaka 565-0871 (Japan)
Fax: (+81)6-6879-7394
E-mail: ogoshi@chem.eng.osaka-u.ac.jp
[b] Prof. Dr. S. Ogoshi
JST, Advanced Catalytic Transformation Program
for Carbon Utilization (ACT-C)
Suita, Osaka 565-0871 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402218.
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be found in the literature, showing that 1,3-enynes facilitate
[2+2] cycloaddition with alkenes under cobalt catalysis.[7a,8]
To elucidate our working hypothesis, we first examined the
reactions of two different types of alkenes with a 1,3-enyne.
Based on the reaction conditions developed by the groups of
Hilt and Cheng,[6] a CoBr2/dppp/Zn/ZnI2 (ratio 1:1:4:4) catalyst
system was used. As expected, styrene (1) reacted with 2-
methyl-1-hexen-3-yne (2) in the presence of the cobalt catalyst
to give desired cyclobutene 3 in 80% yield [Eq. (1)].
CꢀC reductive elimination. Then, C undergoes reductive elimi-
nation to give allene 5.
According to the preliminary results and mechanistic hy-
pothesis, alkenes with no allylic hydrogen atoms have the po-
tential to undergo [2+2] cycloaddition with 1,3-enynes. Sever-
al optimizations of reaction conditions were investigated
based on Equation (1) (for details, see the Supporting Informa-
tion).[13] An equimolar, or a slightly excessive, amount of 1 was
enough to obtain 3 in a sufficient yield. A screening of ligands
revealed that only dppp was suitable for catalytic conversion
into the product. The reaction with dppe, rac-BINAP, or BIHEP
gave cyclobutene 3 in approximately 10% yield.[14] The use of
other mono- or bidentate ligands was unsuccessful. Regarding
cobalt(II) salts, CoBr2, CoI2, and Co(OAc)2 were applicable to
this [2+2] cycloaddition, whereas CoCl2 was less effective.
After further optimizations, we selected the reaction conditions
as follows: CoBr2/dppp/Zn/ZnI2 (1:1:1:2) as the catalyst, 2 mol%
catalyst loading, and CH2Cl2 as the solvent.
Next, the [2+2] cycloaddition of styrene derivatives with
enyne 2 was examined (Scheme 4). Under the optimized con-
ditions, cyclobutene 3 was obtained in 79% isolated yield. The
reaction of para-methylstyrene and para-methoxystyrene gave
6 and 7 in 83 and 80% yield, respectively. Halogen atoms (F,
Cl, and Br) at the para-position were compatible with this [2+
2] cycloaddition, and provided the corresponding cyclobutenes
8–10 in high yields. These halogen substituents, Br and Cl sub-
stituents in particular, can be further functionalized by cross-
coupling reactions. Among the meta-substituted styrenes em-
ployed, the reaction of meta-methylstyrene was comparable to
the corresponding para-substituted example, giving 11 in 83%
yield. On the other hand, electron-withdrawing groups, such
as CF3 and F groups, decreased the reactivity to afford 12 and
13 in 23 and 43% yields, respectively. ortho-Tolyl-substituted
14 was obtained in only 8% yield, probably owing to steric
hindrance. However, neither ortho-methoxy- nor ortho-chloro-
substituents diminished the reactivity and provided com-
pounds 15 (91%) and 16 (51%), respectively. In these cases,
the reactions might be facilitated by chelation to the cobalt
center through an oxygen- or chlorine atom. para-Cyanostyr-
ene and 2-vinylpyridne did not react at all, probably owing to
the deactivation of the catalyst by the strong coordination of
the nitrogen atom. Although the yields were more modest, vi-
nylboronate and vinylsilane reacted to provide cyclobutenes
17 and 18, which can be utilized for further transformations,
such as cross-coupling, allylic substitution, and oxidation. The
scope of 1,3-enynes was also investigated. The reaction of
enynes bearing longer alkyl chains, and chloro- or silyloxy sub-
stituents afforded the corresponding cyclobutenes 19, 20, and
21 in moderate to high yields. With 5 mol% catalyst loading,
1,5-dien-3-yne also reacted to give 1,2-diisopropenylcyclobu-
tene 22 in 49% yield. 1,3-Enynes bearing a naked vinyl group
were also applied to this reaction, affording 23 (50%) and 24
(34%) in the presence of 5 mol% catalyst loading. However,
In contrast to the nickel-catalyzed reaction, the cobalt-cata-
lyzed [2+2] cycloaddition proceeded even at room tempera-
ture and 3 was obtained as a single regioisomer. On the other
hand, the reaction of 1-decene (4) with 2 gave no cyclobutene
under the same conditions; instead, tetrasubstituted allene 5
was obtained as the major product in 76% yield, accompanied
by a small amount of unidentified isomers [Eq. (2)].[10] Although
the cobalt-catalyzed cross-dimerization of alkenes with alkynes
gives (E,E)-dienes,[6] the configuration of the alkenyl moiety of
5 was Z.
The difference in the reactivity of alkenes 1 and 4, under
cobalt catalysis, can be rationalized as described in Scheme 3.
In both cases, the reaction is initiated by the oxidative cycliza-
tion of an alkene with a 1,3-enyne at the cobalt(I) center, form-
ing a h3-butadienyl cobaltacycle (A!B).[11,12] In the reaction of
styrene (1, R=Ph), endo-cyclic b-H elimination could possibly
be suppressed by the h3-butadienyl coordination, and reduc-
tive elimination could occur to give cyclobutene 3. In the case
of 1-decene (4, R=C8H17), however, exo-cyclic b-H elimination
from B takes place to form CoꢀH species C in preference to
a
competing homodimerization of enynes was also ob-
served.[15] Although the yield of the product was not improved,
the slow addition of the enyne through a syringe drive sup-
pressed the formation of the enyne dimer in the case of 23. As
Scheme 3. Proposed mechanism.
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Scheme 5. Electrocyclic ring opening and Diels–Alder reaction of cyclobu-
tene 7. PMP=para-methoxyphenyl.
by the first Diels–Alder reaction, the second Diels–Alder reac-
tion did not take place during this reaction. When 7 was
heated at 608C in the presence of dienophile 26, a direct
Diels–Alder adduct, 28, was obtained as the sole product in
77% yield.[18] Both of the Diels–Alder reactions occurred with
endo selectivity to form 27 and 28 as single diastereomers.
These methodologies enable the synthesis of complex mole-
cules from simple substrates, such as alkenes, 1,3-enynes, and
dienophiles, by short and simple synthetic manipulations.
Finally, we investigated the synthesis of allenes. As shown in
Equation (2), aliphatic alkenes reacted with 1,3-enynes by
means of a 1,4-hydroallylation reaction to afford tetrasubstitut-
ed allenes.[19] 1,4-Hydrovinylation of 1,3-dienes with alkenes
has already been well established by Hilt’s group.[5a–e] The
scope of the substrates is summarized in Scheme 6.
A
10 mol% loading of the CoBr2/dppp/Zn/ZnI2 (1:1:2:4) catalyst
system and two equivalents of alkenes were used. Under the
optimized conditions, allene 5 was obtained in 88% yield.
Other aliphatic alkenes, containing phenyl, phthalimido, and
acetoxy groups, were compatible with this reaction to give the
corresponding allenes 29–32 in moderate to high yields. When
2-methyl-1,5-hexadiene was used as a substrate, only the less-
substituted alkenyl moiety reacted to give 33 and 34. In each
allene synthesis, the Z product was obtained in approximately
90% purity, but also contained unidentified isomers.[10] On the
other hand, alkenes with functional groups at the allylic posi-
tion were found to undergo 1,2-hydroallylation. For example,
allylbenzene and allyloxytrimethylsilane reacted with 2 to give
1,3,6-trienes 35 (46%) and 36 (55%), respectively. In addition,
the configuration of the separated alkenyl groups in 35 and 36
were exclusively E. In this 1,2-hydroallylation, the five-mem-
bered cobaltacycle, rather than the h3-butadienyl isomer,
might be involved as an intermediate because the dimerization
of alkenes with simple alkynes also gives 1,4-dienes with E con-
figuration.[6a] Although the origin of these different modes of
hydroallylation is still unclear, both allenes and 1,3,6-trienes are
useful for further transformations.
Scheme 4. Cobalt-catalyzed [2+2] cycloaddition of alkenes with 1,3-enynes.
pin=pinacolate, PMP=para-methoxyphenyl, TBS=tert-butyldimethylsilyl.
described, this [2+2] cycloaddition allowed the preparation of
cyclobutenes from both unactivated alkenes and 1,3-enynes.
To the best of our knowledge, this is the first example of a tran-
sition-metal-catalyzed [2+2] cycloaddition of simple styrene
derivatives, vinylsilane, and vinylborane with alkynes.[16]
The resultant 1-alkenylcyclobutene is a good precursor for
the cross-conjugated triene, [3]dendralene,[17] by means of the
electrocyclic ring opening of the cyclobutene moiety. Heating
cyclobutene 7 in toluene at 1008C for 24 h afforded triene 25
almost quantitatively (Scheme 5). The reaction proceeded with
outward rotation of the PMP group, resulting in an E configu-
ration of the benzylidene moiety. Triene 25 reacted with N-
methylmaleimide (26) to furnish bicyclic product 27 in 88%
yield. This Diels–Alder reaction took place exclusively at the
less-hindered diene site. Although 27 also has a diene moiety
In conclusion, we have demonstrated a cobalt-catalyzed
cross-dimerization of 1,3-enyne with simple alkenes. A [2+2]
cycloaddition occurred, utilizing alkenes with no allylic hydro-
gen atoms, whereas aliphatic alkenes underwent hydroallyla-
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[2] For reviews on transformations of 1,n-enynes, see: a) C. Aubert, O. Bui-
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75; h) H. Clavier, K. Le Jeune, I. De Riggi, A. Tenaglia, G. Buono, Org. Lett.
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[5] For cobalt-catalyzed acyclic dimerization, see: a) G. Hilt, F.-X. du Mesnil,
S. Lꢃers, Angew. Chem. 2001, 113, 408; Angew. Chem. Int. Ed. 2001, 40,
ˇ
387; b) G. Hilt, S. Lꢃers, Synthesis 2002, 609; c) M. Arndt, M. Dindaroglu,
H.-G. Schmalz, G. Hilt, Org. Lett. 2011, 13, 6236; d) L. Kersten, G. Hilt,
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Adv. Synth. Catal. 2012, 354, 863; e) M. Arndt, M. Dindaroglu, H.-G.
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ˇ
Hilt, M. Dindaroglu, H.-G. Schmalz, Angew. Chem. 2011, 123, 9863;
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[7] a) U. M. Dzhemilev, R. I. Khusnutdinov, Z. S. Muslimov, G. A. Tolstikov,
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Scheme 6. Cobalt-catalyzed hydroallylation of 1,3-enynes with aliphatic al-
kenes. PhthN=phthalimido, TBS=tert-butyldimethylsilyl, TMS=trimethylsil-
yl.
[8] G. Hilt, A. Paul, J. Treutwein, Org. Lett. 2010, 12, 1536.
[9] A. Nishimura, M. Ohashi, S. Ogoshi, J. Am. Chem. Soc. 2012, 134, 15692.
[10] The possible isomers of 5 are listed below.
tion. These reactions are highly chemo- and regioselective and
can provide synthetically useful cyclobutenes and allenes that
were previously difficult to access. We also showed that 1-alke-
nylcyclobutene is a good substrate for conversion into [3]den-
dralene and Diels–Alder cycloadducts. Mechanistic studies in
our laboratory are currently being focused on clarifying the
origin of the reactivities of alkenes in this reaction.
Acknowledgements
[11] Utilizing a cobalt(I) precatalyst, CoCl(PPh3)3, cyclobutene 3 was also ob-
tained in 16% yield without a reducing agent. This observation sug-
gested the involvement of cobalt(I) species in the catalytic cycle. For
details, see the Supporting Information.
This work was supported by a Grant-in-Aid for Scientists (A)
(No. 25708018) and a Grant-in-Aid for Scientific Research on In-
novative Area “Molecular Activation Directed toward Straight-
forward Synthesis” (No. 23105546) from MEXT, and the Asahi
Glass Foundation. A.N. expresses his special thanks for the re-
search fellowships for young scientists from the Japan Society
for the Promotion of Science (JSPS).
[12] a) P. Mçrschel, J. Janikowski, G. Hilt, G. Frenking, J. Am. Chem. Soc. 2008,
130, 8952; b) L. Fiebig, J. Kuttner, G. Hilt, M. C. Schwarzer, G. Frenking,
H.-G. Schmalz, M. Schꢄfer, J. Org. Chem. 2013, 78, 10485.
[13] B. Ma, J. K. Snyder, Organometallics 2002, 21, 4688.
[14] dppe=1,3-bis(diphenylphosphino)ethane;
rac-BINAP=rac-2,2’-bis(di-
phenylphosphino)-1,1’-binaphthyl; BIHEP=2,2’-bis(diphenylphospino)-
1,1’-biphenyl.
Keywords: 1,3-enyenes
·
alkenes
·
allenes
·
cobalt
·
[15] F. Pꢃnner, G. Hilt, Chem. Commun. 2012, 48, 3617.
[16] Gold-catalyzed [2+2] cycloaddition of unactivated highly substituted
alkenes with terminal alkynes has been reported, see: V. Lꢅpez-Carrillo,
A. M. Echavarren, J. Am. Chem. Soc. 2010, 132, 9292.
[17] For a review on dendralenes, see: H. Hopf, M. S. Sherburn, Angew.
Chem. 2012, 124, 2346; Angew. Chem. Int. Ed. 2012, 51, 2298.
[18] At 1008C, the ring opening of 7 competed to give a mixture of 27 and
28. For details, see the Supporting Information.
cyclobutene
[1] For reviews on cycloaddition, see: a) M. Lautens, W. Klute, W. Tam,
Chem. Rev. 1996, 96, 49; b) P. R. Chopade, J. Louie, Adv. Synth. Catal.
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[19] For examples of transition-metal-catalyzed 1,4-addition of 1,3-enyne,
see: a) T. Kusumoto, K. Ando, T. Hiyama, Bull. Chem. Soc. Jpn. 1992, 65,
1280; b) V. Gevorgyan, C. Kadowaki, M. M. Salter, I. Kadota, S. Saito, Y.
Yamamoto, Tetrahedron 1997, 53, 9097; c) J. W. Han, N. Tokunaga, T.
Hayashi, J. Am. Chem. Soc. 2001, 123, 12915; d) C. Lꢃken, C. Moberg,
Org. Lett. 2008, 10, 2505; e) T. Nishimura, H. Makino, M. Nagaosa, T. Hay-
ashi, J. Am. Chem. Soc. 2010, 132, 12865.
Received: February 17, 2014
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COMMUNICATION
&
Synthetic Methods
A. Nishimura, E. Tamai, M. Ohashi,
S. Ogoshi*
&& – &&
One catalyst, two possibilities: In the
presence of a cobalt/1,3-bis(diphenyl-
phosphino)propane (dppp) catalyst, the
[2+2] cycloaddition of 1,3-enynes and
alkenes with no allylic hydrogen atoms
(e.g. styrene) occurred, whereas aliphat-
ic alkenes underwent 1,4-hydroallylation
into 1,3-enynes (see scheme). From
simple substrates, these reactions can
provide cyclobutenes and allenes that
were previously difficult to access in
a highly chemo- and regioselective
manner.
Synthesis of Cyclobutenes and Allenes
by Cobalt-Catalyzed Cross-
Dimerization of Simple Alkenes with
1,3-Enynes
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