Highly Efficient Epoxidation of Allylic Alcohols with Hydrogen Peroxide Catalyzed by Peroxoniobate-Based Ionic Liquids

Abstract

This work reports new kinds of monomeric peroxoniobate anion functionalized ionic liquids (ILs) designated as [A+]­[NbO­(O-O)­(OH)2] (A+ = tetrapropylammonium, tetrabutylammonium, or tetrahexylammonium cation), which have been prepared and characterized by elemental analysis, HRMS, NMR, IR, TGA, etc. With hydrogen peroxide as an oxidant, these ILs exhibited excellent catalytic activity and recyclability in the epoxidation of various allylic alcohols under solvent-free and ice bath conditions. Interestingly, subsequent activity tests and catalyst characterization together with first-principles calculations indicated that the parent [NbO­(O-O)­(OH)2]− anion has been oxidized into the anion [Nb­(O-O)2(OOH)2]− in the presence of H2O2, which constitutes the real catalytically active species during the reaction; this anion has higher activity in comparison to the analogous peroxotungstate anion. Moreover, the epoxidation process of the substrate (allylic alcohol) catalyzed by [Nb­(O-O)2(OOH)2]− was explored at the atomic level by virtue of DFT (density functional theory) calculations, identifying that it is more favorable to occur through a hydrogen bond mechanism, in which the peroxo group of [Nb­(O-O)2(OOH)2]− serves as the adsorption site to anchor the substrate OH group by forming a hydrogen bond, while OOH as the active oxygen species attacks the CC bond in substrates to produce the corresponding epoxide. This is the first example of the highly efficient epoxidation of allylic alcohols using a peroxoniobate anion as a catalyst.

Ionic Liquid Stabilized Niobium Oxoclusters Catalyzing Oxidation of Sulfides with Exceptional Activity

“…Notably, the properties of Nb−OC@[TBA][LA] are distinctly different from those reported mononuclear peroxyniobate anion based ILs [TBA][Nb=O(O−O)(OH) 2 ] and [TBA][NbO(OH) 2 (LA)] . As shown in Figure S2, Supporting Information, the latter two ILs were miscible completely with methanol, and there was no obvious change after adding an equivalent amount of H 2 O 2 .…”

“…93 Nb NMR spectra has been proved to be a powerful tool to gain insight into the possible evolution of active niobium species during the course of the reaction . As shown in Figure , after H 2 O 2 was added to the catalyst (H 2 O 2 /Nb−OC@[TBA][LA]=10:1, molar ratio), the original signals at δ =−849 and −1036 ppm disappeared (Figure b, bottom), and the signal at δ =−1108 ppm was enhanced significantly, accompanied by a new wide peak at δ =−1331 ppm, which can be attributed to forming peroxoniobate species similar to [Nb(O−O) 2 (OOH) 2 ] − . The shifts suggested that the original niobium sites were transformed into new active peroxy niobium species, and the resonance signal at δ =−1108 ppm of the catalyst demonstrated that a part of niobium sites without lactate anion coordination still existed in the Nb oxocluster.…”

“…The IL [TBA][Nb=O(O−O)(OH) 2 ] was prepared according to a previously reported method for the sake of comparsion . Briefly, ionic exchange between KOH (0.156 g, 2.8 mmol) and tetrabutylammonium chloride (TBACl; 0.556 g, 2 mmol) in dry ethanol (10 mL) under N 2 afforded TBAOH.…”

“…This “peroxo way” has been shown to be a valuable method for the synthesis of mixed oxides containing niobium and/or tantalum from well‐defined and pre‐isolated complexes . Recently, our group has developed several kinds of monomeric peroxoniobate anion‐functionalized ILs in this manner, which is the first example of the highly efficient epoxidation of allylic alcohols using a peroxoniobate anion as a catalyst …”

“…[1e] Recently,o ur grouph as developeds everal kinds of monomeric peroxoniobate anion-functionalized ILs in this manner,w hich is the first example of the highly efficient epoxidation of allylic alcohols using aperoxoniobate anion as acatalyst. [3] Organic-inorganic nanostructured materials are aw ide, manifold and excitingc lass of systemst hat derive from an intimate combination, often mediated by the formation of a chemicalb ond, of inorganica nd organic buildingb locks. [4] Among inorganic nanosized building blocks,o xide-based clusters (e.g.,p olyhedrals ilsesquioxanes (POSS), [5] metal oxoclusters, [6] polyoxometalates (POM)), [7] are attracting more and more attention.…”