Dual reactivity of O-α-allenyl esters under palladium(0) catalysis: From carbopalladation/allylic alkylation domino sequence to decarboxylative allenylation

Article abstract of DOI:10.1016/j.jorganchem.2012.03.014

In a mechanistically-oriented study, O-α-allenyl esters have been evaluated as potential substrates for Pd-catalyzed carbopalladation/allylic alkylation domino sequences and decarboxylative allenylation reactions. The domino sequence turned out to be feas

Full text of DOI:10.1016/j.jorganchem.2012.03.014

Journal of Organometallic Chemistry 714 (2012) 53e59
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Dual reactivity of O-a-allenyl esters under palladium(0) catalysis: From
carbopalladation/allylic alkylation domino sequence to decarboxylative
allenylation
,
a,
Claire Kammerer-Pentier a , Alba Diez Martinez b, Julie Oble a, Guillaume Prestat a, Pedro Merino b,
*
Giovanni Poli a
a Institut Parisien de Chimie Moléculaire UMR CNRS 7201, FR2769 Institut de Chimie Moléculaire UPMC, 4 Place Jussieu, case 183, F-75005 Paris, France
b Laboratorio de Síntesis Asimétrica, Departamento de Química Orgánica, Instituto de Síntesis y Catálisis Homogénea (ISQCH), Universidad de Zaragoza-CSIC, E-50009 Zaragoza,
Aragón, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
In a mechanistically-oriented study, O-a-allenyl esters have been evaluated as potential substrates for
Received 20 January 2012
Received in revised form
9 March 2012
Pd-catalyzed carbopalladation/allylic alkylation domino sequences and decarboxylative allenylation
reactions. The domino sequence turned out to be feasible only with electron-deficient aryl iodides,
thereby showing a narrower potential than that previously observed with the malonamide-based series.
Accepted 12 March 2012
The decarboxylative allenylation of O-
with -ketoester-based substrates and using specific basic conditions. Plausible reaction mechanisms are
proposed to account for the observed reactivities.
a-allenyl esters was also tested and turned out to be practicable
b
Keywords:
Palladium
Allene
Ó 2012 Elsevier B.V. All rights reserved.
Lactone
Domino reactions
Decarboxylative allenylation
1. Introduction
allenyl malonate precursor would be highly desirable, too. Indeed,
the oxygenated heterocycles thus formed are valuable intermedi-
ates for the synthesis of natural products [9]. Interestingly, Ma et al.
recently reported the Pd-catalyzed cyclization of monosubstituted
Over the last decades, allenes have aroused an ever-growing
interest in the field of transition-metal catalysis [1] and they are
currently considered as highly versatile and valuable precursors of
carbo- and heterocycles [2]. The reactivity of organylpalladium
species towards 1,2-dienes is particularly interesting since the car-
b-to-d allenyl esters in the presence of various aryl iodides, yielding,
via an endo cyclization mode, the corresponding 8-to-10-
membered lactones (Scheme 2, bottom right) [10]. Such event
prompts us to disclose our results on the 5-exo allene carbopalla-
bopalladation step affords regioselectively an
h
3-allylpalladium
intermediate [3], which may be subsequently trapped by an internal
carbo- [4] or heteronucleophile [5,6]. Such an allene carbopallada-
tion/allylation domino sequence was exploited by some of us, as part
of our research dedicated to the synthesis of pyrrolidin-2-ones via
transition-metal-catalyzed processes [7]. Indeed, a family of trans 4-
dation/allylation domino reaction of O-a-allenyl malonates
(Scheme 2, top right). Additionally, we show that under appropriate
conditions, the decarboxylative allenylation of these substrates is
also feasible (Scheme 2, left).
(a-styryl)
g-lactams was recently prepared in high yields, using
2. Results and discussion
a readily accessible N-a-allenyl malonamide precursor and various
aryl iodides under palladium(0) catalysis (Scheme 1) [8].
From a synthetic point of view, access to the analogous 4-(
styryl) g-lactones via the same domino sequence applied to an O-
The monosubstituted O-a-allenyl malonate 1a [11] and iodo-
a
a
-
-
benzene were chosen as model coupling partners to test the
planned allene carbopalladation/allylic alkylation sequence
(Table 1, entries 1 and 2).
Unfortunately, neither the conditions previously reported by Ma
et al. [4b] for a similar transformation [Pd(OAc)2 cat., PPh3 cat.,
K2CO3, n-Bu4NBr cat., CH3CN] (entry 1), nor those involving initial
preformation of the sodium enolate A [Pd(OAc)2 cat., dppe cat.,
* Corresponding author. Present address: Lehrstuhl für Organische Chemie I,
Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany.
Fax: þ49 33 0 1 44 27 42 87.
E-mail address: claire.kammererpentier@mytum.de (C. Kammerer-Pentier).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.03.014

54
C. Kammerer-Pentier et al. / Journal of Organometallic Chemistry 714 (2012) 53e59
Scheme 1. Preparation of 4-(a-styryl) g-lactams via a Pd-catalyzed allene carbopalladation/allylic alkylation domino sequence.
Scheme 2. New and reported synthetic paths involving O-allenyl esters.
NaH, DMF] (entry 2) afforded the desired
putative intermediate (Scheme 3, top). Instead, extensive
decomposition was observed upon heating the reaction mixture.
Considering the known tendency of O- -allenyl esters to
g
-lactone 2a, via the
catalyst was going to take place, thereby triggering decomposition
or formation of side products (Scheme 3, bottom path).
In order to validate our hypothesis, we repeated the experiment
omitting the aryl halide and using the electron-rich ligand tri-tert-
butylphosphine, so as to boost the oxidative addition process (entry
B
a
oxidatively add onto palladium(0) species [12], we suspected that
a competitive interaction between enolate A and the palladium(0)
3) [13]. After 18 h at rt, 3,4-trans g-lactone 3a bearing a trienyl
Table 1
Screening of the conditions for the cyclization of precursors 1ae1c.
Entry
Substrate
R0
T (ꢀC)
t (h)
Conv. (%)a
Yield of 2 (%)b
Yield of 3 (%)b
1c,d
2d
3e,f
4e
5
1a
1a
1a
1a
1a
1b
1c
H
H
82
60
rt
4
2.5
18
16
5
100
100
n.d.
n.d.
100
100
0
0
0
0
0
0
e
45 (3a)
12 (3a)
0
0
0
NO2
NO2
NO2
NO2
rt
16 (2a)
Traces (2a)
39 (2b)
0
60
60
60
6
7
16
24
a
Consumption of O-
a
-allenyl malonate 1 as determined by 1H NMR spectroscopy.
Yields refer to isolated yields.
b
c
d
e
f
The following conditions were used: Pd(OAc)2 (5 mol%), PPh3 (30 mol%), aryl iodide (1.2 equiv.), K2CO3 (4 equiv.), n-Bu4NBr (10 mol%), CH3CN.
No conversion of 1a was observed at rt.
P(tBu)3 (30 mol%) was used as ligand.
The reaction was carried out in the absence of aryl iodide.

C. Kammerer-Pentier et al. / Journal of Organometallic Chemistry 714 (2012) 53e59
55
Scheme 3. Dual reactivity of O-a-allenyl ester enolate A under palladium(0) catalysis.
substituent was obtained in 45% yield as a single regio- and dia-
stereoisomer. A mechanistic rationale for the formation of 3a is
(12% yield). Unfortunately, further attempts of optimization
improved neither the selectivity, nor the reaction yield.
depicted in Scheme 4. Oxidative addition of the O-
a
-allenyl ester
The influence of the substitution pattern of the allenyl moiety
was next studied (entries 5e7). The trisubstituted allenyl precursor
1b, bearing a gem-dimethyl moiety at the terminal allene position,
enolate A onto the palladium(0) catalyst affords the vinylidene-h3
allyl complex C, which is in equilibrium with the corresponding h1
-
-
dienyl intermediate C0. Coordination of a second molecule of eno-
gave g-lactone 2b in 39% yield (entry 6), whose single crystal X-ray
late A to the electrophilic palladium complex C0 and subsequent
diffraction analysis [17] confirmed its structure and allowed the
unambiguous assignment of the trans configuration between the
newly formed stereocenters (Fig. 1) [18]. Unfortunately, the cyclo-
hexyl precursor 1c failed to cyclize (entry 7).
carbopalladation generates, via D, the
h
3-allyl intermediate E.
Finally, intramolecular trapping of the latter by the internal active
methylene yields the 5-exo cyclization product 3a and releases the
active catalytic species [14].
Further optimization was next performed using precursor 1b
and 1-iodo-4-nitrobenzene (Table 2). A first set of conditions,
involving notably the crown ether 15-crown-5 (Method A), allowed
improving the yield of the sequence to 49% (entry 1). Worthy of
note, omission of the crown ether led to a diminished yield of 26%
(Method B, entry 2). Further experiments revealed that the reaction
scope is limited to electron-poor aryl iodides, such as 1-iodo-4-
(trifluoromethyl)benzene (entry 3). Indeed, in the presence of the
more electron-rich 4-iodotoluene, the reaction gave only traces of
the corresponding lactone 5 (entry 4). Coming back to 1-iodo-4-
nitrobenzene, use of phosphine-free conditions (Method C),
The above result (Table 1, entry 3) confirms the presence of
a concurrent pathway involving the direct oxidative addition of
precursor A onto palladium(0) (Scheme 3, bottom). We reasoned
that an acceleration of the desired oxidative addition step involving
the aryl halide partner might limit the formation of side product 3
arising from the competitive oxidative addition. Accordingly, the
next experiment was run in the presence of the electron-poor 1-
iodo-4-nitrobenzene, which is highly activated toward oxidative
addition (entry 4) [15]. In line with our expectations, the desired 4-
(a
-styryl)
g-lactone 2a [16] was obtained (16% yield) as a single
regio- and trans diastereomer, along with the trienic lactone 3a
previously developed by us for the cyclization of the related N-a-
allenyl malonamides [8], led to the best reaction yield with 1b (53%)
(entry 5). Interestingly, although the catalytic systems involved in
Method A and C are of extremely different natures, they are both
equally effective. It is also worth noting that in all the cases studied,
the O-
a-allenyl malonates afforded exclusively the corresponding
5-exo cyclization products. In contrast, Ma et al. [10] observed the
sole endo products in the cyclization of the related b-to-d allenyl 3-
oxoalkanoates.
The above study shows that the carbopalladation/allylation
sequence of O- -allenyl malonates has a much more limited
efficiency and scope than that of the corresponding malona-
mides [8]. This is likely due to the high propensity of O-
a
a
-
allenyl esters to suffer competitive oxidative addition onto the
palladium(0) catalyst [12]. We thus decided to inspect whether
it was possible to exploit the intrinsic reactive bias of these
substrates to perform Pd-catalyzed decarboxylative allenylations
[19e23] (Scheme 5, bottom). However, in order to avoid the
previously observed carbopalladation of the terminal allene
(Scheme 4 and Scheme 5, top), we directly focused on the gem-
dimethyl allenyl derivatives, and started our investigation
testing precursor 1b.
Scheme 4. Mechanistic rationale for the formation of trienic g-lactone 3a.

56
C. Kammerer-Pentier et al. / Journal of Organometallic Chemistry 714 (2012) 53e59
A plausible mechanism for the formation of g-allenyl ketone 6d
is proposed in Scheme 6. Oxidative addition of the sodium enolate
of precursor 1d onto the palladium(0) catalyst affords the vinyli-
dene-
dianion of F at the terminal position of its electrophilic
cationic fragment generates the allenyl derivative 7 [26]. Eventu-
ally, acidic work-up generates -ketoacid 8 that readily decarbox-
ylates to the -allenyl ketone 6d.
h
3-allyl complex F. Subsequent C-attack of the acetoacetate
h
3-allyl
b
g
3. Conclusion
This study shows that the development of a Pd-catalyzed car-
bopalladation/allylation domino sequence on O- -allenyl malo-
nates, although feasible, suffers a more limited efficiency and scope
than that involving N- -allenyl malonamides. This drawback is
likely due to the proclivity of the O- -allenyl esters to undergo
a
a
a
competitive oxidative addition onto the palladium(0) catalyst. The
intrinsic reactive bias of these substrates was exploited to perform
Pd-catalyzed decarboxylative allenylations. Although
extensive scoping and accurate mechanistic studies have to wait for
further investigations, we have established the feasibility of this
a more
Fig. 1. Solid-state molecular structure of 2b (see Table 1) with thermal ellipsoids set at
50% probability and hydrogen atoms partially omitted for clarity.
transformation in the case of O-a-allenyl b-oxoesters. Interestingly,
Despite several trials in the absence or in the presence of NaH,
and in different solvents (THF, dioxane, DMF) no decarboxylative
allenylation could be detected (Table 3, entries 1e3). As b-oxoesters
are known to afford Pd-catalyzed decarboxylative allylations more
readily than malonates [24], 3-oxobutanoate 1d was next tested
(entries 4e6). Treatment of 1d with Pd(OAc)2/PPh3 in THF at reflux
(entry 4) left once more only unreacted substrate. However, when
comparison of our results with Ma’s ones enlightened the role of
the base in the palladium-catalyzed Carroll-type rearrangement as
well as the influence of the substitution pattern of the starting
substrate.
4. Experimental section
the same experiment was repeated in the presence of NaH, the g-
allenyl ketone 6d was obtained in 38% yield (entry 5). The feasibility
of the planned decarboxylative allenylation transformation was
thus proven. Comparison of entries 4 and 5 clearly indicates that
a base is needed for this process to take place. This point is inter-
esting, as the Pd-catalyzed decarboxylative allylation originally
reported by Tsuji [25a] and Saegusa [25b] as well as the allenylation
procedure recently disclosed by Ma et al. [23] do not require the use
of a base. Not unexpectedly, allene 1d failed to afford 6d when
treated under Ma’s conditions (entry 6).
4.1. Procedure for the allene carbopalladation/allylation domino
sequence, starting from O-a-allenyl malonate 1b (Method B)
Sodium hydride (24 mg, 0.6 mmol, 1.2 equiv., 60% dispersion in
mineral oil) was added to a solution of O- -allenyl malonate 1b
a
(99 mg, 0.5 mmol, 1 equiv.) in THF (2.5 ml) under argon atmo-
sphere. The resulting mixture was stirred at rt for 20 min. In
another flask, triphenylphosphine (13 mg, 50
added to a solution of Pd(OAc)2 (6 mg, 25
mmol, 10 mol%) was
mol, 5 mol%) in THF
m
Table 2
Optimization and scope of the allene carbopalladation/allylation domino sequence with precursor 1b.
Entry
R
Methoda
Conv. (%)b
Product
Yield (%)c
1
2
3
4
5
NO2
NO2
CF3
Me
NO2
A
B
B
B
C
90
85
100
100
100
2b
2b
4
5
2b
49
26
22
Traces
53
a
Method A: Pd(OAc)2 (5 mol%), PPh3 (10 mol%), aryl iodide (1.2 equiv.), NaH (1.2 equiv.), 15-crown-5 (1.2 equiv.), THF, 55 ꢀC, 60 h. Method B: Pd(OAc)2 (5 mol%), PPh3
(10 mol%), aryl iodide (1.2 equiv.), NaH (1.2 equiv.), THF, 55 ꢀC, 16 h. Method C: PdCl2(MeCN)2 (10 mol%), n-BuLi (20 mol%), aryl iodide (1.2 equiv.), NaH (1.2 equiv.), n-Bu4NBr
(20 mol%), DMSO, 55 ꢀC, 2.5 h.
b
Consumption of 1b as determined by 1H NMR spectroscopy.
Yields refer to isolated yields.
c

C. Kammerer-Pentier et al. / Journal of Organometallic Chemistry 714 (2012) 53e59
57
Scheme 5. Reactivity paths of O-
a-allenyl esters under Pd(0) catalysis.
Table 3
Screening of the conditions for the decarboxylative allenylation of 1b and 1d.
4.2. Procedure for the Pd-catalyzed decarboxylative allenylation,
starting from O-a-allenyl 3-oxobutanoate 1d
Sodium hydride (126 mg, 3.14 mmol, 1.3 equiv., 60% dispersion
in mineral oil) was slowly added to a solution of O- -allenyl 3-
a
oxobutanoate 1d (440 mg, 2.42 mmol, 1.0 equiv.) in THF (1.5 ml)
at 0 ꢀC under argon atmosphere. The resulting mixture was stirred
at rt for 10 min. In another flask, triphenylphosphine (127 mg,
0.48 mmol, 20 mol%) and palladium diacetate (37 mg, 0.12 mmol,
5 mol%) were diluted in THF (1.5 ml). The resulting solution was
stirred at rt for 10 min and the solution containing the sodium
enolate of 1d was added via cannula. The resulting yellow mixture
was stirred at 65 ꢀC for 19 h and the reaction was quenched with
a saturated solution of NH4Cl. The aqueous layer was extracted with
diethyl ether. The combined organic layers were washed with
a saturated solution of NaCl, dried on MgSO4 and concentrated in
vacuo. The crude product was purified by flash chromatography on
silica gel eluting with pentane/Et2O 95:5 to afford the expected
product 6d in 38% yield (125 mg) as a colourless oil.
Entry Substrate Base (equiv.) Solvent T (ꢀC) t (h) Conv. (%)a Yield of 6 (%)b
1c
2c
3d
4
1b
1b
1b
1d
1d
1d
e
THF
THF
DMF
THF
THF
55
55
120
reflux 19
reflux 19 100
24
24
12 100
2
0
0
0
0
0
NaH (1.3)
e
100
0
e
5
NaH (1.3)
e
38 (6d)
0
6e
tBuOH 25
0
a
Consumption of O-a
-allenyl esters 1 as determined by 1H NMR spectroscopy.
Yields refer to isolated yields.
The same result was obtained in refluxing dioxane.
Pd(OAc)2 (10 mol%)/dppe (20 mol%) was used as catalytic system.
Pd(dba)2 (5 mol%)/DPEphos (5 mol%) was used as catalytic system.
b
c
d
e
4.3. Single crystal X-ray crystallography
Single crystal X-ray analysis of 2b: Intensity data were collected
on a NONIUS Kappa-CCD four-circle diffractometer by using
a graphite monochromated Mo-Ka radiation. The structure was
solved by direct methods with SHELXS-86 [27] refined by full least-
squares on F and completed with CRYSTALS [28]. All non-H atoms
were refined with anisotropic displacement parameters and H
atoms were simply introduced at calculated positions (riding model
with an overall isotropic temperature factor of 0.088).
(0.5 ml) under argon atmosphere. The appropriate aryl iodide
(0.6 mmol, 1.2 equiv.) was then introduced and the solution con-
taining the sodium enolate of 1b was added via cannula. The
resulting mixture was stirred at 55 ꢀC for 16 h. The reaction was
quenched with a saturated solution of NH4Cl and the aqueous layer
was extracted with diethyl ether. The combined organic layers were
washed with a saturated solution of NaCl, dried on MgSO4 and
concentrated in vacuo. The crude product was purified by flash
chromatography on silica gel.
Crystallographic data of compound: C16H17NO6, M ¼ 319.31,
monoclinic, space group P 21/n, a ¼ 7.3215(7), b ¼ 16.329(19),
Scheme 6. Proposed mechanism for the formation of g-allenyl ketone 6d.

58
C. Kammerer-Pentier et al. / Journal of Organometallic Chemistry 714 (2012) 53e59
(e) F.P.J.T. Rutjes, K.C.M.F. Tjen, L.B. Wolf, W.F.J. Karstens, H.E. Schoemaker,
H. Hiemstra, Org. Lett. 1 (1999) 717e720;
(f) H. Ohno, M. Anzai, A. Toda, S. Ohishi, N. Fujii, T. Tanaka, Y. Takemoto,
T. Ibuka, J. Org. Chem. 66 (2001) 4904e4914;
c ¼ 13.6581(19) Å,
b
¼ 94.407(10), V ¼ 1628.0(3) Å3, Z ¼ 4,
Dcalcd ¼ 1.30 g cmꢁ3
,
l
¼ 0.71073 Å, T ¼ 250(2) K, crystal size ¼ 0.08
$ 0.11 $ 0.15 mm3, 16,283 reflections measured, 4707 unique
(Rint ¼ 0.037) of which 2222 reflections with Fo2 > 3
s
(Fo2) were
(g) S. Ma, H. Xie, J. Org. Chem. 67 (2002) 6575e6578;
(h) S. Ma, J. Zhang, L. Lu, Chem. Eur. J. 9 (2003) 2447e2456;
(i) G.R. Cook, W. Xu, Heterocycles 67 (2006) 215e232;
(j) S. Ma, F. Yu, J. Li, W. Gao, Chem. Eur. J. 13 (2007) 247e254;
(k) S. Ma, Z. Zheng, X. Jiang, Org. Lett. 9 (2007) 529e531;
(l) W. Shu, Q. Yang, G. Jia, S. Ma, Tetrahedron 64 (2008) 11159e11166;
(m) X. Cheng, S. Ma, Angew. Chem. Int. Ed. 47 (2008) 4581e4583;
(n) E.M. Beccalli, G. Broggini, F. Clerici, S. Galli, C. Kammerer, M. Rigamonti,
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considered as observed and used in all calculations. The final reli-
ability factors are R ¼ 0.0404 and Rw ¼ 0.0450.
Acknowledgements
CNRS and UPMC are acknowledged for financial support.
MICINN (Madrid, Spain. Project CTQ2010-19606) and FEDER
Program (EU) are also acknowledged for financial support. A.D.-M.
thanks Government of Aragon (Spain) and CAI-Europe program for
a pre-doctoral fellowship.
(o) E.M. Beccalli, A. Bernasconi, E. Borsini, G. Broggini, M. Rigamonti, G. Zecchi,
J. Org. Chem. 75 (2010) 6923e6932;
(p) Q. Li, X. Jiang, C. Fu, S. Ma, Org. Lett. 13 (2011) 466e469;
(q) J. Cheng, X. Jiang, C. Zhu, S. Ma, Adv. Synth. Catal. 353 (2011) 1676e1682;
(r) W. Shu, Q. Yu, G. Jia, S. Ma, Chem. Eur. J. 17 (2011) 4720e4723.
[6] For examples involving fully intramolecular carbopalladation/allylation
processes, see: (a) R. Grigg, I. Köppen, M. Rasparini, V. Sridharan, Chem.
Commun. (2001) 964e965;
Appendix A. Supplementary material
(b) K. Hiroi, Y. Hiratsuka, K. Watanabe, I. Abe, F. Kato, M. Hiroi, Tetrahedron:
Asymmetry 13 (2002) 1351e1353;
CCDC 861578 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
(c) A. Okano, T. Mizutani, S. Oishi, T. Tanaka, H. Ohno, N. Fujii, Chem. Commun.
(2008) 3534e3536;
(d) S. Inuki, A. Iwata, S. Oishi, N. Fujii, H. Ohno, J. Org. Chem. 76 (2011)
2072e2083.
[7] (a) G. Giambastiani, B. Pacini, M. Porcelloni, G. Poli, J. Org. Chem. 63 (1998)
804e807;
(b) G. Poli, G. Giambastiani, B. Pacini, Tetrahedron Lett. 42 (2001) 5179e5182;
(c) G. Poli, G. Giambastiani, J. Org. Chem. 67 (2002) 9456e9459;
(d) P.-O. Norrby, M.M. Mader, M. Vitale, G. Prestat, G. Poli, Organometallics 22
(2003) 1849e1855;
(e) S. Lemaire, G. Giambastiani, G. Prestat, G. Poli, Eur. J. Org. Chem. (2004)
2840e2847;
Appendix B. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2012.
03.014.
(f) D. Madec, G. Prestat, E. Martini, P. Fristrup, G. Poli, P.-O. Norrby, Org. Lett. 7
(2005) 995e998;
(g) G. Maitro, G. Prestat, D. Madec, G. Poli, Synlett (2006) 1055e1059;
(h) M. Bui The Thuong, S. Sottocornola, G. Prestat, G. Broggini, D. Madec,
G. Poli, Synlett (2007) 1521e1525;
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