Palladium-Catalyzed Chemoselective Protodecarboxylation of Polyenoic Acids

Article abstract of DOI:10.1021/acs.orglett.8b03016

Conditions for the first palladium-catalyzed chemoselective protodecarboxylation of polyenoic acids to give the desired polyenes in good yields are presented. The reactions proceed under mild conditions using either a Pd(0) or Pd(II) catalyst and tolerate a variety of aryl and aliphatic substitutions. Unique aspects of the reaction include the requirement of phosphines, water, and a polyene adjacent to the carboxylic acid.

Full text of DOI:10.1021/acs.orglett.8b03016

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Palladium-Catalyzed Chemoselective Protodecarboxylation of
Polyenoic Acids
Mohammed H. Al-Huniti, Mark A. Perez, Matthew K. Garr, and Mitchell P. Croatt*
Department of Chemistry and Biochemistry, University of North Carolina at Greensboro, 435 Sullivan Science Building,
Greensboro, North Carolina 27402, United States
S
* Supporting Information
ABSTRACT: Conditions for the first palladium-catalyzed chemoselective
protodecarboxylation of polyenoic acids to give the desired polyenes in good
yields are presented. The reactions proceed under mild conditions using
either a Pd(0) or Pd(II) catalyst and tolerate a variety of aryl and aliphatic
substitutions. Unique aspects of the reaction include the requirement of
phosphines, water, and a polyene adjacent to the carboxylic acid.
ienes and polyenes are typically prepared using
Scheme 1. Current Dienoic and Polyenoic Acid
Protodecarboxylation Methods
D
olefination reactions1 or metal-catalyzed couplings.2
However, because of the relative instability of conjugated π-
systems, their large-scale preparation and storage are not
typically viable without the incorporation of a stabilizer.
Polyenoic acids could serve as a concealed form of polyenes if
mild methods existed for their protodecarboxylation. The
existing acid-catalyzed protodecarboxylation methods are not
typically compatible with polyenes due to possible polymer-
ization issues.3 In recent years, metal-catalyzed protodecarbox-
ylations and decarboxylative coupling reactions have received
much attention.4 Unlike protodecarboxylation of aryl4a−c,f,j,k,o,5
carboxylic acids, few methods have been reported for
transition-metal-catalyzed protodecarboxylation of cinnamic
acids (Scheme 1B).4i,k,6 Metal-catalyzed dienoic acid proto-
decarboxylation protocols have been scarcely reported. The
current methods require excess base and high temperatures7 or
give low yields (Scheme 1A).8 Additionally, modern aryl
protodecarboxylation conditions are highly optimized for this
motif; thus, vinylic acids are unreactive5 without modification
of the conditions.9
During our study of the decarboxylative coupling of dienoic
acids and pentadienyl groups (Scheme 1C),10 we observed the
protodecarboxylation of dienoic acids. Because of the limited
methods of protodecarboxylation, we aimed to develop a mild
method which would allow for quick access to polyenes from
shelf stable starting materials (Scheme 1D). This would be a
valuable process since the conjugated polyene motif is widely
used in organic synthesis11 and can be found in several
bioactive natural products,12 such as navenone B, amphotericin
B, rapamycin, and retinoic acid.
2).10c Initially, a palladium catalyst in the absence of solvent
was examined since the polarity difference between the acids
and desired products could minimize the reaction workup.
Scheme 2. Initial Solvent-Free Protodecarboxylation
We began our investigation by exploring the reactivity of 5-
phenyl-2,4-pentadienoic acid (1) in the presence of AgOAc at
140 °C9 or copper oxide at 170 °C.13 Although both of the
conditions successfully protodecarboxylated cinnamic acid,
only the silver-catalyzed reaction showed the desired product
(6% yield, Supporting Information, SI). We subsequently
explored the protodecarboxylation of 1 using conditions
similar to our decarboxylative coupling process (Scheme
Received: September 20, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03016
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Reactions using Pd(II) acetate or Pd(0) complexes at 50 °C
led to no reaction. However, Pd(PPh3)4 in the presence of
water gave a mixture of diene 1a and unanticipated diaryl diene
2a in a moderate overall yield. To explore the formation of
product 2a, we executed the reaction using 15 mol % of
Pd2(dba)3 in the presence of different phosphine ligands. Tris-
p-tolyl- and tris-p-anisylphosphines yielded 1,4-diarylbuta-
dienes 2b and 2c, respectively. These reactions clearly
indicated the source of the second phenyl group in compound
2 was the phosphine ligand.14
Encouraged by the greater formation of diene 1a with the
reaction using triphenylphosphine, we decided to use this
ligand for optimization. Thus, reactions were performed using
Pd(PPh3)4 as a catalyst under acidic or basic conditions in the
absence of water (Table 1, entries 1 and 2). Under these
Surprisingly, the oxidation state of the palladium catalyst
precursor did not have a major effect on the reaction efficiency,
as long as triphenylphosphine and water were added (entries
12−15). No exogenous base was required for Pd(OAc)2-
catalyzed reactions, presumably due to the basicity of the
acetate ligand. A side-by-side comparison determined that the
reaction with Pd(OAc)2 was slightly faster but in some cases
yielded product 2a. We further confirmed the necessity of
palladium in this transformation by examining the reactivity of
other transition metal catalysts (i.e., Cu(I), Cu(II), Zn(II),
Ir(I), and Rh(I), SI), all of which were unreactive.
The optimization reactions in Table 1 were typically run
using 20−50 mg of starting material (0.08−0.2 mmol of acid),
but the reaction was scaled up nicely to 1 g (5.7 mmol) with an
increase in yield while using only 5 mol % catalyst loading
(entry 16). For the ease of material usage, all subsequent
reactions were performed on a smaller scale (0.08−0.2 mmol
of acid) using pyridine (1.2 equiv) as a base and Pd(PPh3)4
(15 mol %) as the catalyst (Table 1, entry 11).
Table 1. Optimization of Dienoic Acid
Protodecarboxylation
With these optimal conditions, the scope of the reaction was
evaluated using other dienoic and polyenoic acids (Scheme 3).
a
Scheme 3. Protodecarboxylation of Various Polyenoic Acids
Pd(PPh3)4
(mol %)
solvent
(0.1 M)
additive
(1.2 equiv)
H2O
(equiv)
yield
(%)
entry
b
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
10
10
10
15
none
none
none
none
MeOH
DCE
DCE
DCE
DCE
DCE
DCE
DCE
NEt3
0
0
25
25
25
0
25
25
25
25
25
25
0
formic acid
formic acid
NEt3
none
none
none
formic acid
NEt3
pyridine
pyridine
PPh3/
pyridine
PPh3/
pyridine
PPh3
(0.4 equiv)
pyridine
pyridine
dec
dec
b
22
b
0
0
b
28
dec
61
64
80 (88)
81
9
10
11
12
c
10
d
c
b
13
14
10
DCE
DCE
0
0
d
c
10
25
78
c
b
15
16
10
5
DCE
DCE
25
25
0
e
(97)
a
Isolated yield with parenthetical NMR yield using dimethyl
b
terphthalate as internal standard. Starting material was recovered.
c
d
Pd(OAc)2 used in place of Pd(PPh)4. Reaction was performed
e
using PPh3 (0.4 equiv) and pyridine (1.2 equiv). Reaction run on 1-g
scale.
conditions, neither product 1a nor 2a was isolated. Using the
same conditions in the presence of water, decomposition was
observed with formic acid, but a 22% yield of diene 1a was
isolated with no observed formation of 2a when Et3N was
added (entries 3 and 4). Further optimization studies of these
solventless conditions indicated a reproducibility problem due
to the inconsistency in mixing the heterogeneous reaction
mixture. Thus, we continued the optimization using DMF,
MeOH, THF, and DCE. Only the reaction performed in DCE
gave the desired product when water was added to the reaction
mixture (Table 1, entry 7). The addition of organic bases led
to an improvement in yield to greater than 60% (entries 9 and
10), whereas the sodium carboxylate of 1a was poorly soluble
and less reactive. Increasing the catalyst loading to 15 mol %
gave the desired product with 80% yield (entry 11).
For comparison, protodecarboxylation using Pd(OAc)2 (Table
1, entry 14) was also examined with various substrates. The
yields were comparable to Pd(0) conditions (within 10%).
Methoxyaryl dienoic acid derivatives (3−6) were efficiently
converted to their respective dienes with yields ranging from
71% to 83%. Similarly, good to excellent yields were observed
with arenes possessing free phenolic (7), chloro (8), and nitro
(9) groups (86%, 90%, and 76%, respectively). When 4-
bromophenyl derivative 11 (mixture of 4Z/4E) was subjected
to protodecarboxylation conditions, a mixture of protodecar-
boxylative products (1Z/1E) were isolated in a moderate yield
(54%). Remarkably, under these mild conditions, no products
B
DOI: 10.1021/acs.orglett.8b03016
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
were observed from palladium insertion into any carbon−
halogen bonds. Furyl and cyclohexyl dienoic acids 12 and 13
reacted under the same conditions to give their corresponding
butadienes in 75% and 61% yields, respectively. Sorbic and
pentadienoic acids reacted to give their decarboxylated diene
derivatives 14a and 15a (1H NMR yields due to the volatility
of 1,3-butadiene and 1,3-pentadiene).
SI). Under these harsher conditions, 13-cis-retinoic acid (19)
gave a mixture of decomposition products, and no reactivity
was observed for benzoic acids.
To exemplify the selectivity of our mild conditions, we
subjected an equimolar mixture of p-methoxycinnamic acid
(22) and 4-(p-methoxyphenyl)-2,4-pentadienoic acid (5) to
our standard reaction conditions (Scheme 4A). The only
With the exception of furyl and 4-bromophenyl derivatives
(11a and 12a), all of the isolated butadiene derivatives adopted
the thermodynamically more stable E-geometry, although the
stereochemistry correlates to that of the starting material
(dienoic acids 6−8 and 11−13 were prepared as a mixture of
4Z/4E). Protodecarboxylation of α-methylpentadienoic acid
derivative (17) at 50 °C resulted in a 15% yield of a mixture of
products (3Z/3E ratio 1:3) and 20% yield using Pd(II) catalyst
(3Z/3E ratio 1:6). Running the reaction with Pd(0) at 80 °C
increased the yield of 17a, but with lessened selectivity (50%,
3Z/3E 1:1.5).
Scheme 4. Selectivity of Protodecarboxylation Reactions
Further extension of the unsaturated acid, by the insertion of
an additional vinyl group, led to the efficient formation of
triene 16a upon protodecarboxylation. Based on the success of
hexatrienoic acid 16, the quintessential polynoic acid, retinoic
acid, was evaluated. all-trans-Retinoic acid (18) yielded the
desired product in 85% yield, but its 13-cis analogue (19,
Figure 1) failed to react. It has been reported that under basic
isolated product was diene 5a, and cinnamic acid 22 was
completely recovered. This result shows that cinnamic acids do
not poison the catalyst, nor do dienoic acids modify the
catalyst to enable the reactivity of 22. Similarly, 1a was isolated
from the reaction of 1 in the presence of benzoic acid 27
(Scheme 4B). To show the complementarity of our conditions,
the same benzoic and dienoic acids were reacted with a silver
catalyst, and the opposite chemoselectivity was observed
(Scheme 4C). It is noteworthy that C(sp2)-carboxylic acids
react under silver or copper protocols in a pattern where
benzoic acids are much more reactive than cinnamic acids,9,13
and dienoic acids are negligibly reactive. This reactivity pattern
is inverted using our palladium conditions.
A possible mechanistic pathway for this Pd(II)-catalyzed
process is shown in Figure 2. Triphenylphosphine attacks16 the
β-position of the electophilic π-complex (A), initially formed
from Pd(II) and diene, to generate phosphonium intermediate
B. The elimination of CO2 and PPh3 from this intermediate
yields vinyl palladium species C. Protodepalladation in the
presence of water furnishes the diene product. A similar 1,2-
addition, 1,2-elimination mechanism has been proposed
previously.17
Figure 1. Unreactive carboxylic acids with protodecarboxylation
reaction conditions.
conditions, 13-cis-retinoic acid undergoes lactonization (δ-
lactone), which would hinder the protodecarboxylation.7b
However, 1H NMR analysis of the reaction mixture and
purified compounds did not indicate the formation of a δ-
lactone product under the mild conditions of our process.
Similar selectivity was observed from the reaction of Z-dienoic
acid 20 (Figure 1), even at 80 °C, despite the reactivity of its
E-analogue (17). On the basis of these data, it is surprising that
protodecarboxylation of compound 3 proceeded smoothly to
give the desired doubly decarboxylated product instead of only
removing the E-carboxylic acid group (Scheme 3). These
results suggest that compound 4 (1E-isomer) is selectively
formed in situ after the first decarboxylation step, which was
confirmed by 1H NMR analysis of the reaction mixture after 8
h.
Acrylic and cinnamic acids 21−26 (Figure 1) failed to react
under our conditions. The lack of reactivity for these
compounds suggests that dienoic acids have unique reactivity
under these conditions.15 Fortunately, increasing the reaction
temperature to 100 °C and replacing DCE with trifluor-
omethylbenzene resulted in the decarboxylation of cinnamic
acid derivatives 22 and 26 (72% and 55% yield, respectively,
The E versus Z selectivity for starting materials can be
rationalized by analyzing the secondary interactions in
phosphonium carboxylate intermediates (B′ and B″, Figure
2). In both intermediates, the conformers positioning the
carboxylate and phosphonium in antiperiplanar orientations
can be achieved. However, in B′, resulting from trans acid,
Pd−π secondary interactions are feasible. This interaction
could lower the energy requirement for the elimination step,
C
DOI: 10.1021/acs.orglett.8b03016
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mpcroatt@uncg.edu.
ORCID
Mitchell P. Croatt: 0000-0002-5643-7215
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding for this project from the National Science Foundation
(CAREER CHE-1351883) and North Carolina Biotechnology
Center (BRG-1205) is gratefully acknowledged. The authors
thank Dr. Franklin J. Moy (UNCG) for assistance with analysis
of NMR data and Dr. Daniel A. Todd (UNCG) for acquisition
of the high-resolution mass spectrometry data at the Triad
Mass Spectrometry Laboratory at the University of North
Carolina at Greensboro.
Figure 2. Proposed mechanism for Pd(II)-catalyzed protodecarbox-
ylation.
allowing for low-temperature protodecarboxylation. In con-
trast, in B″, resulting from the cis-acid, the antiperiplanar
conformation of phosphonium and carboxylate does not allow
for coordination of the palladium to the π-system. Presumably,
the preferred pathway for B″, under the reaction conditions, is
the reformation of the cis-dienoate.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03016.
■
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