Evidence for spontaneous release of acrylates from a transition-metal complex upon coupling ethene or propene with a carboxylic moiety or CO 2

Article abstract of DOI:10.1002/chem.200700532

The development of a new synthetic approach to acrylates based on the formation of alkyl esters of acrylic acids has been studied. A preformed PdCOOMe moiety is used as a model system to investigate the insertion of an olefin into the Pd-C bond. The fast elimination of acrylate is observed. Density functional calculations support the experimental findings and allow the characterization of transition states along the reaction pathway. The first example of olefin/CO₂ coupling with facile release of ethyl acrylate is also presented.

Full text of DOI:10.1002/chem.200700532

DOI: 10.1002/chem.200700532
Evidence for Spontaneous Release of Acrylates from a Transition-Metal
Complex Upon Coupling Ethene or Propene with a Carboxylic Moiety or
CO2
[
a]
[a]
[a]
[b]
Michele Aresta, Carlo Pastore, Potenzo Giannoccaro, Gµbor Kovµcs,
[
a]
[c]
Angela Dibenedetto,* and Imre Pµpai*
Abstract: The development of a new synthetic approach to acrylates based on the
formation of alkyl esters of acrylic acids has been studied.A preformed Pd–
COOMe moiety is used as a model system to investigate the insertion of an olefin
into the PdÀC bond.The fast elimination of acrylate is observed.Density function-
Keywords: acrylates · density func-
tional calculations · homogeneous
catalysis · olefin insertions · poly-
merization
al calculations support the experimental findings and allow the characterization of
transition states along the reaction pathway.The first example of olefin/CO 2 cou-
pling with facile release of ethyl acrylate is also presented.
Introduction
The synthesis of acrylic or methacrylic acid by the direct
coupling of olefins (ethene or propene, respectively) and
CO has attracted the attention of chemists since the early
2
[1–3]
1
980s.
Of the metal systems investigated in these pio-
[1]
[2]
neering studies, Mo(W) and Ni complexes were shown to
Scheme 1.Mode of bonding of the acrylate moieties formed upon reac-
tion of CO with ethene in the presence of Mo and W.
2
promote CÀC coupling processes between CO and C H .
2
2
4
[1]
The Mo and W complexes were found to have a similar
chemistry and afforded dimeric (Mo) or monomeric (W) hy-
dridoacrylate complexes upon coupling ethene and CO2
These complexes afforded propionic acid derivatives upon
treatment with dihydrogen or nBuLi (Scheme 2).In all
cases the original complex was changed into a species that
(
Scheme 1) that were not able to eliminate acrylic acid.
was not able to promote the coupling of ethene and CO for
2
[
a] Prof.M.Aresta, Dr.C.Pastore, Prof.P.Giannoccaro,
Prof.A.Dibenedetto
Department of Chemistry – CIRCC and University of Bari
Via Celso Ulpiani, 27, 70126 Bari (Italy)
Fax : (+39)080-544-3606
E-mail: a.dibenedetto@chimica.uniba.it
[
b] Dr.G.Kovµcs
Research Group of Homogeneous Catalysis
Hungarian Academy of Sciences, University of Debrecen
P.O. Box 7, 4010 Debrecen (Hungary)
[
c] Dr.I.Pµpai
Chemical Research Center of HAS, Institute of Structural Chemistry
1
525 Budapest, P.O.B. 17 (Hungary)
Fax : (+36)1-325-7554
E-mail: papai@chemres.hu
Supporting Information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Scheme 2.Displacement of the acrylate moiety by hydrogenation to form
propionic acid derivatives.
9028
ꢀ 2007 Wiley-VCH Verlag GmbH & Co.KGaA, Weinheim
Chem. Eur. J. 2007, 13, 9028 – 9034

FULL PAPER
any significant number of catalytic cycles.Nickel complexes,
on the other hand, afforded metallacyclic carboxylates, none
of which were able to undergo a b-H shift to generate the
acrylate moiety or to eliminate lactones (Scheme 3).These
metallacycles afforded propionic acid and [L NiCl ]-type
n
2
Scheme 3.Reaction of CO
2
with ethene in the presence of Ni.
Scheme 4.Reaction of a nickel metallacycle upon treatment with dppm.
complexes upon treatment with
HCl.
[4]
We have recently examined
the mechanism of metal-assist-
ed CO /C H coupling reactions
2
2
4
by means of density functional
calculations and have been able
to rationalize some of the
above findings.We have also
Scheme 5.Pd-mediated coupling of an olefin with the methoxycarbonyl moiety and elimination of acrylates.
shown that the activation barri- The saturated derivative is formed in only small amounts.
er of the rate-determining CÀC
coupling step and the stability
of the related intermediates are rather sensitive to the
nature of the chelating ancillary ligand.
Although metalloesters such as 1 or 2 have been known
for a long time as they are formed during the methoxycar-
[6]
Nickel metallacycles have very recently been converted
into hydridoacrylates in reactions with bis(diphenylphosphi-
no)methane (dppm).These hydridoacrylates then evolved
to give dimeric phosphido Ni complexes bearing a bridging
bonylation of olefins, and the elimination of unsaturated
compounds has also been observed in the co-polymerization
[7]
of CO and olefins, the formation of acrylates has never
been investigated in a specific study.We decided to use the
[5]
acrylate, although no evidence for the release of acrylic
acid was obtained in that study either (Scheme 4).
complexes [(LÀL)PdCl(COOMe)] (LÀL=dipyridyl (dipy;
A
H
R
N
1a); 2-(2-(diphenylphosphino)ethyl)pyridine (PÀN; 1b); 1,2-
bis(diphenylphosphino)ethane (dppe; 1c)) as parent com-
pounds as we have investigated these complexes extensively
Although the formation of acrylic acid from ethene and
CO is thermodynamically allowed under standard condi-
2
[8]
tions, the high bond-dissociation energies of the MÀH and
MÀO moieties in the hydridoacrylate intermediates present
substantial kinetic barriers to the elimination of acrylic acid.
We have now developed a new synthetic approach that cir-
cumvents these kinetic barriers and leads to the formation
and elimination of alkyl esters of acrylic acids.
for other purposes. These complexes were treated with
Ag(OSO CF ) (AgOTf) in CH Cl /CH CN to give the re-
spective cationic complexes [(LÀL)PdCOOMe
AHCTREUNG
2
3
2
2
3
A
H
R
N
[9]
AHCTREUNG
1
31
Table 1, while key H and P NMR spectroscopic data for
some of these complexes are given in Tables 2 and 3, respec-
tively.
Results and Discussion
We have used
a preformed
“Pd–COOMe” moiety as
a
model system to investigate the
insertion of olefins (ethene and
propene) into the PdÀC bond
and subsequent acrylate elimi-
nation (Scheme 5).
Chem. Eur. J. 2007, 13, 9028 – 9034
ꢀ 2007 Wiley-VCH Verlag GmbH & Co.KGaA, Weinheim
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9029

A.Dibenedetto, I.Pµpai et al.
rus chemical shift is greatly af-
fected by the positive charge on
the Pd atom.
Treatment of the cationic
complexes 2a–c with ethene or
propene at room temperature
led to the immediate formation
of methyl acrylate (MA) or
methyl methacrylate (MMA),
Table 1.IR spectroscopic data for neutral ( 1a–c) and cationic (2a–c)
complexes [cm ].
respectively, in amounts that depend on the ancillary ligand.
The yield of organic product increases in the order dipy<P-
N<dppe (Table 4).
À1
1
a
2a
1b
2b
1c
2c
C=O
C(O)OC
triflate anion
1633
1055
–
1666
1031
1278
2318
1660
1074
–
1670
1101
1265
2252
1654
1060
–
1660
1024
1261
2293
Formation of the organic compounds was easily detected
by their characteristic smell, and they were quantitatively
determined by GC-MS and characterized by H NMR spec-
1
CH
3
CN
–
–
–
troscopy.The solvent also plays a key role in this reaction,
with the highest yields being obtained in dmf (Table 4).
Ethene undergoes a faster insertion and produces higher
yields of acrylic ester than propene, which gives methyl
methacrylate as a single insertion product.The formation of
minor amounts of the methyl esters of propionic (from
ethene) or 2-methylpropionic acid (from propene) was also
observed.
1
Table 2.The most significant H NMR resonances [ppm] for neutral and
cationic complexes.
1
b
2b
1c
2c
m, -CH
m (CH
s (OCH
m (aromatic)
2
-
)
2.41–2.49
3.46–3.48
3.16
2.72–2.98
3.34–3.44
3.16
2.71–2.92
2.80–2.91
2
3
)
3.48
7.53–8.03
3.20
7.30–7.77
We carried out a more detailed investigation to determine
the reaction mechanism (Scheme 6) and rationalize the for-
mation of the saturated esters.With complex 2c, which af-
fords higher yields, at 250 K we were able to identify the Pd
7.23–7.85
7.23–7.86
1
complex 3c, which bears a coordinated olefin, by H NMR
3
1
Table 3. P NMR spectroscopic data [ppm] for neutral (1) and cationic
2) complexes.
spectroscopy and to isolate and characterize the cyclic spe-
(
cies 4c (see Scheme 6 and the Experimental Section).
Figure 1 shows the H NMR spectroscopic data for coordi-
nated ethene in 3b and 3c.
Ligand
1
2
1
PN
dppe
23.87 (cis to Cl)
32.2 (trans to Cl)
27.57 (trans to Cl)
47.87 (cis to Cl)
23.93
39.12
Species 5c (Scheme 6) could not be isolated as it immedi-
ately releases the acrylate.Hydride 7c could also not be
identified in the reaction mixture, most likely because it
reacts with either the solvent or the cyclic form 4c to afford
the saturated propionic acid derivative (Scheme 6, left part).
This latter reaction is not surprising as we have demonstrat-
ed that cationic hydrides of Pd can behave as proton or hy-
As can be seen from the IR data, the triflate anion is not
coordinated in cationic complexes 2a–c; they bear a coordi-
nated CH CN molecule instead.This coordinating solvent is
3
essential to stabilize the cationic complexes as decomposi-
tion of the complex occurs very easily if chloride elimination
[10]
dride donors.
is carried out in the absence of CH CN.Table 1 also shows
We located possible reaction intermediates by performing
density functional calculations on various levels of molecu-
lar models.According to the gas-phase model (see
Figure 2), the initial step of the reaction is the coordination
of C H to the unsaturated species (a), which leads to the
3
that the formation of the cationic species affects the nC
=
O
and nCÀOMe vibrational frequencies because of a charge redis-
tribution on the metal as a result of chloride elimination.
1
Table 2 lists the H NMR spectroscopic data for neutral
2
4
and cationic complexes 1b,c and 2b,c, respectively.The for-
mation of a positive charge on
ethylene complex (b).C ÀC bond formation between ethyl-
Pd causes a shift of the reso-
nance of both the methylene
[
a]
Table 4.Yield of acrylates (with respect to the starting cationic complexes).
moiety of the ancillary ligand
and the methyl of the methoxy-
2
a
2b
2c
0.1 MPa of
ethene propene
2c
Solvent
MA
0.1 MPa of
ethene propene
0.1 MPa of
P
ethene =3 MPa
3
1
carbonyl moiety.The
P NMR
ethene
propene
PCO2 =3 MPa
signal is also sensitive to elimi-
nation of the halide.Formation
of the cationic complex elimi- CH Cl /CH CN trace
MMA
trace
MA
MMA
< 0.1%
0.1%
MA
MMA
11%
22%
85%
MA
1%
5%
EA
–
–
CH
2
Cl
2
trace
trace
trace
trace
trace
trace
–
–
2
2
3
dmf
trace
1–2%
12%
98%
>60%
nates any possible cis/trans iso-
merism in 1b and the phospho- [a] MA=methyl acrylate; MMA=methyl methacrylate; EA=ethyl acrylate.
9030
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Release of Acrylates from a Metal Complex
FULL PAPER
The gas-phase model predicts a highly endergonic process
À1
for the overall reaction with DG=+12.2 kcalmol .The
metallacyclic species is calculated to be substantially more
stable than the observed products, which leads us to con-
clude that gas-phase calculations do not represent a reliable
model for the acrylate formation.
The solvated model, on the other hand, which explicitly
considers the solvent molecule dmf and includes the effect
of bulk solvent via a dielectric continuum model, resulted in
energetics that are consistent with the experimental findings
(
Figure 3).dmf forms relatively strong bonds with the metal
Scheme 6.Proposed mechanism for methyl acrylate formation.
Figure 3.Calculated free energy profile for the formation of methyl acry-
late (MA) using the solvated model.For the sake of clarity, the dmf mol-
ecule has been omitted for structures B, C, D’ and E as it interacts only
weakly with the metal centre.The full structures are given in the Sup-
porting Information.Letters A–F are correlated to a–f in Figure 3 but
not to a–c in the labeling of the complexes.
1
Figure 1. H NMR spectroscopic data [ppm] for the coordinated ethene
in 3b and 3c.
centre of unsaturated cationic species, including the parent
+
+
[
(LÀL)Pd
A
H
R
U
G
and [(LÀL)PdH]
hydride com-
plexes, thereby giving rise to square-planar complexes A
and F, respectively (see Figure 4).The explored reaction
pathway involves the formation of ethene complex B via the
weakly bonded species A···C H , which is followed by the
2
4
rate determining CÀC coupling process (via TSB-C) to yield
the metallacycle C.The ring opening in C (via TSC-D) occurs
easily because the open form of this species (D) is stabilized
by coordination of dmf to Pd.The b-H shift takes place via
an agostic intermediate D’ and transition state TSD-E, which
allows the formation of a hydride complex that bears the ac-
rylate ester coordinated through its double bond (E).Disso-
ciation of the acrylate is facilitated by the solvent that coor-
dinates to Pd in hydride species F.The overall coupling re-
action is predicted to be clearly exergonic using the solvated
Figure 2.Calculated Gibbs free energy profile for the formation of
methyl acrylate (MA) using the gas-phase model.
À1
ene and COOMe takes place via TSb-c to give the cyclic met-
allalactone species (c).This step is followed by ring opening
model (DG=À3.5 kcalmol ) due to the enhanced stability
of the coordinatively saturated hydride F with respect to A.
As there is no direct solvent stabilization in structure C, the
free-energy of the cyclic species becomes comparable to
that of the products.Our calculations thus underline the im-
portance of the solvent molecules in the reaction, which is
in perfect agreement with the experimental findings.
via TSc-d to give a species with an agostic Pd···
A
H
R
U
G
tion (d), which is followed by a b-hydrogen shift via TSd-e to
give the hydridoacrylate intermediate (e).Finally, dissocia-
tion of the acrylate moiety from e leads to the unsaturated
hydride species f.
Chem. Eur. J. 2007, 13, 9028 – 9034
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9031

A.Dibenedetto, I.Pµpai et al.
To verify whether the Pd hydride formed in the reaction
would be able to initiate a catalytic cycle for possible CO2/
olefin coupling processes, we heated complexes 2a–c at
3
00 K under variable pressures of olefin and CO .We dis-
2
covered that complex 2c gives rise to two clearly distin-
guished classes of acrylates with ethene under these condi-
tions, namely the methyl and ethyl ester of acrylic acid.
While formation of the methyl ester can be rationalized in
terms of Scheme 5 and is supported by density functional
calculations (Figure 4), the ethyl ester must have a different
origin and necessarily requires the reaction of ethene and
CO .Furthermore, the overall molar ratio of acrylic esters/
2
Pd increases well above 1 (>1.6). The yield of methyl ester
is almost quantitative with respect to the amount of Pd com-
plex used, whereas the yield of ethyl ester increases with the
pressure of ethene (Table 4) and the yield of propionic de-
rivatives decreases significantly, approaching zero.The yield
of ethyl ester is zero at 0.1 MPa of the olefin in the absence
of CO2 and higher than 60% with respect to Pd under
À1
Figure 4.Located structures and their relative energies [kcalmol ] for an
assumed CO insertion process.
2
1
3
ried out experiments using ethene and CO in the presence
2
3
MPa of ethene and 3 MPa of CO2.
A logical explanation for the formation of ethyl acrylate
of 2c.The products isolated were CH 2=CHCOOCH3
(almost quantitative yield), which is formed directly from 2c
would be the reaction sequence shown in Scheme 7, which
following the reaction path shown in Scheme 5, and CH =
CH COOEt, which can only be formed by coupling CO2
with ethene, whatever the mechanism.This is the first clear
2
1
3
13
demonstration of the coupling of ethene and CO to afford
2
ethyl acrylate rather than acrylic acid.The yield of more
than 60% is not low, as is the case under mild conditions,
1
3
and the ethyl ester bearing CO can only be formed from
2
1
3
the original Pd complex 2c, ethene and CO .However, to
2
further prove this assumption we treated the cationic com-
1
3
plex 2c with CO and did not observe any incorporation of
2
1
3
CO into the methyl ester.
2
As the amount of esters formed upon coupling ethene
and CO is not low, further investigations (covering both ex-
2
perimental and theoretical aspects) are in progress to eluci-
date the mechanistic details and to improve the conversion
Scheme 7.A putative mechanism for the synthesis of acrylates from CO
and ethene catalysed by “LnPdÀH” species (LÀL=dppe).
2
efficiency of ethene and CO into acrylate.
2
+
includes ethene insertion into [(LÀL)PdH] and subsequent
insertion processes with CO and C H .The insertion of
2
2
4
CO2 into a PdÀMe bond has been reported by Wendt
Conclusion
[11]
et al. to yield a Pd–OCOCH (acetate) fragment instead
3
of a Pd–COOR (alkoxycarbonyl) fragment.Our density
In summary, our results demonstrate that esterification of
the carboxylic moiety prevents the formation of MÀH and
MÀO bonds and makes release of an acrylate or methacry-
functional calculations indicate that although the [(LÀ
+
L)PdEt] intermediate can form easily, the so-called “in-
verse” CO insertion into the PdÀEt bond takes place via a
late moiety from a metal centre possible.Such findings may
be of great interest for developing a new route to acrylates
2
À1
[12]
high energy barrier (58 kcalmol , see Figure 4),
which
practically excludes this reaction pathway.This is further
confirmed by the fact that preformed alkylpalladium com-
from olefins and CO .Ethyl acrylate has been shown to be
2
formed from ethene and CO for the first time.
2
plexes do not react with CO and ethene to afford acrylates
2
under the conditions reported in the Experimental Section.
A more complex and unusual mechanism involving new,
non-trivial intermediates should therefore be proposed for
the formation of ethyl acrylate esters from ethene and CO2.
Theoretical studies along these lines are in progress in our
group.
ExperimentalSection
[
13]
All solvents were RP-grade Aldrich reagents and were distilled
and
stored under dinitrogen (99.98% quality). High-pressure reactions were
carried out in a custom built stainless-steel 4xparallel reactor.IR spectra
were recorded with an FTIR Shimadzu IR Prestige-21 apparatus.GC-MS
analyses were carried out with a Shimadzu 17 A gas chromatograph (ca-
pillary column: 30 m; MDN-5S; diameter: 0.25 mm, 0.25 mm film) cou-
As we were still lacking computational support for a pos-
sible mechanism for the formation of the ethyl esters we car-
9032
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Chem. Eur. J. 2007, 13, 9028 – 9034

Release of Acrylates from a Metal Complex
FULL PAPER
1
3
pled to a Shimadzu QP5050 A mass spectrometer.Quantitative determi-
nations on the reaction solutions were performed with a Hewlett–Pack-
ard 6850 GC-FID (capillary column: 30 m; MDN-5S; diameter: 0.25 mm,
The formation of ethyl acrylate from ethene and CO
2
was evident from
the isotopic labelling of some peaks in the mass spectrum of the product.
As CO contains approximately 3% of C O O, the moieties contain-
2
1
3
13 16 18
0
.25 mm film).
ing oxygen also show peaks that are two mass units higher (given in pa-
rentheses).
Synthesis of neutralcomp el xes : Neutral compounds 1a–c (Figure 1) were
prepared as reported in the literature. The procedure for 1a was opti-
mized as follows: An excess of triethylamine (1.2 mL, 8.75 mmol) was
added to a suspension of [(dppe)PdCl
nol (15 mL) and the reaction mixture was stirred for 10 h at 458C under
an atmosphere of CO.During this time the color of the reaction mixture
changed from white to brown.The solid formed was filtered off, washed
with methanol (2”5mL) and dried under vacuum.Yield: 70% (480 mg).
[
14]
13
+
+
+
C-Ethyl acrylate: m/z 100 (103) [M ], 100 (102) [M ÀH], 86 (88) [M
ÀCH ], 56 (58) [CH =CHCO], 45 (47) [CH CH O], 29, 27.
3
2
3
2
2
] (656 mg, 1.2 mmol) in dry metha-
Computationaldeta si l : Density functional theory was applied at the
B3LYP/SDDP level to characterize the electronic structure of the species
assumed to be involved in the reactions.B3LYP refers to the approximat-
[
15–17]
ed exchange-correlation functional
and SDDP denotes a basis set in-
cluding the Stuttgart–Dresden relativistic small core ECP basis set for Pd
Elemental analysis calcd (%) for C28
H
27ClO
2
P
2
Pd: C 56.12, H 4.54, Cl
and the Dunning/Huzinaga DZ + polarization all-electron basis set for
[
18–21]
5
.91, P 10.35, Pd 17.77; found: C 56.09, H 4.61, Cl 5.88, P 10.29, Pd 17.87.
the lighter atoms.
The chelating dppe ligand in the cationic palladium complexes was re-
placed by H (CH PH (dpe) to keep the quantum chemical calcula-
Complexes 1b,c gave correct elemental analyses and their IR and NMR
spectra were in agreement with the literature data.The IR and NMR
spectroscopic data of compounds 1a–c are reported in Tables 1, 2 and 3.
2
P
A
H
R
U
G
2
)
2
2
tions within reasonable limits.Test calculations carried out for some of
the full structures indicated that the error introduced by this structural
simplification concerning the relative energies of the reaction intermedi-
Synthesis of cationic complexes: The following general procedure was
used to synthesise the cationic complexes 2a–c.The neutral complex
À1
ates is about 2 kcalmol .The structures of all species investigated were
(
1a–c; 0.4 mmol) was dissolved in a mixture of dichloromethane (15 mL)
and CH CN (8 mL) then a stoichiometric amount of a standardized solu-
tion (0.09 m) of AgOTf in CH CN was slowly added.After stirring for
5 min the AgCl formed was separated by filtration and the filtrate was
concentrated under vacuum to half its volume.The solid product formed
fully optimized and the transition states of assumed elementary steps
were located.Vibrational frequencies were calculated for each located
stationary point to characterize their nature and to estimate the zero-
point energy and thermal contributions to the gas-phase free enthalpy of
the reaction components.Thermal corrections were calculated assuming
standard conditions (T=298 K and p=0.1MPa).
3
3
1
was isolated by filtration, washed with CH
vacuo.
2
Cl
2
(2”2mL) and dried in
PdS·C N: Pd 20.68;
NO PPdS·C N: P 4.79,
The solvent effects were included by explicit treatment of a single solvent
molecule (dmf), which was allowed to interact with the metal complexes.
These structures were also fully optimized and the free enthalpies of lo-
2
a: Elemental analysis calcd (%) for C13
found: Pd 20.62.
b: Elemental analysis calcd (%) for C22
Pd 16.45; found: P 4.68, Pd 15.98.
H
14
F
3
N
2
O
5
2 3
H
2
H
21
F
3
5
2 3
H
g
cated stationary points (G ) were obtained as described above.Further-
more, the effect of bulk solvent was estimated in terms of self-consistent
reaction field (SCRF) calculations using the PCM-UA0 solvation
2
27 3 5 2 2 3
c: Elemental analysis calcd (%) for C29H F O P PdS·C H N: P 8.37, Pd
[
22,23]
1
4.38; found: P 8.42, Pd 14.83.
model.
e=39.0 to simulate dmf as a solvent. The reported energetics for the sol-
vated model were computed as G=G +G , where G refers to the free
energies of solvation obtained from the PCM-UA0 calculations.
The dielectric constant in the SCRF calculations was set to
C, H, N analyses were not reliable as the complexes are strongly hygro-
scopic and were weighed in air.CH CN was instead determined by dis-
g
s
s
3
placement from the parent complex with hydrogen chloride (HCl) under
nitrogen and analyzed by quantitative GC analysis.
[
24]
All calculations were carried out with the Gaussian 03 package.
2
2
2
a: CH
b: CH
c: CH
3
CN (%) calcd 8.85; found: 8.38.
3
CN (%) calcd 6.87; found: 6.44.
CN (%) calcd 5.64; found 5.28.
3
IR and NMR spectroscopic data for the cationic complexes (2a–c) are
reported in Tables 1, 2 and 3.
Acknowledgments
Reaction of the cationic complexes with olefins: The general procedure
for the reaction of the cationic complexes with ethene or propene is as
follows.The cationic complex 2a–c was prepared in situ as described
above.After separation of the AgCl by filtration, the filtrate was stirred
under 0.1 MPa of ethene (or propene) at room temperature. The solution
was then analyzed by GC-MS and the acrylate (or methyl methacrylate)
formed was quantitatively determined.When the reaction of the cationic
complex with ethene was carried out in an NMR tube at 250 K formation
of the cationic ethene complexes was supported by the appearance of sig-
nals for coordinated ethene at d=5.39 (3b) and 5.35 ppm (3c).In the
The Italian authors thank Dr.V.Schiralli for experimental assistance, the
University of Bari and MUR (contract 2006031888) for financial support.
The Hungarian authors acknowledge
(K60549).
a Hungarian Research Grant
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PdCH
Ph PCH
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7
]dmf, 400 MHz): d=0.75–1.28 (m, 4H;
-
2 2
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Received: April 4, 2007
Revised: June 21, 2007
Published online: August 15, 2007
[
[
9034
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co.KGaA, Weinheim
Chem. Eur. J. 2007, 13, 9028 – 9034