Mechanistic studies on the selective oxidative carbonylation of MeOH to dimethyl oxalate catalyzed by [Pd(COOMe)n(TsO)2-n(PPh 3)2] (n = 0, 1, 2) using p-benzoquinone as a stoichiometric oxidant

Article abstract of DOI:10.1016/j.jorganchem.2013.11.009

The reactivity of the complexes cis-[Pd(OTs)₂(PPh ₃)₂] (I), trans-[Pd(COOMe)(OTs)(PPh₃) ₂] (II) and trans-[(COOMe)₂(PPh₃)₂] (III), regarding the catalytic oxidative carbonylation of MeOH to dimethyl oxalate (DMO) using benzoquinone (BQ) as a stoichiometric oxidant, has been studied in CD₂Cl₂/MeOH (10/1, v/v) by ¹H and ³¹P{¹H} NMR spectroscopy. I reacts with CO and MeOH at 193 K giving II, which is transformed into III upon addition of a base. The same occurs in the presence of BQ. Instead, if the base is added before admission of CO, [Pd(BQ)(PPh₃)₂] is formed. Starting also from II, complex III is formed only after addition of a base. The base neutralizes TsOH which is formed in the transformation of I to II and III. III is unstable in the presence of 1 equivalent of TsOH and it is transformed into II. At 333 K, under 0.4 MPa of CO, III decomposes with formation of DMO and dimethyl carbonate (DMC) (15%) each), whereas, in the presence of BQ, III is unstable already at 298 K, with formation of only DMO (10%). Catalysis to DMO is observed at 333 K. Thus BQ enhances the reactivity of III and directs the catalysis selectively to DMO. I, II and III have also been used in catalytic experiments in pure MeOH at 298 K, under 0.3 MPa of CO. II and III are active even in the absence of a base (TOF ca. 30 h^-1). I is active only after addition of a base. A catalytic cycle is proposed.

Full text of DOI:10.1016/j.jorganchem.2013.11.009

Journal of Organometallic Chemistry 750 (2014) 74e79
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Mechanistic studies on the selective oxidative carbonylation of
MeOH to dimethyl oxalate catalyzed by [Pd(COOMe)n(TsO)2ꢀn(PPh3)2]
(n ¼ 0, 1, 2) using p-benzoquinone as a stoichiometric oxidant
1
*
Emanuele Amadio , Luigi Toniolo
Department of Molecular Sciences and Nanosystems, University Ca’ Foscari of Venice, Dorsoduro 2137, 30123 Venice, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactivity of the complexes cis-[Pd(OTs)2(PPh3)2] (I), trans-[Pd(COOMe)(OTs)(PPh3)2] (II) and trans-
[(COOMe)2(PPh3)2] (III), regarding the catalytic oxidative carbonylation of MeOH to dimethyl oxalate
(DMO) using benzoquinone (BQ) as a stoichiometric oxidant, has been studied in CD2Cl2/MeOH (10/1, v/
v) by 1H and 31P{1H} NMR spectroscopy. I reacts with CO and MeOH at 193 K giving II, which is trans-
formed into III upon addition of a base. The same occurs in the presence of BQ. Instead, if the base is
added before admission of CO, [Pd(BQ)(PPh3)2] is formed. Starting also from II, complex III is formed only
after addition of a base. The base neutralizes TsOH which is formed in the transformation of I to II and III.
III is unstable in the presence of 1 equivalent of TsOH and it is transformed into II. At 333 K, under 0.4
MPa of CO, III decomposes with formation of DMO and dimethyl carbonate (DMC) (15%) each), whereas,
in the presence of BQ, III is unstable already at 298 K, with formation of only DMO (10%). Catalysis to
DMO is observed at 333 K. Thus BQ enhances the reactivity of III and directs the catalysis selectively to
DMO.
Received 19 September 2013
Received in revised form
2 November 2013
Accepted 4 November 2013
Keywords:
Methanol oxidative carbonylation
Oxalate
Benzoquinone
Palladium
I, II and III have also been used in catalytic experiments in pure MeOH at 298 K, under 0.3 MPa of CO. II
and III are active even in the absence of a base (TOF ca. 30 hꢀ1). I is active only after addition of a base. A
catalytic cycle is proposed.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
reported using PdCl2 [2] and Pd(AcO)2/Co(AcO)2/PPh3 [3] For the
latter system, the function of the phosphine was not reported, for
The catalytic oxidative carbonylation of an alkanol to the cor-
responding carbonate and oxalate can be conveniently performed
in the presence of palladium-based catalysts [1].
example whether it reacted with the acetates forming complexes of
the type M(AcO)2(PPh3)2. As a matter of fact, PPh3 could have
reacted with BQ with formation of PPh3eBQ adduct (betaine) [4e6]
before interacting with the metals. BQ can be used also in
the absence of oxygen. This avoids the formation of water since
BQ is reduced to hydrobenzoquinone (H2BQ) [7]. Recently, we
have reported the use of BQ for the oxidative carbonylation of
MeOH, catalyzed by the pre-formed Pd(II)-PPh3 complexes
[Pd(COOR)nX2ꢀn(PPh3)2] (n ¼ 0, 1, 2; X ¼ Br, Cl, NO2, ONO2, OAc,
OTs) in combination with NEt3. DMO is selectively produced. After
catalysis, the complexes [Pd(BQ)(PPh3)2], [Pd(CO)(PPh3)3] and
[Pd(CO)(PPh3)]3 have been found in the reaction mixture [8]. No
dicarboalkoxy species has been detected, in spite of the fact that the
formation of oxalate is likely to occur through a dicarboalkoxy in-
termediate [9e11].
The use of oxygen implies the formation of water, which causes
consumption of CO and prevents further formation of the product.
The use of triethyl orthoformate as dehydrating agent was pro-
posed by D.M. Fenton et al. for the oxidative carbonylation of
ethanol catalyzed by PdCl2 in combination with a redox couple,
typically Cu(I)/Cu(II) chlorides [2]. The problem of the formation of
water was overcome using an alkyl nitrite in a two step process for
the industrial production of alkyl oxalates catalyzed by Pd/C [1].
The use of BQ as a cooxidant in combination with oxygen was
Abbreviations: BQ, benzoquinone; DMO, dimethyl oxalate; DMC, dimethyl
carbonate.
In addition to being an oxidant, BQ can play other roles by
participating in both the formation and transformation of key in-
termediates into the reaction products by interacting with the
catalytically active metal complex. It can change properties of the
* Corresponding author. Fax: þ39 (0)41 234 8517.
E-mail address: toniolo@unive.it (L. Toniolo).
Present address: ITM-CNR, c/o Department of Chemical Sciences, University of
1
Padua, Via Marzolo 1, 35131 Padua, Italy.
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.11.009

E. Amadio, L. Toniolo / Journal of Organometallic Chemistry 750 (2014) 74e79
75
reaction centre and, consequently, the mechanism and the direc-
tion of the reaction. For example, using PdCl2 or [Pd(CO)Cl]n in the
oxidative carbonylation of MeOH, the corresponding oxalate or
carbonate was formed in the presence or absence of BQ, respec-
tively [12,13]. Another example is the following. trans-[Pd(COOMe)
Cl(PPh3)2]/NEt3 catalyzes the selective oxidative carbonylation of
MeOH to oxalate at 65 ꢁC using BQ as an oxidant, even though this
complex is stable in the absence of BQ [8]. As a matter of fact, it can
be prepared in high yield by carbonylation of trans-[PdCl2(PPh3)2]
in MeOH in the presence of NEt3 at 343 K [14].
To the best of our knowledge, no detailed mechanistic studies
have been reported on the oxidative carbonylation of an alkanol to
oxalate. As above mentioned, cis-[Pd(OTs)2(PPh3)2] (I), trans-
[Pd(COOMe)(OTs)(PPh3)2] (II) and trans-[(COOMe)2(PPh3)2] (III)
have been used as catalyst precursors using BQ as an oxidant [8]. It
is reasonable to suppose that starting from (I), the formation of the
oxalate occurs though the intermediacy of a mono- and a di-
carbomethoxy species of type II and III. Taking advantage of the
fact that these complexes are rather reactive, but stable enough to
be prepared as solid compounds, we took them into consideration
for an NMR study relevant to this catalysis. Hereafter, the results of
this investigation are discussed.
solution already present in the tube as solvent. In both cases the
resulting NMR tube was immediately pressurized at the desiderate
pressure at 193 K.
The multicomponent systems studied and the 1H and 31P{1H}
NMR data of I, II, III and other complexes/compounds identified in
the NMR experiments are reported in Tables 1 and 2, respectively.
2.4. Carbonylation of I in MeOHeNEt3
0.1 mmol of I dissolved in 2 mL of MeOHeNEt3 (Pd/N ¼ 1/6) was
treated with CO at 273 K. The solution, initially light brown, turned
orangeered in a few minutes and, at the same time, a precipitate
was formed. After 200, neither DMO nor DMC were detected by GC.
The NMR and IR spectra of the solid recovered after filtration
(40 mg) showed the presence of [Pd(CO)(PPh3)]3 and III. Upon
adding cold water to the filtrate, a white solid was precipitated
(8 mg), identified as II.
2.5. Oxidative carbonylation of MeOH using I, II, and III as catalyst
precursors
In a glass bottle equipped with a syringe cup for sampling,
6.0$10ꢀ2 mmol of precursor and 6 mmol of BQ were added to 5 mL
of dry MeOH, previously saturated with CO at 298 K under a flux of
the same gas. The bottle was quickly pressurized at 0.3 MPa. After
1 h, NEt3 or PPh3 were added (Pd/N ¼ 1/2, Pd/addedPPh3 ¼ 1/2).
Samples were withdrawn and analyzed by CG every 300 for a period
of 2 h. The results are reported in Table 3.
2. Experimental section
2.1. Reagents
MeOH, NEt3, TsOH$H2O, PPh3, BQ, CD2Cl2 and CD3OD were
purchased from SigmaeAldrich. CD2Cl2 and CD3OD were stored
ꢀ
over 4 A molecular sieves under Ar. Carbon monoxide (purity
3. Results and discussion
higher than 99%) was supplied by SIAD Spa (Italy).
Cis-[Pd(OTs)2(PPh3)2] (I) [15], trans-[Pd(COOMe)(TsO)(PPh3)2]
(II) [16], trans-[Pd(COOMe)2(PPh3)2] [8] were prepared according
to literature procedures.
3.1. Reactivity of I
I reacts with CO at 193 K giving an unidentified species (31P{1H}
23.01 ppm), which reacts with MeOH to yield II, which is trans-
formed into III upon addition of NEt3 (Table 1, system 1.1; Sup-
porting information, Fig. S1). These results have already been
reported [8]. II is formed also in the presence of BQ (Table 1, system
1.2). Above 313 K, III begins to be unstable, at 333 K decomposition
to palladium metal is evident, accompanied with the formation of
DMO and DMC in approximately equal amounts, 15% of each one.
In another experiment, I was treated with BQ and PPh3 at 193 K
and then CO was admitted (Table 1, system 1.3; Supporting infor-
mation, Fig. S2). There was formation of [Pd(BQ)(PPh3)2] [5,6], no
other Pd(0) complex was formed. All PPh3 disappeared because of
the reaction with excess of BQ forming “betaine” [4e6]. At 298 K
the NMR spectra did not change significantly, neither DMO nor
2.2. Instrumentation
NMR spectra were recorded on Bruker AMX 300 spectrometer.
All 1H chemical shifts are reported relative to the residual proton
resonance in the deuterated solvent. 31P{1H} signals were refer-
enced to an 85% aqueous solution of H3PO4. NMR under pressure
was performed using a 5 mm pyrex glass HP-NMR tube with Teflon
head (maximum pressure tolerated 1.3 MPa).
2.3. High pressure NMR experiments
Typically, a solution of 5$10ꢀ3 mmol of the palladium complex
dissolved in CD2Cl2 (0.15 mL) was poured under argon, at r.t., into
the 5 mm pyrex glass HP-NMR tube, previously evacuated by a
vacuum pump. The tube was then quickly placed in a liquid N2/
acetone bath cooled at 193 K. To the cooled solution was added,
under argon flow, a solution containing the desired amount of PPh3
Table 1
Multicomponent systems studied by NMR spectroscopy.
Designation
System
and/or BQ, NEt3, MeOH (30
mL to reach 10% of final volume) in
1.1
1.2
1.3
1.4
2.1
2.2
2.3
3.1
3.2
3.3
3.4
I þ (CO) þ (MeOH) þ (8NEt3)
I þ (MeOH, 5BQ, CO)
0.15 mL of CD2Cl2. The tube was connected with the pressure line by
using the special screw top in Teflon, then purged several times (4e
5) and pressurized with CO or Ar (the maximum pressure used at
this temperature was 0.6 MPa) taking care to shake the tube in
order to favour the solubilization of the gases. The tube was then
heated at the desired temperature in the NMR probe. Further
addition of liquid (such as NEt3 and MeOH) was performed by
injecting the desired amount with a syringe to a depressurized
NMR tube cooled at 193 K. A similar procedure was followed for the
addition of the solid compounds. In this case the solution to be
injected was prepared by solubilising the solid in a little vial cooled
at 193 K under CO or Ar atmosphere using a small part of the
I þ (MeOH, 6PPh3, 10BQ) þ (CO)
I þ (MeOH, 6NEt3, 10BQ) þ (CO)
II þ (MeOH, 1PPh3, CO) þ (8NEt3)
II þ (MeOH, 1PPh3, 10BQ, CO)
II þ (MeOH, 6PPh3, 10BQ, CO)
IIIþ(MeOH, CO)þ(1TsOH) þ (1TsOH) ꢀ (CO)
III
III þ (CO)
III þ (5 BQ, CO) þ (MeOH)
Note: the components that were mixed or added together are in brackets; the value
near the component represents the equivalent with respect to Pd; MeOH 10% in
volume with respect to CD2Cl2. CO in all cases 0.4 MPa.

76
E. Amadio, L. Toniolo / Journal of Organometallic Chemistry 750 (2014) 74e79
Table 2
3.2. Reactivity of II
Relevant 1H and 31P{1H} NMR data of I, II, III and other compounds identified during
the NMR experiments.
At 193 K, II reacts with PPh3 and CO (Table 1, system 2.1) to give
[Pd(COOMe)(PPh3)3](TsO) (Table 1, system 2.1; Supporting infor-
mation, Fig. S3). Only with the subsequent addition of NEt3 further
reactions occur: a PPh3 ligand is displaced from the coordination to
Pd(II) and carbon monoxide and methanol interact giving III. This
complex is stable up to 298 K, but at 313 K (better at 333 K) it de-
composes with formation of [Pd(CO)(PPh3)3] [19], trans-[Pd(COO-
Me)(OMe)(PPh3)2] and of DMO and DMC (ca. 15% each). For the
identification of the methoxy complex see below on the reactivity
of III.
Therefore, as shown in Scheme 1, starting from both I or II, the
dicarbomethoxy complex III is formed only after the addition of
NEt3 (see also Table 1, system 1.1). When the monocarbomethoxy
complex II is formed from I, one equivalent of TsOH is also formed,
so that one equivalent of acid is present before the addition of the
base. In the absence of the base, the acid prevents the formation of
the dicarboxy complex III. As a matter of fact, III, in the presence of
one equivalent of acid, is unstable and gives the mono-
carbomethoxy complex as reported below (Table 1, system 3.1). The
reason why it is necessary to add a base in order to obtain III, is
described below.
The reactivity of II has been studied also with BQ, under con-
ditions close to those of catalysis, except for the presence of NEt3. A
CD2Cl2/MeOH solution of II, PPh3 and BQ was pressurized with CO
at 193 K (Table 1, system 2.2). As shown in Fig. 1, II reacts imme-
diately with CO and MeOH to give III, which starts decomposing at
298 K, giving 20% of DMO. Only upon increasing the temperature
up to 333 K, [Pd(BQ)(PPh3)2], [Pd(CO)(PPh3)]3 are formed together
with ca. 1.5 equivalent of DMO, an amount larger than that of the
starting complex. Thus catalysis is observed. Though not so high,
the catalytic activity is significant considering that, in the absence
of BQ, DMO is formed in 15% yield with respect to III (cfr. system
2.1).
Compound
d
1H NMR [ppm]
d
31P{1H} NMR
[ppm]
cis-[Pd(TsO)2(PPh3)2] (I)
7.48e6.99 (m, 38H, Ar)
2.28 (s, 6H, CH3eTsO)
7.61e6.67 (m, 38H, Ar)
2.36 (s, 3H, COOMe)
2.19 (s, 3H, CH3eTsO)
7.68e7.37 (m, 30H, Ar)
2.55 (s, 6H, COOMe)
38.3 (s)
trans-[Pd(COOMe)(OTs)
(PPh3)2] (II)
18.9 (s)
trans-[Pd(COOMe)2
(PPh3)2] (III)
[Pd(OTs)(PPh3)3](TsO)
21.76 (s)
7.71e7.01 (m, 53 H, Ar) 35.3 (t, 1P); 31.1
2.34 (s, 6 H, CH3eTsO)
7.61e6.67 (m, 38H, Ar)
2.33 (s, 3H, CH3eTsO)
1.91 (s, 3H, COOMe)
6.80 (s, 4H, CH)
6.65 (s, 4H, CH)
9.00 (brs, 2H, OH)
3.73 (s, 6H, CH3)
(d, 2P); JPeP ¼ 13.2 Hz
[Pd(COOMe)(PPh3)3](TsO)
19.9 (d, 2P); 15.7
(t, 1P); JPeP ¼ 39.2 Hz
BQ
H2BQ
DMC
DMO
3.89 (s, 6H, CH3)
PPh3eBQ adduct
[Pd(COOMe)(OMe)(PPh3)2] 2.52 (s, 3H, COOMe)
3.25 (brs, 3H, OMe)
[Pd(CO)(PPh3)]3
[Pd(CO)(PPh3)3]
22.3, 16.4 (s)a
19.7 (s); 20.3 (s)a
28.5 (s)
22.6 (s)a
32.9 (s)a
[Pd(BQ)(PPh3)2]
NMR spectra were taken in CD2Cl2, data at 195 K. Abbreviations: s, singlet; d,
doublet; t, triplet; dt, doublet of triplet; m, multiplet; dt, doublet of triplet; m,
multiplet; brs, broad singlet.
a
In CD2Cl2/MeOH 10%.
DMC were formed at 333 K. [Pd(BQ)(PPh3)2] was formed at 193 K
from I and BQ also in the presence of NEt3 (Table 1, system 1.4).
These results can be summarized as follows. II is formed only in
the absence of a base, otherwise Pd(0) is formed. III is formed only
through II and after addition of a base. Betaine acts as a base
because of the presence of the AreOꢀ moiety. In fact, it reacts with
an acid giving a phosphonium salt [4].
The same results have been obtained also by using a larger
amount of added ligand (Table 1, system 2.3). In fact, in both cases
PPh3 does not interact with the complexes, but rather it is imme-
diately consumed by the reaction with excess BQ forming “betaine”.
It is noteworthy to point out that, in the presence of BQ, III
already gives DMO at 298 K, together with the Pd(0) complexes
[Pd(CO)(PPh3)]3 and [Pd(BQ)(PPh3)2], without formation of DMC
and that catalysis is observed at 333 K. Whereas in the absence of
BQ, III is stable up to 313 K with formation of DMO, DMC, trans-
[Pd(COOMe)(OMe)(PPh3)2] and [Pd(CO)(PPh3)3]. Thus BQ modifies
the reactivity of the system, in particular the reactivity of III toward
the formation of oxalate.
The fact that Pd(0) is formed when the base is present before CO
is admitted can be rationalized as follows. The presence of the base
and MeOH could lead to a methoxy-palladium species and, via b-
elimination, to a palladium hydride [17], unstable in the reaction
conditions [18].
It should be noticed that the NMR experiments were carried out
with MeOH diluted in CH2Cl2. We have found that in MeOH with
dissolved NEt3, in the absence of CD2Cl2, I reacts with CO (0.2 MPa)
at 273 K to give [Pd(CO)(PPh3)]3 [19] together with both II and III.
These results show that in MeOH the carbomethoxy complexes are
formed even when NEt3 is present before the admission of carbon
monoxide, at difference of what observed in a NMR tube in which
MeOH in diluted with CD2Cl2. Thus the concentration of MeOH (and
probably also the polarity of the medium) plays an important role
in directing the reactivity.
3.3. Reactivity of III
A dicarbomethoxy species is likely to be involved in the product-
forming step. Therefore, the study of the stability and reactivity of
III is of paramount importance.
Table 3
Effect of time and base on the activity of the oxidative carbonylation reaction toward
DMO using I, II and III.
As reported above, the formation of III from II requires the
addition of base. The following observations may explain the role of
Time [h]
I
II
III
TONDMO
TONDMO
TONDMO
0.5
1
1.5
2
0
0
5
6
10
14
11a or 9b
21a or 20b
26a
37a
30a
39a
Operative conditions: [Pd] ¼ 1.2$10ꢀ2 mol/L, Pd/BQ ¼ 1/100, PCO ¼ 0.3 MPa, 298 K,
anhydrous MeOH, 5 mL.
a
NEt3 (Pd/NEt3 ¼ 1/2), added after 1 h of reaction.
b
PPh3 (Pd/PPh3 added ¼ 1/2), added after 1 h of reaction. TONDMO ¼ molDMO/molPd
.
Scheme 1. Formation of II and III from I.

E. Amadio, L. Toniolo / Journal of Organometallic Chemistry 750 (2014) 74e79
77
formed in the reaction medium, thus eliminating the cause of
instability of the dicarbomethoxy complex III.
In the experiment relevant to the system 2.1, the formation of a
new complex tentatively formulated as trans-[Pd(COOMe)(O-
Me)(PPh3)2] is reported. This assignment is based on the following
considerations. III, dissolved in CD2Cl2 under Ar (Table 1, system
3.2), starts decomposing at 313 K. Complete decomposition occurs
at 333 K with formation of palladium metal and DMC. Decompo-
sition is accompanied with the decreasing of the intensity of the
signal of the PdeCOOMe moiety at 2.52 ppm from the H(PPh3)/
H(COOCH3) ¼ 30/6 of III to 30/3. In addition, a new broad 1H signal
appears at ca. 3.25 ppm, close to that of the OMe ligand of trans-
[Pd(X)(OMe)(PPh3)2] (X ¼ C6F5, CCl]Cl2) [20] (Fig. 2). DMC could
form through an intramolecular migratory nucleophilic attack of
the COOMe and OMe ligands of [Pd(COOMe)(OMe)(PPh3)2]
(Scheme 3) [18].
In another experiment (Table 1, system 3.3) the stability of III
was tested under CO starting form 193 K. III is stable up to 313 K,
then it decomposes giving DMC and DMO (ca. 10% each) and
Pd(COOMe)(OMe)(PPh3)2. At 333 K III is completely converted to
the methoxy complex.
The reactivity of III with BQ was studied under conditions close
to those of catalysis (Table 1, system 3.4; Supporting information,
Fig. S5). In the presence of BQ and CO the complex is stable up to
298 K, at which temperature new signals appear attributable to
Fig. 1. Selected 31P{1H} (a) and 1H (b) NMR spectra in CD2Cl2 on the reactivity of II,
Table 1, system 2.2.
the base. When one equivalent of TsOH is added to III in CD2Cl2/
MeOH under CO at 193 K (Table 1, system 3.1; Supporting infor-
mation, Fig. S4), II is formed. This complex is stable up to 323 K even
in the absence of CO. By addition of one more equivalent of TsOH, II,
under pressure of CO, is also stable up to 323 K. However, at this
temperature, by replacing of CO with argon, a slow conversion to I
takes place. Considering that I reacts with MeOH and CO giving II,
which is converted to III upon addition of NEt3, the equilibria re-
ported in Scheme 2 may be present in solution. These observations
suggest that the base has the function of subtracting the acid
Fig. 2. Selected 31P{1H} (a) and 1H (b) NMR spectra in CD2Cl2 on the reactivity of III in
Scheme 2. Stability of III and II in the presence of TsOH.
system 3.2.

78
E. Amadio, L. Toniolo / Journal of Organometallic Chemistry 750 (2014) 74e79
system 3.1). Thus starting from II, a dicarbomethoxy species is
formed at least to some extent in the glass bottle, enough to
observe catalysis, in spite of the fact that when III is formed from II,
TsOH is also formed. III is ca. 40% more active than II (TON 14 versus
10, during the first hour of catalysis and before adding a base),
which might be due to the presence of TsOH when starting from the
latter precursor. After 1 h, when NEt3 is added, II and III show a
significant increment in the catalytic activity.
The inactivity of I can be rationalized as follows. Before the
addition of the base, I gives II and TsOH (cfr. reactivity of I). Further
reaction with MeOH and CO to give III does not occur, because it
would yield additional TsOH, which prevents its formation (cfr.
system 3.1).
Scheme 3. Putative pathway for the formation of [Pd(COOMe)(OMe)(PPh3)2] and DMC
from III.
[Pd(BQ)(PPh3)2] and trans-[Pd(COOMe)(OMe)(PPh3)2] and forma-
tion of DMO becomes evident (ca. 10%). Although DMO is formed in
a lower amount than that expected from the disappearance of the
starting complex, it can be stated that BQ destabilizes the dicar-
bomethoxy complex and promotes both the reductive elimination
of DMO as well as the decarbonylation of one PdeCOOMe moiety.
At this point, in order to observe catalysis, MeOH (10% v with
respect CD2Cl2) was added at 193 K, keeping the pressure of CO at
0.4 MPa. No change in the NMR spectra was observed up to 313 K,
But, when after 1 h NEt3 is added, catalysis occurs because III is
formed (cfr. system 1.1). Catalysis is observed also when PPh3 is
used in place of NEt3, because the base betaine is formed. In this
case the role of the base is to neutralise the acid. In the first half an
hour, the increment for II and III is practically the same, from 10 to
26 and from 14 to 30 mol of DMO/mol of Pd, respectively. This
reinforces the above explanation on the retarding effect of TsOH.
The base neutralizes TsOH and favours the formation of III, so that
the two systems become equivalents and show the same increment
of catalytic activity. In this case the role of the base is to neutralise
the acid. In the first half an hour, the increment for II and III is
practically the same, from 10 to 26 and from 14 to 30 mol of DMO/
mol of Pd, respectively. This reinforces the above explanation on the
retarding effect of TsOH. The base neutralizes TsOH and favours the
formation of III, so that the two systems become equivalents and
show the same increment of catalytic activity.
whereas at 333
K [Pd(BQ)(PPh3)2] and trans-[Pd(COOMe)(O-
Me)(PPh3)2] slowly disappeared with concomitant formation of III,
DMO (ca. 1.2 ton) and of hydroquinone H2BQ.
These results give significant insights into the basic aspects of
the catalytic cycle, which are schematized as follows.
i) In order to observe catalysis it is necessary that complex III
(Or any other carbomethoxy species, hereinafter for
simplicity indicated as III) or [Pd(BQ)(PPh3)2] are formed.
ii) Complex III is stable up to 313 K.
3.5. Proposed catalytic cycle
iii) In the presence of BQ complex III is converted to
[Pd(BQ)(PPh3)2] with formation of DMO at ca. 298 K. Thus,
BQ promotes the formation of DMO, probably through a
rearrangement of III into a species having the two carbo-
methoxy ligands in a closer mutual position, more favourable
for the formation of the oxalate. This occurs at a temperature
significantly lower than that necessary to observe catalysis.
iv) Catalysis occurs only at 333 K.
NMR evidences show that the reoxidation of [Pd(BQ)(PPh3)2] is
likely to be the difficult step of the catalysis. It has been reported
that the oxidation of Pd(0)eBQ complexes is acid-induced [21]. In
the present case it has been shown that TsOH prevents the for-
mation of III, through which DMO is likely to be formed. Not only,
but a closer inspection of the activity of III reported in Table 3,
shows that the addition of NEt3 after 1 h catalysis speeds up the
v) These evidences suggest that the reformation of III from
[Pd(BQ)(PPh3)2] is likely to be the difficult step of catalysis.
3.4. Catalytic oxidative carbonylation of methanol using I, II and III
as precursors
In order to demonstrate that I, II and III are effective catalyst
precursors, the oxidative carbonylation of MeOH was performed in
a (robust) glass bottle under relatively mild temperature conditions
(298 K) under 0.3 MPa of CO in pure MeOH. Under these conditions,
decomposition to Pd metal is avoided and catalysis occurs to a
significant extent.
The results are reported in Table 3. In the absence of a base (NEt3
or betaine), only II and III are active. It should be pointed out that
when MeOH is diluted with CD2Cl2 in a NMR tube catalysis occurs
at 333 K and at a lower rate (see Table 1, experiments 2.2 and 3.4).
Catalysis is quite selective to DMO. No DMC is formed even when
the monocarbomethoxy complex II is used, in spite of the fact that
the TsOꢀ anion is weakly coordinating, so that the metal centre
presents an easily available site for the coordination of MeOH,
which in principle could lead to the carbonate.
Since DMO is the only product, it is quite reasonable to suppose
that the product-forming step involves a dicarbomethoxy species.
By NMR spectroscopy, we observed that III is unstable at 298 K
when in the presence of one equivalent of TsOH and gives II (cfr.
Scheme 4. Proposed catalytic cycle for the formation of DMO.

E. Amadio, L. Toniolo / Journal of Organometallic Chemistry 750 (2014) 74e79
79
catalytic activity. In fact, the TON before addition of the base is 14,
Appendix A. Supplementary data
whereas after addition is 39, with an increment of 25 units, almost
double than that observed in the first hour.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.11.009.
Scheme 4 shows a simplified catalytic cycle and the route to III
starting from I and II. As already reported above, precursor I reacts
with MeOH and CO to give II, which, in the presence of the base, CO
and MeOH is converted into III, the catalytic active palladium
species, via the probable formation of intermediate a. BQ activates
III to intermediate b, in which the two carbomethoxy ligands are in
a closer position, in such away to favour their reductive elimination
to DMO and [Pd(BQ)(PPh3)2]. The role of BQ as a promoter of
reductive elimination from palladium complexes in CeC forming
reactions is known [22,23]. The reoxidation of this Pd(0) complex
occurs through the addition of MeOH to the PdBQ moiety with
formation of a ((methoxy)p-hydroxyfenoxy)Pd(II) intermediate c.
An intermediate of this type has been detected by NMR spectros-
copy in the acid-promoted oxidation of Pd(0)eBQ complexes [21].
The subsequent reaction of b with CO and MeOH gives again III, via
the intermediate a, and closes the catalytic cycle.
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Acknowledgements
The financial support of MIUR (Rome) is gratefully acknowledged.