Catalytic Synergism in a C₆₀IL₁₀TEMPO₂Hybrid in the Efficient Oxidation of Alcohols

Abstract

A novel fullerene [5:1]hexakisadduct bearing two 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) radicals and ten 1-propyl-3-methylimidazolium bromide moieties has been synthesized and characterized. Such an C 60 IL 10 TEMPO 2 hybrid has been successfully employed as a catalyst in the selective oxidation of a wide series of alcohols and is highly active at just 0.1 mol% loading. Moreover, it can be easily recovered by adsorption onto a multilayered covalently-linked SILP phase (mlc-SILP) through a "release and catch" approach and reused for up to 12 cycles without loss in efficiency. Interestingly, a catalytic synergistic effect of TEMPO and imidazolium bromide moieties combined in the same hybrid has been clearly shown.Keywords: alcohols; fullerene; ionic liquids; oxidation; TEMPO; 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)In the last decades TEMPO-like catalysts have emerged as powerful, metal-free and environmentally sustainable alternatives in the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds.[1] Nevertheless, their high price prompted scientists examine the development of supported TEMPO catalysts in order to obtain recyclable materials. Hence, TEMPO has been covalently immobilized onto various supports such as silica, [2] magnetic nanoparticles, [3] soluble [4] and insoluble [5] organic polymers, and carbon nanotubes. [6] However, although several of these materials were catalytically active and easily recyclable, they often suffer from the inherent drawback of heterogeneous catalysis, namely the reduced mass transfer that makes reactions slower than in the case of their homogeneous counterparts. [7] In this regard, very recently a new approach called "release and catch" has been highlighted. [8] In this strategy, the catalytic system is non-covalently immobilized on the support, but the catalytic moiety is released in solution over the course of the reaction and it is recaptured at the end of the reaction. In such a manner, the benefits of homogeneous (high catalytic activity and reaction rates) and heterogeneous catalysis (easy separation and recycling) are combined. Interestingly, the "release and catch" approach has found application in organometallic-based catalysts, [9] organocatalysts [10] and metal-based catalysts. [11] In the past few years, we have been engaged in the use of silica-supported ionic liquid-like phase monolayers (SILLP) and multilayers (mlc-SILP) as reversible catalyst reservoirs. Both kinds of materials were excellently employed for organocatalyst recovery and recycling [10,12] as well as in the immobilization of Pd nanoparticles for C À C coupling processes in batch [13] and in flow. [14] On the other hand, we started exploring new recyclable TEMPO-based systems: 10 mol% of ionic liquidtagged TEMPO derivatives were easily recovered with the help of mlc-SILP and recycled in up to 13 consecutive cycles with no loss in activity, [12d] whilst a series of fullerene-TEMPO derivatives with two, four and twelve TEMPO moieties can be quickly ...

Modified Nanocarbons for Catalysis

“…An improved evolution of catalysts 1 – 3 a was achieved by means of the functionalization of a C 60 cage with two TEMPO moieties and ten “ arms ” of 1‐propyl‐3‐methylimidazolium bromide to give system 4 (Figure ) . Catalyst 4 displayed a high activity in low loadings (0.1 mol %) in the oxidation of a series of primary and secondary aliphatic and benzylic alcohols with BAIB as oxidant, giving rise to excellent results (yield >95 %).…”

Functionalised diimidazolium salts: the anion effect on the catalytic ability

“…Furthermore, the incorporation of functional groups in the IL structure minimizes the leakage of the catalyst and can increase the recyclability of the solvent-catalyst media for several catalytic cycles. 14 In this context, as a development of our work aimed to study ILs properties and applications, 15,16 two years ago we synthesized some TSDILs characterized by the presence of 1- 5 Different kinds of TSILs have been considered.…”

“…C-NMR (300 MHz; dmso-d 6 ); δ/ppm:14.1; 22.5; 25.9; 28.8; 28.9; 29.7; 31.6; 49.4; 51.9; 122.2; 122.3; 123.0; 123.1; 123.2; 124.9; 126.5; 126.7; 128.8; 130.1; 130.7; 132.4; 136.6; 136.7; 136.9; 150.6; 150.7. Elemental Anal.…”

Covalently Supported Ionic Liquid Phases: An Advanced Class of Recyclable Catalytic Systems

“…With the aim to develop a more active catalyst than 36 for the above reaction, a fullerene sphere was used as the scaffold in place of the benzene ring, which acted as the scaffold bearing two imidazolium units and one TEMPO unit. In this case, ten imidazolium bromide units and two TEMPO units were linked to the fullerene (catalyst 38 , Scheme ) …”

“…In this case, ten imidazolium bromide units and two TEMPO units were linked to the fullerene(catalyst 38,Scheme 18). [39] Catalyst 38 was used at 0.1 mol %l oadingf or the oxidation of several primary and secondary aliphatic and benzylic alcohols in the presence of BAIB with excellent results. Moreover, catalyst 38 was adsorbed on imidazolium-modified silica 37 and used at 1mol %l oading for several consecutive runs.…”