Metal-ligand cooperation in the cycloisomerization of alkynoic acids with indenediide palladium pincer complexes

Article abstract of DOI:10.1021/cs401029x

Indenediide Pd complexes 1a-c are shown to very efficiently catalyze the cycloisomerization of alkynoic acids into alkylidene lactones via metal-ligand cooperation (TON up to 2000). Complexes 1a-c are competent toward a broad range of alkynoic acids, including functionalized and internal ones, and give access to 5- as well as 6- and 7-membered lactones in excellent yields and with very high selectivities.

Full text of DOI:10.1021/cs401029x

Letter
pubs.acs.org/acscatalysis
Metal−Ligand Cooperation in the Cycloisomerization of Alkynoic
Acids with Indenediide Palladium Pincer Complexes
Noel Nebra, Julien Monot, Rosie Shaw, Blanca Martin-Vaca,* and Didier Bourissou*
́
Universite de Toulouse, UPS, LHFA, 118 Route de Narbonne, F-31062 Toulouse, France and CNRS, LHFA UMR 5069, F-31062
Toulouse, France
S
* Supporting Information
ABSTRACT: Indenediide Pd complexes 1a−c are shown to
very efficiently catalyze the cycloisomerization of alkynoic
acids into alkylidene lactones via metal−ligand cooperation
(TON up to 2000). Complexes 1a−c are competent toward a
broad range of alkynoic acids, including functionalized and
internal ones, and give access to 5- as well as 6- and 7-
membered lactones in excellent yields and with very high
selectivities.
KEYWORDS: pincer complexes, metal−ligand cooperation, palladium, cycloisomerization, noninnocent ligand, homogeneous catalysis
dearomatization process in pyridine- and acridine-based pincer
complexes (complexes of type C)4 This proved extremely
efficient for the activation of a broad range of H−X bonds (X =
H, aryl, OR, NR2, ...) and led to spectacular catalytic
developments.4
Although the concept of metal−ligand cooperative catalysis is
relatively well-established, its development is still in his infancy,
and much remains to be discovered in the field. In particular,
the variety of ligand frameworks able to display noninnocent
character is quite limited, and so far, the scope of concerned
transformations, essentially hydrogenation/dehydrogenation
processes, remains relatively narrow.
he efficiency of cooperative catalysis in biological systems
Thas inspired chemists to design and develop molecular
complexes in which metal and ligands act in concert to promote
chemical processes. Here, the ligands not only modulate the
stereoelectronic properties of the metals but also participate
directly in the catalytic process (they are temporarily
transformed and, thus, exhibit so-called noninnocent charac-
ter). Such metal−ligand cooperation enables activation/
formation of chemical bonds under mild conditions and
represents a valuable alternative to the typical oxidative
addition/reductive elimination pathways.1
This approach was initially developed by Noyori and Shvo
with complexes of types A2 and B,3 whose ligands undergo
reversible amine−amide and alcohol−ketone interconversions.
Over the last two decades, spectacular progress has also been
achieved in metal−ligand cooperative catalysis using pincer
complexes (representative examples are given in Chart 1).4−7
In particular, Milstein discovered an original aromatization/
Recently, we described original pincer complexes of type G
(Chart 2) in which two Ph2PS side-arms support in-plane σ-
Chart 2. Indenediide Pd Pincer Complexes G
Chart 1. Selected Examples of Metal-Ligand Cooperative
Complexes
coordination of the central indenyl moiety (instead of the most
common facial π coordination).8 According to analytical data
and DFT calculations, complexes G are best described as
indenediide Pd(II) complexes. The combination of an
electrophilic Pd center and an electron-rich ligand backbone
was substantiated experimentally by stoichiometric reactions at
either the Pd center or the ligand backbone.9 The noninnocent
Received: November 5, 2013
Published: November 8, 2013
© 2013 American Chemical Society
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character of the indenediide ligand was also illustrated by the
preparation of original polymetallic species.10
again afforded quantitatively the alkylidene lactone 5a (entry
4). Moreover, complete conversion could be attained within 6 h
using only 0.05 mol % of complex 1b (turnover number TON
= 2000), carrying out the reaction at 90 °C and 2 M
concentration (entry 5).
Here, we report on the catalytic applications of such
indenediide Pd complexes involving metal−ligand cooperation.
Complexes of type G are shown to efficiently promote the
cycloisomerization of alkynoic acids (intramolecular addition of
carboxylic acids to alkynes), offering a valuable alternative to
the complexes commonly used to catalyze this transformation
(Pd, Rh, Ru, Pt, Au).11,12 With indenediide Pd complexes, no
external base is needed, the alkylidene lactones are obtained
with very high selectivities, and the reaction is unprecedentedly
broad in scope.
The scope of the reaction was then examined (Table 2).
Several aims were pursued: (i) assess the generality of our
cooperative catalyst and (ii) improve the catalytic trans-
formation in terms of efficiency, selectivity, and scope. First,
different substituents were introduced in the α position to the
n
carboxylic acid. With hex and CO2Et groups, the cyclization
proceeds readily and is actually faster than with the model
substrate 4a (most likely as the result of the Thorpe−Ingold
effect) (entries 2 and 3). The related N-protected amino acid
4d is only sparingly soluble in the reaction solvent and requires
harsher conditions to be cyclized (90 °C over 12 h), but again,
the corresponding γ-alkylidene lactone 5d was obtained in
quantitative yield (entry 4).15 The exclusive formation of 5e
upon cyclization of the propargyl-allyl substrate 4e (entry 5)
illustrates the complete selectivity of the reaction in favor of
alkynes over alkenes. We then varied the linker between the
two reactive moieties. The rigidified substrate 4f was
quantitatively converted into the 3-methylene-3H-isobenzofur-
an-1-one 5f in only 5 min at room temperature (entry 6). The
length of the linker was then modulated, taking into account
that the formation of 6- and 7-membered lactones is
significantly more difficult to achieve than that of 5-membered
lactones. Complex 1b was able to promote the 6-exo cyclization
of 5-hexynoic acid 4g at 90 °C, and the corresponding 6-
membered lactone 5g was obtained in quantitative yield within
12 h (entry 7). This is among the best results obtained so far
for this transformation.12a,b,16
The catalytic activity of indenediide Pd complexes was first
evaluated in the cyclization of 4-pentynoic acid (4a) as the
model reaction. The catalytic tests were carried out in CDCl3 at
room temperature, with 5 mol % of catalyst and in the absence
of any base (Table 1). The reaction proceeds readily and
Table 1. Evaluation of the Catalytic Properties of the
Indenediide Pd Complexes 1a−c in the Cyclization of 4-
Pentynoic Acid 4a
a
b
entry
cat.
Pd mol %
T (°C)
time (h)
conv (%)
We then turned to internal alkynoic acids that are much
more challenging substrates than the terminal ones. At 90 °C,
3-pentynoic acid 4h readily undergoes 5-endo-dig cyclization.
The reaction is complete within 3 h, and lactone 5h is obtained
quantitatively (entry 8). With the homologous substrate 4i,
heating for 24 h at 90 °C was necessary to achieve complete
conversion. Competition between 5-exo and 6-endo cycliza-
tions leads in this case to a 3:1 mixture of the 5- and 6-
membered lactones 5iexo and 5iendo, respectively (entry 9).
Importantly, 5iexo is obtained exclusively in its Z form (as
established by comparing its NMR data with those reported in
the literature).17 This is consistent with activation of the CC
triple bond by π-coordination to Pd and trans addition of the
carboxylate moiety (see below for mechanistic discussion).17
Complex 1b also showed good activity in the cyclization of the
rigid internal alkynoic acids 4j and 4k The reaction requires
only 15−20 min at room temperature. In both cases, it is
completely selective and affords a unique benzannelated
lactone, but with a strong influence of the alkyne substituent:
only 5-exo cyclization with Ph versus 6-endo cyclization with
Cy (entries 10 and 11).18 Last, the formation of 7-membered
lactones was targeted,19 and gratifyingly, the two δ-alkynoic
benzoic acids 4l and 4m were found to be conveniently cyclized
within 7−36 h at 90 °C (entries 12 and 13).
1
2
3
4
1a
1b
1c
1b
1b
5
25
25
25
25
90
1
1
99
99
99
99
99
5
5
1
1
24
6
c
5
0.05
a
All catalytic tests were performed under argon atmosphere starting
b
from 0.2 mmol of 4-pentynoic acid (0.1 M in CDCl3). Conversions
c
were determined by 1H NMR and GC/MS analyses. Reaction carried
out on a 2 M solution of 4-pentynoic acid in CDCl3.
exclusively via 5-exo-dig cyclization. Using the chloropalladate
complex 1a as the catalyst, 5-methylene-dihydrofuran-2-one
(5a) is thereby obtained quantitatively after 1 h (entry 1).13
Practically identical results were obtained with the neutral
trimeric complex 1c (entry 3),14 indicating that the ionic
character of 1a and n-Bu4N+ countercation are not crucial.
Replacing chloride for iodide at palladium did not induce a
significant variation of catalytic activity either. With the
iodopalladate 1b,14 the alkylidene lactone 5a is also obtained
quantitatively within ∼1 h under the same conditions (entry 2).
The study was continued with complex 1b because it is less
soluble in low-polar solvents, which facilitates work-up and
isolation of the formed lactones.13 Recycling tests were then
performed to assess the robustness of the system, and
satisfyingly, no sign of catalyst deactivation was detected up
to at least 10 cycles (after each cycle, the solvent was removed
under vacuum and the lactone 5a was extracted with diethyl
ether).13 To further evaluate catalytic performance, the catalyst
loading was decreased. Using 1 mol %, longer reaction times
were required at 25 °C (24 h), but the cyclization reaction
The fact that complexes 1a−c promote the cyclization of
4a−m without any additive is remarkable and contrasts with
the known Pd-based catalysts that require the presence of 1.5−
10 equiv of a base, typically a tertiary amine.12 This has
certainly to do with the properties of the employed ligand. As
mentioned above, the indenediide backbone is electron-rich. It
can thus behave as an internal base and catch temporarily the
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Table 2. Scope of the Pd-Catalyzed Cyclization of Alkynoic Acids (4a−m)
a
Catalytic reactions performed under argon atmosphere using 0.2 mmol of the corresponding alkynoic acid 4a−m (0.1 M in CDCl3) and 5 mol % of
b
c
the iodopalladate 1b. Conversions were determined by 1H NMR and GC/MS analyses. Isolated yields are given in brackets. The high volatility of
the compound prevented accurate determination of the isolated yield.
acidic proton of the substrate. To corroborate this scheme,
derivatives 1a−c have been compared with the related 2-
indenyl complexes 2a and 3a,b,14,20 in which the ligand
backbone is protonated or methylated at C1 (Table 3). No
reaction at all was observed with 2a and 3a (entries 1 and 2),
and cyclization of 4a proceeds very slowly with 3b (15%
conversion after 24 h at room temperature, entry 3).
activity of the iodopalladate 1b was compared with that of the
corresponding 2-indenyl complex 3b combined with Et3N as an
external base. Cyclization of 4-pentynoic acid 4a proceeds with
the bicomponent system, but it is noticeably slower than with
1b alone: only 51% conversion after 2h with 3b + Et3N (entry
4), although the reaction is complete within 1 h with 1b. The
iodopalladate 1b also displays significantly higher activity than
the bicomponent system 3b + Et3N in the cyclization of 4m, a
more challenging substrate (see Supporting Information Figure
S3),13 corroborating metal−ligand cooperation.
To further corroborate the cooperation between the
electron-rich indenediide backbone and the metal, the catalytic
All these observations strongly argue in favor of an active
participation of the indenediide motif in the catalytic
transformation, and the following simplified catalytic cycle
can be proposed for the cyclization of alkynoic acids catalyzed
by the palladates 1a,b (Scheme 1): (i) activation of the
substrate by protonation of the ligand backbone at C1 and
coordination of either the carboxylate or the alkyne to
palladium (complexes of type I and II, respectively);21 (ii)
cyclization to give an alkenyl complex of type III; and finally,
(iii) liberation of the alkylidene lactone by back-transfer of the
proton from the ligand backbone and regeneration of the
palladate.22 At this point, cyclization reactions were carried out
starting from 4-pentynoic acid labeled with deuterium either at
the carboxylic acid (COOD) or at the alkyne (CCD) moiety.
In both cases, the deuterium labeling was retained, and no
scrambling was detected.13 The obtained alkylidene lactone 5a
was selectively deuterated trans to O in the case of the CO2D
substrate (cis to O in the case of the CCD substrate),
consistent with the proposed scenario.
Table 3. Indenyl Pd Complexes 2a and 3a,b Evaluated in the
Cyclization of 4-Pentynoic Acid 4a
a
b
entry
cat.
Pd mol %
T (°C)
time (h)
conv. (%)
0
1
2
3
4
2a
3a
3b
5
5
5
25
25
25
25
36
24
24
2
c
0
15
51
c
3b + Et3N
5
a
Catalytic tests were performed under argon atmosphere starting from
b
0.2 mmol of 4-pentynoic acid (0.1 M in CDCl3). Conversions were
c
determined by 1H NMR analyses. Reaction carried out with 5 mol %
Pd and 5 mol % Et3N.
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Scheme 1. Simplified Catalytic Cycle Proposed for the
Cyclization of Alkynoic Acids Catalyzed by 1a,b
ACKNOWLEDGMENTS
■
We are grateful to the CNRS and UPS. N.N. thanks the
European Commission for a Marie Curie Intra-European
Postdoctoral Fellowship (PIEF-GA-2009-253112, INDEN). S.
Mallet-Ladeira is acknowledged for the X-ray diffraction
analysis of complex 1c. We also thank J. Lisena for the control
experiments with complex 3a.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data of new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
*E-mail: dbouriss@chimie.ups-tlse.fr.
Notes
■
The authors declare no competing financial interest.
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