Reductive dimerization of dialkyl acetylenedicarboxylate catalyzed by [Rh(binap)(MeOH)2]ClO4 in methanol

Article abstract of DOI:10.1016/S0022-328X(98)00507-5

Cationic Rh(I) complex [Rh(binap)(MeOH)₂]ClO₄ catalyzes reductive dimerization of dialkyl acetylenedicarboxylates 1 to give 1,2,3,4-tetrakis(alkoxycarbonyl)-1,3-butadienes 2 in methanol selectively.

Full text of DOI:10.1016/S0022-328X(98)00507-5

Journal of Organometallic Chemistry 560 (1998) 253–255
Reductive dimerization of dialkyl acetylenedicarboxylate catalyzed by
[Rh(binap)(MeOH)2]ClO4 in methanol
Kazuhide Tani *, Kazuya Ueda, Kenji Arimitsu, Tsuneaki Yamagata, Yasutaka Kataoka
Department of Chemistry, Graduate School of Engineering Science, Osaka Uni6ersity, Toyonaka, Osaka, 560-8531, Japan
Received 16 January 1998; received in revised form 10 February 1998
Abstract
Cationic Rh(I) complex [Rh(binap)(MeOH)2]ClO4 catalyzes reductive dimerization of dialkyl acetylenedicarboxylates 1 to give
1,2,3,4-tetrakis(alkoxycarbonyl)-1,3-butadienes 2 in methanol selectively. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Dimerization; Cationic; Rhodium
It has been well known that oligomerization of
acetylene such as cyclotrimerization leading to benzene
derivatives or linear dimerization of terminal alkynes
leading to ene–yne derivatives is catalyzed by a variety
of metal complexes e.g., cobalt, rhodium, zirconium,
tantalum [1]. Catalytic hydrodimerization of acetylene
leading to 1,3-diene, however, has not so far been
reported to the best of our knowledge; only several
examples of stoichiometric preparation of conjugated-
dienes from acetylenes are known, i.e., formation of
1,3-diene via hydrocupration of terminal acetylenes
([2]a) and the formation of 1,3-diene as a by-product
during the reduction of dimethyl acetylenedicarboxylate
1a with copper(II)/hydrosilanes ([2]b) or sodium hydro-
gen telluride ([2]c), during the reduction of terminal
acetylenes with a cobalt hydride ([2]d), and in the
reaction of 1a with a ruthenium compound ([2]e). Selec-
tive catalytic preparation of ‘open-chain’ conjugated
dienes directly from acetylenes are also very rare. Only
two examples of synthesis of 1,4-dialkyl conjugated
1,3-dienes catalyzed by palladium complexes using te-
tramethyltin or tetramethyltin and alkyl halides as alky-
lating agents have recently been reported [3], though
such direct synthesis of conjugated dienes from alkynes
is highly desirable for forming valuable synthetic inter-
mediates [4]. Herein we report on highly selective hy-
drodimerization of dialkyl acetylenedicarboxylates 1
leading to ‘open-chain’ 1,3-dienes 2 catalyzed by
cationic rhodium(I) diphosphine complexes.
Treatment of diethyl acetylenedicarboxylate 1b in the
presence of a catalytic amount (4 mol%) of [Rh(bi-
nap)(MeOH)2ClO4 [5] (binap=2,2%-bis(diphenylphos-
phino)-1,1%-binaphthyl) under reflux in methanol gave
the hydrodimer, 1,2,3,4-tetrakis(ethoxycarbonyl)-1,3-
butadiene 2b, in 91% yield after 3 h (Eq. 1, Table 1).
The hydrodimer 2b was comprised of the Z,Z- [6,7]
and the E,E-isomer [7] in a ca. 80:20 ratio accompanied
by a small amount of the E,Z-isomer and the cyclic
trimer, 1,2,3,4,5,6-hexakis(ethoxycarbonyl)benzene 3.
Prolonged heating caused isomerization of the hy-
drodimer from the Z,Z-isomer to the E,E-isomer and
after heating of 60 h the isomer ratio, Z,Z:E,E changed
to 9:91. This indicates that the initial product should be
the Z,Z-isomer, which was isomerized to the E,Z- and
the E,E-isomer under the reaction conditions. The
mechanism of the isomerization, however, is not clear
at present. Ester exchange reactions of the hydrodimer
with methanol were also observed and the yield of the
tetraethyl esters 2b decreased to 69%. If the reaction
* Corresponding author. Tel.: +81
6
8506245; e-mail:
tani@chem.es.osaka-u.ac.jp
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PII S0022-328X(98)00507-5

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K. Tani et al. / Journal of Organometallic Chemistry 560 (1998) 253–255
Table 1
Catalytic hydrodimerization of dialkyl acetylenedicarboxylate 1 with [Rh(binap)(MeOH)2]ClO4a
Entry
Substrate
Reaction temp. (°C)
Reaction time (h)
Yield of 2a or 2b (%)b
Z,Z:E,E
1
2
3
4
5
6
1b
1b
1b
1a
1a
1a
20
65
65
20
65
65
28
3
60
70
3
40
91
69c
48
81
54
72:28
80:20
9:91
73:27
61:39
28:72
20
a Conditions: [substrate]=0.25 M in MeOH; [Rh]=0.01 M.
b Byproducts=cyclic trimer 3, vinyl ether 4.
c Ester exchanged products of 2b were also contained as the by-products.
was conducted at 20°C, the starting acetylene was
consumed completely after 28 h but the yield of the
hydrodirner 2b was only 40% (Z,Z:E,E=72:28). The
cyclic trimer 3b and the vinyl ether 4b [8], obtained by
addition of methanol to the acetylene, were obtained as
the by-products. Dimethyl acetylenedicarboxylate 1a
similarly reacted under reflux for 3 h to give the corre-
sponding hydrodimer 2a in 81% yield (Z,Z:E,E:E,Z=
61:39:B1) accompanied by the cyclic trimer 3a in 12%
yield. Prolonged heating induced second reactions and/or
isomerization of the product and the yield of the hy-
drodimer 2a decreased again to 54% (Z,Z:E,E=28:72).
The reaction at 20°C after 70 h gave the hydrodimer 2a
in 48% yield (Z,Z:E,E=73:27). When [Rh(bi-
nap)(MeOH)2]ClO4 prepared in situ from [Rh(bi-
nap)(COD)]ClO4 (COD=cycloocta-1,5-diene) with
dihydrogen in methanol was employed as the catalyst
precursor, the reaction of 1b at 65°C after 20 h gave 2b
in only 40% yield accompanied by 20% of the cyclic
trimer 3b and 35% of the reduction product, diethyl
maleate 5b. This different reactivity between the in situ
catalyst and the isolated methanol complex, [Rh(bi-
nap)(MeOH)2]ClO4, may be due to the presence of the
dinuclear complex, [Rh(binap)]2(ClO4)2 [5], in the former
system. When an in situ catalyst was used after removing
the dinuclear complex by filtration the yield of the
hydrodimer 2b increased to 62%. If [Rh(bi-
nap)(cod)]ClO4 itself was employed as the catalyst pre-
cursor without removing the COD ligand by
hydrogenolysis, 1b did not give the hydrodimer 2b but
the vinyl ether 4b in 34% yield. As the diphosphine
ligand, diphos and other peraryldiphosphines, such as
1,2-bis(diphenylphosphino)benzene and 1,1%-bis(dip-
henylphosphino)ferrocene, were ineffective for the hy-
drodimerization, giving the vinyl ether 4b and the cyclic
trimer 3b as the main products. Only [Rh(bpbp)-
(MeOH)n]+ (bpbp=2,2%-bis(diphenylphosphino)1,1%-
biphenyl), prepared in situ from [Rh(bpbp)(cod)]ClO4
and H2, showed somewhat catalytic activity but 1a gave
the hydrodimer 2a in only 20% yield, accompanied by a
lot of by-products such as 3a, 4a, and 5a in the reaction
at 65°C after 20 h.
The source of the hydrogen was found to be the solvent
methanol with deuterium-labeling experiments. When
the reaction of 1b by [Rh(binap)(MeOH)2]ClO4 was
carried out in d4-methanol the olefinic hydrogen of the
hydrodimer 2b was 100% deuterium, whereas the reac-
tion in CH3OD only 50% of the olefinic hydrogen was
deuterium. Thus, not only the hydroxy hydrogen but also
the methyl hydrogen of methanol were used as the
hydrogen source. The reaction under hydrogen (60 kg
cm−2) at room temperature gave diethyl maleate 5b as
the main product and the hydrodimer 2b was scarcely
obtained. On the basis of these experimental results, a
plausible reaction pathway for formation of the Z,Z-iso-
mer (Z,Z-2), as well as the formation of 3, 4, and 5,
depicted in Scheme 1, is rationally proposed. Consis-
tently, in the reaction mixture, formaldehyde was de-
tected by the chromotropic acid test [9]. Although the
present hydrodimerization needs hydrogen transfer from
methanol, 2-propanol, which is considered as the best
hydrogen source for usual catalytic transfer hydrogena-
tion of carbonyl compounds [10], was not a good solvent
and the yield of the hydrodimer 2b in the reaction in
2-propanol was only 28% accompanied by 56% of the
cyclic trimer 3b and 6% of diethyl maleate 5. Ethanol was

K. Tani et al. / Journal of Organometallic Chemistry 560 (1998) 253–255
255
Scheme 1. A plausible reaction pathway for the catalytic reaction of dialkyl acetylenedicarboxylate with [Rh(binap)(MeOH)2]ClO4.
Rev. 88 (1988) 1081. (c) Sei Otsuka, A. Nakamura, in: F.G.A.
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II, vol. 12, Elsevier, Oxford, 1995, ch. 7.3.
also a good solvent and the hydrodimer 2b was obtained
in 73% yield accompanied by 17% of the cyclic trimcr 3b
and a trace amount of diethyl maleate.
Under similar reaction conditions, ethyl 2-tride-
cynoate gave only addition products of methanol, ethyl
3-methoxy-2-tridecenoate (48%) and ethyl 3,3-
dimethoxytridecanoate (15%) and its hydrolyzed
product, ethyl 3-oxotridecanoate (12%). Terminal
acetylenes, ethyl propynoate and 1-dodecyne, however,
gave a mixture of cyclic trimers, benzene derivatives, in
87 and 71% yields, respectively. Internal acetylenes
without carboxylate substituents such as 6-dodecyne and
1,2-diphenylacetylene did not react under similar condi-
tions. Thus, the present hydrodimerization of acetylene
leading to conjugated dienes is specific for acetylene
dicarboxylates at present. Studies on the detailed mech-
anism and applicability to other substrates are in pro-
gress in our laboratory.
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Acknowledgements
The present study was financially supported in part by
a grant-in-aid for Scientific Research from the Ministry
of Education, Science, Sports, and Culture of Japan.
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