Synthesis of cross-conjugated trienes by rhodiumcatalyzed dimerization of monosubstituted allenes

Article abstract of DOI:10.3762/bjoc.7.67

A rhodium(I)/dppe catalyst promoted dimerization of monosubstituted allenes in a stereoselective manner to give cross-conjugated trienes, which are different from those obtained by a palladium catalyst.

Full text of DOI:10.3762/bjoc.7.67

Synthesis of cross-conjugated trienes by rhodium-
catalyzed dimerization of monosubstituted allenes
Tomoya Miura, Tsuneaki Biyajima, Takeharu Toyoshima
and Masahiro Murakami*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 578–581.
Department of Synthetic Chemistry and Biological Chemistry, Kyoto
University, Katsura, Kyoto 615-8510, Japan
doi:10.3762/bjoc.7.67
Received: 25 January 2011
Accepted: 30 March 2011
Published: 09 May 2011
Email:
Tomoya Miura - tmiura@sbchem.kyoto-u.ac.jp; Masahiro Murakami* -
murakami@sbchem.kyoto-u.ac.jp
This article is part of the Thematic Series "Allene chemistry".
Guest Editor: K. M. Brummond
* Corresponding author
Keywords:
allene; cross-conjugated triene; dimerization; rhodium; stereoselective
© 2011 Miura et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A rhodium(I)/dppe catalyst promoted dimerization of monosubstituted allenes in a stereoselective manner to give cross-conjugated
trienes, which are different from those obtained by a palladium catalyst.
Introduction
Cross-conjugated trienes, known as [3]dendralenes [1], are hand, a palladium-catalyzed dimerization reaction of monosub-
attractive synthetic precursors used for consecutive double stituted allenes produces substituted cross-conjugated trienes 2
[4 + 2] cycloaddition reactions [2-4] to provide a rapid access to in high yield (Scheme 1) [21]. We report here that dimerization
polycyclic carbon frameworks. Thus, a number of methods for of monosubstituted allenes is also catalyzed by a rhodium(I)/
the preparation of the parent 3-methylenepenta-1,4-diene [5] dppe complex to form cross-conjugated trienes 3, which are
and its substituted derivatives [6-17] has been developed. different from those obtained with the palladium catalyst.
Among these, transition-metal-catalyzed dimerization of allenes
presents a unique entry to substituted cross-conjugated trienes. Results and Discussion
For example, a nickel(0)/triphenylphosphine complex catalyzes We initiated our study using undeca-1,2-diene (1a) as the model
a dimerization reaction of 3-methylbuta-1,2-diene to afford 2,5- substrate and a rhodium(I) complex as the catalyst (Table 1).
dimethyl-3,4-bismethylenehex-1-ene [18,19]. The nickel- When 1a was treated with a catalytic amount of [RhCl(cod)]2
catalyzed reaction, however, leads to a complex mixture of (2.5 mol %, cod = cycloocta-1,5-diene) in toluene at 130 °C for
products when monosubstituted allenes such as penta-1,2-diene 12 h, 2a was formed in 40% NMR yield along with another
and 1-phenylpropa-1,2-diene are employed [20]. On the other minor dimerized product (13% NMR yield) and unidentified
578

Beilstein J. Org. Chem. 2011, 7, 578–581.
We propose that the dimerization reaction proceeds through the
pathway outlined in Scheme 2. Initially, two molecules of 1a
coordinate to a rhodium(I) center at the terminal carbon–carbon
double bonds from their sterically less-hindered sides. Oxida-
tive cyclization occurs in a head-to-head manner to form the
five-membered rhodacyclic intermediate A [22-25], which is in
equilibrium with another rhodacyclic intermediate B via σ–π–σ
isomerization. Then, β-hydride elimination takes place with B
to form rhodium hydride C stereoselectively. Finally, reductive
elimination from C yields 3a together with the catalytically
active rhodium(I) complex. It is also conceivable, however, that
oxidative cyclization of two molecules of 1a occurs in a tail-to-
Scheme 1: A new entry to substituted cross-conjugated trienes.
compounds (Table 1, entry 1). The structure of the minor dimer- tail manner to directly furnish B. The other isomer 2a could be
ized product was determined to be (E)-10,11-dimethyleneicos- formed through allylic 1,3-migration of rhodium from C and
8-ene (3a) by 1H and 13C NMR spectroscopy. Thus, the two subsequent reductive elimination.
isomeric dimers, one identical to the isomer obtained by the
palladium-catalyzed reaction and the other a different isomer,
were produced by the rhodium-catalyzed reaction. Next, several
phosphine ligands were examined (Table 1, entries 2–5). To our
delight, the use of the dppe ligand suppressed the formation of
2a and the unidentified compounds, and increased the NMR
yield of 3a to 96% (86% isolated yield, Table 1, entry 4). A
complex mixture of products was obtained when the reaction
temperature was lowered from 130 °C to 90 °C (Table 1,
entry 6). Moreover, the use of [Rh(OH)(cod)]2 and
Rh(acac)(cod) as the precatalyst resulted in a decrease of the
reaction rate (Table 1, entries 7 and 8).
Table 1: Optimization of reaction conditionsa.
Scheme 2: A proposed reaction pathway.
Yield of
2a (%)c
Yield of
3a (%)c
Under the optimized reaction conditions using dppe as the
ligand, various monosubstituted allenes 1b–j were subjected to
the catalytic dimerization reaction (Table 2). In most cases,
essentially one isomer 3 was formed, and the other isomer 2
was barely detectable in the 1H NMR spectrum of the crude
reaction mixture (<5%). Allenes 1b–i possessing a primary
alkyl group reacted well to afford the corresponding products
3b–i in yields ranging from 63% to 90% (Table 2, entries 1–8).
Functional groups such as benzyloxy, siloxy, hydroxy and
cyano groups were tolerated in the alkyl chain under the reac-
tion conditions. Cyclohexylpropa-1,2-diene (1j) possessing a
secondary alkyl group also participated in the dimerization reac-
tion (Table 2, entry 9). On the other hand, 1,1-disubstituted
allenes such as 3-methylbuta-1,2-diene and 3-pentylocta-1,2-
diene failed to undergo the dimerization reaction, in contrast to
the nickel-catalyzed reaction [18,19].
Entry
X
Ligandb
T (°C)
1
2
3
4
5
6
7
8
Cl
Cl
Cl
Cl
Cl
Cl
OH
none
PPh3d
dppm
dppe
dppp
dppe
dppe
130
130
130
130
130
90
40
24
24
<5
17
38
40
44
13
18
37
96 (86)
50
24
130
130
10
acace dppe
<5
aReactions conducted on a 0.4 mmol scale.
bdppm = 1,1-bis(diphenylphosphino)methane, dppe = 1,2-
bis(diphenylphosphino)ethane, dppp = 1,3-
bis(diphenylphosphino)propane.
cNMR yield using mesitylene as an internal standard. Isolated yield
given in parenthesis.
dUsing 10 mol % of PPh3.
eUsing 5.0 mol % of Rh(acac)(cod).
579

Beilstein J. Org. Chem. 2011, 7, 578–581.
and 5a’ were obtained in 75% and 6% yields, respectively. The
major bisadduct 5a resulted from initial addition to the more
congested diene moiety of 3a (site β). When tetracyanoethylene
(6, TCNE), which was a less reactive dienophile than 4, was
used, [4 + 2] cycloaddition also occurred preferentially at site β,
but only once on heating at 60 °C for 24 h.
Table 2: Synthesis of cross-conjugated trienes by the allene dimeriza-
tion reactiona.
Conclusion
In summary, we have developed a new dimerization reaction of
monosubstituted allenes catalyzed by a rhodium(I)/dppe com-
plex, allowing the stereoselective formation of substituted
cross-conjugated trienes. It is interesting that the rhodium cata-
lyst and the palladium catalyst gave different types of cross-
conjugated trienes.
Entry
1
R
R'
3
Yield
(%)b
1
2
3
4
5
6
7
8
9
1b C5H11
1c CH2Ph
1d Cy
H
H
H
H
H
H
H
H
3b
3c
3d
3e
3f
85
78c
83d
70
1e CH2OBn
1f
(CH2)3OBn
78
Experimental
General procedure for rhodium-catalyzed
dimerization of monosubstituted allenes
1g (CH2)3OSiMe2t-Bu
1h (CH2)3OH
3g
3h
3i
90
63
1i
1j
(CH2)3CN
–(CH2)5–
75
3j
60d
To a side-arm tube equipped with a stirrer bar, was added
[RhCl(cod)]2 (4.9 mg, 2.5 mol %) and dppe (7.7 mg, 5 mol %).
The tube was evacuated and refilled with argon three times.
Then, toluene (4 mL) and substrate 1 (0.4 mmol) were added
via syringe and the tube was closed. After heating at 130 °C for
6 h, the reaction mixture was cooled to room temperature,
passed through a pad of Florisil® and eluted with ethyl acetate
(≈ 90–100 mL). The filtrate was concentrated under reduced
pressure and the residue purified by preparative thin-layer chro-
matography to give product 3. Although the isolated 3 was rela-
tively labile, it could be kept at −30 °C for days without any
detectable decomposition or polymerization.
aReactions conducted on a 0.4 mmol scale.
bIsolated yield unless otherwise noted.
cThe product was accompanied by a small amount of an unidentified
impurity.
dNMR yield using mesitylene as an internal standard.
Next, we examined the consecutive double [4 + 2] cyclo-
addition reaction of the cross-conjugated trienes obtained in the
present study. Triene 3a was treated with 4-phenyl-1,2,4-triazo-
line-3,5-dione (4, PTAD), a highly reactive dienophile, in
toluene at 0 °C (Scheme 3). The conversion of 3a was complete
within 1 h, and after chromatographic isolation, bisadducts 5a
Scheme 3: [4 + 2] cycloaddition reaction of 3a with PTAD and TCNE.
580

Beilstein J. Org. Chem. 2011, 7, 578–581.
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Acknowledgements
This work was supported in part by MEXT (Grant-in-Aid for
Challenging Exploratory Research, No. 21656033). T. T. is
grateful for Research Fellowship from JSPS for Young Scien-
tists.
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