Rhodium-catalyzed regio- and stereoselective codimerization of alkenes and electron-deficient internal alkynes leading to 1,3-dienes

Article abstract of DOI:10.1021/ol800995r

(Chemical Equation Presented) A cationic rhodium(I)/H₈-BINAP complex catalyzes codimerization of alkenes bearing no r-hydrogen and electron-deficient internal alkynes, leading to 1,3-dienes in good yields with moderate to excellent regio- and stereoselectivity. The same complex also catalyzes codimerization of an acrylate and phenyl-substituted electron-rich internal alkynes, leading to 1,3-dienes.

Full text of DOI:10.1021/ol800995r

ORGANIC
LETTERS
2008
Vol. 10, No. 13
2829-2831
Rhodium-Catalyzed Regio- and
Stereoselective Codimerization of
Alkenes and Electron-Deficient Internal
Alkynes Leading to 1,3-Dienes
Yu Shibata, Masao Hirano, and Ken Tanaka*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo UniVersity
of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
tanaka-k@cc.tuat.ac.jp
Received April 30, 2008
ABSTRACT
A cationic rhodium(I)/H8-BINAP complex catalyzes codimerization of alkenes bearing no r-hydrogen and electron-deficient internal alkynes,
leading to 1,3-dienes in good yields with moderate to excellent regio- and stereoselectivity. The same complex also catalyzes codimerization
of an acrylate and phenyl-substituted electron-rich internal alkynes, leading to 1,3-dienes.
Transition-metal-catalyzed intermolecular codimerization of
alkenes and alkynes is an attractive method for the synthesis
of substituted dienes in a highly atom-economical fashion.1,2
For example, Trost and co-workers have extensively studied
the ruthenium-catalyzed intermolecular Alder-ene reaction
for the synthesis of substituted 1,4-dienes.3,4 Recently, Hilt
and Treutwein reported a cobalt-catalyzed variant of this
reaction.5 The mechanism for the transition-metal-catalyzed
intermolecular Alder-ene reactions is proposed as shown
in Scheme 1. A transition-metal complex reacts with an
alkene and an alkyne to generate metallacyclopentene A.
ꢀ-Hydride elimination to vinylmetal hydride B followed by
reductive elimination furnishes a 1,4-diene.
On the other hand, transition-metal-catalyzed intermo-
lecular codimerization of alkenes bearing no R-hydrogen and
alkynes giving substituted 1,3-dienes is scarce.6–8 A possible
(1) For reviews, see: (a) Aubert, C.; Buisine, O.; Malacria, M. Chem.
ReV. 2002, 102, 813. (b) Trost, B. M.; Toste, F. D.; Pinkerton, A. B Chem.
ReV. 2001, 101, 2067. (c) Grotjahn, D. B. In ComprehensiVe Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Hegedus, L.,
Eds.; Pergamon Press: Oxford, 1995; Vol. 12, p 741. (d) Schore, N. E.
Scheme 1
Chem. ReV. 1988, 88, 1081
.
(2) For a review of ruthenium-catalyzed alkene-alkyne coupling
reactions, see: Trost, B. M.; Frederiksen, M. U.; Rudd, M. T. Angew. Chem.,
Int. Ed. 2005, 44, 6630
.
(3) (a) Trost, B. M.; Indolese, A. J. Am. Chem. Soc. 1993, 115, 4361.
(b) Trost, B. M.; Indolese, A. F.; Mu¨ller, T. J. J.; Treptow, B. J. Am. Chem.
Soc. 1995, 117, 615. (c) Trost, B. M; Mu¨ller, T. J. J.; Martinez, J. J. Am.
Chem. Soc. 1995, 117, 1888. (d) Trost, B. M.; Machacek, M.; Schnaderbeck,
M. J. Org. Lett. 2000, 2, 1761. (e) Trost, B. M.; Shen, H. C.; Pinkerton,
A. B. Chem.sEur. J. 2002, 10, 2341. (f) Trost, B. M.; Machacek, M. R.
Angew. Chem., Int. Ed. 2002, 41, 4693. (g) Trost, B. M.; Machacek, M. R.;
Ball, Z. T. Org. Lett. 2003, 5, 1895
(4) For a ruthenium-catalyzed Alder-ene reaction of di- and triynes,
see: Hansen, E. C.; Lee, D. J. Am. Chem. Soc. 2005, 127, 3252
.
.
10.1021/ol800995r CCC: $40.75
Published on Web 06/11/2008
2008 American Chemical Society

mechanism for the formation of 1,3-dienes is shown in
Scheme 2. ꢀ-Elimination of the methylene or methine proton
cationic rhodium(I)/H8-BINAP complex-catalyzed regio-
and stereoselective codimerization of alkenes bearing no
R-hydrogen and internal alkynes including electron-deficient
ones giving substituted 1,3-dienes.
The reaction of n-butyl acrylate (1a, 1.1 equiv) with ethyl
2-butynoate (2a) was first examined in the presence of 5
mol % of the cationic rhodium(I)/H8-BINAP complex as a
catalyst, but no reaction was observed at room temperature
(Table 1, entry 1). Fortunately, 1,3-diene 3aa was obtained
Scheme 2
Table 1. Screening of Rh Catalysts for Codimerization of
Acrylate 1a and Electron-Deficient Internal Alkyne 2aa
in metallacyclopentene C or D to dienylmetal hydride E or
F followed by reductive elimination would furnish a 1,3-
diene.9 However, ꢀ-hydride elimination from a cyclic
intermediate is difficult due to the geometrical constraints,
which might deter the formation of a 1,3-diene. Mitsudo,
Watanabe, and co-workers reported one notable success in
achieving such a reaction. They developed the ruthenium-
catalyzed intermolecular codimerization of acrylic acid
derivatives and electron-rich internal alkynes and proposed
ꢀ-elimination of the methylene proton in a ruthenacyclo-
pentene intermediate.6 Murakami, Ito, and co-worker also
realized the ruthenium-catalyzed 1,3-diene formation from
alkenes and electron-rich terminal alkynes by a mechanisti-
cally different approach and proposed the formation of a
vinylidene complex as a key intermediate.7 However, electron-
deficient internal alkynes may not be included in these
ruthenium-catalyzed codimerization reactions.
entry
catalyst
conditions
yield (%)b
1
2
3
4
5
6
7
8c
[Rh(cod)2]BF4/H8-BINAP
[Rh(cod)2]BF4/H8-BINAP
[Rh(cod)2]BF4/Segphos
[Rh(cod)2]BF4/BINAP
[Rh(cod)2]BF4/dppf
[Rh(cod)2]BF4/dppb
[RhCl(cod)]2/2H8-BINAP
[Rh(cod)2]BF4/H8-BINAP
rt, 16 h
0
50
35
9
0
0
80 °C, 3 h
80 °C, 3 h
80 °C, 3 h
80 °C, 3 h
80 °C, 3 h
80 °C, 3 h
80 °C, 8 h
0
71
a Catalyst (0.010 mmol), 1a (0.22 mmol), 2a (0.20 mmol), and CH2Cl2
or (CH2Cl)2 (2.0 mL) were used. b Isolated yield. c 1a: 2 equiv.
Our research group has demonstrated that a cationic
rhodium(I)/H8-BINAP complex is a highly effective catalyst
for chemo- and regioselective cotrimerization of two terminal
alkynes and one electron-deficient internal alkyne leading
to tetrasubstituted benzenes.10,11 This result prompted our
investigation into reactions of alkenes and electron-deficient
internal alkynes in the presence of a cationic rhodium(I)/
BINAP-type bisphosphine complex. Herein, we describe a
in 50% yield with excellent stereoselectivity at elevated
temperature (80 °C, entry 2). Thus, various BINAP-type
bisphosphine ligands were screened (entries 2-4), which
revealed that the use of H8-BINAP furnished 3aa in the
highest yield (entry 2) in accordance with the cotrimerization
of alkynes.10 A cationic rhodium(I) complex and the BINAP-
type bisphosphine ligand are essential to promote this
reaction. The use of a cationic rhodium(I)/dppf or dppb
complex and a neutral rhodium(I)/H8-BINAP complex failed
to catalyze this reaction (entries 5-7). Further improved yield
of 3aa was achieved by using 2 equiv of 1a, although the
reaction time required for completion was prolonged (entry
8).
Next, the scope of this reaction was examined with respect
to both acrylates and internal alkynes as shown in Table 2.
With respect to internal alkynes, not only ethyl 2-butynoate
(2a, entry 1) but also ethyl phenylpropiolate (2b, entry 2)
reacted with 1a to give the corresponding 1,3-diene 3ab in
high yield with excellent regio- and stereoselectivity. Steri-
cally demanding 2-methoxynaphthyl-substituted propiolate
(5) Hilt, G.; Treutwein, J. Angew. Chem., Int. Ed. 2007, 46, 8500.
(6) Mitsudo, T.; Zhang, S.-W.; Nagao, M.; Watanabe, Y. Chem.
Commun. 1991, 598.
(7) Murakami, M.; Ubukata, M.; Ito, Y. Tetrahedron Lett. 1998, 39,
7361.
(8) Recently, ruthenium-catalyzed co-oligomerization of N-vinylamides
with alkenes or alkynes (electron-rich internal alkynes) was reported, and
the authors proposed the formation of a ruthenium hydride species through
activation of sp2 C-H bonds in alkenes or a dmfm ligand, see: Tsujita, H.;
Ura, Y.; Matsuki, S.; Wada, K.; Mitsudo, T.; Kondo, T. Angew. Chem.,
Int. Ed. 2007, 46, 5160.
(9) The formation of 1,3-dienes from 1,6-enynes presumably through
ꢀ-elimination of the methine proton in the metallacyclopentene is a well-
known side reaction in the rhodium-catalyzed Pauson-Khand-type reactions,
see: Schmid, T. M.; Consiglio, G. Chem. Commun. 2004, 2318.
(10) (a) Tanaka, K.; Shirasaka, K. Org. Lett. 2003, 5, 4697. (b) Tanaka,
K.; Toyoda, K.; Wada, A.; Shirasaka, K.; Hirano, M. Chem.sEur. J. 2005,
11, 1145
.
(11) For our accounts of the cationic rhodium(I)/BINAP-type bispho-
sphine complex-catalyzed [2 + 2 + 2] cycloaddition reactions, see: (a)
Tanaka, K. Synlett 2007, 1977. (b) Tanaka, K.; Nishida, G.; Suda, T. J.
Synth. Org. Chem. Jpn. 2007, 65, 862
.
2830
Org. Lett., Vol. 10, No. 13, 2008

2a, giving two regioisomers in good yield (entry 10), while
3,3-dimethyl-1-butene (1g) failed to react with 2a and 2b
(entry 11).12 The reaction of 1-octene (1h) bearing R-hy-
drogen with 2a and 2b was also examined, but both
intermolecular Alder-ene reaction leading to a 1,4-diene and
linear codimerization leading to a 1,3-diene did not proceed
at all (entry 12).12
Table 2. Rh(I)+/H8-BINAP-Catalyzed Codimerization of
Alkenes 1a-h and Electron-Deficient Internal Alkynes 2a-da
Finally, the reactions of acrylate 1a with electron-rich
internal alkynes were also examined. Although high catalyst
loading and prolonged reaction time were required, the
reaction of 1a with diphenylacetylene (2e) proceeded in good
yield with excellent stereoselectivity (Scheme 4). Unfortu-
% yieldb (2E/2Z)
entry
1 (R1, equiv)
2 (R2, CO2R3)
3
4
c
1
1a (CO2n-Bu, 2.0) 2a (Me, CO2Et)
1a (CO2n-Bu, 1.1) 2b (Ph, CO2Et)
71 (2E)
87 (2Z)
85 (2Z)
91 (2Z)
94 (2Z)
-
2
Scheme 4
3
1b (CO2Me, 1.1)
1c (CO2Cy, 1.1)
2b (Ph, CO2Et)
2b (Ph, CO2Et)
4
5d
6
1a (CO2n-Bu, 1.1) 2c (Are, CO2Et)
1a (CO2n-Bu, 5.0) 2d (CO2Me, CO2Me) 76 (1:3)
7
1e (Ph, 2.0)
1e (Ph, 2.0)
1e (Ph, 2.0)
1f (SiMe3, 5.0)
1g (t-Bu, 5.0)
2a (Me, CO2Et)
2b (Ph, CO2Et)
52 (2E)f 19 (-g)f
c
8
50 (2Z)
-
9
2d (CO2Me, CO2Me) 81 (1:2)
10
11
12
2a (Me, CO2Et)
2a or 2b
1h (n-C6H13, 5.0) 2a or 2b
44 (2E)f 21 (-g)f
0 (-)h
0 (-)h
a [Rh(cod)2]BF4 (0.015 mmol), H8-BINAP (0.015 mmol), 1 (0.33-1.50
mmol), 2 (0.30 mmol), and (CH2Cl)2 (1.5 mL) were used. b Isolated yield.
c A separable unidentified mixture of olefins was generated as byproduct.
d Catalyst: 20 mol %. e Ar ) 2-MeO-1-naphthyl. f Isolated as a mixture of
3 and 4. g Not determined. h No reaction was observed.
nately, 4-octyne (2f) failed to react with 1a (Scheme 4).
Unsymmetrical alkyne 2g bearing phenyl and methyl at each
alkyne terminus reacted with 1a in moderate yield, although
two regioisomeric 1,3-dienes were obtained (Scheme 5).
(2c, entry 5) could also participate in this reaction, although
high catalyst loading was required. The reaction of 1a with
dimethyl acetylenedicarboxylate (2d, entry 6) proceeded in
good yield, while moderate stereoselectivity was observed.
With respect to acrylates, not only n-butyl acrylate (1a, entry
2) but also methyl (1b, entry 3) and cyclohexyl (1c, entry
4) acrylates reacted with 2b to give the corresponding 1,3-
dienes in high yields with excellent regio- and stereoselec-
tivity. Interestingly, the reaction of tert-butyl acrylate (1d)
with 2b furnished alkenyl acrylate 5 in good yield, presum-
ably through formation of acrylic acid followed by its
addition to 2b (Scheme 3). The use of electron-rich alkenes
Scheme 5
In conclusion, we have demonstrated that a cationic
rhodium(I)/H8-BINAP complex is an effective catalyst for
the regio- and stereoselective codimerization of alkenes
bearing no R-hydrogen (acrylates, styrene, and vinyltrim-
ethylsilane) and internal alkynes, including electron-deficient
ones, leading to 1,3-dienes. Future studies will focus on
expanding the scope and elucidation of the precise mecha-
nism.
Scheme 3
Acknowledgment. This work was supported partly by a
Grant-in-Aid for Scientific Research [Nos. 20675002 and
19028015 (Chemistry of Concerto Catalysis)] from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan. We thank Takasago International Corp. for the
gift of H8-BINAP and Segphos.
Supporting Information Available: Experimental pro-
cedures and compound characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
bearing no R-hydrogen was also examined.12 The reactions
of styrene (1e) with 2a, 2b, and 2d proceeded in good yields
(entries 7-9), while moderate regioselectivity was observed
(entries 7 and 8). Vinyltrimethylsilane (1f) also reacted with
(12) Although no reaction was observed in entries 11 and 12, the
reactions of 1g and 1h with dimethyl acetylenedicarboxylate (2d) at room
temperature furnished cotrimerization products. The structures of monoenes
and monoynes determine the selectivity between codimerization vs cotri-
merization; see: Shibata, Y.; Noguchi, K.; Hirano M.; Tanaka, K. Org. Lett.
2008, 10, 2825
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