Ionic salen ligands

Fig. 5 Chiral salen ligand with improved solubility in ionic liquids...

In an attempt to increase the ionic liquid/hexane partition coefficient, a new salen ligand appended with an imidazolium salt was developed (Fig. 5) [18]. Unfortunately, modification of the ligand caused a dramatic reduction in the enantioselectivity - down to 57% ee at most - although the reasons for this behavior remain unclear. 

In contrast with salen ligands, ionic liquids were used earlier than fluorinated solvents for biphasic liquid systems with bis(oxazoline)-based complexes. In... 

The immobilization by ionic bonding on Al-MCM-41 was carried out similarly to the immobilization of rhodium-diphosphine complexes as described above. This catalyst was named MCMIHC. For immobihzation via the metal center and covalent bonding of salen, the all-silica MCM-41 was modified with (3-aminopro pyl)triethoxysilane (APTES) [52]. The catalyst obtained by the metal center immobilization was denoted MCM2HC [53], whereas the material obtained by covalent bonding of the salen ligand was named MCM3HC [54]. Detailed procedures are described extensively elsewhere [55]. 

The catalytic asymmetric epoxidation of alkenes offers a powerful strategy for the synthesis of enantiomerically enriched epoxides and enantioselective oxidation reactions in ionic liquids have been summarised previously.[39] Complexes based on chiral salen ligands - usually with manganese(III) as the coordinated metal - often afford excellent yields and enantioselectivities and the catalytic cycle for the reaction is depicted in Scheme 5.5 J40 ... 

Loss of catalytic complex by dissolution from the support This can either occur to physically bound catalysts (physisorbed, entangled in a polymer, hydro-gen-bonded), when the reaction medium has too-good solvent properties. The catalyst complex can also be dissolved from ionically bound species by ion exchange with electrolytes in the reaction mixture, or when the covalent bond to the support is broken (e.g., by hydrolysis). In the case of SIB catalysts, a good solvent such as ethanol can displace a salen-type ligand from the metal. 

Ionic immobilization of a Mn(lll)-salen complex in the pores of Al-MCM-41 has been reported by Hutchings and coworkers [98, 99]. The catalytically active species was formed in situ by the reaction of the ligand (R,Rj-3,5-di-tert-butylsalen with Mn -ion-exchanged Al-MCM-41. The efficacy of this material as a catalyst for the epoxidation of (Zj-stilbene using PhlO as the oxygen donor was then examined (stilbene PhlO catalyst = 7 1 0.13, 25°C, CH2CI2). When the [Mn(salen )]-Al-... 

The salen-based catalysts mentioned above are not soluble in water, which constitutes a limitation this is overcome by the preparation of new amphiphilic salen-type transition metal complexes. Therefore, several bulky salen-type SB ligands containing both tert-butyl and methyl(triphenylphosphonium) substituents have been prepared.122 The introduction of both lipophilic and ionic substituents in the ligands increases the solubility of the complexes of these ligands, which are found to be soluble both in water and in most common organic solvents and this may enhance the catalytic properties of the complexes. 

Ionic liquids can efficiently immobilize transition metal complexes used as catalysts for epoxidation. This phenomenon is encountered not only when specially designed, polar ligands are used, but also for unmodified catalysts. For instance, several unsuccessful attempts have been made to immobilize Jacobsen s chiral Mn(iii)salen epoxidation catalyst 81. Ionic liquids are the first medium to immobilize this complex efficiently without requiring additional modification of the ligand. 

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