J. Chil. Chem. Soc., 57, Nº 2 (2012)
Table 1. Experimental data for the methoxycarbonylation of styrene.
Entry
1
PPh3
--
2
CO [bar]
50
Conversion(%)
traces
b/l ratio (%)
--
Chemoselectivity ester (%)
--
79
86
--
2
50
14(1+2+3)
20(1+2+3)a
Traces
63/37
60/40
--
3
2
50
4
--
2
80
5
80
19(1+2)
74/26
70/30
44/56
92/8
100
98
98
97
94
99
94
96
100
--
6
4
80
25(1+2+3)
58(1+2+3)
97(1+2+3)b
79(1+2+3)b
100(1+2+3)b
80(1+2+3)b
31(1+2+3)c
62(1+2)b,c
Tracesb,d
7
6
80
8
2
80
9
--
2
80
97/3
10
11
12
13
14
50
91/9
--
--
--
--
50
98/2
50
82/18
85/15
--
50
50
Reaction conditions: Solvent: methanol 20 mL; Pd/Subs. ratio 1:100, Pd/p-TsOH ratio 1:10; temperature: 75 ºC; time 24 h; b/l branched ester/lineal ester ratio.
(a) Time 48 h. (b) Pd/HCl ratio 1:10. (c) Using Pd(PPh3)2Cl2 catalyst. (d) Using Pd(CH3CN)2Cl2 catalyst.
The results shown in Table 1, indicate low conversions to products,
under moderated conditions of pressure and temperature in the presence of
p-toluensulfonic acid (p-TsOH) (entries 1-7 and 12), however a significant
improvement is achieved by adding PPh3 (auxiliary ligand) to the catalytic
system but regioselectivities still remain low (entries 2,3,5-7). By considering
the literature antecedents3, 25 we believe that the presence of PPh3 can help to
stabilize the catalyst, by forming the bimetallic complex trans-[(η5-C5H4PPh2)
Re(CO)3](PPh3)PdCl2 which avoid the formation of Pd(0) species, even thought
small amount of palladium black was always observed in these reactions.
Increasing the amount of triphenylphosphine increase the conversion
but a noticeably decrease on the regioselectivity is observed (entries 5-7),
probably the excess of PPh displace the metalloligand forming Pd(PPh )2Cl2
as the catalytic specie, whic3h is known to catalyze the metoxycarbonylati3on of
styrene in moderate conversion and regioselectivity13 (entry 12-13). To prove
the above assumption we evaluate the catalytic properties of Pd(PPh ) Cl2
under the same experimental conditions (entry 12). In view of these result3s,2the
optimized ratio catalyst/auxiliary ligand was 1:2.
formation of neutral species [PdHCl(phosphine)(styrene)] which have been
suggested to be an intermediate for the formation of branched esters.29
With the aim to probe the involvement of our bimetallic complex in
the catalytic reaction, we compared its catalytic activity with the complexes
Pd(PPh3) Cl and Pd(CH3CN)2Cl2 under identical experimental conditions.
In terms2of2conversion, almost a 20% increase was noted when compared
[(η5-C5H4PPh )Re(CO) ]PdCl2(NCMe) (entry 11) vs. Pd(PPh3)2Cl2 (entry 13).
Furthermore, 2a notorio3us improvement was also found in the regioselectivity
to the branched product.
A more dramatic differences were found with the complex Pd(CH CN)2Cl
(entry14)whichundertheexperimentalconditionsused,isinactive.The3differen2t
catalytic behavior of the Pd(II) center coordinated to diphosphinocyrhetrene
ligand (η5-C5H PPh2)Re(CO)3 compared to PPh3 can be attributed to the steric
and electronic4 properties induced by the cyrhetrenyl group bound to the
phosphorous atom. Consequently, the catalytic performance of the palladium
complexes bearing such ligands should be altered, as we observed.
In most of the cases, the chemoselectivities were quite high but the
formation of the ether (3) increase under longer reaction time (entry 3). It is
noteworthy that etherification products are normally formed by acid-catalyzed
ACKNOWLEDGEMENTS
addition of methanol to styrene. 26 Similar results have been previously reported
The authors thank Fondecyt-Chile (Project Nº 3090035 and 1060487).
We also acknowledge the Dirección de Investigación of Pontificia Universidad
Católica de Valparaíso for financial support. D. S. acknowledges MECESUP
and CONICYT for a Doctoral scholarship. We also thank to Professor Dr.
Sergio A. Moya of Universidad de Santiago de Chile for allowing us the use of
a Gas Chromatographer.
27, 28
for other catalytic systems of Pd(II)/phosphines/p-TsOH.
In agreement
3-5, 18
with other Pd-catalytic systems containing monodentate phosphines,
our
system show favorable regioselectivity to form methyl-2-phenylpropanoate
(Table 1).
A speculative explanation of these results can be given by considering
the electron-withdrawing properties of cyrhetrenyl fragment which decrease
the basicity on the phosphorous atom and therefore, forming an electron-poor
catalyst which might lead to an (alkoxycarbonyl)palladium species, which
direct the insertion of the olefin to branched products. We have not ruled out
that the steric factor of the metalloligand can also affect the regioselectivity.
The nature of the acidic promoter, is another important feature which
deserve some comments, our experimental data (Table 1) show that the presence
of 10-fold excess of p-TsOH (entry 5), the conversion was measured to be 14%
and 19% (at CO pressure of 50 (entry 2) and 80 bar (entry 5)), respectively.
In both cases, a moderated regioselectivity to the branched product was also
observed (63% and 74%, respectively). When hydrochloric acid was used as
acid promoter a remarkably increase was noted in the conversion, regio- and
chemoselectivity (entries 10 and 8), at the two different CO pressure. These
results are not surprising since Claver et. al.5,29 and Chaudhari et. al.28 have
reported similar observation for the methoxycarbonylation of styrene by using
other Pd-catalytic systems and HCl as the acid source. Accordingly, we also
believe that the better coordinating capability of Cl- vs. p-TsO- will favor the
REFERENCES
1.- G. Kiss, Chem. Rev. 101, 3435, (2001)
2.- A. Brennfuhrer, H. Neumann, M. Beller, Chem. Cat. Chem. 1, 28, (2009)
3.- I. Del Río, N. Ruiz, C. Claver, L. A. van der Veen, P.W.N.M. van Leeuwen,
J. Mol. Catal. A Chem. 161, 39, (2000).
4.- B. Muñoz, E. Santos Garcia, C. Godard, E. Zangrando, C. Bo, A. Ruiz, C.
Claver, Eur. J. Inorg. Chem. 4625, (2008).
5.- B. K. Muñoz, C. Godard, A. Marinetti, A. Ruiz, J. Benet-Buchholz, C.
Claver, Dalton Trans. 36, 5524, (2007).
6.- I. del Rio, C. Claver, P.W.N.M. van Leeuwen, Eur. J. Inorg. Chem. 2719,
(2001).
7.- P.W.N.M. van Leeuwen, M.A. Zuideveld, B.H.G. Swennenhuis, Z. Freixa,
P.C.J. Kamer, K. Goubitz, J. Fraanje, M. Lutz, A.L. Spek, J. Am. Chem.
Soc. 125, 5523, (2003).
8.- C. Bianchini, A. Meli, W. Oberhauser, S. Parisel, O.V. Gusev, A.M.
1102