Abstract

Background: Although, Wittig reaction is mainly reported with aldehydes and phorphorus ylides, unfortunately, aldehyde oxidation mainly drops the efficiency of the reaction trailed by the decomposition or polymerization. There are several one-pot methods that have been testified to achieve an efficient Wittig reaction, but these protocols mainly hurt due to the requirement of toxic, oxidizing mediators and the inevitability of the process activation step, for example, the use of toxic Martin’s reagent to oxidize alcohol during the Wittig olefination reaction.

Methods: The exclusive physiochemical properties of imidazolium ionic liquids like low viscosity, negligible vapor pressure and high thermochemical stability make them a perfect candidate for biphasic nanocatalysis. Herein, we wish to report 1-n-butyl-3- methylimidazolium hexafluorophosphate (bmim.PF6) for the synthesis and stabilization of Ru (0) and Rh (0) nanoparticles (NPs). Further, we used these metal NPs for Wittig reaction as an effective catalytic system in liquid-liquid biphasic condition.

Results: We created stable transition nanoparticles of [M0]n type in a dry ionic liquid medium under hydrogen atmosphere. The synthetic protocol of transition nanoparticles is very simple; they were prepared in dry bmim.PF6 reaction medium. The dry ionic liquid was used as a reaction medium because the presence of water may cause ionic liquid decay and may create transition metal phosphates along with hydrogen fluoride. The physiochemical properties of all the synthesized nanoparticles were analyzed by advanced analytical methods. These analyses confirmed that the particle size of well dispersed Ru (0) and Rh (0) nanoparticles ranged from 2.1 to 5.5nm. The Ru (0) and Rh (0) nanoparticles were used as catalysts under mild reaction conditions (without using any oxidizing agents) to obtain Wittig olefination reaction product in good yield and acceptable distereoselectivity in contrast to conventional solvent systems. In addition, easy product isolation and 8 times reuse of Ru (0) nanoparticles catalyst are the main results of this study.

Conclusion: In summary, we synthesized and stabilized the Ru (0) and Rh (0) NPs using bmim.PF6 ionic liquid. We obtained narrow size distributed, agglomeration free and stable NPs of Ru (0) and Rh (0) with practical size ranging from 2.1 to 5.5 nm. Both the NPs were found active to catalyze Wittig olefination reaction in ionic liquid with respect to the conventional solvent system. The Ru (0) nanoparticle was recorded as an outstanding catalyst for the Wittig type elimination reaction. It was found active for 19 different types of alcohol and ylide. In most of the results, we obtained the Wittig reaction yield more than 65% with satisfactory diastereoselectivity. Additionally, no aqueous work-up is compulsory throughout the product isolation step. Simply diethyl ether extraction method (5 x 2mL) was applied to recover the reaction product without loss of catalytic systems. Ionic liquid immobilized Ru (0) NPs were recycled up to 8 runs without any important loss of product yield.

Keywords: Catalyst recycling, ionic liquid, nanocatalysts, Rh (0) NPs, Ru (0) NPs, wittig olefination.

Graphical Abstract