LETTER
α,β-Alkynyl and Unsymmetrical Maleate Esters
985
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It can be seen that electron-donating (OMe and Me) and
electron-withdrawing (CF3) substituted aromatic or het-
erocyclic aromatic substrates show excellent selectivity
towards Z-isomer formation, giving very good yields of
the desired product (Table 2, entries 17–20).
The catalyst showed remarkable recyclability, being used
in up to six consecutive cycles without loss of activity or
selectivity (Figure 1). We also performed the reaction on
larger scale (5 mmol of 1a) and carried out ICP-AES anal-
ysis of the bulk reaction, finding palladium leaching to be
below detectable levels.
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O
C
O
H
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1a
2a
1st run
nd run
rd run
93%
92%
92%
91%
89%
88%
TBAI
2
3
4th run
5th run
6th run
Pd/C
O2
O
3a
O
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Figure 1 Recycle study of oxidative alkoxycarbonylation of termi-
nal alkyne
In summary, we have developed a heterogeneous, recy-
clable protocol for the preparation of α,β-alkynyl esters
and unsymmetrical maleate esters by oxidative alkoxycar-
bonylation of terminal alkynes.12 The reaction proceeds
under mild conditions using simple starting materials. The
catalytic system has competitive advantages in that it
eliminates the use of phosphine ligands, bases, and acids,
and enables easy catalyst recovery allowing several cycles
of reuse without significant loss in catalytic activity or se-
lectivity.
(7) Gadge, S. T.; Khedkar, M. V.; Lanke, S. R.; Bhanage, B. M.
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(9) See Table 1 of the Supporting Information for catalyst
screening and loading.
Acknowledgment
We are grateful to Dr. C.V. Rode from the National Chemical La-
boratory, Pune for providing HRMS analysis of products. S.T.G. is
grateful to the Council of Scientific and Industrial Research, New
Delhi for providing a Research Fellowship.
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Supporting Information for this article is available online at
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References and Notes
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(12) Oxidative Alkoxycarbonylation of Terminal Alkynes;
General Procedure: To a 100 mL stainless steel autoclave,
the alk-1-yne (1 mmol), the alcohol (0.5 mL), 10% Pd/C (10
mol%), TBAI (0.6 mmol), and 1,4-dioxane (10 mL) were
added. The autoclave was closed and pressurized with
oxygen (1 atm) and CO (5 atm) without flushing. The
reaction mixture was stirred (550 rpm) and heated at 80 °C
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Synlett 2013, 24, 981–986