Mechanistic study of the palladium - Phenanthroline catalyzed carbonylation of nitroarenes and amines: Palladium - Carbonyl intermediates and bifunctional effects

Article abstract of DOI:10.1002/chem.200801882

Palladium - phenanthroline complexes catalyze both the nitroarene carbonylation reaction and the amine oxidative carbonylation reaction to give, depending on the conditions, carbamates and ureas. There is evidence that the key step in both processes is the amine carbonylation. Here, we show that when the reaction is run in methanol key intermediate compounds have the general formula [Pd(RPhen)(COOMe)₂] (1) (RPhen = l,10-phenanthroline or one of its substituted derivatives). The kinetics of the reaction of 1 with toluidine in the presence of a carboxylic or phosphorus acid is firstorder with respect to complex, acid, and toluidine. A CO atmosphere is also required for the reaction to proceed. Acid dimerization was shown not to be influential under the concentration conditions examined, but reaction between the acid and toluidine is not negligible and a correction has to be applied. Diphenylphosphinic acid is more effective than any carboxylic acid in promoting this reaction, as also observed under catalytic conditions. A series of equilibria and an irreversible acid-assisted proton transfer explain the observed data. Formation of an adduct between complexes of the kind 1 and CO was spectroscopically observed when RPhen = 2,9-Me₂Phen. Several analogous complexes were also spectroscopically characterized and the X-ray structure of [Pd(2,9Me₂Phen)Cl₂(CO)] was solved. This shows an asymmetric coordination of the nitrogen ligand. Kinetic measurements were also conducted under catalytic conditions. An Eyring plot shows that the effect of the acidic promoter is to decrease the ?S * value, whereas no positive effect is observed on ?H*. A temperature-dependent correction for the reaction between the acid and aniline and phenanthroline present under the reaction conditions has to be applied. Comparison of the results obtained under stoichiometric and catalytic conditions strongly supports the view that 1 is involved even in the latter and that the acid is acting as a bifunctional promoter.

Full text of DOI:10.1002/chem.200801882

DOI: 10.1002/chem.200801882
Mechanistic Study of the Palladium–Phenanthroline Catalyzed
Carbonylation of Nitroarenes and Amines: Palladium–Carbonyl
Intermediates and Bifunctional Effects
[
a]
[a]
[a]
[a]
Fabio Ragaini,* Michela Gasperini, Sergio Cenini, Lucia Arnera,
[
a]
[b, c]
[b]
Alessandro Caselli, Piero Macchi,
and Nicola Casati
Dedicated to the Centenary of the Italian Chemical Society
Abstract:
Palladium–phenanthroline
influential under the concentration
conditions examined, but reaction be-
tween the acid and toluidine is not neg-
ligible and a correction has to be ap-
plied. Diphenylphosphinic acid is more
effective than any carboxylic acid in
promoting this reaction, as also ob-
served under catalytic conditions. A
series of equilibria and an irreversible
acid-assisted proton transfer explain
the observed data. Formation of an
adduct between complexes of the kind
1 and CO was spectroscopically ob-
spectroscopically characterized and the
X-ray structure of [Pd(2,9-
Me Phen)Cl (CO)] was solved. This
complexes catalyze both the nitroarene
carbonylation reaction and the amine
oxidative carbonylation reaction to
give, depending on the conditions, car-
bamates and ureas. There is evidence
that the key step in both processes is
the amine carbonylation. Here, we
show that when the reaction is run in
methanol key intermediate compounds
2
2
shows an asymmetric coordination of
the nitrogen ligand. Kinetic measure-
ments were also conducted under cata-
lytic conditions. An Eyring plot shows
that the effect of the acidic promoter is
°
to decrease the DS value, whereas no
°
positive effect is observed on DH . A
have the general formula [Pd
A( RPhen)-
H
U
G
R
N
U
G
temperature-dependent correction for
the reaction between the acid and ani-
line and phenanthroline present under
the reaction conditions has to be ap-
plied. Comparison of the results ob-
tained under stoichiometric and cata-
lytic conditions strongly supports the
view that 1 is involved even in the
latter and that the acid is acting as a bi-
functional promoter.
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(COOMe) ] (1) (RPhen=1,10-phenan-
2
throline or one of its substituted deriv-
atives). The kinetics of the reaction of
served when RPhen=2,9-Me Phen.
2
1
with toluidine in the presence of a
Several analogous complexes were also
carboxylic or phosphorus acid is first-
order with respect to complex, acid,
and toluidine. A CO atmosphere is also
required for the reaction to proceed.
Acid dimerization was shown not to be
Keywords: carbonylation · isocya-
nates · N ligands · nitroarenes ·
palladium
Introduction
[
a] Prof. Dr. F. Ragaini, Dr. M. Gasperini, Prof. Dr. S. Cenini,
Dipl.-Chem. L. Arnera, Dr. A. Caselli
Dipartimento di Chimica Inorganica
Metallorganica e Analitica “Lamberto Malatesta” and ISTM-CNR
Universitꢀ degli Studi di Milano
via Venezian 21, 20133 Milano (Italy)
Fax : (+39)025-031-4405
The carbonylation of organic nitro compounds is a process
with high potential synthetic and industrial interest, because
many products can be obtained from nitro compounds and
[1–3]
CO, such as isocyanates, carbamates, and ureas.
Ureas
and carbamates are important final products and intermedi-
ates in the synthesis of pesticides, fertilizers, and dyes and
mono- and diisocyanates are important intermediates in the
manufacturing of pesticides, polyurethane foams, plastics,
synthetic leather, adhesives, and coatings. Furthermore, the
urea functionality is also present in several pharmaceutical
E-mail: fabio.ragaini@unimi.it
[
b] Dr. P. Macchi, Dr. N. Casati
Dipartimento di Chimica Strutturale e Stereochimica Inorganica
and ISTM-CNR
Universitꢀ degli Studi di Milano
via Venezian 21, 20133 Milano (Italy)
[
c] Dr. P. Macchi
Present address: Departement fꢁr Chemie und Biochemie
Universitꢂt Bern, Freiestrasse 3
[4]
candidates.
The classical method for the production of isocyanates re-
quires the intermediate reduction of the nitro compound to
amine, followed by reaction with phosgene. However, phos-
gene is a very toxic and corrosive material. Recently (2006),
3
012 Berne (Switzerland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801882.
8064
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Chem. Eur. J. 2009, 15, 8064 – 8077

FULL PAPER
a large plant producing toluenediisocyanate (TDI) was shut
in Italy mainly because of pressure from the local popula-
tion and authorities against the use of phosgene close to a
populated area. Thus, it is not surprising that enormous
effort has been applied to the development of phosgene-
free routes to isocyanates. Among these, the catalytic car-
bonylation of nitro compounds, particularly of aromatic
ones, represents one of the most interesting alternatives, al-
though the direct carbonylation of nitro compounds to the
corresponding isocyanates has proved to be a difficult reac-
tion. However, in the presence of an alcohol, carbamates
can be obtained more easily and with a high selectivity
tional activation process may be operative and we decided
as a working hypothesis to investigate the promoting activity
of a series of substances that may have better bifunctional
activation ability than carboxylic acids. This working hy-
pothesis proved fruitful and allowed us to markedly improve
the efficiency of the catalytic system in terms of both reac-
tion rate and catalyst life by using phosphorus acids as pro-
[9,29,31–33,43,44]
moters.
However, the simple fact that phospho-
rus acids are better than carboxylic acids does not prove per
se that this occurs for the reason supposed by us. We thus
decided to investigate the mechanism of the palladium–phe-
nanthroline catalyzed carbonylation of nitroarenes with the
dual aim of clarifying the role of the promoter and finding
clues on how to further improve catalyst performance. Very
little conclusive evidence has been reported in the literature
about the mechanism of this reaction, partly because of the
instability of palladium–carbonyl complexes, which are
surely intermediates in the reaction. Our study involved a
large number of kinetic and equilibrium measurements
under both stoichiometric and catalytic conditions, and also
led to the isolation of a class of palladium–carbonyl inter-
mediates whose importance goes well beyond their involve-
ment in the carbonylation of nitroarenes. The results strong-
ly support our initial assumption that a bifunctional effect
by the acid promoter is operating.
[Eq. (1)]:
cat:
ArNO þ ROH þ 3 CO ƒ! ArNHCOOR þ 2 CO
ð1Þ
2
2
If no alcohol is added, but at least an equimolar amount of
aromatic amine with respect to the nitroarene is present, di-
arylureas are produced [Eq. (2)]:
cat:
ArNO þ ArNH þ 3 CO ƒ! ArNHCðOÞNHAr þ 2 CO
2
2
2
ð2Þ
At high temperature, diarylureas react with alcohols when
the latter are employed as solvents to afford carbamates and
arylamines [Eq. (3)]:
Lengthy mathematical elaboration of the data has been
placed in the Supporting Information and only the main re-
sults are discussed in the text.
ArNHCðOÞNHAr þ ROH ! ArNHCOOR þ ArNH2
ð3Þ
Results and Discussion
Moreover, it was shown several years ago that in the case of
ruthenium-catalyzed reactions, diarylureas are intermediate-
ly formed during the reaction even when alcohols are pres-
Kinetic study of the reaction under catalytic conditions and
identification of the reaction pathway: During our previous
synthetic studies, we found that the kinetics of the catalytic
reaction (nitrobenzene and methanol as substrates) is zero
order with respect to nitroarene, first order with respect to
CO pressure, and also depends on aniline and diphenylphos-
[
5–8]
ent,
and we recently showed conclusively that this is the
[9]
case even for the present palladium system (see below).
Both carbamates and ureas are important industrial prod-
ucts themselves, but can also be thermally cracked to the
corresponding isocyanates, thus providing a phosgene-free
[31,32]
phinic acid concentrations.
A kinetic study with respect
to aniline had not been performed, but it had been shown
that if no aniline is initially added, the reaction has an in-
duction period that it is not observed otherwise. We have
now performed such a study and the results are shown in
Figure 1. Note that a very high catalytic ratio and a short re-
action time had to be employed to keep the conversion low
and allow a better comparison. Diphenylphosphinic acid
was chosen as the acidic promoter because it is the one that
was employed in most other reactions investigated in this
paper.
The plot in Figure 1 shows a clear first-order dependency
of the rate on aniline concentration up to a point at which a
sudden flattening is observed. Such a result is typical of a
change in the identity of the rate-determining step of the
cycle. At relatively low aniline concentrations, the slow step
involves a reaction of this reagent, but when its concentra-
tion is raised significantly, this step is accelerated to the
point that another step of the cycle becomes rate-determin-
[10]
route to these important intermediates [Eq. (4)]:
In recent years the catalytic system based on palladium–
phenanthroline complexes has emerged as the most active
[
9,11–43]
and promising for possible industrial application.
eral groups have reported that the addition of acidic co-cat-
Sev-
[14,24,25,30,42]
alysts (carboxylic acids
or phenanthrolinium
) markedly improves both rate and selectivity of
[15,38–40]
salts
the carbonylation of nitroarenes to give isocyanates or car-
bamates. Some years ago we were intrigued by van Leeu-
[24]
wenꢄs observation
that no correlation was present be-
tween the acidity of the carboxylic acid employed as pro-
moter and the rate increase. This suggested that a bifunc-
Chem. Eur. J. 2009, 15, 8064 – 8077
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8065

F. Ragaini et al.
Figure 2. Nitrobenzene conversion as a function of the analytical diphe-
nylphosphinic acid concentration. Conditions: [PhNO
2
]=0.982m, mol
in
Figure 1. Nitrobenzene conversion as a function of the initial aniline con-
centration. Conditions: [PhNO ]=0.982m, mol ratios PhNO
Ph P(O)OH/Phen/[Pd(Phen) [BF =14800:1300:300:1, in MeOH
1 mL)+2,2-dimethoxypropane (34 mL). PCO =60 bar, at 1708C for 1 h.
The empty circle corresponds to a reaction in which Et N ([Et N]=
.015m), but no aniline, was added. The straight line corresponds to the
ratio PhNO /PhNH /Phen/[Pd(Phen) [BF =14500:140:300:1,
2
2
A
H
U
G
R
N
N
2
]
A
H
U
G
R
N
N
4 2
]
2
2
/
MeOH (1 mL)+2,2-dimethoxypropane (34 mL). PCO =60 bar, at 1708C
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
]
A
H
U
G
R
N
N
4 2
]
for 1.5 h. The straight line corresponds to the best fit through the data
(
2
points (R =0.947).
3
3
0
2
best fit through the first five data points (R =0.989).
low-concentration range and at constant aniline concentra-
tion and the results are shown in Figure 2.
ing. Two things are worthy of note: 1) The experimental
The kinetics are also clearly first order with respect to
acid concentration and the positive intercept indicates that
the reaction can proceed even in the absence of the acid, as
has been known for many years. Again it should be noted
that the acid concentration employed in typical catalytic re-
[31,32]
conditions optimized in the synthetic studies
correspond
to an aniline concentration=0.023m, well within the linear
[45]
region of the plot.
2) In all experiments the amount of
aniline at the end of the reaction was very close to the
amount added initially, so that the difference can be neglect-
ed. However, it is important that a small amount of aniline
was found even at the end of the reaction in which it had
not been initially added. This point is associated with a very
low conversion (1.5%) and yet it is clear that at least part of
the observed reactivity is due to the presence of the aniline
formed during the reaction.
[45]
actions lies within the linearity range.
It is also worth
mentioning that an approximately first-order dependence of
the rate of same carbonylation reaction on benzoic acid con-
centration at low concentrations is also apparent from the
data reported by van Leeuwen, although only three data
points are within the useful range (see Figure 1 in ref. [24]).
This supports the generality of our results.
Because the catalyst is initially added as a palladium(II)
complex, but the species active in the nitroarene activation
is surely zerovalent, it might be argued that aniline is not di-
rectly involved in a carbonylation step, but it merely acts as
a base facilitating the palladium reduction by CO. If enough
aniline is present to make this process fast, then a further in-
crease in its amount is not influential. Such an explanation
is unlikely to be correct because it is our experience that
palladium reduction under high CO pressure is very fast and
palladium metal is formed in a few minutes if an oxidant
We recall from previously published data that, if the reac-
tion is run at temperatures lower than 1108C and in the
presence of a sufficient amount of aniline, diphenylurea is
[9]
formed even when working in methanol as solvent. Only a
trace amount of carbamate is formed under these condi-
tions. We have independently tested that the alcoholysis re-
action [Eq. (3)] proceeds appreciably, at least in the case of
diphenylurea and methanol, only at temperatures higher
[9]
than 1208C.
Altogether, the data mentioned above show that, analo-
gously to several other catalytic systems for the same reac-
(
nitroarene in our case) is not present. In any case, we
[1,46]
tested this possibility by running a reaction in which another
base, triethylamine, was added, which is typically used to
help palladium reduction. The result (3.5% conversion) is
shown in Figure 1 as an empty circle and clearly corresponds
to a much lower conversion than that determined in the
presence of aniline at the same concentration. Thus, it clear
that the catalytic reaction is effectively first order with re-
spect to aniline concentration under the optimized reaction
conditions.
tion,
the catalytic cycle involves the intermediate reduc-
tion of the nitroarene to aniline and only at a later stage is
the latter carbonylated (Scheme 1).
As far as diphenylphosphinic acid is concerned, we had
previously shown that an initial rise in the catalytic rate
upon increasing acid concentration is observed, followed by
a flattening or even decrease in the rate over a limiting acid
amount, however, the number of data points in the low-con-
centration regime was too low to allow a kinetic order to be
determined. Thus, we ran a new series of reactions in the
Scheme 1.
In this process, the oxidation state of palladium changes
from zero to two and then (in the aniline carbonylation
step) to zero again. We have shown elsewhere
[1,46]
that this
reaction pathway renders the nitroarene carbonylation reac-
8066
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Chem. Eur. J. 2009, 15, 8064 – 8077

Carbonylation of Nitroarenes and Amines
FULL PAPER
tion very akin to the oxidative carbonylation of
intermediate formation of aniline, have shown that the spe-
cies responsible for the carbonylation step is a bis-carbal-
[4,47,48]
amines.
The carbonylative step is the same and the oxi-
dative step differs in the fact that oxygen has to be added
when amines are employed as substrates, whereas nitroar-
enes act as oxidants for palladium themselves. Note that the
palladium–phenanthroline system has also been successfully
employed to catalyze the oxidative carbonylation of
koxy complex, [Ru AHCTUNGERTNNUNG( DPPE)(CO) ACHTGNUTRENNU(GN COOMe) ]. Palladium
2 2
carbalkoxy complexes are easily formed under a variety of
conditions from different palladium(II) or palladium(0) pre-
cursors in the presence of CO and alcohols. Although bases
are often added to increase the yield, these are not necessa-
[49–53]
[65]
amines,
thus strengthening this connection.
ry.
Palladium carbalkoxy complexes have already been
It should be mentioned that a scenario in which the nitro-
arene is intermediately reduced to aniline is not the only
possible one. A metallacyclic complex A has been isolated
from the reaction mixture under relatively mild conditions
and shown to afford a carbamate after treatment at high
temperature with an alcohol under CO pressure. This occurs
best if in the presence of an acid, without requiring the in-
implicated as intermediates in different carbonylation reac-
[66]
tions such as olefin/CO copolymerization
carbonate and oxalate synthesis. All of these studies have
prompted several workers in the field to postulate that the
and dimethyl-
[67]
known [Pd ACHUTNGREUNNG( Phen) CAHTUNGTRNNEUN(G COOMe) ] (1a, Phen=1,10-phenanthro-
2
line) complex is the corresponding active species when pal-
ladium–phenanthroline complexes are used as precata-
[54]
[24,28,68]
termediate formation of aniline (Scheme 2).
lysts.
This assumption has been supported by several
pieces of experimental evidence. In particular:
1
) Although isolation or observation of 1a under the actual
reaction conditions proved impossible because of the
very low catalyst concentration and the forcing experi-
mental conditions, several years ago we reported that re-
action of palladium(0) complexes of the kind [Pd-
AHCTUNGTRENNUGN( Phen)(L)] (L=dibenzylideneacetone, benzoquinone)
Scheme 2.
with nitrosobenzene, an intermediate in the reduction of
nitrobenzene, afforded an insoluble, polymeric, complex
[28]
with an apparent composition [Pd Phen ACHTUNGNRTEUNG( PhNO) ].
2 3
2
The structure of this complex and of some close analogues
was solved almost at the same time by three different re-
When this complex was subjected to typical catalytic
conditions, 1a was obtained, together with the typical or-
ganic products deriving from the carbonylation reaction.
Thus, 1a can be formed under these reaction conditions.
[55–57]
search groups.
We are currently studying the effect of
different reaction conditions on the reaction mechanism,
and data have been obtained showing that a “direct carbon-
ylation” pathway (not involving the intermediate formation
of aniline), likely involving complex A as an intermediate,
plays a role in certain cases, although its contribution to the
2) The reactivity of [Pd ACHUTNRGENNUG( Phen) ACHTUNGTRENNUNG( COOMe) ] (1a) towards
2
amines, including aniline, has already been partly investi-
gated in the context of oxidative carbonylation reactions
of amines, and the compound does indeed afford substi-
[58,59]
[49]
reaction is always a minor one.
However, the first-order
tuted ureas when reacted with amines.
Most impor-
dependence of the reaction rate on aniline concentration
and the very low intercept at zero aniline concentration un-
equivocally indicate that any reaction pathway involving a
direct isocyanate or carbamate formation from A plays only
a very minor role, if any, under the conditions in which car-
bamate synthesis is typically performed. Complex A, if
formed, may be just an intermediate in the formation of ani-
line or perhaps a dormant state of the catalytic cycle. The
former possibility is worth consideration, although it has not
been previously proposed, because related ruthenium and
rhodium metallacycles play indeed this role. However, in-
vestigating such a possibility is beyond the scope of this
study and we will focus on the carbonylative step of the re-
action, that is, the conversion of aniline into phenylisocya-
nate. This is the step of the cycle that is common to the oxi-
dative carbonylation of amines, so the results are relevant to
the mechanisms of both reactions.
tantly, the reaction is accelerated by the presence of
[49]
acids, as found for the catalytic system.
Under the
proper experimental conditions, the reaction is fast even
at room temperature, that is, under conditions much
milder than those employed in catalysis. In this paper we
will also provide more quantitative evidence to support
this point, showing that the effect of several variables is
also the same for the model reaction and the catalytic
cycle.
3) The use of complex 1a as catalyst for the carbonylation
reactions of nitroarenes to carbamates was first reported
[1]
[40]
in a patent, and we have more recently compared its
catalytic activity with that of a standard precatalyst such
as [Pd
ACHTUNGTNERNUN(G Phen) ] ACHTUNGTENRNUN[G BF ] . The conversion was about 25%
2 4 2
higher when 1a was employed as catalyst in place of [Pd-
[28]
AHCTUNGTRENNUG( Phen) ] ACHTUNGTNRENUN[G BF ] .
2 4 2
Extensive studies by Gladfelter and his group on the
All of the aforementioned evidence encouraged us to in-
mechanism of the [Ru
A
H
U
G
R
N
N
(DPPE)(CO) ] (DPPE=bis-(diphe-
vestigate into more detail the reactivity of 1a, the results of
3
nylphosphino)ethane) catalyzed carbonylation of nitroar-
which are reported below.
[6,60–64]
enes to carbamates,
which also proceeds through the
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F. Ragaini et al.
Kinetic investigations of the reaction of [Pd
A
H
U
G
R
N
U
G
in Scheme 4, in which the acid is proposed to act as a bifunc-
tional catalyst in transferring a proton from nitrogen to
oxygen, thus rendering the amine attack irreversible.
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(COOMe) ] (1a) with amines in the presence of acidic pro-
2
moters: As previously mentioned, it has been reported that
1
a reacts with aniline to afford diphenylurea, methanol, and
metallic palladium, and the reaction is accelerated by the
[49]
addition of acids, including carboxylic acids. The following
reaction sequence was proposed to account for the observed
reactivity (Scheme 3; here and in the following, NꢀN repre-
sents Phen or a substituted phenanthroline). The intermedi-
ately formed isocyanate is immediately trapped by excess
amine and is not observable. The competitive trapping of
the same product by methanol to yield the carbamate is
[6]
much slower under these experimental conditions.
Scheme 3.
Scheme 4.
However, we found that, contrary to that reported, no re-
action at all occurs between 1a and an amine, either in the
presence or the absence of a carboxylic acid, if the reaction
is not performed under a CO atmosphere. Moreover, no re-
action was observed between benzoic acid and 1a in the ab-
sence of both the amine and CO. These observations are
clearly in contrast with the mechanism shown in Scheme 3.
A kinetic study of the reaction between 1a, p-toluidine, and
benzoic acid showed the reaction to be first order with re-
spect to 1a, toluidine, and acid and to strongly depend also
on the amount of dissolved CO, although experimental diffi-
culties prevented us from determining a precise reaction
Further evidence for bifunctional activation by carboxylic
or phosphorus acids is presented below. Note that we have
indicated proton transfer to the carboxylic oxygen atom of
the carbalkoxy group rather than to that bearing the methyl
group, because the former is in general the more basic of
the two for compounds of this kind. More specifically, in the
case of another palladium–carbalkoxy complex, [Pd-
A
H
U
G
R
N
U
G
A
H
U
G
R
N
N
3
ꢀ
2
C(O )
[70]
ic features.
Several more carboxylic acids and one phosphorus acid
were tested as promoters in the stoichiometric reaction. The
values of the apparent kinetic constants for the reaction pro-
moted by the tested acid and according to the kinetic Equa-
tion (5) are reported in Table 1:
[69]
order with respect to this reagent.
Toluidine was em-
ployed in the kinetic study in place of aniline to allow the
1
reaction to be better followed by H NMR. Note that the
acid was not consumed during the reaction and a catalytic
(
with respect to 1a) amount was sufficient to bring about
ꢀ
d½1 aꢁ=dt ¼ kapp½1 aꢁ
ð5Þ
the reaction, even if under normal catalytic conditions the
acid is added in large excess with respect to palladium in
order to optimize the efficiency of the reaction with respect
to the metal. Details of the kinetic study are reported in the
Supporting Information.
Table 1. Apparent first-order constants for the reaction of [Pd
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(Phen)-
[
a]
A
H
U
G
R
N
N
2
] (1a) with toluidine.
[
b]
ꢀ1
k
app
A
H
U
T
E
N
G
)
[min
]
pK
a
The rate dependence from four different reagents can
only be accounted for by a series of rapid equilibria preced-
ing the rate-determining step. For the kinetics in all reagents
to be first order, these pre-equilibria should also be shifted
towards the reagents. This is also in accordance with the fact
that no intermediate is observable by IR or NMR spectro-
scopic studies during the reaction. All the kinetic and reac-
tivity data are consistent with the reaction sequence shown
0.0316
0.0179
0.0277 (0.0296)
0.0273
0.0339
0.0294 (0.0722)
2.92
3.45
4.19
4.43
4.76
2.30
6
4
2,4,6-Me
COOH
,5-(MeO)
CH COOH
Ph P(O)OH
3
C
6
H
2
COOH
COOH
[
b]
C
3
6
H
5
2
C
6
H
3
3
[b]
2
ꢀ2
[
a] Experimental conditions: [1a]=2.69ꢅ10 m, initial molar ratios 1a/
acid/toluidine=1:0.08:26, at 273 K in CDCl
same temperature overnight. [b] See text for the meaning of kcorr
3
saturated with CO at the
.
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Carbonylation of Nitroarenes and Amines
FULL PAPER
Note that all experiments were run at the same toluidine
and acid concentrations and the kapp value also includes the
actual CO concentration, which is not exactly known, al-
though it was kept constant (Supporting Information). Also
to note that at the typical acid concentration employed
during the kinetic studies the equilibrium is essentially com-
pletely shifted towards the monomer side and thus, dimer
formation can be safely neglected in the kinetic study. This
is consistent with the fact that a clean first-order kinetics
with respect to the acid concentration was observed. If
dimer formation were substantial, a first-order rate depend-
ence on the total acid concentration should not have been
observed.
reported in the table are the pK values of the acids em-
a
ployed.
All acids catalyzed the reaction, although with different
efficiencies. As observed by van Leeuwen for catalytic reac-
tions, there is no correlation between the pK of the acid
a
and its promoting activity, although the promoting order
under stoichiometric conditions is also different from that
under catalytic conditions. The feature that most puzzled us
at the beginning is that diphenylphosphinic acid was not the
best promoter and the apparent kinetic constant was quite
close to that of benzoic acid, despite the fact that the former
is a much better promoter than the latter under catalytic
To analyze the possible role of acid deprotonation by tol-
uidine, we measured the decrease in the intensity of the car-
bonyl band of different solutions of benzoic acid in CHCl3
upon addition of the same amounts of toluidine employed
in the kinetic measurements. Details of the mathematical
and experimental procedure are given in the Supporting In-
formation. From the calculations, an equilibrium constant
[31,32]
ꢀ
+
ꢀ5
conditions.
However, it must be considered that the
K =[PhCOO ]
A
H
U
G
R
N
U
G
ACHTUNGTNERNUNG[ PhCOOH] ACTHUNGTRENNUN[G TolNH ]=1.38ꢅ10
2
eq
3
acids can be involved in two types of equilibria: self-dimeri-
zation and deprotonation by toluidine. Thus, we undertook
a quantitative study of the effect of these reactions on the
results of our kinetic study.
was determined.
In all kinetic experiments, toluidine is present in a very
large molar excess with respect to benzoic acid (typically
200-fold). Thus, the free toluidine concentration is essential-
ly unaffected by the extent of acid deprotonation. However,
the extent of acid deprotonation is not completely negligible
and is within the range 4.7–9.4% for the kinetic experiments
at different acid concentrations. Although such a variation
should in general affect the linearity of the rate dependence
on acid concentration, the extent of deprotonation is small
enough that when the apparent kinetic constants are cor-
rected for the actual amount of free acid, the plot kcorr vs.
Dimerization and deprotonation of benzoic and diphenyl-
phosphinic acids under the conditions of the kinetic study:
To discuss the quantitative aspects of the promoting activity
of different acids on both the stoichiometric reactions of 1a
and the catalytic reactions it is necessary to determine in
which form they are present under the reaction conditions.
It is well known that, even in the absence of bases, carboxyl-
ic acids are present in solution as monomer–dimer equilibri-
2
[PhCOOH] remains linear (the correlation coefficient R
even marginally increases from 0.9898 to 0.9909).
[71]
um mixtures (Scheme 5).
Having determined the equilibrium constant for the ben-
zoic acid–toluidine reaction, it is also possible to determine
the extent of acid deprotonation for the kinetic experiments
run at a variable toluidine and fixed acid concentrations.
The amount of dissociated acid ranges from 5.1 to 6.9% for
the concentrations investigated. Even the results of these ki-
netic experiments can be corrected accordingly, and again it
was found that the small correction resulted in an even
better linearity of the k vs. acid concentration (Supporting
Information). The value of the individual apparent first-
order kinetic constants changes slightly. In Table 1, the cor-
rected value for the experiment reported is also included as
kcorr. The correction is small in this case and the new value is
only 6.8% higher than the uncorrected one.
Scheme 5.
An analogous equilibrium also exists for dialkyl- and dia-
rylphosphinic acids and this is even more shifted towards
the dimer side.
as aniline (or toluidine) and phenanthroline, part of the acid
may be present in the deprotonated form. We recall that in
order to be able to measure the kinetics of the stoichiomet-
ric reactions of 1a the experiments had to be performed
under much milder conditions (08C) than those employed in
catalytic experiments (120–1808C), and in a different solvent
[72,73]
Moreover, in the presence of bases such
The extent of deprotonation for the other carboxylic acids
employed was not measured, but is not expected to differ
significantly from that observed for benzoic acid.
(
CDCl instead of methanol). Thus, the two situations must
3
be investigated independently. In this paragraph we only
consider the conditions of the kinetic studies, whereas we
will analyze the catalytic conditions later.
The extent of dimerization and deprotonation for diphe-
nylphosphinic acid cannot be measured by IR spectroscopy.
3
1
Thus, we resorted to P NMR spectroscopy. Data and dis-
cussion are again reported in the Supporting Information.
The conclusion is that in the solution containing both the
acid and toluidine, only 40.7% of the acid is in the protonat-
ed form. If we correct the kapp value in Table 1 for the frac-
tion of active free acid, the obtained kcorr value (0.0722) be-
comes the highest. Note that the second-best acid, acetic
We decided to analyze the benzoic acid monomer–dimer
equilibrium in CHCl by IR spectroscopy at different acid
3
concentrations. Indeed, it is known that the position of the
ꢀ
1
nCOO stretching frequency around 1700 cm is sensitive to
[74,75]
dimer formation.
Full data series and a discussion are
reported in the Supporting Information. Here, it is sufficient
Chem. Eur. J. 2009, 15, 8064 – 8077
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www.chemeurj.org
8069

F. Ragaini et al.
[
103–106]
acid, is less acidic than benzoic acid and thus, even if the
extent of deprotonation has not been measured in this case,
it is surely very low and the kcorr value would be very close
has been provided in only a few cases,
kinetic evidence is available in others.
whereas only
To the best of
[
107,108]
our knowledge, in no case has such an adduct been isolated
in the solid state. Moreover, calculations show that the
structure of these intermediates may not be trivial,
making the structural characterization of one of them espe-
cially interesting.
In an attempt to render the carbonyl adduct more stable,
we turned to carboxylate complexes. Note that the carbome-
thoxy group can be considered an isomer of the acetate ion
and that palladium phenanthroline complexes with carboxyl-
ate or halide anions also react with amines and CO to give
ureas and are probable intermediates in related carbonyla-
tion reactions in which no alcohol is present (see also the
last paragraph in this paper). Thus, the use of these com-
plexes can be considered to closely mimic the behavior of
1a,b. As a first attempt, we investigated the reactivity of
(
less than 6.8% higher) to the uncorrected one. Thus, the
[109–113]
apparent discrepancy between the results of the kinetic and
catalytic experiments with respect to the promoting abilities
of diphenylphosphinic and other carboxylic acids is solved
and the phosphorus acid is shown to be better than any
tested carboxylic acid under both conditions.
Characterization of carbon monoxide adducts: We now
focus on the identity of the proposed carbonyl adduct 2. As
mentioned above, when 1a is employed as substrate, no in-
termediate is observable by IR or NMR spectroscopies. Spe-
cifically, no absorption is observed in the IR spectrum
ꢀ
1
within the 2135–2000 cm region, in which the band associ-
ated with a CO group terminally bound to a neutral or cat-
ionic palladium(II) complex would be expected. Even when
[Pd ACHUNTGRENNU(G Neoc) ACHTUTGNRENNUN(G TMB) ] (1c, TMB=2,4,6-trimethylbenzoate) be-
2
[76]
2
1
a was treated with CO alone in CH Cl
formation of
cause the increased steric hindrance even of the carboxylate
should further favor the formation of 2. When 1c was treat-
ed with CO in CH Cl as described for 1b, an absorption
was observed within a few minutes at 2085 cm , very close
to the one attributed to 2b, but a band at 1878 cm was
also present from the beginning, which then grew at the ex-
pense of the 2085 cm band over several hours. During this
time the absorption at 1689 cm due to the coordinated car-
boxylate decreased in intensity, while the absorption of free
2
metallic palladium slowly occurred, but again no intermedi-
ate was observable by IR spectroscopy.
2
2
ꢀ
1
An extensive chemistry has been developed for platinu-
m(II) complexes with 2,9-dimethyl-1,10-phenanthroline (ne-
ocuproine, Neoc), showing that the presence of the two
ortho methyl groups stabilizes pentacoordinate complexes
ꢀ
1
ꢀ
1
[
77–93]
ꢀ1
by destabilizing the starting square-planar complex,
a limited number of related palladium complexes have also
been characterized.
and
[78,94–100]
ꢀ1
Note that in some cases the ad-
trimethylbenzoic acid at 1734 cm correspondingly grew.
ducts are not really pentacoordinate, but one bite of neocu-
proine detaches to afford a complex best described as dis-
torted square planar.
Performing the reaction in CDCl in an NMR tube al-
3
lowed us to assess that the formation of 2c is largely incom-
plete at room temperature after several minutes and almost
all of the palladium is still in the form of 1c when formation
of a third species (3c) starts to be observable. Compound 2c
is not stable even at 223 K, however, at this temperature it
When [Pd
ACHTUNGTRENNUNG( Neoc) ACHTUNGTRENNUNG( COOMe) ] (1b) was reacted with CO in
2
dry CH Cl an absorption immediately appeared at
2
2
ꢀ
1
2085 cm in the IR spectrum, indicating the presence of a
1
CO terminally bound to a palladium(II) complex (2b). The
intensity of this absorption slowly increased and reached a
maximum after two hours, however, a second absorption at
accumulated to a sufficient extent that its H NMR signals
could be unambiguously assigned. Both the neocuproine
and trimethylbenzoate signals show a symmetrical situation
and overall the spectrum is very similar to that of 1c, except
for a small shift of all signals. No broadening of the peaks is
observed, which implies that either the complex is truly pen-
tacoordinate or the exchange of the two nitrogen atoms is
fast on the NMR time scale even at this temperature. The
signals of 3c could also be assigned. They show a symmetri-
cal environment for the neocuproine ligand and that the
complex has a ratio of trimethylbenzoate/neocuproine 1:1.
Its formation is associated with the formation of one equiva-
lent of trimethylbenzoic acid. Although an univocal charac-
terization of 3c has not been possible up to now due to its
instability, the available data clearly points to a dimeric pal-
ladium(I) complex with one neocuproine ligand per palladi-
um atom and at least one bridging CO group. A structure of
ꢀ
1
1
884 cm , typical of a CO group in a bridging position
(
complex 3b), also developed during this time. Over a
longer timescale (several hours) both absorptions disap-
peared and metallic palladium was formed. Performing the
reaction at lower temperatures resulted in a slowing down
of all reactions, but the compound responsible for the
ꢀ
1
2
085 cm absorption was never the only species in solution.
Pentacoordinate complexes are often proposed to be in-
termediates in palladium-catalyzed reactions, and kinetic or
spectroscopic evidence in support of their formation has
been presented in some cases. However, the true nature of
such intermediates usually remains elusive. For example,
recent calculations suggest that the spectroscopically and
ꢀ
electrochemically observable [P PdRX ] (P=phosphane,
2
2
R=alkyl or aryl, X=halogen) may indeed be tetracoordi-
nate complexes in which the halide is held in the proximity
of the metal by attractive interactions with the phosphane li-
the kind [(Pd
ACHTUNGERTNNUNG( Neoc) ACHTUNREGTNNUNG( TMB)) ACHTGUNETRNNUG( m-CO) ] (n=1 or 2) would be
2 n
consistent with all available data, however, no precedent
exists for such a compound for comparison.
Many attempts to grow crystals of 2c suitable for an X-
ray structure determination failed because of decomposition
of the complex during the time necessary for crystal growth.
[114,115]
[101]
gands.
In the field of carbonylation reactions, pentacoor-
dinate CO adducts have often been proposed as intermedi-
[102]
ates,
however, spectroscopic evidence for their formation
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Carbonylation of Nitroarenes and Amines
FULL PAPER
However, during one of these attempts crystals of 1c were
obtained. A description of its structural features is reported
in the Supporting Information. A strong steric hindrance is
evident and this results in a distortion from planarity of the
neocuproine ligand that remains tightly bonded to the palla-
dium atom.
um pseudo-pentacoordinate structures, the geometric distor-
tion from a tetracoordinate square planar toward square
pyramid is much more frequent than toward trigonal bipyra-
mid, especially if the fifth coordination is not tight. Devia-
tions from square planarity would imply a modification of
the identical Cl1-Pd1-Cl2 and N2-Pd1-C1 angles to the
values expected in a bipyramidal geometry (note that the
apical sites are not predetermined). Indeed, the two angles
have very similar values and, accordingly, the asymmetric
In the hope of preparing a more stable analogue of 2b,c
several other [Pd
pared in which a different form of steric-hindrance (pivalate,
d), connection of the two carboxylate groups (2,2-diethyl-
malonate, 1e) or a change in their electron-withdrawing
ACHTUNGTRENNUNG( Neoc)(carboxylate) ] complexes were pre-
2
1
binding of the neocuproine is pronounced, (Dd=dPdꢀN10
ꢀ
dPdꢀN2 =0.476(2) ꢆ, see caption to Figure 3), although still
[116]
+
power (trifluoroacetate, 1 f) were present.
All of these
quite far from that observed in [Pd ACHTUNGTNERGUN( Phen) CH ] (Dd=
2 3
[117]
complexes reacted with CO to give a complex of type 2 (CO
0.70 ꢆ).
Note that a sixth coordination is in principle
ꢀ
1
IR stretching: 2d, 2085; 2e, 2090; 2 f, 2137 cm ), however,
all of them were even less stable than 2c. Absorptions at-
tributable to 3d–f were also observed during decomposition
of the complexes.
available at the Pd, and a weak interaction with the neocu-
proine methyl group (C15) is observed, similar to what
occurs in other pentacoordinate complexes of palladium and
platinum. The role of transition-metal atoms acting as hy-
drogen-bond acceptor, and, therefore, distinctive from an
agostic-type interaction, has been widely discussed by Bram-
Finally, we prepared [Pd ACHTUNGTRENN(UNG Neoc)Cl ] (1g) and found that
2
ꢀ
1
its addition product with CO, 2g (nCO =2130 cm ) was rela-
tively more stable than all other related derivatives. Indeed,
although decomposition in solution at room temperature oc-
curred over several hours, at 223 K the complex was stable
long enough for some crystals suitable for X-ray structure
analysis to be grown by diffusion of diethyl ether into a
CH Cl solution of 2g at this temperature. Its structure was
[118]
mer.
Theoretical calculations (to be discussed in a future
publication) show, however, that the conformation of the
methyl group is such that the H–Pd distance is actually
maximized. Therefore, this interaction deserves more accu-
rate theoretical and spectroscopic investigations that are cur-
rently underway in our laboratories, together with a theoret-
ical and experimental determination of the molecular elec-
tron density with the purpose of analyzing all the interesting
bonding features we addressed.
2
2
determined by single-crystal X-ray diffraction at T=100 K
Figure 3); details of the experiment and refinement are
given in the Supporting Information. CCDC 688642 and
88643 contain the supplementary crystallographic data for
(
6
Compound 2g is comparable with some Pt analogous
known since the early nineties. In particular, [PtI2-
this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[119]
A
H
U
G
R
N
U
G
has a quite similar molecular geometry, al-
though the asymmetry is significantly smaller (Dd=0.31 ꢆ).
As a consequence, it is slightly closer to a bipyramidal ar-
rangement than 2g (in fact, in [PtI ACTHUNGTERNN(UNG Neoc)CO] the N-Pt-CO
2
angle is more bent than the I-Pt-I one). The complex [PtCl-
[120]
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
is, instead, an almost regular bipyra-
3
mid (Cl and CH are apical sites; Dd=0.02 ꢆ).
3
The detachment of one of the two nitrogen atoms of the
phenanthroline has important chemical consequences, be-
cause it is very likely that the same will occur during the cat-
alytic cycle, at least in our system. To the best of our knowl-
edge, this is the first time that the detachment of a nitrogen
atom of a phenanthroline ligand has been observed to occur
during a catalytic cycle, although detachment of a nitrogen
atom of other chelating ligands upon CO binding has been
proposed to occur in at least two cases based on kinetic evi-
Figure 3. ORTEP view of 2g, with selected labeling. The main interatom-
ic distances [ꢆ] and angles [8]: Pd1ꢀC1 1.880(2); Pd1ꢀN2 2.078(1); C1ꢀ
O1 1.118(2); Pd1ꢀCl1 2.323(1); Pd1ꢀCl2 2.321(1); H15bꢀPd1 2.85(2);
[121,122]
dence.
The unequivocal characterization of 2g leads
credence also to all other palladium–CO adducts proposed
to be formed during different catalytic reactions, but also
questions their true pentacoordinate nature. The detach-
ment of one nitrogen atom also suggests that hemilabile li-
gands may be more appropriate ligands than symmetrical
phenanthrolines for both the catalytic carbonylation reac-
tion of nitroarenes and the oxidative carbonylation of
amines. Attempts to test this hypothesis are in progress in
our laboratories.
Pd1
ꢀ
N10 2.554(1); O1-C1-Pd1 179.1(1); C1-Pd1-N2 172.78(5); Cl1-Pd1-
Cl2 171.76(1); N2-Pd1-N10 73.1(1); N10-Pd1-C1 114.1(1).
As we can appreciate from Figure 3, 2g is not a truly pen-
tacoordinate complex, but is better viewed as square planar,
with distortion toward a square pyramid due to a fifth inter-
action occurring with N10 of the neocuproine ligand. This is
in keeping with the observation that among all the palladi-
Chem. Eur. J. 2009, 15, 8064 – 8077
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8071

F. Ragaini et al.
Detection of free isocyanate: In Schemes 3 and 4 we have
indicated the primary organic product as an arylisocyanate.
Formation of this type of compound is commonly assumed
in carbonylation reactions of amines, even with other metals
previously described with the rate of the catalytic reactions
discussed in the next paragraph it is necessary to know to
which extent the acid is deprotonated under catalytic condi-
tions.
(
e.g., ref. [6]), but to the best of our knowledge, it has never
Diphenylphosphinic acid was chosen for this part of the
study because the most efficient phosphorus acids (phenyl-
phosphonic and phosphoric acids) are multiprotic and this
would have made calculations unsolvable with the available
data. The approach we used to solve the problem is similar,
but more complex, to that described for the analogous de-
been directly observed. Because excess amine is always
present and the reaction of amines with arylisocyanates is
fast, only ureas are directly observed and this also occurred
[123,124]
in our case.
Thus, it may also be proposed that the ob-
served urea derives from the reaction of a coordinated iso-
cyanate moiety directly with external amine, without free
isocyanate being formed at any stage of the reaction.
protonation in CDCl . Here, it must be considered that
3
excess phenanthroline is also added to the reaction mixture.
The complete data and all the mathematical elaborations
are described in detail in the Supporting Information, how-
ever, a few comments are necessary. First, it should be con-
sidered that under typical optimized catalytic conditions, the
molar amount of acid is higher than the combined amounts
of aniline and phenanthroline. This implies that complete
deprotonation of the acid can never be achieved. At the
concentrations employed in the present study, the lowest
molar fraction of free acid achievable (complete protonation
of both aniline and phenanthroline) is 0.53. At room tem-
perature (298.4 K), the experimental molar fraction of free
To allow detection of free isocyanate, in case it is indeed
formed, we decided to use a sterically hindered amine, 2,6-
diisopropylaniline. In fact, steric hindrance on the amine is
expected to slow down its reaction with the palladium com-
plex to some extent, and to slow down even more the fol-
lowing formation of the diarylurea, because in the latter
case two sterically hindered molecules have to react, result-
ing in a sort of “squaring” of the steric retardation. This ap-
proach (Scheme 6) was successful and 2,6-diisopropylpheny-
lisocyanate could be directly observed in the reaction mix-
ture by IR spectroscopy (typical isocyanate absorption at
ꢀ
1
2
289 cm ) or trapped by the addition of excess methanol to
acid is 0.67 (K =1.17), indicating that deprotonation occurs
eq
give the corresponding carbamate, observed by GC-MS.
to a large extent. However, at 407.9 K this value increases
to 0.898 (K =0.0319) and at the optimal temperature for
eq
catalytic reactions (443 K), the calculated molar fraction of
free acid is 0.924 (K =0.016). As is apparent from the equi-
eq
librium-constant values, which vary by almost two orders of
magnitude in the series, if the acid had been in defect with
respect to the bases, the differences would have been even
more impressing. In any case, it emerges that although de-
protonation proceeds extensively at room temperature, free
acid and bases are almost exclusively found at high tempera-
ture. Although this should not be surprising against a theo-
retical background, the extent of the variations is probably
generally underestimated in the literature, in which acids
are among the most often employed catalysts (or co-cata-
lysts) and “buffered” conditions are also extremely
common. The results reported here show that the composi-
tion, and hence the reactivity, of weak acid–weak base sys-
tems changes considerably as the temperature changes, and
this should be considered when investigating the effect of
different reaction temperatures. The results also indicate
that the identity of the ideal acidic catalyst, or co-catalyst,
for a given reaction should also change as temperature
changes, with higher temperatures requiring stronger acids.
Our reaction is accelerated by an increase in the concen-
trations of both a base (aniline or other aromatic amines, if
different nitroarenes are employed) and an acid. At low
temperatures these two reagents deactivate each other, but
the extent of this deactivation is strongly reduced by an in-
crease in the temperature, an effect that cannot be mim-
icked by any variation in the absolute or relative concentra-
tions of the two reagents.
Scheme 6.
Deprotonation of diphenylphosphinic acid under catalytic
conditions: As described above, we investigated the extent
of deprotonation of benzoic and diphenylphosphinic acids
under the conditions of the kinetic study. However, catalytic
reactions are performed under very different conditions
(
120–1808C instead of 08C, methanol as solvent in place of
CDCl , higher acid concentrations, very different acid/
3
amines ratios). To link the results of the kinetic experiments
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Carbonylation of Nitroarenes and Amines
FULL PAPER
Kinetic investigations under catalytic conditions: Supporting
evidence for a bifunctional effect of the acid: The kinetic
study on the reaction of 1a with toluidine and several acids
showed that the latter do not interact with 1a until CO and
toluidine have reacted with the complex, in full agreement
with the postulated bifunctional effect. However, evidence
must be provided that the stoichiometric reactions discussed
are relevant to the actual catalytic cycle. The need for a
thermodynamically unfavorable (in the case of unsubstituted
phenanthroline) CO coordination to 1a is in full agreement
sence and presence of diphenylphosphinic acid are obtained.
Even from these data it is immediately evident that the posi-
tive effect of the acid is not on the activation enthalpy,
which is apparently even higher in the presence of the pro-
moter, but rather on the activation entropy.
We have shown in this work that the kinetics of the cata-
lytic reactions is first order with respect to both aniline and
diphenylphosphinic acid under typical catalytic concentra-
tions. At this point we can use the data obtained for the di-
phenylphosphinic acid–aniline equilibrium as a function of
temperature to correct the measured rates for the amount
of free acid and amine present in each experiment. The in-
terested reader can find the corresponding calculations in
the Supporting Information. Although the calculations are
straightforward, it is necessary to assume that the aniline
concentration is approximately constant during the catalytic
reaction. The experimental results show indeed that at the
end of the reaction the amount of aniline in solution is very
close to that added initially, but the low absolute amounts
[32]
with the observed
first-order dependence of “catalytic”
reaction rate on CO pressure and constitutes the first evi-
dence that the two processes are associated. To further sub-
stantiate the proposed reaction pathway, we determined the
°
°
DH and DS for the catalytic reaction both in the absence
and in the presence of diphenylphosphinic acid following
the Eyring approach. To this aim, we ran two series of cata-
lytic reactions with nitrobenzene as substrate and at differ-
ent temperatures; the first in the absence of any acidic pro-
moter and the second in the presence of diphenylphosphinic
acid. Because the kinetics of the catalytic reaction have al-
ready been shown to be zero order with respect to nitroben-
°
cause a relatively large uncertainty in the calculated DH
°
°
and DS values. In any case, the corrections (DH =6.32ꢅ
4
ꢀ1
°
ꢀ1 ꢀ1
10 Jmol and DS =ꢀ63.2 Jmol K ) go in the direction
[32]
°
zene (the only reagents consumed in detectable amounts
being CO and methanol present in large stoichiometric ex-
cesses), the kinetic constants are the same as the turnover
of equalizing the two DH values and to increase the differ-
°
ence in the DS values, further stressing that the promoting
effect is of entropic origin.
ꢀ
1
frequency values (expressed in s ).
We can now go back to the probable structure of the tran-
sition state for the rate-determining proton transfer in the
stoichiometric reaction of 1a with amines, both in the pres-
ence and absence of diphenylphosphinic acid. Looking at
Scheme 4 and taking into account the results of the X-ray
structure determination, we can represent the transition
state for the diphenylphosphinic acid-promoted process as
in Scheme 7A. On the other hand, in the absence of the
acid and in methanol as solvent the generation of an equally
long four-atom chain for the proton transfer would require
the involvement of two methanol molecules, as in
Scheme 7B.
The results of the catalytic reactions performed are shown
in Table S9 (Supporting Information). Note that the abso-
lute concentrations of phenanthroline, aniline (initial), and
diphenylphosphinic acid (when present) are the same as
those employed during the experiments for the determina-
tion of the extent of deprotonation of diphenylphosphinic
acid during catalytic reactions. These concentrations are also
very close to those used under optimized catalytic condi-
tions. With respect to the latter, however, more catalyst was
employed to reach significant conversions even at the lowest
temperatures and in the absence of promoter. Because of
this, the conversion in the experiment at 1708C and in the
presence of diphenylphosphinic acid reached 100% and no
kinetic constant value could be determined for it.
Before analyzing these data we must point out some limi-
tations of this approach. In its original formulation, the
Eyring equation should be applied only to an elementary re-
[125]
action. It has been shown in the literature
that extension
to catalytic processes is possible. However, in this case the
°
°
determined DH and DS values will also include a contri-
bution from any equilibrium reaction occurring before the
rate-determining step. With this limitation in mind, we first
applied the Eyring approach to the crude TOF values re-
ported in Table S9. Very good linear plots for both the “un-
Scheme 7.
2
promoted” and the “promoted” reactions (R =0.973 and
Such hydrogen-bonded chains are very often found, espe-
cially for water in most biologically relevant proton-transfer
processes (in which much longer chains are sometimes in-
volved), although their importance is usually neglected in
organometallic chemistry.
0
.9966, respectively, for the reactions in the absence and
presence of diphenylphosphinic acid, Figure S12, Supporting
Information) were obtained.
°
4
ꢀ1
From these plots the values DH =5.17ꢅ10 Jmol ,
°
ꢀ1 ꢀ1
°
4
ꢀ1
°
DS =ꢀ149 Jmol K , and DH =6.57ꢅ10 Jmol , DS =
111 Jmol K , respectively, for the reactions in the ab-
Both transition states are highly organized and will be
characterized by strongly negative activation entropies, how-
ꢀ
1
ꢀ1
ꢀ
Chem. Eur. J. 2009, 15, 8064 – 8077
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8073

F. Ragaini et al.
ever, structure B requires the structuring of one more mole-
cule and should be associated with a more negative DS .
complexes have never been tested as catalysts and they may
be especially active.
°
This is exactly what we found as the result of the Eyring
analysis of the “catalytic” reaction, and further supports that
the series of reactions in Scheme 4 is indeed closely connect-
ed to what is occurring under the much more forcing cata-
lytic conditions.
To test this possibility we synthesized [Pd
ACHTUNGTRENNUNG( Phen)-
AHCTUNGTREG(NNUN O PPh ) ] (4) by an exchange reaction similar to those em-
2
2 2
ployed to synthesize other carboxylato complexes
(Scheme 9).
Investigation of a possible alternative: Although all of the
data discussed in this paper is consistent with the mechanis-
tic scenario proposed, it should be mentioned that a com-
pletely different alternative may exist for the promoting role
of phosphorus acids. It is well known that nitroarene car-
bonylation can be catalyzed by palladium–phenanthroline
complexes even in the absence of methanol if a stoichiomet-
Scheme 9.
[1,9]
ric amount of amine is present.
Ureas are the products,
and the intermediate in the carbonylation step must clearly
be different in this case because no carbalkoxy complex can
be present in the absence of any alcohol. Carboxylate com-
plexes such as those synthesized in this work (1b–f) can be
employed for this reaction and the data reported in this
paper suggest that they operate with a mechanism similar to
that shown in Scheme 4, the carboxylate being a regioisomer
of the carbalkoxy group. In this case, a carboxylic acid mole-
cule would be eliminated in the proton transfer in place of
one of methanol and one of CO (Scheme 8).
Compound 4 was employed as catalyst in two reactions
employing, respectively, Ph P(O)OH and H PO as promot-
2
3
4
ers and the results were compared with those of two reac-
tions run under exactly the same conditions, but employing
[Pd ACHTUNGTERNNGN(U Phen) ] ACHTUNGRTENN[GU BF ] as catalyst. All reactions were run at a
2 4 2
very high catalytic ratio, 15000, to maximize differences in
rate. The results are reported in Table 2.
2 2 2 2
Table 2. Comparison between [Pd ACHTUNGTRNENUGN( Phen) AHCTUNGTERUNNGN( O PPh ) ] and [Pd ACHTUNGTRENNUNG( Phen) ]-
[
a]
A
H
U
G
R
N
U
G
4 2
] as catalysts.
Promoter
2
PhNO conv. Carbamate
[
b]
[c]
[
%]
select. [%]
[Pd
[Pd
[Pd
[Pd
A
T
N
T
E
N
N
(Phen)(OP(O)Ph
2
)
)
2
2
]
]
H
H
Ph
Ph
3
3
PO
PO
4
4
28.2
51.5
73.3
74.4
82.3
84.3
A
H
U
G
R
N
U
G
2
]
A
H
U
G
R
N
N
4
]
2
A
H
U
G
R
N
U
G
2
2
P(O)(OH) 20.5
P(O)(OH) 34.8
A
H
U
G
R
N
U
G
2
]
A
H
U
G
R
N
N
4
]
2
2
ꢀ
3
[
a] Experimental conditions: catalyst=1.26ꢅ10 mmol, mol ratios
/PhNH /promoter/Phen/Pd=15000:400:1350:300:1, CO =60 bar,
708C in MeOH (15 mL)+2,2-dimethoxypropane (0.5 mL) for 1.5 h.
b] Calculated with respect to the starting PhNO . [c] Calculated with re-
spect to the converted PhNO
PhNO
1
[
2
2
P
2
2
.
In both cases the conversions are clearly better when the
traditional precatalyst is employed. This excludes that [Pd-
AHCTUNGTNERNUG( Phen)(OP(O)Ph ) ] is an active intermediate and indirectly
2 2
further supports the reaction scenario previously outlined in
this paper.
Scheme 8.
Conclusion
We have investigated the reactivity of [Pd AHCUTNGREUNNG( Phen) HCATUNGTRENNUN(G COOMe) ]
2
[
1,24]
In previous works
we and others have shown that the
(1a) towards amines starting from the working hypothesis
that it may be an intermediate in the reductive carbonyla-
tion reaction of nitroarenes under our optimized conditions
and that its formation may also be relevant to some oxida-
tive carbonylation reactions of amines. The data obtained
showed that not only 1a reacts with amines in the expected
way, but a number of features of the “stoichiometric” reac-
presence of carboxylates retards the catalytic reaction with
respect to non-coordinating anions. Moreover the fact that
diphenylphosphinic acid did not react with 1a,b in the ab-
sence of CO and amine also makes unlikely the formation
of diphenylphosphinate–palladium complexes as catalytical-
ly active intermediates. However, phosphorus acids-derived
8074
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Chem. Eur. J. 2009, 15, 8064 – 8077

Carbonylation of Nitroarenes and Amines
FULL PAPER
tion parallel the observations made under catalytic condi-
tions. In particular, corresponding features include:
Acknowledgements
We thank Heros Larici, Carlo Cognolato, and Francesco Ferretti for per-
forming some experiments and the MiUR (PRIN2007HMTJWP_004) for
financial support.
1
) The kinetics of the stoichiometric reaction is first order
with respect to amine and the kinetics of the catalytic re-
action is first order with respect to amine up to a satura-
tion point.
[1] S. Cenini, F. Ragaini, Catalytic Reductive Carbonylation of Organic
Nitro Compounds, Kluwer Academic, Dordrecht, 1996.
2] A. M. Tafesh, J. Weiguny, Chem. Rev. 1996, 96, 2035–2052.
3] F. Paul, Coord. Chem. Rev. 2000, 203, 269–323.
4] D. J. Dꢇaz, A. K. Darko, L. McElwee-White, Eur. J. Org. Chem.
007, 4453–4465.
[5] a) J. H. Grate, D. R. Hamm, D. H. Valentine, U.S. Patent US
705883, 1987; b) J. H. Grate, D. R. Hamm, D. H. Valentine, U.S.
2
) The rate of the stoichiometric reaction depends on CO
concentration and the rate of the catalytic reaction is
first order with respect to CO pressure. Interestingly,
even the rate of the oxidative carbonylation of 4-chloro-
aniline by a similar catalytic system is faster at higher
[
[
[
2
[51]
4
pressure.
Patent US 4629804, 1986; c) J. H. Grate, D. R. Hamm, D. H. Valen-
tine, U.S. Patent US 4600793, 1986; d) J. H. Grate, D. R. Hamm,
D. H. Valentine, U.S. Patent US 4603216, 1986.
3
4
) The rate of the stoichiometric reaction is first order with
respect to the acidic promoter and the rate of the catalyt-
ic reaction is first order with respect to the acidic pro-
moter up to a saturation point.
) Diphenylphosphinic acid is a better promoter than any
tested carboxylic acid for the stoichiometric reaction and
diphenylphosphinic and other phosphorus acids are
better promoters than any tested carboxylic acid under
catalytic conditions.
[6] J. D. Gargulak, W. L. Gladfelter, J. Am. Chem. Soc. 1994, 116,
3792–3800.
7] F. Ragaini, A. Ghitti, S. Cenini, Organometallics 1999, 18, 4925–
933.
8] F. Ragaini, S. Cenini, J. Mol. Catal. A 2000, 161, 31–38.
9] M. Gasperini, F. Ragaini, C. Remondini, A. Caselli, S. Cenini, J.
Organomet. Chem. 2005, 690, 4517–4529.
10] a) Chem. Week, March 9, 1977, 43; b) R. Rosenthal, J. G. Zajacek,
U.S. Patent US 3962302, 1976 [Chem. Abstr. 1976, 85, 177053a];
c) R. Tsumura, U. Takaki, A. Takeshi, Ger. Offen. 2635490, 1977
Chem. Abstr. 1977, 87, 23929v]; d) E. T. Shawl, H. S. Kesling, Jr.,
U.S. Patent US 4871871, 1989 [Chem. Abstr. 1990, 112, 138777w].
[11] E. Alessio, G. Mestroni, J. Mol. Catal. 1984, 26, 337–340.
12] E. Alessio, G. Mestroni, J. Organomet. Chem. 1985, 291, 117–127.
13] A. Bontempi, E. Alessio, G. Chanos, G. Mestroni, J. Mol. Catal.
[
4
[
[
[
5
) Ditolylurea is the only observable product under stoi-
chiometric conditions, but the intermediate formation of
an isocyanate has been proved by use of a sterically hin-
dered amine, and diarylureas are the products of the cat-
alytic reactions if the temperature is kept at or under
[
[
[
1
008C (carbamates are formed only at higher tempera-
1
987, 42, 67–80.
tures by urea alcoholysis). The formation of free isocya-
nates as reaction intermediates has often been pro-
[
[
[
14] S. Cenini, F. Ragaini, M. Pizzotti, F. Porta, G. Mestroni, E. Alessio,
J. Mol. Catal. 1991, 64, 179–190.
15] R. Santi, A. M. Romano, F. Panella, G. Mestroni, A. Sessanta o
Santi, J. Mol. Catal. A 1999, 144, 41–45.
[126]
posed.
6
) Kinetic data suggest a bifunctional role of the acid for
the stoichiometric reaction and Eyring plots for catalytic
reactions show that the effect of the acidic promoter is
16] E. Drent, P. W. N. M. Van Leeuwen, Eur. Pat. Appl. EP 86281, 1983
[
Chem. Abstr. 1984, 100, 6109].
[17] E. Drent, Eur. Pat. Appl. EP 250037, 1987 [Chem. Abstr. 1988, 109,
6236].
°
to decrease the DS value, whereas no positive effect is
°
[18] E. Drent, Eur. Pat. Appl. EP 224292, 1987 [Chem. Abstr. 1987, 107,
observed on DH . An alternative explanation has been
1
99074].
19] E. Drent, Eur. Pat. Appl. EP 231045, 1987 [Chem. Abstr. 1988, 109,
28605].
[20] E. Drent, Pure Appl. Chem. 1990, 62, 661–669.
21] R. Santi, A. M. Romano, F. Panella, C. Santini, J. Mol. Catal. A
997, 127, 95–99.
22] P. Wehman, G. C. Dol, E. R. Moorman, P. C. J. Kamer, P. W. N. M.
van Leeuwen, J. Fraanje, K. Goubitz, Organometallics 1994, 13,
4856–4869.
shown to be inconsistent with the experimental data.
[
1
All of these observations strongly suggest that the stoi-
[
chiometric reactions we investigated are indeed relevant to
the actual catalytic reactions, and that the reaction scheme
proposed is a good description of the carbonylative step of
the catalytic cycle.
1
[
During our study, we had to take into account several
problems whose importance is not limited to the presently
investigated reaction. The real identity of a carbon monox-
ide–palladium complex has been elucidated by X-ray dif-
fraction analysis and will surely represent a reference for
other palladium-catalyzed carbonylation reactions. The im-
portance of deprotonation reactions between a weak acid
and a weak base and the strong dependence of this reaction
on reaction temperature have been quantitatively investigat-
ed. This is also an often neglected problem in catalysis and
we hope that the availability of quantitative data, even just
for one reaction, will stimulate other researchers to take
into account this problem in their studies.
[23] P. Wehman, V. E. Kaasjager, W. G. J. Delange, F. Hartl, P. C. J.
Kamer, P. W. N. M. van Leeuwen, J. Fraanje, K. Goubitz, Organo-
metallics 1995, 14, 3751–3761.
[
24] P. Wehman, L. Borst, P. C. J. Kamer, P. W. N. M. van Leeuwen, J.
Mol. Catal. A 1996, 112, 23–36.
[25] P. Wehman, P. C. J. Kamer, P. W. N. M. van Leeuwen, Chem.
Commun. 1996, 217–218.
[
26] P. Wehman, L. Borst, P. C. J. Kamer, P. W. N. M. van Leeuwen,
Chem. Ber. Recueil 1997, 130, 13–21.
[
27] P. Wehman, H. M. A. van Donge, A. Hagos, P. C. J. Kamer,
P. W. N. M. van Leeuwen, J. Organomet. Chem. 1997, 535, 183–193.
[28] E. Gallo, F. Ragaini, S. Cenini, F. Demartin, J. Organomet. Chem.
999, 586, 190–195.
29] F. Ragaini, S. Cenini, C. Querci, Belgian Pat. Appl. BE 1013489,
002 [Chem. Abstr. 2005, 142, 281948].
30] M. Gasperini, F. Ragaini, S. Cenini, E. Gallo, J. Mol. Catal. A 2003,
04–205, 107–114.
1
[
[
2
2
Chem. Eur. J. 2009, 15, 8064 – 8077
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8075

F. Ragaini et al.
[
[
[
[
31] F. Ragaini, C. Cognolato, M. Gasperini, S. Cenini, Angew. Chem.
003, 115, 2992–2995; Angew. Chem. Int. Ed. 2003, 42, 2886–2889.
32] F. Ragaini, M. Gasperini, S. Cenini, Adv. Synth. Catal. 2004, 346,
3–71.
33] M. Gasperini, F. Ragaini, C. Cazzaniga, S. Cenini, Adv. Synth.
Catal. 2005, 347, 105–120.
[57] F. Paul, J. Fischer, P. Ochsenbein, J. A. Osborn, Organometallics
1998, 17, 2199–2206.
[58] The best method to quantify the contribution of a direct carbonyla-
tion pathway is to run a reaction with equimolar amounts of
2
6
5
[D ]nitrobenzene and undeuterated aniline, at a temperature (no
more than 1008C) such that neither diphenylurea alcoholysis nor
exchange of the amino moieties of urea with free amine can occur.
If only a direct carbonylation occurs, the final product should be
34] S. Yamada, K. Moriya, T. Fuchigami, N. Wakasa, Jpn. Kokai
Tohhio Koho JP 2003183241, 2003 [Chem. Abstr. 2003, 139,
6
9060].
5
exclusively Ph-[D ]NHC(O)NHPh. On the other hand, if the inter-
[
35] S. Yamada, K. Moriya, T. Fuchigami, N. Wakasa, Jpn. Kokai
mediate formation of aniline is involved, a mixture of all possible
ureas (undeuterated, penta-, and decadeuterated) should be ob-
tained, with a composition depending on the nitrobenzene conver-
sion. When we performed a carbonylation under conditions similar
to those described in the caption of Figure 1, but at 908C and for
Tohhio Koho JP 2003183240, 2003 [Chem. Abstr. 2003, 139,
6
9059].
36] F. Shi, Y. He, D. Li, Y. Ma, Q. Zhang, Y. Deng, J. Mol. Catal. A
006, 244, 64–67.
37] J. Stapersma, K. Steernberg, Eur. Pat. Appl. EP 296686, 1988
Chem. Abstr. 1989, 111, 133802].
38] R. Santi, A. M. Romano, P. Panella, G. Mestroni, It. Pat. Appl.
MI96A 002071, 1996.
39] R. Santi, A. M. Romano, P. Panella, G. Mestroni, It. Pat. Appl.
MI96 A 002072, 1996.
[
[
[
[
[
2
5 2 2
30 min, using equimolar amounts of [D ]PhNO and PhNH as sub-
[
strates, the final urea contained 88.0% undeuterated and 12.0%
pentadeuterated urea. Considering that the nitrobenzene conver-
sion was 6.4%, this deuteration degree is too high to be accounted
for by the deuterated aniline that is liberated in solution, and evi-
dences a contribution by a direct carbonylation pathway. However,
even under these conditions such a contribution accounts for less
then 12% of the total reaction and a pathway involving the inter-
mediate formation of aniline is strongly dominant.
40] R. Santi, A. M. Romano, R. Garrone, P. Panella, It. Pat. Appl.
MI97A 000433, 1997.
[
[
41] E. Alessio, G. Mestroni, Eur. Pat. EP 0169650 (A2), 1986.
42] P. Leconte, F. Metz, Eur. Pat. Appl. EP 330591, 1989 [Chem. Abstr.
[59] Complex A is a likely candidate as an intermediate when the reac-
tion is run in an aprotic solvent and isocyanates are produced.
However, this is a much slower reaction than the synthesis of car-
1
990, 112, 78160].
[
43] M. Gasperini, F. Ragaini, S. Cenini, E. Gallo, S. Fantauzzi, Appl.
Organomet. Chem. 2007, 21, 782–787.
ꢀ
1
bamates: TOF=58 h at 1808C and 40 bar (ref. [14]) compared to
ꢀ
1
[
44] a) Phosphorus acids are well known to be very good bifunctional
promoters. For some examples, see: b) L. M. Litvinenko, N. M.
Oleinik, Russ. Chem. Rev. 1978, 47, 401–415; L. M. Litvinenko,
N. M. Oleinik, Usp. Khim. 1978, 47, 777–803; c) A. P. Kozlov, S. S.
Sazhnev, G. A. Kozlova, Yu. S. Andreiichikov, Russ. J. Org. Chem.
about 6000 h at 1708C and 100 bar for the present system
(ref. [31]).
[60] J. D. Gargulak, A. J. Berry, M. D. Noirot, W. L. Gladfelter, J. Am.
Chem. Soc. 1992, 114, 8933–8945.
[61] J. D. Gargulak, W. L. Gladfelter, Organometallics 1994, 13, 698–
705.
1
996, 32, 1523–1528; A. P. Kozlov, S. S. Sazhnev, G. A. Kozlova,
Yu. S. Andreiichikov, Zhur. Org. Khim. 1996, 32, 1573–1578;
d) G. V. Semenyuk, N. G. Razumova, J. Org. Chem. USSR 1986, 22,
[62] S. J. Skoog, J. P. Campbell, W. L. Gladfelter, Organometallics 1994,
13, 4137–4139.
9
1
96–997; G. V. Semenyuk, N. G. Razumova, Zhur. Org. Khim.
986, 22, 1109–1110; e) G. V. Semenyuk, N. G. Razumova, J. Org.
[63] J. D. Gargulak, M. D. Noirot, W. L. Gladfelter, J. Am. Chem. Soc.
1991, 113, 1054–1055.
Chem. USSR 1984, 20, 421–424; G. V. Semenyuk, N. G. Razumova,
Zhur. Org. Khim. 1984, 20, 468–471; f) G. D. Titskii, O. P. Stepko,
L. M. Litvinenko, J. Org. Chem. USSR 1975, 11, 1009–1014; G. D.
Titskii, O. P. Stepko, L. M. Litvinenko, Zhur. Org. Khim. 1975, 11,
[64] S. J. Skoog, W. L. Gladfelter, J. Am. Chem. Soc. 1997, 119, 11049–
11060.
[65] G. Cavinato, L. Toniolo, A. Vavasori, J. Mol. Catal. A 2004, 219,
233–240.
1
021–1026; g) L. M. Litvinenko, G. D. Titskii, O. P. Stepko, Dokl.
[66] J. Liu, B. T. Heaton, J. A. Iggo, R. Whyman, J. F. Bickley, A. Stein-
er, Chem. Eur. J. 2006, 12, 4417–4430.
[67] F. Rivetti, U. Romano, J. Organomet. Chem. 1978, 154, 323–326.
[68] P. Giannoccaro, N. Ravasio, M. Aresta, J. Organomet. Chem. 1993,
451, 243–248.
Phys. Chem. 1972, 202, 133–136; L. M. Litvinenko, G. D. Titskii,
O. P. Stepko, Dokl. Acad. Nauk USSR 1972, 202, 1127–1130.
45] In the following we will see that aniline can be protonated by di-
phenylphosphinic acid, so that the effective concentration of both
is altered. However, at 1708C the protonation equilibrium is essen-
tially shifted towards the neutral compounds (see below) and the
small correction that should be applied does not alter the linearity
of the first part of the plot.
[
3
[69] Complex 1a decomposes under a CO atmosphere in CDCl at
room temperature or in methanol even at low temperatures at a
rate not negligible with respect to that of the reaction with tolui-
dine. Thus, the kinetic study was performed in CDCl at 08C, a
3
[
[
46] F. Ragaini, S. Cenini, J. Mol. Catal. A 1996, 109, 1–25.
47] B. Gabriele, G. Salerno, M. Costa, G. P. Chiusoli, Curr. Org. Chem.
temperature at which the complex is stable in the absence of the
other reagents, and even the reaction with toluidine is very slow
relative to when benzoic acid is also present. To observe reproduci-
ble kinetics, it was necessary to saturate the solvent with CO by
stirring it overnight at 08C before use. Stirring for just a few hours
gave reaction rates for otherwise identical reactions that were
roughly proportional to the presaturation time.
2
004, 8, 919–946.
48] B. Gabriele, G. Salerno, M. Costa, Top. Organomet. Chem. 2006,
8, 239–272.
[
1
[
[
49] P. Giannoccaro, J. Organomet. Chem. 1994, 470, 249–252.
50] B. L. Goodall, W. Terlouw, Eur. Pat. Appl. EP 395144 A1, 1990
[
Chem. Abstr. 1991, 114, 142898].
[70] R. Bertani, G. Cavinato, L. Toniolo, G. Vasapollo, J. Mol. Catal.
1993, 84, 165–176.
[71] D. H. Williams, T. F. Gale, B. Bardsley, J. Chem. Soc. Perkin Trans.
2 1999, 1331–1334.
[72] R. E. Asfin, G. S. Denisov, K. G. Tokhadze, J. Mol. Struct. 2002,
608, 161–168.
[73] J. R. Ferraro, D. F. Peppard, J. Phys. Chem. 1963, 67, 2639–2643.
[74] D. P. N. Satchell, J. L. Wardell, Trans. Faraday Soc. 1965, 61, 1199–
1201.
[
51] B. L. Goodall, W. Terlouw, A. M. Wullink-Schelvis, Eur. Pat. Appl.
EP 393789 A1, 1990 [Chem. Abstr. 1991, 114, 163776].
52] V. L. K. Valli, H. Alper, Organometallics 1995, 14, 80–82.
53] F. Shi, J. Peng, Y. Deng, J. Catal. 2003, 219, 372–375.
54] P. Leconte, F. Metz, A. Mortreux, J. A. Osborn, F. Paul, F. Petit, A.
Pillot, J. Chem. Soc. Chem. Commun. 1990, 1616–1617.
55] A. Sessanta o Santi, B. Milani, G. Mestroni, E. Zangrando, L. Ran-
daccio, J. Organomet. Chem. 1997, 545–546, 89–91.
[
[
[
[
[
56] N. Masciocchi, F. Ragaini, S. Cenini, A. Sironi, Organometallics
[75] M. H. Abraham, P. P. Duce, R. A. Schulz, J. J. Morris, P. J. Taylor,
D. G. Barratt, J. Chem. Soc. Faraday Trans. 1 1986, 82, 3501–3514.
1
998, 17, 1052–1057.
8076
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Chem. Eur. J. 2009, 15, 8064 – 8077

Carbonylation of Nitroarenes and Amines
FULL PAPER
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
76] All solvents have to be carefully dried, otherwise formation of met-
[107] W. P. Mul, H. Oosterbeek, G. A. Beitel, G.-J. Kramer, E. Drent,
Angew. Chem. 2000, 112, 1919–1922; Angew. Chem. Int. Ed. 2000,
39, 1848–1851.
[108] C. Bianchini, A. Meli, W. Oberhauser, Dalton Trans. 2003, 2627–
2635.
[109] B. A. Markies, P. Wijkens, A. Dedieu, J. Boersma, A. L. Spek, G.
van Koten, Organometallics 1995, 14, 5628–5641.
[110] K. E. Frankcombe, K. J. Cavell, B. F. Yates, R. B. Knott, Organo-
metallics 1997, 16, 3199–3206.
[111] K. E. Frankcombe, K. J. Cavell, B. F. Yates, R. B. Knott, J. Phys.
Chem. 1996, 100, 18363–18370.
[112] J. H. Groen, A. de Zwart, M. J. M. Vlaar, J. M. Ernsting,
P. W. N. M. van Leeuwen, K. Vrieze, H. Kooijman, W. J. J. Smeets,
A. L. Spek, P. H. M. Budzelaar, Q. Xiang, R. P. Thummel, Eur. J.
Inorg. Chem. 1998, 1129–1143.
[113] S. A. Macgregor, G. W. Neave, Organometallics 2003, 22, 4547–
4556.
allic palladium occurs very quickly in all cases.
77] F. P. Fanizzi, G. Natile, M. Lanfranchi, A. Tiripicchio, G. Pacchioni,
Inorg. Chim. Acta 1998, 275–276, 500–509.
78] V. G. Albano, G. Natile, A. Panunzi, Coord. Chem. Rev. 1994, 133,
6
7–114.
79] N. Margiotta, V. Bertolasi, F. Capitelli, L. Maresca, A. G. G. Moli-
terni, F. Vizza, G. Natile, Inorg. Chim. Acta 2004, 357, 149–158.
80] V. De Felice, F. Giordano, I. Orabona, D. Tesauro, A. Vitagliano, J.
Organomet. Chem. 2001, 622, 242–250.
81] V. G. Albano, M. Monari, I. Orabona, F. Ruffo, A. Vitagliano,
Inorg. Chim. Acta 1997, 265, 35–46.
82] R. Romeo, S. Carnabuci, M. R. Plutino, A. Romeo, S. Rizzato, A.
Albinati, Inorg. Chem. 2005, 44, 1248–1262.
83] P. Papadia, A. Ciccarese, J. A. Miguel-Garcia, P. M. Maitlis, F. P.
Fanizzi, J. Organomet. Chem. 2005, 690, 2097–2105.
84] F. P. Fanizzi, N. Margiotta, M. Lanfranchi, A. Tiripicchio, G. Pac-
chioni, G. Natile, Eur. J. Inorg. Chem. 2004, 1705–1713.
85] F. P. Fanizzi, G. Natile, M. Lanfranchi, A. Tiripicchio, F. Laschi, P.
Zanello, Inorg. Chem. 1996, 35, 3173–3182.
86] R. J. H. Clark, F. P. Fanizzi, G. Natile, C. Pacifico, C. G. van Rooy-
en, D. A. Tocher, Inorg. Chim. Acta 1995, 235, 205–213.
87] F. P. Fanizzi, M. Lanfranchi, G. Natile, A. Tiripicchio, Inorg. Chem.
[114] In the literature, a complex was reported whose cationic part has
+
the structure [(Pd
A
H
U
G
R
N
U
G
2
A
H
U
G
R
N
G
A
T
N
T
N
U
G
ꢀ
1
spectrum shows an absorption at 1880 cm very close to that of 3c
(ref. [116]). Thus, the composition [(Pd(Neoc))2 A( m-CO)(m-TMB)]-
[TMB] may also be proposed for 3c, however, the observation of
C
T
N
T
E
N
G
AHCTUNGTRENNUNG
only one set of signals for the trimethylbenzoate anion even at
1
1
994, 33, 3331–3339.
223 K in the H NMR spectrum would imply that a fast exchange
88] V. G. Albano, V. De Felice, M. Monari, G. Roviello, F. Ruffo, Eur.
J. Inorg. Chem. 2005, 416–422.
89] V. De Delice, A. De Renzi, M. L. Ferrara, A. Panunzi, J. Organo-
met. Chem. 1996, 513, 97–104.
90] V. G. Albano, M. Monari, I. Orabona, A. Panunzi, G. Roviello, F.
Ruffo, Organometallics 2003, 22, 1223–1230.
91] S. A. De Pascali, D. Migoni, P. Papadia, A. Muscella, S. Marsi-
gliante, A. Ciccarese, F. P. Fanizzi, Dalton Trans. 2006, 5077–5087.
92] P. Papadia, A. Ciccarese, J. A. Miguel-Garcia, P. M. Maitlis, F. P.
Fanizzi, J. Organomet. Chem. 2005, 690, 2097–2105.
between the free and the coordinated trimethylbenzoate anion is
occurring, which is quite surprising for an anion in a bridging posi-
tion.
[115] R. Garrone, A. M. Romano, R. Santi, R. Millini, Organometallics
1998, 17, 4519–4522.
[116] [Pd AHCUTNGRENNU(G Neoc) ACTHGNUTRENUNN(G malonate)] was also synthesized, but its very low solu-
bility prevented us from studying its reactivity with CO.
[117] B. Milani, A. Marson, E. Zangrando, G. Mestroni, J. M. Ernsting,
C. J. Elsevier, Inorg. Chim. Acta 2002, 327, 188–201.
[118] L. Brammer, Dalton Trans. 2003, 3145–3157.
93] V. De Felice, B. Giovannitti, A. Panunzi, F. Ruffo, D. Tesauro,
Gazz. Chim. Ital. 1993, 123, 65–69.
94] S. Hansson, P. O. Norrby, M. P. T. Sjogren, B. Akermark, M. E.
Cucciolito, F. Giordano, A. Vitagliano, Organometallics 1993, 12,
[119] F. P. Fanizzi, L. Maresca, G. Natile, M. Lanfranchi, A. Tiripicchio,
G. Pacchioni, J. Chem. Soc. Chem. Commun. 1992, 333–335.
[120] V. De Delice, M. L. Ferrara, F. Giordano, F. Ruffo, Gazz. Chim.
Ital. 1994, 124, 117–119.
4
940–4948.
[121] R. E. Rꢁlke, J. G. P. Delis, A. M. Groot, C. J. Elsevier, P. W. N. M.
van Leeuwen, K. Vrieze, K. Goubitz, H. Schenk, J. Organomet.
Chem. 1996, 508, 109–120.
[
[
[
[
[
95] V. G. Albano, C. Castellari, M. E. Cucciolito, A. Panunzi, A. Vita-
gliano, Organometallics 1990, 9, 1269–1276.
96] A. De Renzi, G. Morelli, A. Panunzi, A. Vitagliano, Gazz. Chim.
Ital. 1987, 117, 445–447.
97] F. P. Fanizzi, F. P. Intini, L. Maresca, G. Natile, M. Lanfranchi, A.
Tiripicchio, J. Chem. Soc. Dalton Trans. 1991, 1007–1015.
98] N. Desmarais, C. Adamo, B. Panunzi, V. Barone, B. Giovannitti,
Inorg. Chim. Acta 1995, 238, 159–163.
[122] J. H. Groen, M. J. M. Vlaar, P. W. N. M. van Leeuwen, K. Vrieze,
H. Kooijman, A. L. Spek, J. Organomet. Chem. 1998, 551, 67–79.
[123] In at least one case (ref. [124]), it has been claimed that formation
of isocyanates could be observed by gas chromatography. However,
in our experience ureas easily thermally crack at the typical tem-
perature of a GC injector to give isocyanates and amines, and
these products can be observed and identified if a GC-MS instru-
ment is used, even when pure ureas are injected into the gas chro-
matograph.
99] V. De Felice, M. E. Cucciolito, A. De Renzi, F. Ruffo, D. Tesauro,
J. Organomet. Chem. 1995, 493, 1–11.
[
[
100] V. De Felice, V. G. Albano, C. Castellari, M. E. Cucciolito, A.
De Renzi, J. Organomet. Chem. 1991, 403, 269–277.
101] S. Kozuch, C. Amatore, A. Jutand, S. Shaik, Organometallics 2005,
[124] B. Gabriele, G. Salerno, R. Mancuso, M. Costa, J. Org. Chem.
2004, 69, 4741–4750.
2
4, 2319–2330.
[125] J. J. Rooney, J. Mol. Catal. A 1995, 96, L1–L3.
[102] G. K. Anderson, R. J. Cross, Acc. Chem. Res. 1984, 17, 67–74.
[103] P. E. Garrou, R. F. Heck, J. Am. Chem. Soc. 1976, 98, 4115–4127.
[104] F. Ozawa, N. Kawasaki, H. Okamoto, T. Yamamoto, A. Yamamoto,
Organometallics 1987, 6, 1640–1651.
[126] Isocyanates can be the final reaction products under certain condi-
tions, but only in the absence of alcohols. Formation of anilines as
intermediates in these systems is unlikely, so they are mechanisti-
cally distinct from those investigated here.
[
105] H.-K. Luo, Y. Kou, X.-W. Wang, D.-G. Li, J. Mol. Catal. A 2000,
1
51, 91–113.
[
106] D. H. Nguyen, G. Laurenczy, M. Urrutigoꢈty, P. Kalck, Eur. J.
Inorg. Chem. 2005, 4215–4225.
Received: September 12, 2008
Published online: March 9, 2009
Chem. Eur. J. 2009, 15, 8064 – 8077
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8077