Palladium(I) Carbonyl Cation-Catalyzed Carbonylation of Olefins and Alcohols in Concentrated Sulfuric Acid

Article abstract of DOI:10.1021/jo990449e

A new palladium catalyst was found to exhibit high catalytic activity for carbonylation of olefins and alcohols. cyclo-Bis(μ-carbonyl)dipalladium(I) cation (1) with bridging CO ligands is formed by reductive carbonylation of palladium sulfate, PdSO₄, in concentrated H₂SO₄. When an olefin or alcohol is added, complex 1 changes to a new complex (2) with terminal CO ligands, and tertiary carboxylic acids are obtained in high yields at room temperature and atmospheric pressure of CO. IR and ¹³C NMR studies suggest that complex 2 may be tentatively formulated to be [Pd₂(CO)₂]²⁺, in which the terminal CO ligands are chemically equivalent. Complex 1 is a catalyst precursor, and complex 2 functions as an active species for the carbonylation of olefins and alcohols. The catalytic behavior of the palladium carbonyl catalyst supports the recently proposed reaction mechanism involving an olefin-metal-CO complex as an intermediate for the catalytic carbonylation of olefins and alcohols in strongly acidic solution.

Full text of DOI:10.1021/jo990449e

6306
J . Org. Chem. 1999, 64, 6306-6311
P a lla d iu m (I) Ca r bon yl Ca tion -Ca ta lyzed Ca r bon yla tion of Olefin s
a n d Alcoh ols in Con cen tr a ted Su lfu r ic Acid
Qiang Xu,*,† Yoshie Souma,† J unya Umezawa,‡ Mutsuo Tanaka,† and Hisako Nakatani†
Osaka National Research Institute, AIST, MITI, 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577, J apan,
and Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology,
5-16-1, Ohmiya, Asahiku, Osaka 535, J apan
Received March 11, 1999
A new palladium catalyst was found to exhibit high catalytic activity for carbonylation of olefins
and alcohols. cyclo-Bis(µ-carbonyl)dipalladium(I) cation (1) with bridging CO ligands is formed by
reductive carbonylation of palladium sulfate, PdSO4, in concentrated H2SO4. When an olefin or
alcohol is added, complex 1 changes to a new complex (2) with terminal CO ligands, and tertiary
carboxylic acids are obtained in high yields at room temperature and atmospheric pressure of CO.
IR and 13C NMR studies suggest that complex 2 may be tentatively formulated to be [Pd2(CO)2]2+
,
in which the terminal CO ligands are chemically equivalent. Complex 1 is a catalyst precursor,
and complex 2 functions as an active species for the carbonylation of olefins and alcohols. The
catalytic behavior of the palladium carbonyl catalyst supports the recently proposed reaction
mechanism involving an olefin-metal-CO complex as an intermediate for the catalytic carbony-
lation of olefins and alcohols in strongly acidic solution.
In tr od u ction
that the CO vibrational frequencies are considerably
higher than that of free CO, for which the term nonclas-
sical metal carbonyl was previously proposed, and its
definition has recently been revised.13d,e Although there
is an argument on the classification of classical and non-
classical metal carbonyls,8h,13b,e there is no doubt that the
metal carbonyl cations have remarkably reduced π-back-
bonding. Of great interest is the catalytic activity for
carbonylation related to the high reactivity of CO origi-
nating from the reduced metal f CO π-back-bonding.14,15
Recently, the more than 100-year-old chemistry of the
metal carbonyls has achieved remarkable develop-
ments.1,2 Many new highly reduced metal carbonyl anions
have been obtained through chemical reduction in basic
solvents.3 Our knowledge of relatively stable neutral
carbonyl complexes has been dramatically enriched by
the synthesis of the first metalloidal and alkaline earth
metal carbonyls, [Cp*Si(CO)]4 and [Cp*Ca(CO)],5 respec-
tively, in solution and by the isolation and X-ray struc-
tural characterization of the first f-block carbonyl com-
plex, [(C5Me4H)3U(CO)].6 In particular, the focus of much
attention for the past years has been a new and very
intriguing class of late-metal carbonyl complexes: ho-
moleptic carbonyl cations and their cationic derivatives
of the electron-rich metals in groups 8-12 isolated in
superacidic media,7-12 including [Pd(CO)4][Sb2F11]2,9a,e cis-
Pd(CO)2(SO3F)2,9b,d and [c-Pd2(µ-CO)2](SO3F)2.9c In con-
trast to typical metal carbonyl complexes, the new family
of metal carbonyls has a distinguishing characteristic in
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Willner, H.; Schaebs, J .; Hwang, G.; Mistry, F.; J ones, R.; Trotter, J .;
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Bodenbinder, M.; Willner, H.; Aubke, F. Inorg. Chem. 1993, 32, 4667.
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* To whom correspondence should be addressed. Tel.: +81-727-51-
9652. Fax: +81-727-51-9629. E-mail: xu@onri.go.jp.
† Osaka National Research Institute.
‡ Osaka Institute of Technology.
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10.1021/jo990449e CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/30/1999

Carbonylation of Olefins and Alcohols in H2SO4
J . Org. Chem., Vol. 64, No. 17, 1999 6307
There has been a large number of reports dealing with
the carbonylation of olefins, many of which are catalyzed
by metal carbonyls.16 In Roelen17 and Reppe18 reactions,
the typical metal carbonyls, such as Co2(CO)8 and Ni-
(CO)4, are used to catalyze the carbonylation of olefins
to produce aldehydes and carboxylic acids, which requires
high temperature and high pressure. Another carbony-
lation, the Koch reaction, gives tertiary carboxylic acids
in strong acids such as H2SO4, HF, H3PO4, or BF3‚H2O,
in which no metal catalysts are used but high CO
pressure is necessary.19 We previously reported that the
addition of Cu(I) and Ag(I) carbonyl catalysts caused the
Koch-type reaction to proceed under much milder
conditions;at ambient temperature and pressure.20
Recently, we have also found that the gold(I) carbonyl
cation is an excellent catalyst for carbonylation of olefins,
which is the first application of gold(I) carbonyls to
organic syntheses.21
three palladium carbonyl complexes, [Pd(CO)4][Sb2F11]2,
cis-Pd(CO)2(SO3F)2, and [c-Pd2(µ-CO)2](SO3F)2, have been
prepared in superacidic media and well characterized by
Willner et al.9 The reductive carbonylation of the mixed-
valency compound PdIIPdIV(SO3F)6 at 25 °C produces
almost exclusively cis-Pd(CO)2(SO3F)2.9b When the reac-
tion is carried out in fluorosulfuric acid, an isomer
mixture of the composition Pd(CO)2(SO3F)2 is obtained.9d
Single crystals of both cis-Pd(CO)2(SO3F)2 and [c-Pd2-
(µ-CO)2](SO3F)2 are obtained by recrystallization of the
isomer mixture from fluorosulfuric acid under slightly
different conditions.9c,d The cyclic and completely planar
cation with symmetrically bridging carbonyl ligands,
[c-Pd2(µ-CO)2]2+, has also been recently isolated as the
salt of [Sb2F11]-.9e The homoleptic carbonyl cation, [Pd-
(CO)4]2+, has been isolated with the most weakly coor-
-
dinating anion [Sb2F11
]
by the solvolysis of Pd(CO)2-
(SO3F)2 in liquid antimony(V) fluoride in the presence of
CO.9a
Starting with the ingenious invention of the industrial
process for acetaldehyde production by the air oxidation
of ethylene catalyzed by PdCl2 and CuCl2, the so-called
Wacker process, remarkable progress has been made in
organic synthesis using Pd compounds both as stoichio-
metric reagents and catalysts.22,23 Many significant reac-
tions are catalyzed or promoted by Pd(0) and Pd(II)
compounds, including oxidative carbonylation of olefins
by Pd(II) and hydrocarboxylation of olefins by Pd(0),
whereas little is known about the catalytic activities of
Pd(I) compounds.22 Most of the reactions by Pd catalysts
are carried out in organic solvents, while a few of them
proceed in acidic media such as TFA, HCl, and HBF4.24,25
In this paper, we wish to report a new palladium
carbonyl catalyst in concd H2SO4, with which olefins and
alcohols react with CO to produce tertiary carboxylic
acids in high yields at atmospheric pressure and room
temperature. Evidence for the formation of a new Pd(I)
carbonyl complex without bridging CO ligands is pre-
sented. This work extends the family of the highly active
metal carbonyl catalysts for carbonylation of olefins from
group 11 to group 10 elements. The discovery of the
palladium carbonyl catalyst sheds light on the reaction
mechanism of the metal carbonyl cation-catalyzed car-
bonylation in strongly acidic media.
We have found a remarkably facile route for preparing
palladium carbonyls suitable for the catalytic carbony-
lation of olefins and alcohols. The commercial palladium
sulfate, PdSO4, is reduced by CO to form the cyclo-bis-
(µ-carbonyl)dipalladium(I) cation, [c-Pd2(µ-CO)2]2+ (1), in
concd H2SO4 solution at 25 °C (eq 1). The brown color
fades away during the CO uptake, and a clear orange-
red solution is obtained if the concentration of Pd in 96%
H2SO4 is below 0.15 M. The CO uptake is 1.5 mol per
mole of PdSO4, and hence, the CO/Pd stoichiometric ratio,
the number of moles of absorbed CO per mole of Pd, is
exactly 1.0, because the reduction of Pd(II) to Pd(I) is
accompanied by the oxidation of 0.5 mol of CO to CO2,
which is dissolved in the H2SO4 solution. When the
concentration of Pd in 96% H2SO4 is above 0.15 M, a
heterogeneous suspension is formed.
Complex 1 is also formed in concd H2SO4 using pal-
ladium oxide, PdO, or palladium acetate, Pd(CH3COO)2,
as the starting material, and the resulting solutions are
suitable for the catalytic carbonylation of olefins and
alcohols. Carbonyl derivatives of palladium with lower
oxidation state tend to be formed in media of lower
acidity. For example, the reaction of PdSO4 with CO in
the so-called magic acid, HSO3F/SbF5 (1:1), resulted in
the formation of [Pd(CO)4]2+, while [Pd(CO)4][Sb2F11]2, the
salt obtained from SbF5,9a was immediately converted
into complex 1 when dissolved in concd H2SO4. Palladium
sulfate is reduced to Pd metal by directly exposing it to
CO or by allowing it react with CO in sulfuric acid of
concentration below 70%.
Resu lts a n d Discu ssion
F or m a t ion of P a lla d iu m (I) Ca r b on yl Ca t ion ,
[c-P d 2(µ-CO)2]2+ (1), in Con cen tr a ted H2SO4. So far,
(14) Rack, J . J .; Strauss, S. H. Catal. Today 1997, 36, 99.
(15) For a recent review, see: Xu, Q.; Souma, Y. Top. Catal. 1998,
6, 17.
(16) For leading references, see: (a) Falbe, J ., Ed. New Syntheses
with Carbon Monoxide; Springer-Verlag: Berlin, 1980. (b) Colquhoun,
H. M.; Thompson, D. J .; Twigg, M. V. Carbonylation: Direct Synthesis
of Carbonyl Compounds; Plenum Press: New York, 1991.
(17) Roelen, O. (to Ruhrchemie, A. G.), German Patent no. 849,-
548, 1938.
Complex 1 in concd H2SO4 solution exhibits IR absorp-
tion at 1972 cm-1 (Figure 1a), which is close to that for
the bridging CO groups in solid [c-Pd2(µ-CO)2](SO3F)2.9c
The unusually high CO stretching frequencies, νCO, about
200 cm-1 higher than for bidentate bridging CO groups
in usual transition-metal carbonyl compounds, suggest
substantially reduced metal-to-CO π-back-bonding. Com-
plex 1 survives after continuous evacuation for over 24
(18) Reppe, W. Liebigs ann. Chem. 1953, 582, 1.
(19) Koch, H. Brennst. Chem. 1955, 36, 321.
(20) (a) Souma, Y.; Sano, H.; Iyoda, J . J . Org. Chem. 1973, 38, 2016.
(b) Souma, Y.; Sano, H. J . Org. Chem. 1973, 38, 3633. (c) Souma, Y.;
Sano, H. Chem. Lett. 1973, 1059. (d) Souma, Y.; Sano, H. Bull. Chem.
Soc. J pn. 1974, 47, 1717. (e) Souma, Y.; Iyoda, J .; Sano, H. Inorg. Chem.
1976, 15, 968. (f) Souma, Y.; Iyoda, J .; Sano, H. Bull. Chem. Soc. J pn.
1976, 49, 3291.
(21) Xu, Q.; Imamura, Y.; Fujiwara, M.; Souma, Y. J . Org. Chem.
1997, 62, 1594.
(22) For a recent review, see: Tsuji, J . Palladium Reagents and
Catalysts; J ohn Wiley and Sons: Chichester, 1995.
(23) Maitlis, P. M. The Organic Chemistry of Palladium; Academic
Press: New York, 1971.
(24) Miller, D. G.; Wayner, D. D. M. J . Org. Chem. 1990, 55, 2924.
(25) Fujiwara, Y.; Takaki, K.; Taniguchi, Y. Synlett 1996, 591.

6308 J . Org. Chem., Vol. 64, No. 17, 1999
Xu et al.
Ta ble 1. P a lla d iu m (I) Ca r bon yl Ca tion -Ca ta lyzed
Ca r bon yla tion of Olefin s a n d Alcoh ols in Con cd H2SO4
a
substrate
1-hexene
tert-carboxylic acid
yieldb/%
2,2-dimethylpentanoic
2-methyl-2-ethylbutanoic
others
48
23
1
1-octene
1-decene
2,2-dimethylheptanoic
2-methyl-2-ethylhexanoic
2-methyl-2-propylpentanoic
others
2,2-dimethylnonanoic
2-methyl-2-ethyloctanoic
2-methyl-2-propylheptanoic
2-methyl-2-butylhexanoic
others
37
17
9
2
29
15
9
3
2
cyclohexene
t-butanol
1-methylcyclopentanecarboxylic
2,2-dimethylpropanoic
others
52
70
2
1-hexanol
1-octanol
2,2-dimethylpentanoic
2-methyl-2-ethylbutanoic
others
2,2-dimethylheptanoic
2-methyl-2-ethylhexanoic
2-methyl-2-propylpentanoic
others
57
27
2
47
23
12
2
1-decanol
2,2-dimethylnonanoic
2-methyl-2-ethyloctanoic
2-methyl-2-propylheptanoic
2-methyl-2-butylhexanoic
others
38
20
11
5
F igu r e 1. Infrared spectra of palladium(I) carbonyl cations
in 96% H2SO4 solution at room temperature: (a) cyclo-bis(µ-
carbonyl)dipalladium(I) cation at 0.1 M; (b) after addition of
1-hexene equimolar to Pd in the solution of (a); (c) after
addition of 2.0 mol of 1-hexene per mole of Pd in the solution
of (a).
3
cyclohexanol
1-methylcyclopentanecarboxylic
62
a
PdSO4/substrate ) 1.0 mmol/5.0 mmol, 96% H2SO4 10 mL,
b
CO 1 atm, rt. Based on substrate.
very important position in synthetic chemistry.22 Many
significant reactions are catalyzed or promoted by Pd(0)
and Pd(II) compounds, whereas few Pd(I) catalysts have
been reported so far to our knowledge. It has been
reported that Cu(I), Ag(I), and Au(I) carbonyl cations
catalyze the carbonylation of olefins and alcohols in
strong acids.15,20,21 This paper reveals that, in the pres-
ence of Pd(I) carbonyl cations, olefins and alcohols react
readily with CO to give tert-carboxylic acids in high yields
at room temperature and atmospheric pressure (eq 2).
F igu r e 2. 13C NMR spectra of palladium(I) carbonyl cations
in 96% H2SO4 solution at room temperature: (a) 13C-enriched
cyclo-bis(µ-carbonyl)dipalladium(I) cation at 0.1 M; (b) after
addition of 5 mol of tert-butyl alcohol per mole of Pd in the
solution of (a) under 13CO at 1 atm.
Table 1 summarizes the results of the palladium(I)
carbonyl cation-catalyzed carbonylation of olefins and
alcohols under atmospheric pressure of CO and at room
temperature in 96% H2SO4. Strong acids and superacids
have been used as useful reaction media and catalysts
in organic syntheses and industrial processes,26 including
cationic carbonylation of methane and its substituted
derivatives27 and formylation of aromatic compounds.28
In concd H2SO4, the olefin is protonated, or the alcohol
is protonated and dehydrated, to form a carbocation
h or on standing at 60 °C for 3 days, indicating relatively
high stability of the bridging CO ligands. Two resonances
appear in the 13C NMR spectrum of the 13C-labeled
complex 1 in concd H2SO4 at ambient temperature and
atmospheric pressure of 13CO, a signal at 125 ppm due
to 13CO2, and an intense resonance at 167 ppm due to
[c-Pd2(µ-CO)2]2+ (Figure 2a). The resonance at 167 ppm
remains a singlet even at a temperature down to -10
°C, the freezing point of concd H2SO4, excluding a CO
exchange process and indicating that the CO ligands are
chemically equivalent.
(26) (a) Olah, G. A. Friedel-Crafts and Related Reaction; Wiley-
Interscience: New York, 1964. (b) Olah, G. A.; Prakash, G. K. S.;
Sommer, J . Superacids; Wiley-Interscience: New York, 1985.
(27) (a) Bagno, A.; Bukala, J .; Olah, G. A. J . Org. Chem. 1990, 55,
4284. (b) Olah, G. A.; Bukala, J . J . Org. Chem. 1990, 55, 4293.
P a lla d iu m (I) Ca r bon yl Ca tion -Ca ta lyzed Ca r bo-
n yla tion of Olefin s a n d Alcoh ols. Palladium catalysts,
most of which work in organic solvents, have assumed a

Carbonylation of Olefins and Alcohols in H2SO4
J . Org. Chem., Vol. 64, No. 17, 1999 6309
Ta ble 2. IR a n d 13C NMR Da ta of Ca r bon yl Der iva tives
of P a lla d iu m in Differ en t Oxid a tion Sta tes
intermediate, which isomerizes to a tert-carbocation via
Wagner-Meerwein rearrangement prior to the carbony-
lation. The carbonylation of carbocations leads to the
formation of acylium cations, which react with water to
give tertiary carboxylic acids or with alcohols to give the
corresponding esters.15,20 Without the palladium(I) car-
bonyl catalyst, olefins and alcohols are converted to the
carboxylic acids in yields as low as 10% and 15%,
respectively, under atmospheric pressure of CO and at
room temperature due to the oligomerization as a com-
peting reaction. As shown in Table 1, addition of the
palladium carbonyl catalyst drastically enhances the rate
of carbonylation, which is completed in 1-2 h, and tert-
carboxylic acids are formed in high yields. As proposed
recently, an olefin-metal-carbonyl complex seems to act
as an intermediate, which accounts for the high catalytic
activity of the carbonylation of olefins in strong acid (vide
infra).
compda
ν
CO(IR)/cm-1 δ (13C)/ppm
ref
Pd(CO)b
Pd(CO)2
Pd(CO)3
Pd(CO)4
2044
2040
2053
2066
1972
1977
2006
2019, 1994
2167, 2144
2166
2173, 2152
2186, 2163
2227, 2208
2248
32
32
32
32
b
b
b
[c-Pd2(µ-CO)2]2+ (1)c
[c-Pd2(µ-CO)2](SO3F)2
167
162
this work
9c
9e
35
this work
36
37
37
9b,d
9a,e
[c-Pd2(µ-CO)2][Sb2F11
]
2
[Pd2(CO)2(PPh2py)2Cl2]d
[Pd2(CO)2]2+ (2)c
177
d
Pd2(CO)2Cl4
cis-Pd(CO)2(C6Cl5)2
cis-Pd(CO)2(C6F5)2
cis-Pd(CO)2(SO3F)2
e
e
145
144
[Pd(CO)4][Sb2F11
]
2
a
Unless otherwise stated, spectra are obtained on solid com-
pounds. In krypton matrix. c In concentrated H2SO4. In dichlo-
romethane solution. e Stable only below -30 °C under CO pres-
sure.
b
d
F or m a tion of New P a lla d iu m (I) Ca r bon yl Ca tion
w ith Ter m in a l CO Liga n d s a n d Its Role in Ca ta lytic
Ca r bon yla tion of Olefin s a n d Alcoh ols. When an
olefin or alcohol was added dropwise in a concd H2SO4
solution of complex 1 under atmospheric CO, tertiary
carboxylic acids were obtained in high yields (Table 1),
accompanied by the conversion of complex 1 to a new
palladium carbonyl complex (2), which may be tentatively
formulated as [Pd2(CO)2]2+ (vide infra). For example,
when 1-hexene was added to the concd H2SO4 solution
of complex 1, the IR absorption due to bridging CO (1972
cm-1) decreased, and two new bands appeared at 2167
and 2144 cm-1 and grew concomitantly with the same
intensity (Figure 1b).29 The CO stretching frequencies,
higher than 2143 cm-1 for free CO, indicate that complex
2 should be attributed to a cationic palladium carbonyl
complex with terminal CO ligands, in which π-back-
bonding is reduced.7,13 At 1-hexene/Pd ) 2 mmol/1 mmol,
the IR absorption of bridging CO completely disappeared,
and only the two bands of terminal CO ligands were
observed (Figure 1c). Attempts to record a Raman
spectrum were unsuccessful, quite possibly because of the
limited concentration of the complexes in concd H2SO4.
The two IR absorptions at 2167 and 2144 cm-1 survived
after continuous evacuation for 1 h or after heating at
60 °C for 1 day, but they disappeared after continuous
evacuation for over 1 day or on standing at 80 °C for 1
day. These facts indicate that the metal-CO bonds in
complex 2 are relatively stable in comparison with those
in the dicarbonyl cations of the metals in group 11,
[M(CO)2]+ (M ) Cu, Ag, and Au) in concd H2SO4, of which
the CO ligands are reversibly absorbed and released.
Similar results were obtained when use was made of the
other substrates listed in Table 1. No change was
observed when trimethylacetic acid (TMAA) was added
in the concd H2SO4 solution of complex 1, indicating that
the conversion of complex 1 to complex 2 is not caused
by tertiary carboxylic acids, the products of carbonylation
of olefins and alcohols.
The resonance at 167 ppm in the 13C NMR spectrum
of complex 1 disappeared and a new resonance at 177
ppm was observed when an olefin or alcohol was added
into the concd H2SO4 solution of 13C-labeled complex 1
under an atmosphere of 13CO. Figure 2b shows the 13C
NMR spectrum of the solution in which tert-butyl alcohol
was added.30 At higher fields, only the resonances for the
tertiary butyl groups were observed, ruling out that
complex 2 is olefin-coordinated. The additional intense
13C resonance at 199.1 ppm should be assigned to the
-COOH group in TMAA, the products of carbonylation
from tert-butyl alcohol, in the strongly acidic medium,31
whereas a signal at 185.7 ppm is expected for the
-COOH group in pure carboxylic acids.20a The signal at
199.1 ppm remained a singlet even at a temperature
down to -10 °C, the freezing point of concd H2SO4,
suggesting that only TMAA dissolved in concd H2SO4
existed, excluding the possibility that TMAA is involved
in complex 2 as a ligand.
Table 2 shows a comparison of IR and 13C NMR data
for palladium carbonyl derivatives in different oxidation
states. The CO stretching frequencies (νCO) for terminal
CO ligands in Pd(0) carbonyl complexes in a noble gas
matrix are much lower than that for free CO (2143 cm-1),
due to a large degree of metal-to-CO π-back-bonding.32
In contrast to several dinuclear Pd(I) complexes with
(30) Carbonylation of t-BuOH produces 2,2-dimethylpropanoic acid
(TMAA), of which the 13C NMR spectrum for the -COOH group is the
simplest of all of the tertiary carboxylic acids. The same 13C resonance
at 177 ppm was observed for the CO ligands when use was made of
the other substrates listed in Table 1.
(28) (a) Tanaka, M.; Souma, Y. J . Chem. Soc., Chem. Commun. 1991,
1551. (b) Tanaka, M.; Iyoda, J .; Souma, Y. J . Org. Chem. 1992, 57,
2677. (c) Tanaka, M.; Souma, Y. J . Org. Chem. 1992, 57, 3738. (d)
Tanaka, M.; Fujiwara, M.; Ando, H.; Souma, Y. J . Org. Chem. 1993,
58, 3213. (e) Tanaka, M.; Fujiwara, M.; Ando, H. J . Org. Chem. 1995,
60, 2106. (f) Tanaka, M.; Fujiwara, M.; Ando, H. J . Org. Chem. 1995,
60, 3846. (g) Tanaka, M.; Fujiwara, M.; Xu, Q.; Souma, Y.; Ando, H.;
Laali, K. K. J . Am. Chem. Soc. 1997, 119, 5100. (h) Tanaka, M.;
Fujiwara, M.; Xu, Q.; Ando, H.; Raeker, T. J . J . Org. Chem. 1998, 63,
4408.
(29) When an olefin or alcohol was added, a small amount of a purple
colloidal substance, probably due to Pd(0) particles, was formed in the
solution. By allowing the purple solution to stand for several days,
the colloidal substance was precipitated and left a yellow clear solution
of complex 2, for which the IR absorption bands at 2167 and 2144 cm-1
remained unchanged.
(31) Similar deshielding of the 13C resonance (194 ppm) has been
observed for the -COOH group of TMAA adsorbed in an H-type zeolite
(H-ZSM-5), for which possible attributions have been proposed but not
defined. (see: Stepanov, A. G.; Luzgin, M. V.; Romannikov, V. N.;
Zamaraev, K. I. J . Am. Chem. Soc. 1995, 117, 3615.) By determining
the H2SO4-concentration dependence of the 13C chemical shift, δ(13C),
of the -COOH group of TMAA dissolved in H2SO4 solutions at room
temperature, we conclude that the deshielding of δ(13C) originates from
the protonation of the -COOH group of TMAA in the strong acids.
δ(13C) of the -COOH group of TMAA dissolved in water (0% H2SO4)
is 184.4 ppm (close to 185.7 ppm for pure TMAA), which shifts to 185.1,
191.2, and 199.1 ppm with increasing H2SO4 concentration to 50, 80,
and 96%, respectively.
(32) (a) Darling, J . H.; Ogden, J . S. Inorg. Chem. 1972, 11, 666. (b)
Darling, J . H.; Ogden, J . S. J . Chem. Soc., Dalton Trans. 1973, 1079.

6310 J . Org. Chem., Vol. 64, No. 17, 1999
Xu et al.
terminal ligands of isoelectronic isocyanides,33 the car-
bonyl derivatives of monovalent palladium without bridg-
ing CO ligands reported so far are rare.34 An example is
[Pd2(CO)2(2-diphenylphosphinopyridine)2Cl2],35 in which
the coordination of the donor ligands, PPh2py and Cl-1
,
to Pd(I) gives rise to νCO well below 2143 cm-1. Few
mononuclear palladium(II) carbonyl derivatives have
been reported so far.34 The νCO for terminal CO ligands
in Pd(II) carbonyl complexes without donor ligands are
above 2200 cm-1, rather higher than for free CO (2143
cm-1), due to reduced π-back-bonding. The coordination
of donor ligands lowers νCO as shown by Pd2(CO)2Cl4,36
cis-Pd(CO)2(C6Cl5)2, and cis-Pd(CO)2(C6F5)2.37 The CO
vibrational numbers (2167 and 2144 cm-1) for complex
2 are considerably higher than those of the zerovalent
palladium carbonyl complexes and significantly lower
than those of the divalent palladium carbonyl cations
without donor ligands.
The 13C NMR resonance at 177 ppm remained a singlet
even at a temperature down to the freezing point of concd
H2SO4, excluding a CO exchange process between differ-
ent species. Hence, only chemically equivalent CO ligands
exist in complex 2. So far, a trend has been well
established for the 13C chemical shift of the terminal CO
groups in metal carbonyl complexes.7b The chemical shift
of free 13CO at δ ) 184 ppm relative to (CH3)4Si (TMS)
can be regarded as a limit for cationic metal carbonyl
derivatives with smaller values, as well as typical metal
carbonyls and highly reduced carbonylmetalates with
higher resonance frequencies and correspondingly higher
δ values. The 13C chemical shift for complex 2 (177 ppm)
is located in the range where those for the Pd(I) carbonyl
cation (1) and the Cu(I), Ag(I), and Au(I) carbonyl
cations7,15 are located, in contrast with the chemical shifts
upfield from 145 ppm for divalent palladium carbonyl
cations (Table 2).
The above IR and 13C NMR results suggest that
complex 2 is a cationic carbonyl complex of monovalent
palladium with chemically equivalent and terminal CO
ligands. Complex 2 was found to be ESR silent and hence
a diamagnetic dinuclear Pd(I) complex, in which the odd
electrons on each palladium atom are spin-paired. Com-
plex 2 can, therefore, be tentatively formulated to be [Pd2-
(CO)2]2+, of which the isolation is expected. In well-
characterized dinuclear Pd(I) complexes of isocyanides,
such as [(CH3NC)6Pd2]2+, the two square planes with the
Pd ions centered are nearly perpendicular.33 The obser-
vation of the two IR absorptions at 2167 and 2144 cm-1
with the same intensity suggests that the two chemically
equivalent CO ligands in complex 2 do not occupy the
axial coordination sites, as in Pd2(CO)2(PPh2py)2Cl2.35
F igu r e 3. Plot of the yield of tertiary C7 acids versus the
amount of 1-hexene added in 10 mL of 96% H2SO4 solution of
0.1 M cyclo-bis(µ-carbonyl)dipalladium(I) cation at room tem-
perature and atmospheric pressure of CO.
2 mol of 1-hexene per mole of Pd are added, where
complex 1 has been completely converted to complex 2
as observed by IR (Figure 1c). This fact leads to the
conclusion that complex 1 is a catalyst precursor and that
complex 2 acts as the active species for the catalytic
carbonylation of olefins and alcohols in concd H2SO4.
We can provide a qualitative discussion on the catalytic
activity of the Pd(I) carbonyl catalyst and on the reaction
mechanism of the metal carbonyl cation-catalyzed car-
bonylation of olefins and alcohols in strongly acidic
solution by comparing the metal-CO bonds with those
in the monovalent copper, silver, and gold carbonyl
cations. Barnes et al. calculated (ab initio) M+-CO bond
energies to be 29.0, 19.2, and 31.5 kcal/mol and (CO)-
M+-CO bond energies to be 30.8, 19.5, and 27.6 kcal/
mol for M ) Cu, Ag, and Pd, respectively.38 Veldkamp
and Frenking calculated (MP2 level of theory) (CO)xAg+-
CO bond energies to be 21, 27, and 12 kcal/mol and
(CO)xAu+-CO bond energies to be 45, 50, and 9 kcal/mol
for x ) 0, 1, and 2, respectively.39 The calculations
predicted the stability of the metal monocarbonyl cations
to be in the order of Ag < Cu < Pd < Au and the stability
of the metal dicarbonyl cations to be in the order of Ag
< Pd < Cu < Au. The calculations, which neglected the
effects from the counteranions and solvents, also pre-
dicted that the dicarbonyl cations are more stable than
the corresponding monocarbonyl cations for Cu, Ag, and
Au. It has, however, been found that in HSO3F or concd
H2SO4 solution the dicarbonyl cations of Cu(I), Ag(I), and
Au(I) are so unstable that the reversible CO ligands are
lost even after a brief evacuation for several seconds,
while the corresponding monocarbonyl cations are so
stable that they remain unchanged after continuous
Figure 3 illustrates that the catalytic activity of the
Pd(I) carbonyl catalyst does not decline even if more than
(33) (a) Otsuka, S.; Tatsuno, Y.; Ataka, K. J . Am. Chem. Soc. 1971,
93, 6705. (b) Doonan, D. J .; Balch, A. L.; Goldberg, S. Z.; Eisenberg,
R.; Miller, J . S. J . Am. Chem. Soc. 1975, 97, 1961. (c) Rettig, M. F.;
Kirk, E. A.; Maitlis, P. M. J . Organomet. Chem. 1976, 111, 113. (d)
Goldberg, S. Z.; Eisenberg, R. Inorg. Chem. 1976, 15, 535. (e) Boehm,
J . R.; Balch, A. L. Inorg. Chem. 1977, 16, 778. (f) Lindsay, C. H.;
Benner, L. S.; Balch, A. L. Inorg. Chem. 1980, 19, 3503. (g) Rutherford,
N. M.; Olmstead, M. M.; Balch, A. L. Inorg. Chem. 1984, 23, 2833.
(34) Dixon, K. R.; Dixon, A. C. Comprehensive Organometallic
Chemistry II; Pergamon: New York, 1995; Vol. 9, Chapter 4.
(35) Maisonnat, A.; Farr, J . P.; Balch, A. L. Inorg. Chim. Acta 1981,
53, L217.
(37) Uso´n, R.; Fornie´s, J .; Toma´s, M.; Menjo´n, B. Organometallics
1985, 4, 1912.
(36) (a) Calderazzo, F.; Dell’Amico, D. B. Inorg. Chem. 1981, 20,
1310. (b) Dell’Amico, D. B.; Calderazzo, F.; Zandona, N. Inorg. Chem.
1984, 23, 137.
(38) Barnes, L. A.; Rosi, M.; Bauschlicher, C. W. J . Chem. Phys.
1990, 93, 609.
(39) Veldkamp, A.; Frenking, G. Organometallics 1993, 12, 4613.

Carbonylation of Olefins and Alcohols in H2SO4
J . Org. Chem., Vol. 64, No. 17, 1999 6311
evacuation for 1 day.15,21 The stabilization effects from
the counteranions may account for the great stability of
the monocarbonyl cations.
Exp er im en ta l Section
Rea gen ts. Commercial reagents PdSO4 (Wako Pure Chemi-
cal), H2SO4 (96%, Kanto Chemical Co.), CO (Nippon Sanso),
and 13CO (13C enrichment 99%, ICON) were used for the
preparation and characterization of palladium(I) carbonyls.
Reagents (special grade, Wako Pure Chemical) 1-hexene,
1-octene, 1-decene, cyclohexene, tert-butyl alcohol, 1-hexanol,
1-octanol, 1-decanol, and cyclohexanol were used for the
carbonylation without further purification.
P r ep a r a tion of th e p a lla d iu m (I) ca r bon yl ca tion s was
carried out using a 200-mL three-necked flask connected to a
CO gas buret, similar to the equipment used in the previous
studies.21 A mixture of 202.5 mg (1.0 mmol) of PdSO4 and 10
mL of 96% H2SO4 in the three-necked flask was stirred and
sonicated (40 kHz, 35 W) for 1 h. The three-necked flask was
evacuated by a rotary pump to remove air, and then carbon
monoxide was introduced into the gas buret. The mixture of
PdSO4 and H2SO4 was stirred vigorously to react with carbon
monoxide for about 1 day, and cyclo-bis(µ-carbonyl)dipalla-
dium(I) cation (1) was formed in an orange-red clear solution.
Complex 2 was formed by adding dropwise an olefin or alcohol
to the concd H2SO4 solution of complex 1.
Ca r bon yla tion of olefin s a n d a lcoh ols was carried out
by a method similar to that described in the previous papers.21
Using a syringe, an olefin or alcohol was added dropwise to
the concentrated H2SO4 solution of complex 1. After the
reaction was finished, the reaction mixture was poured over
ice-water. The products were extracted with chloroform and
analyzed by GC, NMR, and IR.
In contrast with the high catalytic activities of the
dicarbonyl cations of the metals in group 11, the corre-
sponding monocarbonyl cations were found to be catalyti-
cally inactive.15 Originally, it was considered that the
unstable metal polycarbonyl cations act as a CO carrier
that transports CO from the gas phase to the H2SO4
solution,16a,20a resulting in high CO concentration in the
solution and consequently accelerating the carbonylation
of olefins and alcohols via the pathway of the original
Koch reaction. However, complex 2, from which an
evacuation for 17 h is required to remove the CO ligands,
is so stable that it cannot act as a mere CO carrier to
transport CO from the gas phase and readily release the
CO ligands to the H2SO4 solution. As shown in Table 1,
the conversions of olefins and alcohols to tertiary car-
boxylic acids by the palladium carbonyl catalyst are
comparable to those by Cu(I), Ag(I), and Au(I) carbonyl
catalysts, despite the considerably high stability of the
Pd-CO bonds in the Pd(I) carbonyl complex. The obser-
vations for the Pd(I) carbonyl catalyst conflict with the
CO carrier model and support the recently proposed
reaction mechanism involving an olefin-metal-CO in-
termediate for the carbonylation in strongly acidic me-
dia.21 A recent theoretical study (B3LYP level of theory)
on copper(I), silver(I), and gold(I) carbonyl cation-
catalyzed carbonylation has indicated that the carbony-
lation of ethylene, propylene, and isobutene prefers to
proceed via olefin-metal(I)-dicarbonyl intermediates.40
The mechanism of the metal carbonyl cation-catalyzed
carbonylation in strong acids is of interest and requires
more investigation, such as kinetic studies (currently
underway).
Identification of 2,2-dimethylpropanoic (TMAA), 2,2-di-
methylpentanoic, 2-methyl-2-ethylbutanoic, 2,2-dimethylhep-
tanoic, 2-methyl-2-ethylhexanoic, 2-methyl-2-propylpentanoic,
2,2-dimethylnonanoic, 2-methyl-2-ethyloctanoic, 2-methyl-2-
propylheptanoic, 2-methyl-2-butylhexanoic, and 1-methylcy-
clopentanecarboxylic acids was carried out by comparison of
retention times and “spiking” with authentic samples. The
reagent 2,2-dimethylpropanoic acid from Nakarai Chemicals,
Ltd., was used as one of the authentic samples. The other
authentic samples were prepared by the carbonylation of
olefins catalyzed by the Cu(I) carbonyl catalyst.
Con clu sion s
Ch a r a cter iza tion of th e P a lla d iu m (I) Ca r bon yl Ca t-
ion s. Both complexes 1 and 2 were characterized by IR and
NMR. The infrared spectra were obtained on thin films
between two silicon disks on a J ASCO FT/IR-230 spectrometer,
and the background of concd H2SO4 was subtracted. 13C NMR
spectra were obtained on a J EOL J NM-AL400 spectrometer
operating at 100.40 MHz. Liquid samples were contained in
sample tubes of 5 mm o.d., in which coaxial inserts of CDCl3
as an external reference and a lock were placed. 13C enriched
carbon monoxide was used for 13C NMR measurements.
Chemical shifts are given in δ unit (parts per million) down-
field from tetramethylsilane.
In summary, we have found a remarkably facile
procedure to form the cyclo-bis(µ-carbonyl)dipalladium-
(I) cation (1), [c-Pd2(µ-CO)2]2+, from palladium(II) sulfate,
oxide, and acetate, respectively, in concd H2SO4 solution.
A new palladium(I) carbonyl complex (2) with terminal
CO ligands was formed by adding an olefin or alcohol to
a concd H2SO4 solution of complex 1. Complex 2 was
suggested to be a carbonyl cation of monovalent pal-
ladium, which may be tentatively formulated to be [Pd2-
(CO)2]2+. In the concentrated H2SO4 solution of Pd(I)
carbonyl cations, the olefin or alcohol reacted with CO
to give tert-carboxylic acids in high yields at room
temperature and atmospheric pressure. It was concluded
that complex 1 is a catalyst precursor, whereas complex
2 functions as an active species for the carbonylation of
olefins and alcohols. The catalytic behavior of the pal-
ladium carbonyl catalyst supported the recently proposed
reaction mechanism involving an olefin-metal-CO in-
termediate for the catalytic carbonylation of olefins and
alcohols in strong acids.
Ack n ow led gm en t. Professor H. Willner is acknowl-
edged for providing the compound [Pd(CO)4][Sb2F11 2
]
and for valuable discussions. The authors express their
thanks to Professors S. Suzuki and T. Eguchi for ESR
measurements and for helpful discussions.
J O990449E
(40) Mogi, K.; Sonoda, T.; Sakai, Y.; Xu, Q.; Souma, Y. unpublished
results.