Palladium(0)-catalyzed decarboxylation of buta-2,3-dienyl 2'-alkynoates: Approach to the synthesis of 2-alkynoates: Buta-1,3-dienes

Article abstract of DOI:10.1021/ol702577g

The Pd(PPh₃)₄-catalyzed decarboxylation of buta-2,3-dienyl 2'-alkynoates allows the rapid construction of 2-alkynyl buta-1,3-dienes. The carboncarbon bond-forming reaction occurs at the central position of an allene moiety.

Full text of DOI:10.1021/ol702577g

ORGANIC
LETTERS
2008
Vol. 10, No. 3
433-436
Palladium(0)-Catalyzed Decarboxylation
of Buta-2,3-dienyl 2 -Alkynoates:
′
Approach to the Synthesis of 2-Alkynyl
Buta-1,3-dienes
So Hee Sim, Hee-Jun Park, Sang Ick Lee, and Young Keun Chung*
Intelligent Textile System Research Center, and Department of Chemistry, College of
Natural Sciences, Seoul National UniVersity, Seoul 151-747, Korea
ykchung@plaza.snu.ac.kr
Received October 23, 2007
ABSTRACT
The Pd(PPh3)4-catalyzed decarboxylation of buta-2,3-dienyl 2′-alkynoates allows the rapid construction of 2-alkynyl buta-1,3-dienes. The carbon−
carbon bond-forming reaction occurs at the central position of an allene moiety.
The development of catalytic versions of C-C bond-forming
processes is a topic of great importance in modern organic
chemistry.1 In this context, transition-metal-catalyzed decar-
boxylative couplings2 continue to attract a great deal of
attention because they allow the use of readily available
carboxylic acids or esters under neutral conditions to produce
a C-C coupling product and CO2 as the only byproduct. In
particular, palladium-catalyzed decarboxylative couplings
have been widely used in many useful reactions, such as a
decarboxylative Heck coupling,3 aldol addition,4 decarboxy-
lative alkylation,5 decarboxylative aza-Michael addition-
allylation,6 decarboxylative cross-coupling,7 and decarbo-
xylative protonation.8
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Nakamura, E. Angew. Chem., Int. Ed. 2005, 44, 7248-7251. (h) Burger,
E. C.; Tunge, J. A. J. Am. Chem. Soc. 2006, 128, 10002-10003. (i) Knight,
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7738-7739. (b) Trost, B. M.; Bream, R. N.; Xu, J. Angew. Chem., Int. Ed.
2006, 45, 3109-3112. (c) Patil, N. T.; Huo, Z.; Yamamoto, Y. J. Org.
Chem. 2006, 71, 6991-6995. (d) Wang, C.; Tunge, J. A. Org. Lett. 2006,
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10.1021/ol702577g CCC: $40.75
© 2008 American Chemical Society
Published on Web 01/03/2008

Allenynes are very interesting substrates because of their
high degree of unsaturation.9 Thus, they have been used as
substrates in various catalytic reactions, including cycloi-
somerization,10 [2 + 2] cycloaddition reaction,11 ring-closing
metathesis,12 Pauson-Khand reactions,13 and ene-type reac-
tions.14 Recently, we found that there has been no example
of palladium-catalyzed decarboxylation that involves buta-
2,3-dienyl 2′-alkynoates. Furthermore, if we control the
decarboxylation reaction, 2-alkynyl buta-1,3-dienes instead
of allenynes would be obtained. Recently, Shi reported15 the
synthesis of 2-alkynyl buta-1,3-dienes by the reaction of the
corresponding diiodides from methylenecyclopropanes with
substituted alkynes via Sonogashira coupling in the presence
of Pd(PPh3)4/CuI catalysis. Formation of 2-alkynyl buta-1,3-
dienes by the reaction of cross-metathesis of 1,3-diynes with
an olefin was also reported by Lee.16 2-Ethynyl buta-1,3-
diene has been used as the diene component in Diels-Alder
addition reactions.17 Recently, π-conjugated polymers have
been widely explored as advanced materials for electronic
and photonic applications.18 Many chromophores have
2-phenylethynyl buta-1,3-diene as a substructure.19 Herein
we report a new approach to conjugated dienynes that relies
upon the palladium-catalyzed decarboxylation of allenyl
alkynoates.
Decarboxylation was studied using 4-methylpenta-2,3-
dienyl 3-phenylpropiolate (1a) as a model substrate and Pd2-
(dba)3 as a catalyst in toluene at 75 °C (eq 1). After 2 h of
reaction time, 2-phenylethynyl buta-1,3-diene (1b) was
obtained in 43% yield.
Thus, we initially screened the reaction parameters, such
as the palladium catalyst, the reaction solvent, the reaction
temperature, and the reaction time, for the decarboxylation
of 1a (Table 1).
Table 1. Pd-Catalyzed Decarboxylation of 1aa
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Chem. Soc. 2006, 128, 11348-11349. (b) Matsubara, S.; Yokota, Y.;
Oshima, K. Org. Lett. 2007, 9, 2071-2073. (c) Olivier, R.; Abdelkhalek,
R.; Franc¸oise, H.; Jacques, M. Eur. J. Org. Chem. 2002, 3986-3994.
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923. (b) Brummond, K. M.; Chen, H. In Modern Allene; Krause, N., Hashmi,
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T (°C)
yield
(%)b
entry
catalyst
solvent [t (h)]
1
2
2.5 mol % of Pd2(dba)3
2.5 mol % of Pd2(dba)3/dppe(1:1)
5 mol % of Pd(PPh3)4
toluene 75 [2]
43
toluene 75 [0.5] >99
toluene 75 [0.5] >99
toluene 75 [0.5] 99
toluene 25 [24] n.r.c
toluene 45 [24] 34 (37)
3
4
1 mol % of Pd(PPh3)4
5
1 mol % of Pd(PPh3)4
6
1 mol % of Pd(PPh3)4
7
1 mol % of Pd(PPh3)4
THF
DCE
75 [24] 12 (74)
75 [24] 47 (50)
toluene 75 [6]
8
1 mol % of Pd(PPh3)4
9
1 mol % of Pd(OAc)2/PPh3 (1:1)
67
10
1 mol % of [C3H5]PdCl-NHC(Ipr) toluene 75 [24] n.r.
a 1a (0.12 g, 0.54 mmol) in 5 mL of solvent was used. b Isolated yield.
c n.r. ) no reaction.
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A.; Aubert, C.; Fensterbank, L.; Malacria, M. Angew. Chem., Int. Ed. 2006,
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An addition of dppe to Pd2(dba)3 slightly increased the
catalytic activity (entry 2). However, as Table 1 shows, Pd-
(PPh3)4 was the best catalyst of those examined. The loading
of the catalyst can be lowered to 1 mol % with a quantitative
yield (entry 4). When the solvent and the reaction temper-
ature were screened using Pd(PPh3)4 as a catalyst (entries
4-8), the best yield (>99%) was obtained in toluene at
75 °C for 0.5 h (entry 4). The Pd(OAc)2/PPh3 (1:1) system
shows a slightly lower activity (entry 9). Complex (π-C3H5)-
Pd(IPr)Cl (IPr ) N,N′-bis(2,6-diisopropylphenyl)imidazol-
(11) (a) Mukai, C.; Hara, Y.; Miyashita, Y.; Inagaki, F. J. Org. Chem.
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434
Org. Lett., Vol. 10, No. 3, 2008

Table 2. Pd(PPh3)4-Catalyzed Decarboxylation of Buta-2,3-dienyl 2′-alkynoatesa
1
a Buta-2,3-dienyl 2′-alkynoate (0.54 mmol) in 5 mL of solvent was used. b Isolated yield. c The ratio was determined by H NMR
2-ylidene) has no catalytic activity (entry 10). When the
reaction was carried out in toluene at 100 °C, the reaction
time could be shortened to 10 min. Thus, the optimized
reaction conditions were established to be as follows: 1 mol
% of Pd(PPh3)4, toluene, and 100 °C.
ratio of the two isomers (E/Z) was determined by the NOE
study of the products (Figure 1). The overall yields were
77-99% with an E/Z ratio ranging from 1.2:1 to 14:1. To
examine the temperature effect on the E/Z ratio of the
product, the reaction temperature was lowered to 70 °C. For
the substrates in entries 5 and 7, the E/Z ratio was insensitive
to the reaction temperature. However, the effect of the
reaction temperature on the E/Z ratio was quite dramatic for
entry 6.
We next investigated the palladium-catalyzed decarboxy-
lation of various buta-2,3-dienyl 2′-alkynoates under the
optimized reaction conditions (Table 2). The reaction time
was highly dependent upon the substrate and ranged from
15 min to 1 h. The catalytic amount of Pd(PPh3)4 induced
the decarboxylation of buta-2,3-dienyl alkynoates to produce
carbon-carbon bond-coupling products of 2-alkynyl buta-
1,3-dienes. The yields of 2-alkynyl buta-1,3-dienes were
satisfactory, except for the case of 4-iso-propyl-5-methyl-
hexa-2,3-dienyl 3-phenylpropiolate in entry 3. However,
lowering the reaction temperature to 70 °C and increasing
the reaction time to 3 h dramatically increased the yield from
38 to 68%.
Figure 2. Other buta-2,3-dienyl 2′-alkynoates tested for Pd-
catalyzed decarboxylation.
For entry 6, lowering the reaction temperature to 70 °C
led to a dramatic change of the E/Z ratio from 2:1 to 19:1.
The dominant existence in the E isomer compared to the Z
isomer was presumably due to steric effects.
Unfortunately, our attempts to isolate major products for
the substrates given in Figure 2 failed presumably due to
decomposition under our reaction conditions.
Figure 1. NOE studies of the products.
For buta-2,3-dienyl 2′-alkynoates bearing unsymmetric
substituents on the allenic carbon (entries 5-8), the isomeric
The present novel palladium(0)-catalyzed carbon-carbon
bond-coupling reaction is characterized by its rapidness and
Org. Lett., Vol. 10, No. 3, 2008
435

is unique among the reported palladium-catalyzed carbon-
carbon bond-forming reactions that occur at the central
position of an allene moiety. This observation suggests an
intermediacy of a π-allylpalladium(II) complex. The pre-
sumed reaction pathway based on the proposition of Tunge1f
is shown in Scheme 1. The reaction is initiated by the
When a crossover reaction using a mixture of 1a and 10a
was carried out in the presence of Pd(PPh3)4, complete
crossover was observed: a mixture of four possible crossover
products was obtained in 93% yield with a ratio of 1.6:1.0:
0.6:0.7 (eq 2).
Scheme 1
In conclusion, we have developed an efficient synthesis
of 2-alkynyl buta-1,3-dienes from buta-2,3-dienyl 2′-
alkynoates. Utilizing this and related methods for the
synthesis of π-conjugated polymers will be the subject of
future reports.
Acknowledgment. This work was supported by the Korea
Research Foundation grant funded by the Korean Govern-
ment (MOEHRD) (KRF-2005-070-C00072) and the SRC/
ERC program of MOST/KOSEF (R11-2005-065). S.H.S.
thanks the Brain Korea 21 fellowships and Seoul Science
Fellowships.
oxidative addition of buta-2,3-dienyl 2′-alkynoate to Pd-
(PPh3)4 to produce a π-allylpalladium(II) complex with a
free or ion-paired carboxylate (I). Nucleophilic addition of
carboxylate anion to the π-allylpalladium cation followed
by decarboxylation generates a π-allylpalladium complex
intermediate (III).
A reductive elimination produces 2-alkynyl buta-1,3-diene
and the palladium(0) catalyst. The result of the cross-coupling
experiment shown in eq 2 is compatible with the presumed
reaction pathway involving I.
Supporting Information Available: Experimental details
and spectroscopic characterization of all isolated compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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