A new ruthenium-catalyzed cleavage of a carbon-carbon triple bond: Efficient transformation of ethynyl alcohol into alkene and carbon monoxide

Article abstract of DOI:10.1021/ja036246w

We report a new and efficient ruthenium-catalyzed reaction that transforms ethynyl alcohol into alkene and carbon monoxide. The most efficient catalysts are TpRu(PPh₃)(CH₃CN)₂PF₆ (10 mol %) and lithium triflate

Full text of DOI:10.1021/ja036246w

Published on Web 07/09/2003
A New Ruthenium-Catalyzed Cleavage of a Carbon-Carbon Triple Bond:
Efficient Transformation of Ethynyl Alcohol into Alkene and Carbon Monoxide
Swarup Datta, Chia-Lung Chang, Kuo-Liang Yeh, and Rai-Shung Liu*
Department of Chemistry, National Tsing-Hua UniVersity, Hsinchu, Taiwan, 30043, Republic of China
Received May 21, 2003; E-mail: rsliu@mx.nthu.edu.tw
Scheme 1 a
Metal-catalyzed cleavage of carbon-carbon bonds is an interest-
ing topic in organic reactions.1 Cleavage of the carbon-carbon triple
bond of alkynes is very difficult because this functionality normally
gives products containing an unstrained CdC or C-C bond during
catalytic transformation.1,2 Most studies on alkyne cleavage have
focused on stoichiometric organometallic reactions,3,4 including
oxidative cleavage of alkyne to carboxylic acid,3 alkyne cleavage
across metal-metal multiple bonds,4 water-assisted splitting of
alkyne into alkane and coordinated CO,5 as well as alkyne-ligand
scission on metal complexes.6 There have been very few reported
examples for the catalytic cleavage of organic alkyne via a
nonmetathesis pathway.7 Jun et al. reported catalytic cleavage of
alkynes through rhodium-catalyzed hydroiminoacylation.8 Very
recently, Yamamoto reported the cleavage of diynes8 via ruthenium-
catalyzed hydroamination.8 These reactions require additional
promoters 2-aminopyridine or 2-aminophenol to facilitate cleavage
of the alkyne.8 Because of the importance of carbon-carbon bond
cleavage, we sought to identify a new pathway for the cleavage of
alkyne in the absence of organic promoters. In this report, we
describe a new ruthenium-catalyzed reaction in which ethynyl
alcohol is split into alkene and carbon monoxide. The mechanism
of this new process was elucidated on the basis of the results of an
a (a) 10 mol % catalysts, 100 °C in toluene 16 h for entries 1-6, and 3
h for entries 7-10. (b) 20 mol % Lewis acid was used. (c) Isolated yields
from the silica column. (d) CO was determined from GC with 35% and
72% yields in entries 1 and 7, respectively.
isotope-labeling experiment.
We sought to achieve cleavage of alkynes via generation of a
metal vinylidene intermediate in a catalytic reaction.5 We examined
various ruthenium and rhodium catalysts (Scheme 1), because they
readily form metal vinylidene species in the presence of alkynes.5
TpRu(PPh3)(CH3CN)2PF69 (10 mol %, Tp ) tris(1-pyrazolyl)borate)
is the most active for the transformation of ethynyl alcohol (1a)
(100 °C, toluene, 16 h) into alkene in an isolated yield of 33%.
GC-analysis of gaseous mixtures confirmed the formation of carbon
monoxide (35%), indicative of the catalytic reaction depicted in
Scheme 1. Under the same conditions, C5H5Ru(PPh3)(CH3CN)2PF6
and other catalysts (entries 2-6) including TpRu(PPh3)2Cl, TpRu-
(PPh3)(CH3CN)Cl, RuCl2(PPh3)3, and ClRh(PPh3)3 failed to give
significant amounts of alkene 1b but rather gave uncharacterized
black tar. The efficiency of this process can be further improved
with Lewis-acid catalysts (entries 7-10) with LiOTf (20 mol %)
being the most effective: the reaction time is reduced (toluene 100
°C, 3 h), and alkene 1b is obtained in an isolated yield of 75%.
Carbon monoxide is found in ca. 72% yield according to GC-
analysis.
Table 1. Ruthenium-Catalyzed Cleavage of Ethynyl Alcohols
We further examined the scope of organic substrates, and the
results are summarized in Table 1. A mixture of TpRu(PPh3)(CH3-
CN)2PF6 (10 mol %) and LiOTf (20 mol %) catalysts was
employed. This method also works for tertiary ethynyl alcohol 2a
to give 1,1-disubstituted 2b in 52% yield. It can be extended to
phenyl groups containing electron-rich 1,2-(methylenedioxy) and
diphenylamino substituents 3a-5a (entries 2-4) and electron-
withdrawing cyano and methoxycarbonyl groups 6a-7a (entries
5, 6), and the corresponding alkenes 3b-7b were obtained in yields
a 10 mol % TpRu(PPh3)(CH3CN)2PF6, 20 mol % LiOTf, toluene (0.15
M substrate), 110 °C, 5 h. b Yields were reported after separation from the
silica column.
of 74-91%. This new reaction is also compatible with substrates
bearing 2-naphthyl and oxygen, nitrogen, and sulfur heteroaryl
functionalities (entries 7-10), and alkene products 8b-11b were
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10.1021/ja036246w CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Deuterium Labeling for Cleavage of Alkyne
catalytic system. It is also likely to arise from the formation of Z/E
(1/2) isomers of species D and E.
In summary, we have reported a new and efficient catalytic
cleavage of organic alkynes in which ethynyl alcohol is split into
alkene and CO by ruthenium complex.12 This reaction is syntheti-
cally useful because it can tolerate various functional groups. The
mechanism of this catalytic reaction was elucidated with an isotope-
labeling experiment. The development of new catalytic processes
based on this principle is under investigation.
Acknowledgment. The authors wish to thank the National
Scheme 3
Science Council, Taiwan, for support of this work.
Supporting Information Available: Experimental procedures and
spectral data of compounds 1a-17a and 1b-17b (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
References
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