A new ruthenium-catalyzed hydrogen-transfer reaction: Transformation of 3-benzyl but-1-ynyl ethers into 1,3-dienes and benzaldehyde

Article abstract of DOI:10.1021/ja012623y

(Chemical Equqtion Presentation) We report a ruthenium-catalyzed reaction for various 3-benzyl but-1-ynyl ethers with suitable functionalities. Treatment of these substrates with TpRu(PPh₃)(CH₃CN)₂PF₆ (8.0 mol %

Full text of DOI:10.1021/ja012623y

Published on Web 05/17/2002
A New Ruthenium-Catalyzed Hydrogen-Transfer Reaction: Transformation of
3-Benzyl But-1-ynyl Ethers into 1,3-Dienes and Benzaldehyde
Kuo-Liang Yeh, Bo Liu, Ching-Yu Lo, Heh-Lung Huang, and Rai-Shung Liu*
Department of Chemistry, National Tsing-Hua UniVersity, Hsinchu, Taiwan, ROC
Received November 30, 2001
Scheme 1
The metal-catalyzed hydrogen-transfer reaction between two
organic substrates has attracted considerable attention because it is
an energy-saving and environmentally friendly process.1-4 This
reaction avoids the use of conventional oxidants and reductants.
Several hydrogen-transfer reactions have been shown to be useful
in organic synthesis, and notable examples include the asymmetric
hydrogenation of ketones and imines from formic acid or 2-propanol
using chiral ruthenium1a,2a,b and aluminum catalysts.3 Despite their
Scheme 2
synthetic importance, the scope of metal-catalyzed hydrogen-transfer
reactions is rather limited. Many of them deal exclusively with the
hydrogenation of a CdX (X ) CR2, NR, O) or CtC bond by
alcohols and formic acid.1-4 An accepted mechanism of these
catalytic reactions is shown in eq 1 (Scheme 1). We report here a
new ruthenium-catalyzed reaction to effect the efficient conversion
of various 3-benzyl but-1-ynyl ethers to 1,3-butadiene and benzal-
dehyde (eq 2). The direct synthesis of butadiene derivatives from
but-1-yn-3-ol and its analogues is synthetically challenging and to
the best of our knowledge has not yet been described. This
Table 1. Catalytic Transformation of 3-Benzyl But-1-y-nyl Ethersa
transformation is normally achieved by stoichiometric LiAlH4
reduction5 or hydrogenation on Lindlar catalyst6 to give allylic
alcohols, followed by a tedious dehydration at elevated tempera-
tures.7 The mechanism (based on the results of the isotopic labeling
experiment) of this new catalytic reaction appears to be interesting..
We first examined the catalytic transformation of various but-
1-yn-3-ol derivatives 1a-1e with TpRuPPh3(CH3CN)2PF6 catalyst
to study the structural effect of the substrates. This catalyst was
prepared directly by heating TpRu(PPh3)2Cl with LiPF6 in CH3CN.8
The reaction was performed with ruthenium catalyst (8.0 mol %)
in dichloroethane (80 °C, 12 h, [1a-1e] ) 1.5 M), and the results
are summarized in Scheme 2. No products were found for the
alcohol 1a and silyloxy derivative 1b in the presence of benzyl
alcohol (2.0 equiv) with exclusive recovery of unreacted 1a and
1b. A similar treatment on the methoxy 1c gave the diene 2 (E/Z
) 6.0, 53%) and benzaldehyde (51%), and 38% of unreacted 1c
was recovered. The yields of diene 2 (83%, E/Z ) 6.0) and
benzaldehyde (81%) were significantly improved when benzyl ether
a Conditions: (a) 8.0 mol % catalyst, 80 °C, 12 h (entries 1-11); (b) 15
mol % catalyst, 80 °C, 48 h; (c) the yields of diene and benzaldehyde were
1d was used alone. p-Methoxybenzyl ether derivative 1e is even
more reactive and less catalyst (5 mol %) sufficed for the reaction.
In the reaction involving enyne 1f and benzyl alcohol (eq 2), a
black tar and small amount of benzaldehyde (5%) were formed
with complete consumption of enyne 1f, showing a weak indication
of transfer hydrogenation. The disubstituted alkynyl derivative 1g
was recovered in 92% under the same catalytic conditions.
Table 1 shows various benzyl but-1-ynyl ethers that were
catalyzed by TpRuPPh3(CH3CN)2PF6 catalyst.. The reactions were
performed with 8.0 mol % of the catalyst with heating in
dichloroethane (1.5 M, 80 °C, 12 h) except for entry 12 which
involved a higher loading of catalyst (15 mol %) for a longer time
reported after separation on a silica column.
(80 °C, 48 h). The yields of diene products and benzaldehyde were
calculated after purification from a silica column. Entries 1 and 2
shows two examples of the syntheses of dienes 15 and 16 bearing
a C13H27 and phenyl group, respectively, in yields of 75 and 72%.
This catalytic approach was also applicable to the efficient synthesis
(78% yield) of trisubstituted diene 17 (entry 3) with an isomeric
ratio of E/Z ) 5.6. Similarly, the outer diene 18 (entry 4) was
obtained in 58% yield. We prepared the substrates 7 and 8 bearing
an alkene and alkyne group, respectively (entries 5 and 6). The
corresponding dienes 19 and 20 were produced smoothly without
hydrogenation of the saturated carbon-carbon bond by the benzyl
group. This catalytic system also worked well for diene 21 bearing
* To whom correspondence should be addressed. E-mail: rsliu@mx.nthu.edu.tw.
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10.1021/ja012623y CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Labeling Experiment for Hydrogen-Transfer Reaction
yield) under mild conditions (10 mol %, 80 °C, 12 h). The driving
force for easy cleavage of the ether ring of 27 is the formation of
ruthenium-allenylidene intermediate F which subsequently under-
goes hydrogen transfer from its tethered alcohol to the allenylidene
functionality.
Scheme 4
In summary, we report a new catalytic reaction involving a
tandem dealkoxylation and transfer hydrogenation. The mechanism
of this catalytic reaction was examined by deuterium experiments.
This catalytic reaction is synthetically useful because it tolerates
some oxygen and nitrogen functionalities. Further modification of
the catalyst and the application of this reaction are under investiga-
tion.
Acknowledgment. We thank National Science Council, Taiwan,
for support of this work.
Supporting Information Available: Experimental procedures for
the syntheses and spectral data of new compounds 1a-1e,2-28 (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
a ketone group. It also works well for dienes 22 and 23 comprising
a dioxolane and a 2,2-dimethyloxolane group respectively; the yields
were reasonable (68-69%, Z/E ) 6.3-8.4). The acidic fluorenyl
proton of compound 12 did not inhibit the catalytic reactivity, and
the diene 24 (81%) was also produced smoothly. A nitrogen-
containing diene 25 was obtained in 70% yield following this
catalytic protocol. A higher loading of catalyst (15 mol %) was
used to complete the reaction with compound 14 tethered to a nitrile
group, and the yield of the corresponding diene 25 was 52% after
workup.
Scheme 3 shows the results of deuterium-labeling experiments
to elucidate the reaction mechanism. Deuterium migrations of ether
1d to the diene 2 proceeded regiospecifically. The C1-cis-proton
of the diene arose primarily from the benzyl CH2 protons of the
diene 1d (entry 1). The terminal acetylene proton of 1d was
transferred to the C2-proton of the diene with 56% deuterium content
(entry 2a). In the presence of one equimolar amount of PhCH2OD,
the deuterium content of this proton was increased to 76%. The
site of the C3-proton was unchanged after the reaction (entry 3).
We also prepared 1d bearing a deuterated C4-methylene group, and
a deuterium was found mainly at the C4-diene and partly at the
C2-proton, whereas the remaining deuterium could not be found at
one specific diene proton exclusively, including the C1-trans
position.9
On the basis of the results of deuterium experiments, we propose
a plausible mechanism that involves the formation of ruthenium-
allenylidenium A10 which undergoes transformation into a more
stable Fisher-carbenium B.11,12 The benzyl group of B was activated
by ruthenium after dissociation of its second CH3CN, yielding
species C bearing an oxonium group, which further induces
ionization of its C4-hydrogen to lead to the formation of ruthenium
hydride species bearing an alkenyl vinylidenium species D. Hydride
insertion at the C1-carbon forms a 1,3-butadien-1-yl species.
Decomplexation of this species with a proton and CH3CN regener-
ates ruthenium catalyst and diene. This mechanism accounts for
the results of deuterium-labeling experiment except for the result
in entry 3. In this case, deuterium is expected to be located in the
C1-trans position of diene 2. This discrepancy may be due to the
proton exchange with diene9 and benzene protons or residue water
at reaction condition (80 °C, 12 h) (Scheme 4).
References
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C11H18D2 (found 154.1703, 14.88%; calcd
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A useful application of this ruthenium catalyst is to transform
the cyclic alkynyl ether 27 into the functionalized diene 28 (93%
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