Recyclable Catalyst Reservoir: Oxidation of Alcohols Mediated by Noncovalently Supported Bis(imidazolium)‐Tagged 2,2,6,6‐Tetramethylpiperidine 1‐Oxyl

Abstract

Bis(imidazolium)-tagged 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) catalysts were adsorbed on different supports such as silica gel, silica gel modified with highly cross-linked polymeric imidazolium networks, and highly cross-linked polymeric imidazolium networks entrapping magnetic particles. These systems\ud provided a convenient tool for the oxidation of both primary and secondary alcohols working as recyclable reservoirs for the bis(imidazolium)-tagged TEMPO catalysts. By using EPR spectroscopy it was demonstrated that the catalyst was released as the corresponding oxoammonium salt in the solution during the recycling step, thus promoting the oxidative process in a homogeneous fashion. After solvent removal, the catalyst was readsorbed on the support allowing an easy recovery and recycle of the catalytic material up to 13 consecutive cycles with no loss in activity. The bis(imidazolium)-tagged\ud TEMPO catalyst could be used in only 1 mol% both for the oxidation of benzylic and aliphatic alcohols. The catalytic material was highly recyclable if used on silica or imidazolium-modified\ud silica gel in 10 mol% loading. Loading could be scaled down to 1 mol% and the catalyst proved to be recyclable up to\ud 8 cycles only with imidazolium-modified silica gel. Such a “catalyst-sponge-like” system permits to combine the benefits of homogeneous and heterogeneous catalysi

A Simple Procedure for the Oxidation of Alcohols Using [Bis(acetoxy)iodo]benzene and a Catalytic Amount of Bromide Ions in Ethyl Acetate

“…[20][21][22][23] Recently, we described the oxidation of alcohols by using new imidazolium bromide-based TEMPO catalysts. 24 In particular, we observed a synergic effect when fullerenebased catalyst 1 was used (Figure 1). 25 These reactions were carried out with PhI(OAc) 2 (1.1 equiv).…”

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

“…The above approach can be also employed for the immobilization of other imidazolium‐tagged catalysts such as 4‐hydroxy‐TEMPO (TEMPO=2,2,6,6‐tetramethylpiperidine‐1‐oxy, catalyst 36 ) with the aim to increase the affinity toward an imidazolium‐modified silica gel (material 37 , Scheme ) . Again, the system was designed to act as a catalyst reservoir.…”

“…The above approachc an be also employedf or the immobilization of other imidazolium-tagged catalysts such as 4-hydroxy-TEMPO( TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxy, catalyst 36)w itht he aim to increase the affinity toward ani midazolium-modified silica gel (material 37,S cheme 17). [38] Again, the system was designed to act as ac atalyst reservoir.I ndeed, during the reaction, catalyst 36 was dissolved in the reaction medium. When the reaction was complete, the solvent was removed,c ausing the re-adsorption of the catalysto nto support 37,whereas the final product was extracted by using asuitable solventt hat does not dissolve the adsorbed catalyst.…”

Supported Ionic Liquids: A Versatile and Useful Class of Materials

“…With the aim to obtain a new recyclable catalytic system active in the oxidation of alcohols, 4‐hydroxy‐TEMPO was modified with a (bis)imidazolium‐tag to give compound 33 . In such a way, the affinity of catalyst 33 towards imidazolium‐modified support materials was improved and it was also possible to employ the “ release and catch ” approach . Best results were obtained when imidazolium‐modified silica gel 35 was used as support to adsorb 33 giving rise to material 36 (Scheme ).…”