Enantioselective Catalysis in Fluorinated Media – Synthesis and Properties of Chiral Perfluoroalkylated (Salen)manganese Complexes

Abstract

Chiral (salen)manganese complexes, which are selectively soluble in perfluorocarbons, were synthesized and tested as epoxidation catalysts in fluorous‐organic two‐phase systems. The immiscibility of the perfluorocarbons with regular organic solvents allowed a quick and effective separation of the catalyst from the products. These unprecedented perfluoroalkylated salen complexes were found to be efficient and chemoselective catalysts in the presence of several oxygen donors, but enantioselectivities were generally poor. An interesting exception to this behaviour was observed in the asymmetric epoxidation of indene that provides indene oxide with 70–92% enantiomeric excess.

Use of Ionic Liquids as Solvents for Epoxidation Reactions Catalysed by a Chiral Katsuki-Type Salen Complex: Enhanced Reactivity and Recovery of Catalyst

“…However, in order to take advantage of relatively easy phase separation and catalyst recovery, it was necessary to modify the salen ligand by incorporation of a perfluorinated alkyl chain to help to provide better solvation of catalyst in the FBS solvent. Moreover, a reasonable enantioselectivity was obtained for only one substrate, indene [16]. Therefore, we decided to investigate alternative approaches.…”

“…Due to the limitations of solid-supported catalysts, a two-phase liquid system, known as Fluorous Biphase System (FBS), was used by Pozzi and his co-workers [16]. However, in order to take advantage of relatively easy phase separation and catalyst recovery, it was necessary to modify the salen ligand by incorporation of a perfluorinated alkyl chain to help to provide better solvation of catalyst in the FBS solvent.…”

Application of the Fluorous-Biphase Concept to Palladium-Catalyzed Sonogashira Couplings

“…So far, a number of reactions have been successfully evaluated under FBS conditions, including hydroformylation [4], hydroboration [5], hydrosilylation [6], epoxidation [7] [8], and the Rh-catalyzed cyclopropanation of alkenes [9], oxidations of aldehydes [7], thioesters [7], and alkanes [10], as well as Wacker oxidations [11], Pdcatalyzed allylic substitutions [12], and Negishi [13] and Heck reactions [14].…”

Fluorous biphasic hydrolytic kinetic resolution of terminal epoxides

“…Several salen ligands with a high fluorine content (F > 60%) and bearing at least four C 7-C 8 perfluoroalkyl substituents (R F ) have been previously synthesised in our laboratory [6][7][8][9]19]. These heavily fluorinated chiral ligands can be divided into two subsets, characterised by the presence of electron-withdrawing R F or bulky t-butyl substituents in the 3,3 0 -positions, respectively.…”

“…However, modifications of the catalyst structure, which must be specifically tailored for use in these reaction media, should be taken in due account. The importance of the ligand design was first demonstrated in the case of the FB asymmetric epoxidation of alkenes catalysed by fluorous chiral (salen) manganese(III) complexes [6][7][8][9]. Other fluorous chiral ligands and complexes have been subsequently reported as well as the use of fluorous techniques in several asymmetric reactions [10].…”