Mechanistic studies on the selective oxidative carbonylation of MeOH to dimethyl oxalate catalyzed by [Pd(COOMe)n(TsO)2−n(PPh3)2] (n = 0, 1, 2) using p-benzoquinone as a stoichiometric oxidant
Graphical abstract
Detailed mechanistic studies on the oxidative carbonylation of an alkanol to oxalate using p-benzoquinone as a stoichiometric oxidant are reported.
Introduction
The catalytic oxidative carbonylation of an alkanol to the corresponding carbonate and oxalate can be conveniently performed in the presence of palladium-based catalysts [1].
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 proposed 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 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 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 PPh3–BQ adduct (betaine) [4], [5], [6] 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)nX2−n(PPh3)2] (n = 0, 1, 2; X = 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 intermediate [9], [10], [11].
In addition to being an oxidant, BQ can play other roles by participating in both the formation and transformation of key intermediates into the reaction products by interacting with the catalytically active metal complex. It can change properties of the reaction centre and, consequently, the mechanism and the direction 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, respectively [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.
Section snippets
Reagents
MeOH, NEt3, TsOH·H2O, PPh3, BQ, CD2Cl2 and CD3OD were purchased from Sigma–Aldrich. CD2Cl2 and CD3OD were stored over 4 Å molecular sieves under Ar. Carbon monoxide (purity 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.
Instrumentation
NMR spectra were recorded on Bruker AMX 300 spectrometer. All 1H chemical shifts are reported relative to the residual
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 transformed into III upon addition of NEt3 (Table 1, system 1.1; Supporting 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
Conclusions
It has been demonstrated that not only the carbomethoxy complexes but also both BQ and the base play a role of paramount importance in the oxidative carbonylation of MeOH. In particular, it has been shown that BQ, in addition of being the oxidant, plays also an important role in promoting the reductive elimination of DMO from III.
Acknowledgements
The financial support of MIUR (Rome) is gratefully acknowledged.
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Present address: ITM-CNR, c/o Department of Chemical Sciences, University of Padua, Via Marzolo 1, 35131 Padua, Italy.