Bottom-up synthesis of indium(0) nanoparticles and its application for the allylation of benzaldehyde. An experimental and theoretical study.

Abstract

In recent years, the synthesis of metal nanoparticles has attracted significant attention because of their unique properties. Although many strategies for the preparation of noble- and transition-metal nanoparticles have been published, the synthesis of indium nanoparticles (InNPs) has been scarcely reported. Most of the bottom-up methods require the use of indium salts and strong reducing agents such as sodium metal, zinc power, alkalides/electrides, or decomposition of organometallic complexes. Regrettably, some of them provide little control over particle size and size distribution of the InNPs, and the presence of stabilizing agents is generally mandatory. Recently we have been working on the simple, mild, and efficient synthesis of very reactive, monodisperse (4.0 ± 1.5 nm) spherical InNPs by fast reduction of indium(III) chloride with lithium powder and a catalytic amount of 4,4´-di-tert-butylbiphenyl (DTBB) in THF at room temperature, and in the absence of any anti-agglomeration additive or ligand. We explored the above mentioned InNPs-based reactive system for the allylation of a variety of aldehydes and ketones in a one-pot procedure, by adding allyl bromide over a suspension of InNPs followed by the addition of the corresponding carbonyl compound. For most of the compounds tested, the homoallylic alcohol product was obtained in good yields. In order to extend the scope of this indium-mediated protocol, herein, we report a comparative study on the allylation of benzaldehyde with a series of substituted allyl bromides promoted by both InNPs and In powder, in THF at room temperature. As expected, indium nanoparticles demonstrated to be much more efficient than granular indium yielding the corresponding allylic alcohols almost quantitatively. Additionally, computational studies have been applied in order to explain the differences in reactivity observed. Based on the experimental data and the DFT studies, we have proposed a possible reaction mechanism that implies the formation of a cyclic six-membered Zimmermann-Traxler-type transition state and the participation of allylindiums sesquihalides as intermediates in the indium-mediated allylation.