to form the product and palladium(0) species, which is then
oxidized by silver(I) to regenerate the palladium(II) to complete
the catalytic cycle. As can be seen in Scheme 2, the silver
oxide plays two different roles in the catalytic cycle: first as
the oxidant to oxidize the palladium(0); and second as the
decarboxylative agent to generate the desired alkynyl metal
species.
In conclusion, an efficient palladium-mediated decarboxyl-
ative coupling of aryl boronic acids with alkynyl carboxylic
acids was successfully established. The reaction described here
is mild, general, and efficient. This method provides easy
access to a wide variety of unsymmetrical alkynes.
We gratefully acknowledge the Nanyang Technological
University, the Ministry of Education (No. M45110000),
and A*STAR S.E.R.C. (No. 0721010024) for the funding of
this research.
Scheme 1 Consecutive reactions for the synthesis of unsymmetrical
diarylalkyne.
alkynyl carboxylic acids, both cyclic and acylic substrates can
be effectively coupled to furnish the desired products in good
to excellent yields (Table 3, entries 7–10). Owing to the
difficulty of direct introduction of the 1-propynyl group, it is
worth mentioning that the reaction employing 2-butynoic acid
as the coupling partner can also afford the corresponding
product, albeit in moderate yield (Table 3, entry 7).
Notes and references
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M. Santelli, Tetrahedron Lett., 2005, 46, 1717.
3 (a) K. Sonogashira, in Metal-Catalyzed Cross-Coupling Reactions,
ed. F. Diederich and P. J. Stang, Wiley-VCH, Weinheim, 1998,
pp. 203–229; (b) K. Sonogashira, J. Organomet. Chem., 2002, 653,
46; (c) E. Negishi and L. Anastasia, Chem. Rev., 2003, 103, 1979;
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To demonstrate the efficiency of this method for the synthesis
of unsymmetrical alkynes, Sonogashira reaction followed by
our decarboxylative coupling method was attempted using
iodobenzene, propiolic acid and 4-methoxyphenyl boronic
acid as substrates. After simple workup of the reaction between
iodobenzene and propiolic acid (without purification), the
intermediate was subjected directly to the decarboxylative
coupling to provide the desired unsymmetrical alkyne in
70% overall yield (Scheme 1).
A proposed mechanism based on our results is shown in
Scheme 2.9 First, transmetalation between arylboronic acids
and palladium acetate occurs to form aryl palladium species I.
Then the alkynyl group is transfered from the metal–silver
species to the metal–palladium species to form the intermediate
II which contains two organic ligands in the coordination
sphere of the palladium center. Reductive elimination ensues
6 (a) H. F. Chow, C. W. Wan, K. H. Low and Y. Y. Yeung, J. Org.
Chem., 2001, 66, 1910; (b) S. K. Kang, S. K. Yoon and Y. M. Kim,
Org. Lett., 2001, 3, 2697.
7 (a) J. Moon, M. Jeong, H. Nam, J. Ju, J. H. Moon, H. M. Jung and
S. Lee, Org. Lett., 2008, 10, 945; (b) J. Moon, M. Jang and S. Lee,
J. Org. Chem., 2009, 74, 1403.
8 For decarboxylative coupling, see: (a) A. G. Myers, D. Tanaka and
M. R. Mannion, J. Am. Chem. Soc., 2002, 124, 11250;
(b) D. Tanaka, S. P. Romeril and A. G. Myers, J. Am. Chem.
Soc., 2005, 127, 10323; (c) L. J. Goossen, G. Deng and L. M. Levy,
Science, 2006, 313, 662; (d) L. J. Goossen, B. Zimmermann and
T. Knauber, Angew. Chem., Int. Ed., 2008, 47, 7103;
(e) L. J. Goossen, N. Rodriguez, B. Melzer, C. Linder, G. Deng
and L. M. Levy, J. Am. Chem. Soc., 2007, 129, 4824;
(f) L. J. Goossen, N. Rodriguez and C. Linder, J. Am. Chem.
Soc., 2008, 130, 15248; (g) H. P. Bi, L. Zhao, Y. M. Liang and
C. J. Li, Angew. Chem., Int. Ed., 2009, 48, 792.
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Chem., 2007, 3476; (c) L. J. Goossen, B. Zimmermann and
T. Knauber, Angew. Chem., Int. Ed., 2008, 47, 7103.
Scheme 2 Proposed reaction mechanism.
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4779–4781 | 4781