Barbier-type reaction mediated with tin nano-particles in water
Graphical Abstract
Introduction
The classical Barbier-type reaction has not been used as extensively as Grignard-type reaction even though the latter involves an extra step to prepare the organometallic reagents.1 This is because many side reactions can also be mediated by the metal at the same condition. In order to extend the application of Barbier-type reactions and fully take its advantage, Barbier-type reactions in aqueous media were developed in the recent years.2 The importance of this type of reaction has been gradually recognized not only because the tedious protection–deprotection processes can be simplified for certain functional groups containing acidic hydrogen atoms, but also because there is a growing public interest in Green Chemistry.3 Many metal mediators as well as their salts4 have been used in the Barbier-type allylation reactions to enhance the reaction yield and improve the stereoselectivity.5 The dimension of metal particles should affect on metal-mediated allylation of carbonyl compounds in aqueous media because the key intermediate is believed to be generated on the metal surface.2b However, the metal particles smaller than the regular powder, for example, nanometer-scale particles,6 have been not been well studied as a reagent in organic reactions.7, 8 As suggested by our previous study,7 the yield of the Barbier-type allylation reaction can be improved by applying nano-scale metal particles. In order to further test this idea and explore the mechanism of Barbier-type reactions in water, we recently studied the allylation mediated by tin nano-particles with different sizes (Scheme 1). The details of this study are described below.
Section snippets
TEM and XRD of 20-nm and 100-nm tin particles
In order to further study the mechanism of Barbier-type allylation reaction mediated by metal nano-particles and the effect of the size of nano-particles on this reaction, tin nano-particles with different sizes were prepared. Tin nano-particles with an average diameter of 20-nm were prepared by γ-radiation (method A), and tin nano-particles with an average diameter of 100-nm were prepared conveniently by reduction of SnCl2 with KBH4 in water at the presence of cetyl trimethylaminoium bromide
Conclusion
In conclusion, the allylation reaction mediated with tin nano-particles of different size in water has been systematically investigated. More importantly, the in situ generated allylation intermediates (4 and 5 in Scheme 3) have been directly observed using 1H NMR. A mechanism involving the allyltin intermediates is proposed. Further research is in progress in our laboratory to control the regioselectivity and stereoselectivity in allylation reactions by adjusting the dimension of the metal
Experimental
Analytical thin-layer chromatography (TLC) plates were commercially available. Solvents were reagent grade unless otherwise noted. Tin powder (150 mesh, 99.99%, 100 g packing) was freshly opened for use. Carbonyl compounds were further purified by redistillation or recrystallization from commercial chemicals when necessary.
Acknowledgements
The authors are grateful to the National Natural Science Foundation of China (No. 50073021 and 20472078) and Science Foundation of Anhui Province ((No. 01046301) for the support.
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Present address: Department of Chemistry, Columbia University, New York, NY 10027, USA.