An NMR study on the mechanism of ethene hydromethoxycarbonylation catalyzed by cationic Pd(II)–PPh3 complexes
Graphical abstract
The reactivity of cis-[Pd(H2O)2(PPh3)2](TsO)2, trans-[Pd(COEt)(TsO)(PPh3)2] and trans-[Pd(COOMe)(TsO)(PPh3)2] is studied by 1H and 31P{1H} NMR spectroscopy under conditions that mime the catalytic ethene hydromethoxycarbonylation. All the key intermediates of the Pd–H cycle are detected.
Introduction
Pd(II)–phosphine complexes are efficient catalysts for the carbonylation of ethene which yields a wide spectrum of products, ranging from monocarbonylated ones to perfectly alternating CO–ethene high molecular weight polyketones (PKs) [1], [2], [3]. In MeOH as a solvent, catalysis may start from a Pd–H initiator and/or a Pd–OMe one. In the case of the copolymerization, catalysis undergoes through both initiators, as proved by end-group analysis of the copolymer [4]. The crossover between the hydride and the other mechanism has been proven by a multinuclear NMR study [5]. When the copolymerization process is interrupted just after the incorporation of only one molecule of each monomer, methyl propanoate (MP) is formed. In this case, several studies have shown that the “hydride” cycle plays a major role, if not the only one [6], [7], [8], [9], [10], [11]. This is rather surprisingly being the two reactions closely related. In the hydride cycle, the olefin inserts into a Pd–H bond giving a Pd–alkyl intermediate, which inserts CO generating a Pd-acyl intermediate, which undergoes methanolysis with production of the ester and regeneration of the starting hydride. This route has gained wide acceptance also using other olefins [12], [13], [14], [15]. The other mechanism is initiated by the formation of a Pd–OMe species. Successive insertion of CO and of the olefin, followed by protonolysis with MeOH of the resulting intermediate, yields the product and regenerates the Pd–OMe initiator [16], [17], [18].
Using the system [Pd(TsO)2(PPh3)2](I)/PPh3 for the EHMC, the catalytic activity is significantly higher in the presence of a hydride source such as H2O, H2 and TsOH [19]. After catalysis, the propionyl complex trans-[Pd(COEt)(TsO)(PPh3)2] (II), related to the hydride mechanism, was isolated [20]. II reacts with MeOH to give the expected MP in an almost stoichiometric amount and is active in the EHMC and in the HMC of other olefins. These facts prove that, though it is sufficiently stable to be isolated, is also reactive enough to enter the catalytic cycle [20].
The activity of the carbomethoxy complex trans-[Pd(COOMe)(TsO)(PPh3)2] (III), related to the other cycle, has also been studied [21], [22]. Significant catalytic activity occurs only in the presence PPh3, TsOH and of H2O. MP is formed together with light CO–ethene co-oligomers having only keto- and ester-ending groups and no dimethyl succinate (DMS) or higher diesters are formed. These results are also in favour of the hydride route. In addition, (III) and the corresponding carbomethoxy complexes with in place of TsO−, are rather reluctant to insert ethene into the Pd–COOMe bond [22], [23]. Nevertheless, it has been found that [Pd(COOMe)X2(PPh3)2] (X = coordinating or non-coordinating anion) catalyze the oxidative carbonylation of ethene in MeOH using benzoquinone as a stoichiometric oxidant, yielding DMS and dimethyl oxalate, together with minor amounts of MP and dimethyl carbonate. The formation of DMS unambiguously proves that ethene inserts into a Pd–COOMe bond [24].
Direct evidences that a Pd–H species is an effective initiator for the catalytic EHMC were briefly reported using the [Pd(SO4)(PPh3)]/H2SO4/PPh3 catalytic system [23]. Taking advantage that I, II and III can be prepared as solid complexes, to handle, we undertook a detailed NMR investigation on their reactivity, under conditions that mime those used in the catalytic EHMC, i.e. in the presence of PPh3, H2O and TsOH. The results are hereafter discussed.
Section snippets
Reagents
Carbon monoxide and ethene (purity higher than 99%) were supplied by SIAD Spa (Italy). MeOH, PPh3, TsOH.H2O, CD2Cl2 were purchased from Aldrich Chemicals. Pd(AcO)2 and PdCl2 were purchased from Chimet SpA (Italy). MeOH, and CD2Cl2 were stored together with molecular sieves and under argon. The other chemicals were used as received. cis-[Pd(H2O)2(PPh3)2](TsO)2.2(H2O) (I,H2O) [25], trans-[Pd(COEt)(TsO)(PPh3)2] (II) [20] and trans-[Pd(COOMe)(TsO)(PPh3)2] (III) [23] were prepared according to
Reactivity of I
Actually, in place of I, the corresponding aquo complex cis-[Pd(H2O)2(PPh3)2](TsO)2.2(H2O) (I,H2O) was used, because this complex is well characterized, easier to handle and because H2O can be a reagent.
Conclusions
In summary, the reactivity of catalyst precursor I,H2O, the propionyl complex II and of the carbomethoxy complex III has been studied by 1H and 31P{1H} NMR spectroscopy under conditions that mime EHMC, i.e. in the presence of PPh3, H2O and TsOH. I,H2O reacts with CO and MeOH giving III and III′ in the presence of PPh3. These carbomethoxy complexes are unstable in the presence of H2O and TsOH and are converted into hydride IV, which is the initiator of the catalytic EHMC. The elementary steps of
Acknowledgements
The financial support of MIUR (Rome) is gratefully acknowledged. E. Amadio thanks the Erasmus programme for giving him the opportunity to develop part of the investigations at TUM in Munich under the guidance of H.P. Härter.
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Present address: Department of Chemical Sciences, University of Padua, via Marzolo 1, Padua, Italy.