Asymmetric Epoxidation of Alkenes in Fluorinated Media, Catalyzed by Second-Generation Fluorous Chiral (Salen)manganese Complexes

Abstract

Keywords: Biaryls / Biphasic catalysis / Epoxidations / Fluorinated ligands / ManganeseThe synthesis of sterically hindered chiral (salen)manganese complexes bearing perfluoroalkyl ponytails and their use in asymmetric epoxidation reactions are described. For better understanding of the relative influences of steric and electronic effects on the enantioselectivity of the fluorous catalysts, the epoxidation of 1,2-dihydronaphthalene and benzosuberene was first studied under homogeneous conditions. It was shown that the presence of sterically demanding tertbutyl groups and, to a lesser degree, the displacement of the electron-withdrawing perfluoroalkyl substituents from the ligand core provide ees higher than those attainable with first generation fluorous chiral (salen)manganese complexes featuring perfluoroalkyl substituents in the key positions (3,3Ј

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.…”

“…GC analyses were performed on a Hewlett-Packard 5890 instrument (column: 30 m  0:32 mm HP-5 5% phenyl methyl siloxane or 30 m  0:25 mm HP chiral-20% permethylated b-cyclodextrin column.). HPLC analysis was performed using an Agilent The parent fluorous salen ligand (1.3 g, 0.58 mmol), synthesised according to a literature procedure [9], was dissolved in degassed trifluorotoluene (10 ml) and refluxed. Co(OAc) 2 Á4H 2 O (0.21 g, 1.14 mmol) suspended in ethanol (10 ml) was added dropwise to the salen solution under nitrogen.…”

“…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].…”

A Green Protocol for Asymmetric Epoxidation of Olefins Catalyzed by Carbon Dioxide Soluble Chiral Salen-Mn(III) Complexes in Supercritical CO2

“…11 The 1 and 2 contain the sterically hindered t-butyl group at C3 and C3' positions that is required to obtain the high degree of enantioselectivity in chiral Salen-Mn(III) complex catalyzed epoxidations of alkenes, 12 and the perfluorinated alkyl group at C5 and C5' position.…”

“…The solubility of Salen-Mn(III) complexes 1, 2 and metal free ligand (4) of complex 1 prepared according to the literature procedure 11 in supercritical CO 2 was studied and the results were presented in the Initially, we studied the epoxidation of styrene (1 eq) with PhIO (2 eq) in the presence of 5 mol% of 1 in a 10 mL view cell, at 35 o C and 250 bar pressure in supercritical CO 2 (Table 1) . Styrene oxide was obtained in 66% yield with 21% ee.…”