Carbonylation of terminal alkynes catalysed by Pd complexes in combination with tri(2-furyl)phosphine and methanesulfonic acid
Introduction
The carbonylation of 1-alkynes (Scheme 1) is a versatile reaction which allows to synthesise important chemical intermediates such as α,β-unsaturated carboxylic acids and their derivatives [1], [2], [3].
The successful application of this one-step transformation depends on the availability of highly efficient catalysts. In this connection, the catalytic system recently disclosed by Drent [3], [4] (Pd(OAc)2 in combination with methanesulfonic acid and 2-pyridyldiphenylphosphine) is notable since it works under mild conditions providing very high activity and almost complete selectivity towards the branched product. A key role is played by the 2-pyridyl substituent present on the phosphorus ligand [3], [4]. It has been proposed that the pyridyl moiety allows a fast transfer of protons to the metal centre [3], [4], [5]. Deeming this subject worth of further investigation, we have thought it interesting to study the catalytic activity of phosphine ligands bearing heteroaromatic substituents not containing nitrogen atoms in the ring. In particular, we have tested the use of tertiary phosphines bearing 2-furyl substituents (Fig. 1), which, for instance, have been advantageously used as ligands in the palladium catalysed cross-coupling of alkylstannanes with aryl halogenides [6].
Section snippets
Results and discussion
In order to investigate the catalytic efficiency of 2-furyl substituted tertiary phosphines, we have chosen as model reaction the methoxycarbonylation of phenylacetylene (Scheme 2). The catalysts were prepared in situ by mixing together Pd(OAc)2, the phosphorus ligand and CH3SO3H. The ligands tested, namely 2-furyldiphenylphosphine [7] and tri(2-furyl)phosphine [8] (Fig. 1), were synthesised according to literature methods.
The reactions were carried out in a magnetically stirred stainless steel
Mechanistic considerations
Usually, two mechanisms, namely the “metal–carbalkoxy” [10], [11] or the “metal–hydride” [12], [13] based routes, are invoked in the carbonylation of alkynes and alkenes; moreover, Sperrle [14] showed that, at least in the alkoxycarbonylation of alkenes, both mechanisms are likely to operate simultaneously.
As far as his catalytic system is concerned, Drent [3], [4] proposed a metal–carbalkoxy pathway on the basis of studies of the effects brought about by the steric bulk of the phosphine.
Conclusions
In the present work, we have shown that Pd(OAc)2 in combination with tri(2-furyl)phosphine and methanesulfonic acid is an effective catalyst for the carbonylation of 1-alkynes. As a matter of fact, even if the tri(2-furyl)phosphine does not match in efficiency 2-pyridyldiphenylphosphine, it appears that it is much more effective than triphenylphosphine. Finally, this work shows that tertiary phosphines bearing heteroaromatic substituents are, for reasons, which need to be further considered,
Experimental
All the operations were carried out under argon in Schlenk-type glassware. Commercial solvents (C. Erba) were purified following methods described in the literature [16]. 2-Furyldiphenylphosphine [7], tri(2-furyl)phosphine [8] and Pd2(dba)3 [17] were synthesised according to literature methods. Phenylacetylene (Aldrich) was distilled prior to use, 2-pyridyldiphenylphosphine (Aldrich), methanesulfonic acid (Aldrich), CH3OD (Aldrich) and Pd(OAc)2 (Engelhard) were used as received. High purity CO
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