Palladium-catalyzed tandem coupling reaction of alkyne, conjugated diene, and triethylborane

Article abstract of DOI:10.3987/COM-12-S(N)70

Pd(0) catalyst promotes three-component coupling reactions of allylic alcohols and vinylcyclopropanes with terminal alkynes and triethylborane to provide (E)-1-substituted 2-ethyl-1,4-pentadienes and (E)-1-substituted 2-ethyl-1,4-heptadienes involving gem

Full text of DOI:10.3987/COM-12-S(N)70

HETEROCYCLES, Vol. 86, No. 1, 2012
171
HETEROCYCLES, Vol. 86, No. 1, 2012, pp. 171 - 180. © 2012 The Japan Institute of Heterocyclic Chemistry
Received, 28th June, 2012 Accepted, 3rd August, 2012, Published online, 8th August, 2012
DOI: 10.3987/COM-12-S(N)70
PALLADIUM-CATALYZED TANDEM COUPLING REACTION OF
ALKYNE, CONJUGATED DIENE, AND TRIETHYLBORANE
Daiki Takushima, Masahiro Fukushima, Hideaki Satomura, Gen Onodera,
and Masanari Kimura*
Graduate School of Engineering, Nagasaki University, 1-14 Bunkyo-machi,
Nagasaki, 852-8521, Japan. E-mail; masanari@nagasaki-u.ac.jp
Dedicated to Prof. Dr. Ei-ichi Negishi on the occasion of his 77th birthday.
Abstract – Pd(0) catalyst promotes three-component coupling reactions of allylic
alcohols and vinylcyclopropanes with terminal alkynes and triethylborane to
provide (E)-1-substituted 2-ethyl-1,4-pentadienes and
(E)-1-substituted
2-ethyl-1,4-heptadienes involving geminal ethylation and allylation at the
acetylenic terminal carbon atom with high regio- and stereoselectivities.
1,3-Butadiene and bis-dienes can act as allylating agents to undergo the
multi-component coupling reactions, accompanied by dimerization of the diene
moieties, to provide 1,4,9-decatrienes and pyrrolidines with excellent regio- and
stereoselectivities.
Multi-component coupling reactions are among the most efficient and useful strategies for construction of
complex molecules in synthetic chemistry.1 Specifically, allylic alkylations are convenient and effective
methods for transformation of allyl halides and allyl acetates to valuable unsaturated hydrocarbons and
fine chemicals. We have previously developed the direct activation of allyl alcohols, promoted by a
combination of a Pd catalyst and triethylborane, to form a !"allylpalladium intermediate.2 This species
serves as an allyl cation equivalent for a wide variety of soft nucleophiles, such as amines, polyphenols,
and active methylene compounds.3 Recently, we reported that indole serves as an effective nucleophile
toward C-allylic alkylation to give 3-allylindole and 3,3-diallylindolenine in excellent yields (Scheme 1).4
Furthermore, we extended the efficient tandem cyclization to afford pyrroloindole alkaloid frameworks
by means of allylic alkylation of tryptamine with 1,3-butadiene via bis-!-allylpalladium key
intermediates.

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HETEROCYCLES, Vol. 86, No. 1, 2012
Our methods for the Pd/triethylborane system are also useful for nucleophilic allylation toward various
kinds of electrophiles. For example, the !-allylpalladium species undergoes an allyl-ethyl exchange
reaction with triethylborane, providing allyl diethylborane as an allyl anion equivalent, which reacts with
electrophiles such as aldehydes, acetals, and aldimines to provide homoallyl alcohols and
homoallylamines.5 Thus, the Pd/triethylborane system works well for the generation of both allyl cation
and allyl anion equivalents directly from allylic alcohols to achieve amphiphilic allylic alkylations.6
We report herein that a similar reaction system using a Pd(0) catalyst and triethylborane promotes allylic
alcohols and vinylcyclopropanes to undergo coupling reactions with terminal alkynes to provide
(E)-1-substituted 2-ethyl-1,4-pentadienes and (E)-1-substituted 2-ethyl-1,4-heptadienes involving geminal
ethylation and allylation at the acetylenic terminal carbon atom with high regio- and stereoselectivities
(Scheme 2). It is noteworthy that triethylborane serves not only as an ethylating agent for the coupling
reaction but also as an activator for the oxidative addition of allylic alcohols toward Pd(0) catalyst to form
the !-allylpalladium intermediate. Furthermore, 1,3-butadiene and bis-dienes can act as allylating
agents accompanied by dimerization of the diene moieties to undergo the multi-component coupling
reactions, providing 1,4,9-decatrienes and pyrrolidines possessing dienyl and alkenyl side chains in
excellent regio- and stereoselectivities.
Pd catalyst
N
H
OH
Et3B
N
H
N
NH2
Pd catalyst
Et3B
NH
N
H
N
H
H
Scheme 1. Pd-Et3B Promoted Allylic Alkylation of Indole and Tryptamine
with Allyl Alcohol and Butadiene
E1
Pd catalyst
E1
HO
E2
R
R
E2
R
Et3B
Et
1
Et
2
R
Et
3
Scheme 2. Pd-Catalyzed Coupling Reaction of Terminal Alkyne, Allylating Agent, and Et3B

HETEROCYCLES, Vol. 86, No. 1, 2012
173
Pd(acac)2
R
PPh3
Ar
R
(1)
X
OH
Et3B, THF
Et
1
50 ˚C
Table 1. Pd-Catalyzed Coupling Reaction of Allylic Alcohol, Alkyne, and Et3Ba
run
1
X
H
allyl alcohol
HO
yield (%)
1a: 81
HO
Ph
2
3
4
5
6
7
8
9
1b: 84
1b: 70
1c: 72
1d: 50
1e: 65
1f: 75
1g: 71
1h: 84
H
H
HO
Ph
H
HO
HO
H
H
OH
HO
p-OMe
p-Me
p-F
Ph
HO
Ph
HO
Ph
a The reaction was undertaken in the presence of Pd(acac)2 (0.025 mmol),
PPh3 (0.05 mmol), alkyne (1 mmol), allylic alcohol (2 mmol), and Et3B (2.4
mmol) in dry THF (0.5 mL) at 50 ˚C for 72 h under nitrogen
atmosphere.
The results of the coupling reactions of various kinds of terminal alkynes, substituted allylic alcohols and
triethylborane are summarized in Table 1. Reactions of substituted phenylacetylenes with a wide
structural variety of substituted allylic alcohols and triethylborane were conducted in the presence of
Pd(acac)2 catalyst and PPh3 ligand at 50 ˚C under nitrogen atmosphere. Both "-phenylallyl alcohol and
cinnamyl alcohol underwent the coupling reactions at the less substituted allylic position to afford the
linear type product 1b as a single product with high regio- and stereoselectivities (runs 2 and 3, Table 1).
#-Methallyl alcohol and 2-cyclohexen-1-ol participated in the coupling reaction in a similar manner (runs
4 and 5, Table 1). Divinyl carbinol serves as a 2,4-pentadienyl synthetic equivalent to provide

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HETEROCYCLES, Vol. 86, No. 1, 2012
1,3,6-heptatriene 1e as a single isomer in reasonable yield (run 6, Table 1). Pd-catalyzed
three-component coupling reactions of "-phenylallyl alcohol with several kinds of substituted
phenylacetylenes were also conducted. Phenylacetylenes substituted at the para positions with electron
donating and fluorine groups participated in the coupling reactions without the deterioration of the
reactivity and the yield (runs 7-9, Table 1).
Pd(acac)2
E1
PPh3
E1
E2
R
(2)
R
E2
Et3B, Et3N
THF, rt
Et
2
Table 2. Pd-Catalyzed Coupling Reaction of Vinylcyclopropane, Alkyne, and Et3Ba
E1
E2
R
Ph
yield (%)
2a: 74
2b: 71
2c: 55
2d: 51
2e: 60
2f: 40
run
1
CO2Me
CO2Me
CO2Me
CO2Me
CO2Et
COMe
CO2Me
CO2Me
CO2Me
CO2Me
COMe
COMe
2
(p-F)Ph
n-Bu
3
4
BzO(CH2)2-
Ph
5
6
Ph
a The reaction was undertaken in the presence of Pd(acac)2 (0.025 mmol),
PPh3 (0.05 mmol), alkyne (1 mmol), cyclopropane (1 mmol), Et3B (3
mmol), Et3N (1 mmol) in dry THF (0.5 mL) at room temperature for 72 h
under nitrogen atmosphere.
Next, we examined similar coupling reactions of vinylcyclopropanes, which were prepared from dimethyl
malonate, #-ketoester, and acetylacetone with 1,4-dichloro-2-butene, in the presence of terminal alkynes,
triethylborane, and Pd(0) catalyst at room temperature (eq 2, Table 2).7 In all cases, triethylamine was
indispensable to achieve the coupling reactions. In the absence of triethylamine, the desired reaction did
not proceed at all and the vinylcyclopropanes were recovered almost quantitatively. Aromatic and
aliphatic substituted terminal alkynes were also useful for the coupling reaction with vinylcyclopropane
derived from dimethyl malonate in reasonable to good yields (runs 1-4, Table 2). Vinylcyclopropanes
with #-ketoester and #-diketone skeletons reacted with phenylacetylene to give the corresponding
coupling products 2e and 2f, respectively (runs 5 and 6, Table 2). All of these olefin geometries with
respect to the main chain were assigned as E-stereoselectivities.

HETEROCYCLES, Vol. 86, No. 1, 2012
175
Pd(acac)2
PPh3
R
R
(3)
Et3B, Et3N
THF, rt
Et
3
Table 3. Pd-Catalyzed Coupling Reaction of Butadiene, Alkyne, and Et3Ba
R
yield (%)
run
1
3a: 74
Ph
2
3
4
3b: 71
3c: 60
3d: 56
(p-Me)Ph
(p-F)Ph
n-Hept
a The reaction was undertaken in the presence of Pd(acac)2 (0.025 mmol),
PPh3 (0.05 mmol), alkyne (1 mmol), butadiene (2 mmol), Et3B (2.4 mmol),
Et3N (3 mmol) in dry THF (0.5 mL) at room temperature for 72 h under
nitrogen atmosphere.
1,3-Butadiene undergoes dimerization to serve as a bis-!-allylpalladium complex under the Pd(0)
catalytic system.8 The results of the coupling reaction with aromatic and aliphatic terminal alkynes,
1,3-butadiene, and triethylborane in the presence of Pd catalyst are presented in Table 3. Triethylamine
was required to complete the multi-component coupling reaction. Irrespective of the kinds of terminal
alkynes, the reaction proceeded smoothly at room temperature to give (1E,4E)-1,4,9-decatrienes 3 as a
single isomer.
A plausible reaction mechanism for the multi-component coupling reaction with an alkyne, triethylborane,
and 1,3-butadiene is proposed in Scheme 3. Pd(0) catalyst promotes the dimerization of butadiene to
afford !1,!3-bis-allylpalladium intermediates I and II serving as Lewis acids to increase the acidity of the
coordinated terminal alkynes.9 The abstraction of the activated acetylenic terminal proton by
triethylamine proceeds to generate !-allylpalladium alkynylborate intermediate IV, which would undergo
[1,2] ethyl group transfer from the B atom to the acetylide carbon atom to form !-allylpalladium
vinyldiethylborate complex V with excellent regio- and stereoselectivities. Allylvinylpalladium VI
would be formed through the transmetallation of complex V, and decatriene 3 would be formed with the
liberation of Pd(0) catalyst via reductive elimination.

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HETEROCYCLES, Vol. 86, No. 1, 2012
R
2 X
3
Et
Pd0
Pd
R
Pd0
Et
VII
Pd
Et
H
R
Pd
L
Pd
L
VI
II
I
R
BEt2
Et
R
Et3N
R
H
R
Pd
H
BEt2
Pd
V
Et3N
III
R
R
Pd
H
BEt3
Et3B
R
IV
Scheme 3. Plausible Reaction Mechanism for Tandem Coulping Reaction
A series of these reaction systems were applied to the coupling reaction of a bis-diene tethered by
tosylamide for the efficient synthesis of pyrrolidines via tandem cyclizations. The coupling reactions of
the bis-diene, various kinds of terminal alkynes, and triethylborane were carried out in the presence of Pd
catalyst in THF at room temperature (eq 4). Regardless of the aromatic and aliphatic substituents on the
terminal alkynes, the pyrrolidine frameworks were constructed successfully (Table 4). It is also
significant that triethylamine accelerated the multi-component coupling reaction involving tandem
cyclization of the diene moiety giving rise to trans-3-allyl-4-pentadienyl pyrrolidine 4 in a 7:1 ratio with
exclusive trans selectivities (entries 2 and 4, Table 4). This method might be one of the most useful and
convenient strategies for the highly efficient regio- and stereocontrolled synthesis of heterocyclic
compounds.

HETEROCYCLES, Vol. 86, No. 1, 2012
177
Pd(acac)2
PPh3
R
TsN
R
TsN
(4)
Et
Et3B, Et3N
THF, rt
4
Table 4. Pd-Catalyzed Tandem Reaction of Bis-diene, Alkyne, and Et3Ba
yield (%) [ratio]
4a: 86 [7:1]
run
1
R
(p-Me)Ph
2
(p-F)Ph
(p-Br)Ph
n-Propyl
n-Bu
4b: 86 [7:1]
3
4c: 41 [single]
4d: 50 [single]
4e: 41 [single]
4f: 63 [single]
4g: 70 [single]
4
5
6
c-Propyl
(CH2)2OTHP
7
a The reaction was undertaken in the presence of Pd(acac)2 (0.025 mmol),
PPh3 (0.05 mmol), alkyne (1 mmol), bis-diene (0.5 mmol), Et3B (2.4 mmol),
Et3N (3 mmol) in dry THF (0.5 mL) at room temperature for 72 h under
nitrogen atmosphere.
In summary, Pd(0) catalyst promotes three-component coupling reactions of allylic alcohols and
vinylcyclopropanes with terminal alkynes and triethylborane to provide (E)-1-substituted
2-ethyl-1,4-pentadienes and (E)-1-substituted 2-ethyl-1,4-heptadienes involving geminal ethylation and
allylation at the acetylenic terminal carbon atom with high regio- and stereoselectivities. Under similar
conditions, Pd-catalyzed three-component coupling reactions of 1,3-butadiene, terminal alkynes, and
triethylborane involving dimerization of butadiene gave (1E,4E)-deca-1,4,9-triene with excellent regio-
and stereoselectivities. Furthermore, a bis-diene undergoes the tandem coupling reaction accompanied
by intramolecular dimerization of the diene moiety via bis-!$allylpalladium to afford
trans-3-allyl-4-pentadienylpyrrolidiene with excellent regio- and stereoselectivities. Applications of the
multi-component coupling reaction for the efficient synthesis of physiologically active molecules such as
prostanoids are being pursued in our laboratory.
ACKNOWLEDGEMENTS
Financial support from the Ministry of Education, Culture, Sports, Science, and Technology, Japanese
Government (Scientific Research on Innovative Areas, “Molecular Activation Directed toward
Straightforward Synthesis”), is gratefully acknowledged.

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REFERENCES AND NOTES
1. E.-i. Negishi and J. A. Miller, J. Am. Chem. Soc., 1983, 105, 6761; S. Bräse and A. D. Meijere,
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5803.
7. General procedure for the Pd-catalyzed multi-component coupling reaction (run 1, Table 2): A 25
mL of two-necked round-bottomed flask, equipped with a rubber septum and an air condenser at the
top of which is attached a three-way stopcock fitted a nitrogen balloon, is charged with Pd(acac)2
(7.6 mg, 0.025 mmol) and PPh3 (13.1 mg, 0.05 mmol). The apparatus is purged with nitrogen and
the flask is charged with freshly distilled THF (0.5 mL). Into this solution, phenylacetylene (102
mg, 1 mmol), vinylcyclopropane (184 mg, 1 mmol), triethylamine (140 µL, 1 mmol), and Et3B (3.0
mL of 1 M hexane solution; 3.0 mmol) are successively added while stirring the solution with a
magnetic stirrer. The stirring is continued for 72 h at room temperature. After the reaction
completes, the reaction mixture is diluted with ethyl acetate (20 mL). The organic phase is washed
with sat. NaHCO3 (2 x 20 mL) and brine (2 x 20 mL), and then dried over magnesium sulfate,

HETEROCYCLES, Vol. 86, No. 1, 2012
179
filtered, and concentrated. The organic phase was dried (MgSO4) and concentrated in vacuo to give
a brown oil, which was subjected to column chromatography over silica gel (hexane/EtOAc = 20/1,
v/v) to give 2a (234 mg, 74%, Rf = 0.51; hexane/EtOAc = 4/1, v/v).
Dimethyl 2-[(2E,5E)-5-benzylidenehept-2-enyl]malonate (2a): IR (neat) 3022 (w), 2962 (s), 2875
1
(w), 1736 (s), 1647 (w), 1599 (w), 1435 (s), 1197 (s), 1153 (s), 1028 (m), 920 (w), 700 (s) cm-1; H
NMR (CDCl3, 400 MHz) ! 1.05 (t, J = 7.6 Hz, 3 H), 2.21 (q, J = 7.6 Hz, 2 H), 2.65 (t, J = 7.0 Hz, 2
H), 2.84 (d, J = 6.8 Hz, 2 H), 3.46 (t, J = 7.0 Hz, 1 H), 3.72 (s, 6 H), 5.48 (dt, J = 15.4, 6.8 Hz, 1 H),
13
5.60 (dt, J = 15.4, 7.0 Hz, 1 H), 6.20 (s, 1 H), 7.15-7.31 (m, 5 H); C NMR (CDCl3, 100 MHz) !
12.8, 23.8, 31.8, 39.7, 51.8, 52.3, 125.1, 125.7, 126.9, 127.7, 128.2, 131.3, 138.0, 142.9, 168.9.
High-resolution MS, calcd for C19H24O4: 316.1675. Found m/z (relative intensity): 316.1678 (M+,
100), 317 (24), 285 (11).
1-[(1E,4E)-2-Ethyldeca-1,4,9-trienyl]benzene (3a): IR (neat) 3022 (w), 2966 (s), 2930 (s), 2856 (s),
1641 (m), 1599 (m), 1493 (s), 1443 (m), 970 (s), 912 (s), 698 (s) cm-1; 1H NMR (CDCl3, 400 MHz) !
1.07 (t, J = 7.6 Hz, 3 H), 1.49 (quint, J = 7.4 Hz, 2 H), 2.04–2.09 (m, 4 H), 2.25 (q, J = 7.6 Hz, 2 H),
2.85 (d, J = 5.5 Hz, 2 H), 4.94 (d, J = 10.2 Hz, 1 H), 5.00 (d, J = 17.1 Hz, 1 H), 5.42–5.56 (m, 2 H),
13
5.81 (ddt, J = 17.1, 10.2, 6.7 Hz, 1 H), 6.24 (s, 1 H), 7.12-7.31 (m, 5 H); C NMR (CDCl3, 100
MHz) ! 12.9, 23.8, 28.8, 32.0, 33.2, 40.1, 114.3, 124.9, 125.8, 127.9, 128.1, 132.0, 138.4, 143.8.
High-resolution MS, calcd for C18H24: 240.1878, Found m/z (relative intensity): 240.1867 (M+, 100),
211 (73), 241 (45).
(3R*,4R*)-3-Allyl-4-[(1E,4E)-4-ethyl-7-(tetrahydro-2H-pyran-2-yloxy)hepta-1,4-dienyl]-1-tosyl-
pyrrolidine (4g): IR (neat) 3076 (w), 2939 (s), 2872 (s), 2359 (s), 2341 (s), 1641 (s), 1599 (s), 1441
(m), 1346 (s), 1163 (s), 1032 (s), 814 (m), 667 (s) cm-1; 1H NMR (CDCl3, 400 MHz) ! 0.93 (t, J = 7.6
Hz, 3 H), 1.49-1.57 (m, 6 H), 1.79-1.85 (m, 2 H), 1.99 (q, J = 7.6 Hz, 2 H), 2.22 (br dd, J = 9.6, 6.5
Hz, 1 H), 2.24 (t, J = 8.5 Hz, 1 H), 2.30 (q, J = 7.1 Hz, 2 H), 2.44 (s, 3 H), 2.67 (d, J = 6.8 Hz, 2 H),
2.88 (dd, J = 9.8, 8.5 Hz, 1 H), 2.94 (t, J = 9.8 Hz, 1 H), 3.36 (t, J = 9.6 Hz, 2 H), 3.47-3.51 (m, 2 H),
3.68 (dt, J = 16.8, 7.3 Hz, 1 H), 3.85 (td, J = 15.1, 7.3 Hz, 1 H), 4.58 (t, J = 6.8 Hz, 1 H), 4.95 (d, J =
10.5 Hz, 1 H), 4.98 (d, J = 17.1 Hz, 1 H), 5.09 (dd, J = 15.2, 8.5 Hz, 1 H), 5.13 (t, J = 7.1 Hz, 1 H),
5.40 (dt, J = 15.2, 6.8 Hz, 1 H), 5.62 (ddd, J = 17.1, 10.5, 7.8 Hz, 1 H), 7.31 (d, J = 8.2 Hz, 2 H), 7.70
13
(d, J = 8.2 Hz, 2 H); C NMR (CDCl3, 100 MHz) ! 13.0, 19.6, 21.5, 23.2, 25.5, 28.4, 30.7, 35.5,
39.7, 44.0, 47.5, 52.6, 53.0, 62.3, 67.2, 98.7, 116.5, 121.0, 127.4, 129.5, 131.9, 134.1, 135.4, 141.2,
143.1. High-resolution MS, calcd for C28H41NO4S: 487.2756. Found m/z (relative intensity):
487.2759 (M+, 100).
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Compounds. In Handbook of Organopalladium Chemistry for Organic Synthesis; ed. by E.-i.
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9. A. Krotz, F. Vollmüller, G. Stark, and M. Beller, Chem. Commun., 2001, 195; M. Kimura, K.
Toyama, Y. Yamaguchi, and T. Kohno, Heterocycles, 2010, 80, 787.