Silver-catalyzed functionalization of esters by carbene transfer: The role of ylide zwitterionic intermediates

Article abstract of DOI:10.1002/cctc.201402241

The reaction of esters with ethyl diazoacetate catalyzed by the complex [F₂₇-Tp^4Bo,3CF2CF3Ag(acetone)] generates α-(acyloxy)acetates in moderate to high yields. This is a novel transformation in the context of carbene-transfer reactions from diazo compounds that, according to experimental and theoretical data, is suggested to occur through zwitterionic intermediates.

Full text of DOI:10.1002/cctc.201402241

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DOI: 10.1002/cctc.201402241
Silver-Catalyzed Functionalization of Esters by Carbene
Transfer: The Role of Ylide Zwitterionic Intermediates
[a]
[a]
[b]
[a]
[c, d]
Riccardo Gava, M. ꢀngeles Fuentes, Maria Besora, Tomꢁs R. Belderrain, Kane Jacob,
[b, e]
[c, d]
[a]
[a]
Feliu Maseras,*
Michel Etienne,*
Ana Caballero,* and Pedro J. Pꢂrez*
The reaction of esters with ethyl diazoacetate catalyzed by the
4
Bo,3CF2CF3
complex [F –Tp
Ag(acetone)] generates a-(acyloxy)ace-
2
7
tates in moderate to high yields. This is a novel transformation
in the context of carbene-transfer reactions from diazo com-
pounds that, according to experimental and theoretical data, is
suggested to occur through zwitterionic intermediates.
Catalytic carbene-transfer reactions from diazo compounds
[
1]
constitute a significant transformation in organic synthesis.
Unsaturated or saturated substrates (Scheme 1) are effectively
transformed, usually under mild conditions, into value-added
products. This process is based on the formation of a strongly
electrophilic metallocarbene intermediate that reacts with nu-
cleophiles to give the desired products. One of the most signif-
icant examples is the functionalization of CÀH bonds of nonac-
Scheme 1. General transition-metal-catalyzed carbene transfer to saturated
and unsaturated substrates. Nu=Nucleophile.
4
Bo,3CnF2n+1
[F –Tp
N
Ag(L)] (N=6n+15; n=1, 2, 3, 4, 6; L=acetone
[
2,3]
[5b,c]
tivated alkanes.
Our group has been involved in this area
or THF).
The diazo compound used as the carbene source
for the last decade and has developed a family of copper and
silver complexes supported by tris(pyrazolyl)borate or tris(inda-
is ethyl diazoacetate (EDA). Thereby, nonactivated alkanes RÀH,
including methane, despite their large CÀH bond dissociation
x
[6]
zolyl)borate ligands with various substituents, Tp , that have
energies, have been converted into esters RÀCH CO Et
2
2
[
4,5]
[7]
been employed as catalysts in such reactions.
The most effi-
(Scheme 2).
cient catalysts are those based on silver associated with li-
gands bearing electron-withdrawing groups, which thus lead
to complexes in which the metal has strong electrophilic
Given that this strategy employs the substrate as the solvent
in most cases, a consecutive double insertion of the carbene
group into the same molecule of the alkane has never been
observed. Therefore, we decided to study the influence of
Br3
[5a]
character. This is the case of [Tp Ag(acetone)]
and
x
Tp AgL complexes on the reaction of esters RÀCH CO Et with
2
2
[a] R. Gava, Dr. M. ꢀ. Fuentes, Prof. Dr. T. R. Belderrain, Dr. A. Caballero,
EDA with the aim of observing such unprecedented double in-
sertion. Instead, to our surprise, a novel transformation was
found: the formation of a-(acyloxy)acetates by using
Prof. Dr. P. J. Pꢁrez
Laboratorio de Catꢂlisis Homogꢁnea, Unidad Asociada al CSIC
CIQSO—Centro de Investigaciꢃn en Quꢄmica Sostenible and Departamento
de Quꢄmica y Ciencia de los Materiales
Campus de El Carmen s/n, Universidad de Huelva
4
Bo,3CnF2n+1
[F –Tp
N
Ag(L)] complexes as catalysts. This reaction has
[1]
been reported with amides, esters, and aldehydes, but to the
2
1007-Huelva (Spain)
E-mail: perez@dqcm.uhu.es
ana.caballero@dqcm.uhu.es
[
b] Dr. M. Besora, Prof. Dr. F. Maseras
Institute of Chemical Research of Catalonia (ICIQ)
Av. Paꢅsos Catalans, 16, 43007 Tarragona (Spain)
E-mail: fmaseras@iciq.es
[c] K. Jacob, Prof. Dr. M. Etienne
CNRS; LCC (Laboratoire de Chimie de Coordination)
2
05, route de Narbonne, BP44099, F-31077 Toulouse Cedex 4 (France)
E-mail: michel.etienne@lcc-toulouse.fr
[
d] K. Jacob, Prof. Dr. M. Etienne
Universitꢁ de Toulouse; UPS, INPT; LCC
F-31077 Toulouse Cedex 4 (France)
[e] Prof. Dr. F. Maseras
Departament de Quꢄmica, Universitat Autꢆnoma de Barcelona
0
8193 Bellaterra (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402241.
Scheme 2. The catalytic functionalization of an alkane by silver-catalyzed
carbene transfer from ethyl diazoacetate.
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[
9,10]
best of our knowledge, the use of esters for such a purpose is
unknown.
aration of several antibiotics and natural products.
These
compounds are commonly prepared by condensation of
a halo ester with a carboxylic acid in anhydrous potassium car-
bonate. To our knowledge, their formation upon direct reac-
tion of a diazo compound and an ester is unknown, despite
the fact that other related substrates such as aldehydes, ke-
tones, and amides have been employed as reactants with sev-
Results and Discussion
In a first set of experiments, we treated ethyl propionate
(
4 mL), as the model substrate and the reaction solvent, with
Br3
[1]
EDA (0.25 mmol) in the presence of either [Tp Ag(acetone)] or
eral diazo reagents.
4
Bo,3CF2CF3
[
F –Tp
Ag(acetone)] (0.01 mmol) as the catalyst
The formation of alcohols and phenols during the course of
the reaction could be related to the presence of water in the
reaction medium. To obtain information on the mechanism
that controls this transformation, two experiments were per-
formed. First, the reaction of ethyl propionate with EDA by
2
7
(Scheme 3). After 2 h, the diazo compound was completely
4
Bo,3CF2CF3
using [F –Tp
Ag(acetone)] as the catalyst was per-
2
7
formed in the presence of added D O (Scheme 5a). The
2
2
2
H NMR spectrum of this reaction showed a doublet ( J
=
H,D
2.5 Hz) centered at 4.6 ppm, which was assigned to the
ÀOC(H)DCOOÀ group. A singlet located at 5.4 ppm corre-
sponding to EtOD was also observed. In line with these results,
1
2
the H NMR spectrum showed a 1:1:1 triplet ( J =2.5 Hz) at
H,D
4
.56 ppm corresponding to the ÀOC(H)DCOOÀ group (see the
Scheme 3. Reaction of ethyl propionate with EDA catalyzed by several
Tp Ag complexes.
x
Supporting Information). In the second experiment, we em-
ployed the deuterated ester CH COOC D as the substrate
3
6
5
consumed, as inferred by analysis of the reaction mixture by
(Scheme 5b); C D OH was the only deuterated compound ob-
6 5
1
2
GC and H NMR spectroscopy. NMR spectroscopy studies
served in the H NMR spectrum at the end of the reaction, in
agreement with the role of water in this reaction.
showed that 2-ethoxy-2-oxoethyl propionate was the product
formed from the carbene-transfer reaction with both
Br3
4Bo,3CF2CF3
[
Tp Ag(acetone)] and [F –Tp
Ag(acetone)].
27
This product was formed in 17 and 66% yield (deter-
1
mined by H NMR spectroscopy), respectively, and all
of the EDA was consumed. Some insertion of the car-
bene group was observed into the alkyl chain of the
ester to account for the entire initial amount of the
diazo compound. GC and NMR spectroscopy studies
also confirmed the presence of ethanol as a stoichio-
metric byproduct relative to the amount of the
(acyloxy)acetate derivative formed.
To examine the scope of this transformation,
a series of aliphatic and aromatic esters were treated
4
Bo,3CF2CF3
with EDA in the presence of [F –Tp
Ag-
27
(
acetone)] as the catalyst. After 2 h, analysis of the
1
crude reaction mixture by GC and H NMR spectros-
copy showed the formation of the corresponding a-
(
acyloxy)acetate (RCO CH CO Et) in moderate to high
2 2 2
4
Bo,3CF2CF3
Scheme 4. Carbene-transfer reaction to esters catalyzed by F27–Tp
Ag(acetone).
yields (Scheme 4). The remaining amount of the ini-
tial EDA led to products resulting from carbene inser-
tion into the alkyl CÀH bonds of the esters. In the
case of aromatic esters, the Buchner reaction (addi-
x
General conditions: [Tp AgL] (0.01 mmol), EDA (0.25 mmol), ester (4 mL), 2 h, RT. Reac-
tions were quantified by GC by using calibration curves.
tion to the aromatic double bond followed by ring expansion
[
1]
to cycloheptatriene) was also observed to a minor extent. a-
Acyloxy)acetates were the major products in all cases. Notably,
EtOH was detected during the synthesis of 1a–1 f, whereas for
g, phenol and p-fluorophenol were observed in the crude re-
(
1
action mixture.
a-(Acyloxy)acetates are normally employed as intermediates
in the synthesis of a-hydroxy b-keto esters (Chan rearrange-
Scheme 5. Proposed mechanism for the formation of a-(acyloxy)acetates
from esters and EDA.
[
8]
ment), a significant transformation in the context of the prep-
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The products formed in this reaction are identical to those
that would be formed from the direct reaction of the parent
organic acids of the esters and ethyl diazoacetate by direct,
metal-catalyzed carbene transfer to the OH bond. Given the
[5c]
high electrophilic character of those silver complexes, we
wondered if the metal center would induce ester hydrolysis
and subsequent functionalization of the acid. To test our hy-
pothesis, methyl acetate was stirred with catalytic amounts of
4
Bo,3CF3
[
(
F –Tp
Ag(acetone)] for 24 h. NMR spectroscopy studies
2
1
1
13
H and C) showed no evidence of hydrolysis after that time,
and the ester remained unchanged in solution and no acetic
acid was detected (Scheme 6a). Alternatively such hydrolysis
Scheme 7. Proposed mechanism for the formation of a-(acyloxy)acetates
from esters and EDA.
that basis and that of the experiments shown in Schemes 5
and 6, a plausible reaction pathway would involve the forma-
tion of a silver metallocarbene that reacts with the ester to
give a carbonyl ylide (Scheme 7). Attack of H O would deliver
2
one H to the ylide moiety, whereas the elimination of the R
group of the ester with the OÀH group would afford the ob-
served alcohols. However, some questions arise from this
proposal:
1
2
) Is the ylide a true intermediate?
x
) What is the fate of the electrophilic silver moiety Tp Ag
after carbene transfer?
3) Does it influence hydrolysis of the ylide and ethanol
formation?
To address the intimate nature of this reaction and to pro-
Scheme 6. Mechanistic experiments.
vide answers to the above questions, a DFT study was per-
formed. We computationally studied a model system consist-
ing of AcOR (R=ethyl, phenyl) plus methyl diazoacetate plus
TpAg plus water. With respect to the experimental system, Tp
could be involved in an equilibrium lying to the left, and con-
sequently, small, undetectable amounts of the acid would be
available. To evaluate this possibility, a second experiment was
4
Bo,3CF2CF3
[Tp=hydrotris(pyrazolyl)borate] replaces F –Tp
and
27
4
Bo,3CF3
performed in which methyl acetate, [F –Tp
Ag(acetone)],
methyl diazoacetate replaces ethyl diazoacetate. Scheme 8
summarizes the computational results. Only the most repre-
sentative intermediates and transition states (TS) are men-
tioned. Detailed energy profiles are given in the Supporting
Information.
21
and [D ]MeOH were stirred at room temperature for 72 h. Mon-
4
2
itoring of the reaction by H NMR spectroscopy showed no in-
corporation of any OCD into methyl acetate (Scheme 6c); this
3
confirmed the stability of the ester in the presence of the
silver complex. Therefore, we can exclude the participation of
the free acid in this transformation. However, for the sake of
completeness, we performed the reaction of acetic and pro-
pionic acids with EDA in the presence and absence of silver
complexes. We found that with the added catalyst, ethyl diazo-
acetate was completely consumed and the corresponding
products were obtained (Scheme 6c), whereas the direct reac-
tion without the catalyst provided the products in lower yields
Reaction from metallocarbene I1 to product P takes place
through two key intermediates, I2 and I4. Intermediate I2 is
a zwitterionic complex that can be viewed formally as the co-
ordination of the hypothetical ylide molecule to the silver com-
plex. Intermediate I4 appears after incorporation and cleavage
À
+
of the water molecule and contains the OH and H fragments
bound to the zwitterion mentioned above. The mechanism is
further complicated by the presence of intermediate I3, an ep-
oxide that can be reached from I2, but which seems to be
a dead end.
[11]
and without complete consumption of the diazo reagent. In
any case, this transformation has no impact on the transforma-
tion of the esters on the basis of the above experiments.
As mentioned above, intermolecular reactions of esters and
diazo compounds such as these are unprecedented, in contrast
to the known reactions of aldehydes, amides, and ketones
The relative energies of intermediates and transition states,
also reported in Scheme 8, are compatible with a reaction oc-
curring at room temperature. The case of ethyl acetate (R=Et)
is detailed here. Intermediate I2 should convert mostly into I4
[
1,12]
À1
with diazo reagents.
In the latter examples, the formation
rather than into I3 (TS2–4, 12.2 kcalmol ; TS2–3, 13.1 kcal
À1
À1
of carbonyl ylide intermediates from a metal–carbene species
has been invoked to explain the catalytic transformation. On
mol ). With such close barriers (separated by 0.9 kcalmol ),
part of the molecules would convert into epoxide I3, but they
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ing this metallocarbene. One of
them is
a cycloaddition that
forms CÀN and CÀO bonds at
the same time to result in a five-
membered ring. The other one is
an S 2-like transition state in
N
which the nitrogen atom of EDA
is substituted by the oxygen
atom of the ester. Both of these
two alternative processes are un-
favorable with very high ener-
À1
gies, 45.7 and 52.8 kcalmol , re-
spectively, above the separate
reactants. So, the formation of
metallocarbene I1 is expected,
and the reaction should proceed
afterwards through the path
outlined in Scheme 8.
Scheme 8. Computationally postulated mechanism for the formation of a-(acyloxy)acetates from esters and EDA.
Energies given in [kcalmol ].
À1
would evolve also into the main product by overcoming a bar-
À1
rier of 23.1 kcalmol (from I3 to TS2–3). For phenyl acetate,
À1
a slightly higher barrier of 27.1 kcalmol should be overcome
from I3. Although this value could be lowered by method or
model improvement, we consider the overall mechanism to be
correct.
The most complex part of Scheme 8 is the transformation of
Figure 1. Computed transition states for the direct reaction between TpAg
and other species in the medium. Energies given in [kcalmol ].
I2 into I4. The highest energy point for this step is TS2–4; this
does not correspond to water splitting but to the dissociation
of the zwitterion, which is then replaced by water as a ligand
on silver. Remarkably, the whole process takes place in the co-
ordination sphere of silver. There is no free ylide in solution,
and water splitting is assisted by the silver complex. Uncata-
À1
Conclusions
4
Bo,3CF2CF3
We found that the silver complex [F –Tp
Ag(acetone)]
27
À1
lyzed water cleavage would cost over 40 kcalmol . More de-
catalyzes the reaction of alkyl and aryl esters with ethyl diazoa-
cetate to give a-(acyloxy)acetates, and this constitutes a novel
reaction based on the strategy of metal-catalyzed decomposi-
tion of diazo compounds and subsequent carbene transfer.
Mechanistic studies showed the involvement of carbonyl ylide
intermediates as well as the metal center for the incorporation
of a water molecule to afford the final products.
tails on this alternative mechanism are provided in the Sup-
porting Information. Thus, the silver catalyst plays a double
role: it is required for the generation of carbene I1 and for the
water-splitting process.
An alternative pathway could involve the coordination of
the diazo reagent to the silver center by the N donor, which
would therefore activate the methine carbon atom on the
ester for electrophilic attack. In this case, no metallocarbene in-
termediate would be formed, and the catalysis would be trig-
I
gered by the Lewis acid nature of Ag . We are not confident in
Experimental Section
this route because a well-known Lewis acid such as scandium-
General methods
(
III) trifluoromethanesulfonate [Sc(OTf) ] does not catalyze the
3
reaction of EDA with ethyl acetate and because the reactions
described in this contribution are not catalyzed to the same
All preparations and manipulations were performed either under
an oxygen-free nitrogen atmosphere by using conventional
Schlenk techniques or inside a dry box. All the esters and ethyl di-
azoacetate (EDA) were purchased from Aldrich or Alfa Aesar and
were employed without further purification. The complexes
I
extent with copper analogues, despite the higher acidity of Cu
I
relative to that of Ag . For the sake of completeness, we also
examined this possibility computationally. We computed the
three transition states shown in Figure 1. The first of them cor-
responds to the reaction of the diazoacetate with the TpAg
Br3
[5a]
4Bo,3CF2CF3
[5b,c]
[
Tp ]Ag(thf) and [F –Tp
]Ag(COMe2)
were prepared
27
according to literature procedures. NMR spectroscopy experiments
were run with Agilent Technologies 400 and 500 MHz spectrome-
catalyst to lead to metallocarbene I1, and its free energy is
1
13
ters. Chemical shift values for H and C are reported as d values
À1
1
8.4 kcalmol above the separate reactants. The other two
(ppm) relative to internal SiMe . GC studies were performed by
4
transition states correspond to possible pathways not involv-
using Varian 3900 and Varian Saturn 2100T instruments.
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Catalytic reactions with esters
Acknowledgements
4
Bo,3CF2CF3
General
procedure:
[F –Tp
]Ag(COMe2)
(10.4 mg,
2
7
We thank the MINECO (CTQ2011-28942-CO2-01, and CTQ2011-
27033), Junta de Andalucꢄa (P10-FQM-06292 and P12-FQM-
01765), the AGAUR (2009SGR0259), and the ICIQ Foundation for
financial support. R.G. and M.A.F. thank MINECO and MEC for FPI
and FPU fellowships, respectively. M.E. and K.J. thank the CNRS
for support.
0
.01 mmol) was dissolved in the corresponding ester (4 mL) and
EDA (27 mL, 0.25 mmol) was added in one portion. The solution
was stirred for 2 h at RT until complete consumption of EDA, as
verified by GC. Reaction yields were determined by GC by using
calibration curves performed with synthesized samples; these data
were corroborated by NMR spectroscopy by using 1,4-dimethoxy-
benzene as an internal standard.
Keywords: carbenes
·
diazo
compounds
·
ester
functionalization · silver · zwitterions
Reaction of ethyl propionate and EDA in D O
2
4
Bo,3CF2CF3
[1] M. P. Doyle, M. A. McKervey, T. Ye in Modern Catalytic Methods for Organ-
ic Synthesis with Diazo Compounds, Wiley, 1998.
[
F –Tp
]Ag(COMe ) (10.4 mg, 0.01 mmol) was dissolved in
2
2
7
the desired ester (4 mL) and then D O (10 mL, 0.55 mmol) and EDA
27 mL, 0.25 mmol) were added at once. The solution was stirred
for 2 h at RT until complete consumption of EDA, as detected by
GC. The product was characterized by NMR spectroscopy.
2
[
[
[
2] M. M. Dꢄaz-Requejo, A. Caballero, M. R. Fructos, P. J. Pꢂrez in Alkane C-H
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Pꢂrez, J. Am. Chem. Soc. 2002, 124, 896–897; b) A. Caballero, M. M.
Dꢄaz-Requejo, T. R. Belderrain, M. C. Nicasio, S. Trofimenko, P. J. Pꢂrez, J.
Am. Chem. Soc. 2003, 125, 1446–1447; c) A. Caballero, M. M. Dꢄaz-Re-
quejo, T. R. Belderrain, M. C. Nicasio, S. Trofimenko, P. J. Pꢂrez, Organo-
metallics 2003, 22, 4145–4150.
Synthesis of [D ]phenyl acetate
5
A flask was charged with [D ]phenol (1 g, 0.01 mol) and dry di-
chloromethane (5 mL) as the solvent. Acetyl chloride (0.78 mL,
.011 mol) was added over 0.5 h at 08C, and then triethylamine
2.5 mL, 0.018 mol) was incorporated into the mixture, which was
stirred overnight. The mixture was then transferred to a separatory
funnel and extracted with a saturated solution of sodium bicarbon-
ate. The combined organic layer was washed with a saturated
sodium chloride solution and then dried with magnesium sulfate.
After filtration, the volatiles were removed under vacuum. The
product was characterized by NMR spectroscopy.
6
0
(
[
5] a) J. Urbano, T. R. Belderrain, M. C. Nicasio, S. Trofimenko, M. M. Dꢄaz-Re-
quejo, P. J. Pꢂrez, Organometallics 2005, 24, 1528–1532; b) E. Despag-
net-Ayoub, K. Jacob, L. Vendier, M. Etienne, E. ꢀlvarez, A. Caballero,
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sio, M. Etienne, P. J. Pꢂrez, Science 2011, 332, 835–838.
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[
8] S. D. Lee, T. H. Chan, K. S. Kwon, Tetrahedron Lett. 1984, 25, 3399–3402.
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Computational details
The reaction was modeled by considering the reactants methyl di-
azoacetate (model of ethyl diazoacetate) and the real substrates
ethyl acetate and phenyl acetate. The catalyst Tp Ag was simpli-
F21
[
[
[
10] a) J. D. White, S. C. Jeffrey, J. Org. Chem. 1996, 61, 2600–2601; b) J. D.
fied to Tp by removing the second rings and the CF CF groups.
2
3
White, S. C. Jeffrey, Tetrahedron 2009, 65, 6642–6651.
[
13]
Calculations were performed by using the Gaussian 09 package.
11] For a review on Brønsted acid-catalyzed diazo compound decomposi-
tion see: D. J. Miller, C. J. Moody, Tetrahedron 1995, 51, 10811–10843.
12] M. S. Kharasch, T. Rudy, W. Nudenberg, G. Büchi, J. Org. Chem. 1953, 18,
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261.
[14]
The B97D method was used together with the standard 6-31+
[15]
G(d,p) basis sets to describe all atoms, except for Ag for which
the large core scalar relativistic pseudopotentials by Dolg et al.
[16]
were used coupled to a double-z quality basis set. Full geometry
optimizations were performed in methyl acetate, which was used
as the model solvent under the SMD approach (e=6.9).
[17]
The
[
nature of the stationary points was characterized by a vibrational
analysis performed within the harmonic approximation at 298 K
and 101.3 kPa. Transition states were identified by the presence of
one imaginary frequency and minima by a full set of real frequen-
cies. Isomers of the organic fragments were searched by using
2
[
[
16] W. Küchle, M. Dolg, H. Stoll, H. Preuss, Mol. Phys. 1991, 74, 1245.
17] A. V. Marenich, C. J. Cramer, D. G. Truhlar, J. Phys. Chem. B 2009, 113,
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[
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Macromodel and reoptimized with Gaussian 09 at the level de-
scribed above; the geometries of the lower organic energy isomers
were used to search manually the isomers for the metal com-
plexes. All energies presented correspond to free energies [kcal
Received: April 15, 2014
Published online on && &&, 0000
À1
mol ] in solution.
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5
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These are not the final page numbers!

COMMUNICATIONS
R. Gava, M. ꢀ. Fuentes, M. Besora,
T. R. Belderrain, K. Jacob, F. Maseras,*
M. Etienne,* A. Caballero,* P. J. Pꢁrez*
&& – &&
Silver-Catalyzed Functionalization of
Esters by Carbene Transfer: The Role
of Ylide Zwitterionic Intermediates
Transferring out: The hitherto unre-
ported reaction of esters with ethyl
diazoacetate catalyzed by the complex
the context of carbene-transfer
reactions from diazo compounds that,
according to experimental and
4
Bo,3CF2CF3
[
F –Tp
Ag(acetone)] generates
theoretical data, is suggested to occur
through zwitterionic intermediates.
2
7
a-(acyloxy)acetates in moderate to high
yields. This is a novel transformation in
ꢃ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 5
&
6
&
ÞÞ
These are not the final page numbers!