Novel palladium(II) complex containing a chelating anionic N-O ligand: Efficient carbonylation catalyst

Article abstract of DOI:10.1021/ol9912884

(matrix presented) A novel palladium(II) complex containing chelating anionic pyridine-2-carboxylato and labile tosylato ligands is a highly efficient catalyst for the carbonylation of organic alcohols and olefins to carboxylic acids/esters. Carbonylation of primary, secondary, and tertiary alcohols as well as linear and functionalized terminal olefins was studied. In all cases remarkable activity and selectivity were observed. The catalyst is stable under reaction conditions even in the absence of excess phosphine ligands.

Full text of DOI:10.1021/ol9912884

ORGANIC
LETTERS
2000
Vol. 2, No. 2
203-206
Novel Palladium(II) Complex Containing
a Chelating Anionic N−O Ligand:
Efficient Carbonylation Catalyst
S. Jayasree, A. Seayad, and R. V. Chaudhari*
Homogeneous Catalysis DiVision, National Chemical Laboratory, Pune-411 008, India
rVc@ems.ncl.res.in
Received November 30, 1999
ABSTRACT
A novel palladium(II) complex containing chelating anionic pyridine-2-carboxylato and labile tosylato ligands is a highly efficient catalyst for
the carbonylation of organic alcohols and olefins to carboxylic acids/esters. Carbonylation of primary, secondary, and tertiary alcohols as well
as linear and functionalized terminal olefins was studied. In all cases remarkable activity and selectivity were observed. The catalyst is stable
under reaction conditions even in the absence of excess phosphine ligands.
Catalytic carbonylation of organic substrates is a very
promising and clean route for the synthesis of carbonyl
compounds1 such as carboxylic acids, esters, ketones, amides,
and amino acids. In general, group VIII metal complexes
have been used and a specific class of reaction requires
unique catalytic properties and operating conditions. Even
though many carbonylation catalysts have emerged in the
past few decades,2 the search for new catalysts and processes
continues because of the economical and environmental
demands. Our interest has been in the development of novel
palladium-based catalytic systems for liquid-phase carbony-
lation reactions to form carboxylic acid/ester products.3 Here,
we report the novel palladium complex 14 containing the
chelating anionic N-O ligand (pyridine-2-carboxylato) and
labile TsO (tosylato) ligand as an efficient catalyst for
carbonylation of a variety of alcohols and olefins, providing
high selectivity to carboxylic acid/ester. Complex 1 was
prepared5 by reacting Pd(OAc)2 with 1 equiv of pyridine-
2-carboxylic acid and 1-2 equiv of TsOH and PPh3. An IR
spectrum of the complex showed carbonyl stretching vibra-
tions at 1668 cm-1 (νCdO) and 1330 cm-1 (νO)C-O) and a
Pd-N stretching vibration at 568 cm-1.
(1) (a) Colquhoun H. M.; Thompson, D. J.; Twigg M. V. Carbonyla-
tion: Direct Synthesis of Carbonyl Compounds; Plenum: New York, 1991.
(b) Wu, G. G.; Wong, Y.; Poirier, M. Org. Lett. 1999, 1, 745. (c) Beller,
M.; Eckert, M.; Geissler, H.; Napierski, B.; Rebenstock, H. P.; Holla, E.
W. Chem. Eur. J. 1998, 4, 935. (d) Imada, Y.; Alper, H. J. Org. Chem.
1996, 61, 6766. (e) Pisano, C.; Consiglio, G.; Sironi, A.; Moret, M. J. Chem.
Soc., Chem. Commun. 1991, 421.
(2) (a) Applied Homogeneous Catalysis with organometallic Compounds;
Cornils, B., Herrmann, W. A., Eds.; VCH: Weinhiem, 1996; Vols. 1 and
2. (b) Marr, A. C.; Ditzel, E. J.; Benyei, A. C.; Lightfoot, P.; Cole-Hamilton,
D. J. Chem. Commun. 1999, 1379. (c) Goedheijt, M. S.; Reek, J. N. H.;
Kamer, P. C. J.; van Leeuwen, P. W. N. M. Chem. Commun. 1998, 2431.
(3) (a) Seayad, A.; Jayasree, S.; Chaudhari, R. V. Catal. Lett. 1999, 61,
99. (b) Jayasree, S.; Seayad, A.; Chaudhari, R. V. Chem. Commun. 1999,
1067. (c) Seayad, A.; Jayasree, S.; Chaudhari, R. V. Org. Lett. 1999, 1,
459.
1H NMR and elemental analysis6 of complex 1 was
consistent with the given formulation. A 31P NMR spectrum
(4) Chaudhari, R. V.; Jayasree, S.; Seayad, A. Indian Patent Appl. 3697
DEL 98, March 1999; U.S. Appl. No. 09/291,929, 31-03-1999.
(5) Equimolar amounts of Pd(OAc)2 and pyridine-2-carboxylic acid and
1 or 2 equiv of TsOH and PPh3 in chloroform were vigorously stirred or
shaken at room temperature for a few minutes. After all the components
were dissolved and the solution turned yellow, the product was isolated as
an yellow oil by addtion of n-hexane or diethyl ether. The oily product
was washed several times with diethyl ether and hexane and was kept under
vacuum, forming an yellow fluffy solid.
10.1021/ol9912884 CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/31/1999

showed a strong signal at δ 34.58 ppm along with a weak
signal at δ 36.19 ppm, which may be the trans (N trans to
PPh3) and cis (N cis to PPh3) isomers, respectively.7 At room
temperature, complex 1 was highly soluble in most of the
polar solvents, moderately soluble in less polar solvents, and
hygroscopic. Crystals suitable for X-ray crystallography were
not obtained by common techniques, and an oil of the
complex was found to separate out in all the cases.
140 °C for methanol;10 TOF ) 2-3 h-1 at 90 °C for benzyl
alcohol11).
Among the secondary and tertiary alcohols, the important
reactions investigated were the carbonylation of 1-(4-
isobutylphenyl)ethanol (IBPE) to 2-(4-isobutylphenyl)pro-
pionic acid (Ibuprofen)12 and carbonylation of tert-butyl
alcohol to pivalic acid. Using complex 1 as the catalyst
precursor, high TOFs (up to 804 h-1) (entry 4, Table 1) and
up to 99% regioselectivity for Ibuprofen were achieved for
the carbonylation of IBPE at moderate reaction conditions
(115 °C/5.4 MPa). Here LiCl was used as the halide
promoter. The catalytic activity was found to increase with
promoters (TsOH/LiCl) and IBPE concentrations as well as
with CO partial pressure. The Ibuprofen selectivity remained
in the range of 85-99% under these conditions even at lower
CO partial pressures such as 1.36 MPa. These results are a
significant improvement over the conventional catalyst
system PdCl2(PPh3)2/HCl (TOF ) 50-70 h-1 at 130 °C)12a
which gave high selectivity (>95%) only at high CO
pressures of >15 MPa. However, the TOFs were comparable
to those of the catalyst system PdCl2(PPh3)2/TsOH/LiCl (TOF
) 850 h-1 at 115 °C) reported in our previous work.3a
The carbonylation of alkyl and aryl alcohols as well as
olefins has been carried out to investigate the activity and
chemo- as well as regioselectivity of the catalyst toward the
formation of carboxylic acids (Scheme 1). Complex 1
Scheme 1
showed improved activity compared to that of the conven-
tional catalysts in all the cases. A unique advantage of
complex 1 as a catalyst is that it is stable without using excess
ligands in addition to giving a high TOF8 and selectivity at
milder conditions. In addition, only small amount of catalyst
(0.2 mol %) and short reaction times were required in most
cases. Table 1 presents the typical results of carbonylation9
of various alcohols and olefins. All the reactions except the
hydrocarbomethoxylation of styrene (entry 7, Table 1) were
carried out in methyl ethyl ketone as a solvent using TsOH
and LiX as promoters, where X is Cl- or I-.
Under similar conditions, carbonylation of tert-butyl
alcohol gave pivalic acid (60%) and isovaleric acid (32%)
with a TOF of 30 h-1 (entry 2, Table 1). Even though the
hydrocarbalkoxylation of tert-butyl alcohol to isovaleric acid
esters was reported earlier,13 to the best of our knowledge
this is the first report in which pivalic acid is demonstrated
to be synthesized by transition metal catalyzed carbonyla-
tion.14
Remarkable improvement in catalytic activity [1-hexene
(TOF ) 613 h-1), 1-dodecene (TOF ) 400 h-1), styrene
(TOF ) 2600 h-1), and 4-isobutylstyrene (TOF ) 1313 h-1)
(Table 1)] and regioselectivity were also observed for
carbonylation of aliphatic and aryl-substituted terminal
olefins9 using complex 1 as the catalyst precursor. Aryl-
substituted terminal olefins yielded the corresponding 2-aryl-
propionic acids almost regiospecifically. In the case of
1-hexene and 1-dodecene, both branched and linear car-
boxylic acids were obtained with i/n ratio of 3.26 and 2.41,
respectively.
Carbonylations of primary, secondary, and tertiary alcohols
are demonstrated. Methanol on carbonylation gave acetic acid
(50%) and methyl acetate (20%) along with dimethyl ether
(28%) and methyl iodide (2%) with a TOF of 41 h-1.
Similarly, carbonylation of benzyl alcohol yielded phenyl-
acetic acid (29%) and benzyl phenylacetate (41%) together
with dibenzyl ether (25%) and benzyl iodide (5%) (TOF )
25 h-1). In both the cases, LiI was used as a promoter and
the reaction rates were much higher than the earlier reported
catalyst systems based on palladium (TOF ) 5-7 h-1 at
The carbonylation of a functionalized olefin, 4-pentenoic
acid (Scheme 2), using complex 1 gave 2-methylglutaric acid
(6) Data for 1: IR (KBr) 1668 vs (νCdO), 1604 s (νCdC), 1330 s
(νOdC-O), 568 s cm-1 (νPd-N). 31P (121. 1 MHz, CDCl3) δ 34.58 s (N trans
1
to PPh3), δ 36.19 w (N cis to PPh3). H (300 MHz, CDCl3) δ 2.3 s (3H,
tolyl CH3), δ 7-7.9 m (Ph and pyridil).13C (75.5 MHz, CDCl3) δ 21 s
(tolyl CH3), δ 142 s (OdC-O). Anal. Calcd for C31H26NO5PPdS‚H2O:
C, 54.75; H, 4.15; N, 2.06; S, 4.71. Found: C, 55.18; H, 4.23; N, 1.91; S,
4.38. The presence of water was detected by 1H NMR and IR spectroscopy.
(7) Jin, H.; Cavell, K. J.; Skelton, B. W.; White, A. H. J. Chem. Soc.,
Dalton. Trans. 1995, 2159.
Scheme 2
(8) TOF ) turnover frequency ) number of moles of carbonylation
product formed per mole of catalyst per hour.
(9) In a typical experiment,15 the substrate (28.08 mmol) and the required
amount of complex 1, promoters, water, and solvent were charged to a
stirred pressure reactor and the reaction was carried out at 5.4 MPa of CO
partial pressure at 115 °C under 1000 rpm for a specified time. After the
reaction, the reactor was cooled to room temperature and the products were
analyzed by gas chromatography ((FFAP capillary column 25 m × 0.2 mm,
FID) and further confirmed by GC-MS, IR, MS, and 1H NMR. Hydro-
carbomethoxylation of styrene (14.5 mmol) was carried out using the same
method but at 75 °C and 3.4 MPa of CO pressure in methanol as solvent.
and adipic acid with high rates (TOF ) 674 h-1) and
isolated15 yields of 60% and 25%, respectively (entry 10,
(10) Yang, J.; Haynes, A.; Maitlis, P. M. Chem. Commun. 1999, 179.
(11) Lin, Y. S.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1998, 71, 723.
204
Org. Lett., Vol. 2, No. 2, 2000

Table 1. Carbonylation of Alcohols and Olefins Using Palladium Complex 1a as Catalyst
Table 1). These results are highly promising for the synthesis
of dicarboxylic acids.
(Scheme 3). These reactions are efficiently carried out using
cationic palladium complexes, which are the most active
Hydrocarbalkoxylation of olefins to carboxylic acid esters
in the absence of halide promoters is of much current interest
Scheme 3
(12) (a) Elango, V.; Davenport, K. G.; Murphy, M. A.; Mott, G. N.;
Zey, E. G.; Smith, B. L.; Moss, G. L. Eur. Pat. Appl. 400 892, 1990. (b)
Jang, E. J.; Lee, K. H.; Lee, J. S.; Kim, Y. G. J. Mol. Catal. A: Chem.
1999, 138, 25. (c) Seayad, A.; Kelkar, A. A.; Chaudhari, R. V. Stud. Surf.
Sci. Catal. 1998, 113, 883.
(13) Zhou, H.; Lu, S.; Li, H.; Chen, J.; Fu, H.; Wang, H. J. Mol. Catal.
A: Chem. 1997, 116, 329.
Org. Lett., Vol. 2, No. 2, 2000
205

catalysts reported for such reactions.16 Here, the hydrocar-
bomethoxylation of styrene was carried out using 1 in the
presence of TsOH in methanol yielding methyl 2-phenyl-
propionate and methyl 3-phenylpropionate with a TOF of
150 h-1 (i/n ) 0.82) (entry 7, Table 1), which is significantly
higher compared to that of cationic complexes such as
[Pd(MeCN)2(PPh3)2](BF4)2 and [Pd(PhCN)2(PPh3)2](BF4)2
reported in the literature (TOF in the range of 10-15 h-1 at
80 °C)16a and is comparable to that of the catalyst system
Pd(OAc)2/4PPh3/TsOH (TOF ) 200 h-1 at 75 °C) as
previously reported by us.16b,c
The reusability of complex 1 as a catalyst was checked
by carrying out the recycle by adding fresh styrene to the
final reaction mixture (of entries 6 and 7) after attaining
complete conversion, and no significant loss in catalytic
activity and selectivity was observed. All these observations
indicate that complex 1 is highly active toward carbonylation
and the labile sites provide easy coordination of substrates,
thereby providing high reaction rates. It can act as an
alternative for classical neutral palladium complexes such
as PdCl2(PPh3)2 as well as cationic complexes such as
Pd(PPh3)2(OTs)2 or [Pd(PhCN)2(PPh3)2][BF4]2. Any of the
two catalytic cycles involving Pd0/PdII or PdII/PdIV seems to
be possible for the carbonylation of alcohols and olefins in
the presence of LiX promoter, in which case the reaction
proceeds through the corresponding halo derivatives as
intermediates.3 In the case of hydrocarbomethoxylation of
styrene, the active catalytic species might be a palladium
hydrido complex that activates the olefin by insertion to the
Pd-H bond as described in our previous reports.16c,17
Although more studies are required to further investigate the
catalytic properties and reaction mechanism, it is noteworthy
that this novel palladium complex is a versatile catalyst for
the carbonylation of a variety of substrates.
In conclusion, we have shown that a novel palladium
complex consisting of chelating anionic pyridine-2-carboxy-
lato and labile tosylato ligands is a highly active catalyst for
carbonylation of a variety of organic alcohols and olefins.
The catalyst is stable under the reaction conditions even in
the absence of excess phosphine ligands and also provides
high turnovers and selectivity. The kind of new complex
described here also has additional advantageous properties
such as easy accessibility and possible derivatization to form
catalysts of desired steric, electronic, and physical properties.
One such potential derivative is its water-soluble analogue,
studies on which are currently in progress at our laboratory.
Acknowledgment. S.J. and A.S. thank the CSIR (Council
of Scientific and Industrial Research), India, for a research
fellowship.
(14) Conventional method for the synthesis of pivalic acid is by Koch
carbonylation using concentrated H2SO4 and BF3 as catalysts. (a) Koch,
H.; Brennstoff-Chem. 1955, 36, 321. (b) Brilman, D. W. F.; van Swaaij,
W. P. M.; Versteeg, G. F. Chem. Eng. Sci. 1999, 54, 4801.
(15) The detailed procedure is given in ref 3c.
OL9912884
(16) (a) Oi, S.; Nomura, M.; Aiko, T.; Inoue, Y. J. Mol. Catal. A: Chem.
1997, 115, 289. (b) Seayad, A.; Kelkar, A. A.; Chaudhari, R. V.; Toniolo,
L. Ind. Eng. Chem. Res. 1998, 37, 2180. (c) Seayad, A.; Kelkar, A. A.;
Toniolo, L.; Chaudhari, R. V. J. Mol Catal. A: Chem. In press.
(17) Jayasree, S.; Seayad, A.; Chaudhari, R. V. Catal. Lett. 1999, 58,
213.
206
Org. Lett., Vol. 2, No. 2, 2000