Perfluorocarbon Soluble Crown Ethers as Phase Transfer Catalysts

Abstract

Fluorous derivatives of dibenzo-18-crown-6 ether were readily prepared by means of metal-catalyzed cross-coupling reactions, and then successfully applied as catalysts in representative solid-liquid phase transfer catalysis (PTC) reactions, which were performed in standard organic solvents, such as chlorobenzene and toluene, and also in fluorous solvents, such as perfluoro-1,3-dimethylcyclohexane (PFDMC). It was clearly shown that properly designed fluoroponytailed crown ethers can promote the disintegration of the crystal lattice of alkali salts and transfer anions from the solid surface into an apolar, non-coordinating perfluorocarbon phase. Far from being a simple chemical curiosity, this unprecedented observation has relevant implications in the design of PTC scenarios of wide applicability. This new paradigm represents an advance in crown ether chemistry, and their use as recyclable phase transfer catalysts.

Fluorous phase transfer catalysts: From onium salts to crown ethers

“…In analogy to liquid-liquid-liquid PTC systems [54], at the end of the reaction, the immiscible fluorous phase containing the catalyst can be easily recovered and reused without resorting to any specific treatment, such as cumbersome fluorous solvent or solid-phase extraction techniques. Indeed, we have proven this more simplified PTC concept by using tetrafluoroponytailed substituted dibenzo-18-crown-6 derivatives, and the promising results obtained, have been submitted for publication [56]. Another viable option we are currently pursuing can be represented by the use of thermally stable, low melting fluorous polyethers as catalysts in gas-liquid PTC, with the consequences of having environmental and economical advantages that were described in Section 2.…”

“…This approach was successfully applied to other PTC nucleophilic substitution and oxidation reactions, such as the nucleophilic displacement reaction of n-octyl methanesulphonate (or noctyl bromide) with solid KCN to provide n-octyl cyanide, the challenging nucleophilic aromatic substitution reaction of 4-nitrochlorobenzene with KOMe to give 4-nitroanisole, and the KOH promoted aerobic oxidation of fluorene to fluorenone (Scheme 12) [56].…”

Di(1H,1H,2H,2H-perfluorooctyl)-dibenzo-18-crown-6: A “light fluorous” recyclable phase transfer catalyst

“…The fluorous ponytails were introduced by an efficient, palladiumcatalysed Heck reaction [18] between dibromo-dibenzo-18-crown-6 and the appropriate 1H,1H,2H-perfluoroalkene to give the desired intermediates, (12) and (13), as a mixture of cis-4,4 0 -and trans-4,5 0 -isomers. The di(1H,1H,2H,2H-perfluoroalkyl)-dibenzo-18-crown-6 ether derivatives, (14) and (15), were then obtained in excellent yields by the subsequent reduction of the alkenes by a rhodium-catalysed hydrogenation.…”

“…However, the catalytic activity of this ''heavy fluorous'' PTC was clearly reduced compared to the parent, non-fluorinated PTC due to its' low solubility in toluene. More recently, Pozzi reported the synthesis of a series of perfluorocarbon-soluble dibenzo-18-crown-6-ethers that contain four fluorous ponytails (5) [12]. Pozzi demonstrated for the first time that PTC catalysis could take place with organic substrates in a perfluorocarbon solvent and good catalytic activity and excellent recycling results were achieved over four cycles in a Finkelstein reaction between 1-bromooctane and potassium iodide in perfluoro-1,3-dimethylcyclohexane.…”

“…However, since the PTC catalytic activity of the perfluoroalkylated diaza-18-crown-6 ether (6) was much lower than the parent, non-fluorinated PTC catalyst, 4,13-diaza-18-crown-6, in the aliphatic nucleophilic substitution, we were interested in developing a more catalytically-active PTC using the same ''light fluorous'' approach. Since 18-crown-6-ethers are better PTC catalysts than diaza-18-crown-6-ethers, we decided to functionalise the aromatic rings of dibenzo-18-crown-6 ether with perfluoroalkyl and perfluoroalkylamino groups and in this paper we report the synthesis and catalytic applications of a small series of ''light fluorous'' dibenzo-18-crown-6-ethers [15]; work which was already in progress when Pozzi reported the syntheses of his ''heavily-fluorinated'' crown ethers [12].…”