Highly efficient Heck olefin arylation in the presence of iminophosphine–palladium(0) complexes
Graphical abstract
In the presence of complex 1 Ton of up to 20,000 are obtained in 2 h.
Introduction
The Heck arylation reaction is one of the most important methods for the formation of new CC bonds (Scheme 1) [1].
The most commonly used catalysts are palladium species in combination with phosphines [2] or other neutral ligands such as heterocyclic carbenes [3], and nitrogen [4] or sulfur ligands [5]. Alternatively, there is a wide variety of phosphorus [3], [6], nitrogen [7] and sulfur [8] containing palladacycles which are able to efficiently catalyse the reaction. For an industrial application, the development of “ligand-free” palladium catalytic systems appears particularly interesting [1], [9].
Recently, we have reported that the iminophosphine–palladium(0) complex 1 (Fig. 1) is an highly active catalyst for the Stille [10] and Suzuki [11] cross-coupling reactions. Spurred by the analogies existing between all these reactions, we have extended our studies to assess the catalytic activity of 1 in the Heck arylation reaction. Herein we wish to report the results of our investigations on the catalytic activity of 1 in the coupling of aryl bromides with two model olefins such as styrene and butyl acrylate.
Section snippets
Experimental
All reactions, unless otherwise stated, were carried out under an inert atmosphere (argon). Commercial solvents (Aldrich or Fluka) were purified before the use according to standard procedures [12]. Bromobenzene, 4-bromotoluene, 4-chloroacetophenone, chlorobenzene, styrene, butyl acrylate, n-butylamine and thiophenol (Aldrich) were distilled before the use. 4-Bromoacetophenone (Aldrich) was recrystallised from methanol [12]. 4-Bromoanisole, 2-bromo-1,3,5-trimethylbenzene, triphenylphosphine
Results and discussion
To test the catalytic activity of 1 we used as the model reactions the coupling of 4-bromoacetophenone with styrene or butyl acrylate (Scheme 2).
Preliminary experiments in polar solvents such as dimethylacetamide (DMA) or N-methylpyrrolidinone (NMP) showed that the reaction requires relatively high temperatures (T > 120 °C) in order to proceed with significant rates (see Table 1). At 140 °C the catalyst activity appears very good and the coupling of 4-bromoacetophenone with styrene proceeds giving
Conclusions
In conclusion, the iminophosphine–palladium complexes are highly active catalyst precursors for the Heck coupling of aryl bromides with terminal olefins. The catalyst efficiency compares well with that of other highly active catalysts, even if the catalysis appears particularly sensitive to steric hindrance on the aryl group. Further studies are in progress in order to determine the effect of the structure of the catalyst (effect of the nature of the groups Z and Y in Fig. 1) and gain evidences
Acknowledgements
This work has been carried out with the Financial Support of the MIUR (COFIN).
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