Polymeric catalysts for chemo‐ and enantioselective epoxidation of olefins: New crosslinked chiral transition metal complexing polymers

Abstract

Polymeric analogs of well-known chiral Mn(III)-salen complexes were synthesized and were used as recyclable catalysts for asymmetric epoxidation of olefins. For this purpose two different monomers, 2 and 3, bearing chiral Mn(III)-salen moieties were synthesized. The monomer 3 carries a bulky substituent closer to the Schiff base moiety, while monomer 2 lacks such a substituent. These metal complexed chiral monomers were subsequently copolymerized with ethylene glycol dimethacrylate producing insoluble crosslinked functional matrices that possess macroporous morphology. Chemo-and enantioselective catalytic activities of these two polymers were evaluated for epoxidation of olefins. Both polymers catalyzed the epoxidation of a variety of olefins at room temperature in the presence of iodosylbenzene (PhIO) as the terminal oxidant with yields comparable to the homogenous system. In terms of their enantioselective catalytic activity, polymer P-2 (obtained from 3) performed better than polymer P-1 (obtained from 2). Unfortunately, while the homogeneous systems are reported to offer over 80% enantioselectivity, with the present polymeric catalysts, enantioselectivity to a maximum of 30% were observed. Unlike the homogeneous system, use of an external nitrogenous donor played a very insignificant role in influencing enantioselectivity.

Utilisation of homogeneous and supported chiral metal(salen) complexes in asymmetric catalysis

“…The synthesis of the first polymer-supported chiral Mn(salen) complexes along with their application as recyclable asymmetric catalysts was reported independently by Sivaram et al 32 and Minutolo et al 33 Monomeric Jacobsen-type units (containing two polymerisable vinyl groups) were copolymerised with styrene and divinylbenzene to yield the corresponding crosslinked polymers 25-27 as a monolithic compact block.…”

Thermally latent reaction of hemiacetal ester with epoxide catalyzed by recyclable polymeric catalyst consisting of salen‐zinc complex and polyurethane main chain

“…The resulting solution was evaporated under reduced pressure, and then the products were isolated 8, 9. The reaction of 2 (0.150 g, 1.00 mmol) and 3 (0.230 g, 1.00 mmol) with recovered poly(Zn/ 1 NHCOO ) (35.2 mg, 60.0 μmol, 3.00 mol % to 2 and 3 ) was conducted at 60–150 °C for 3 h under a nitrogen atmosphere, and the reaction and the catalyst recovering processes were repeated for several times.…”

“…Many polymeric salen‐metal complexes have been prepared by the metal insertion method into the polymeric ligands. The heterogeneous polymeric salen‐metal complexes catalyze the asymmetric epoxidation of olefins,8–10 hydrolytic resolution of asymmetric epoxides with water,11–16 and addition of dimethyl zinc to benzaldehyde 17, 18. Main‐chain type polymers bearing a binaphthyl salen‐metal structures were prepared from metal ions and carboxylate‐functionalized building blocks, and applied to asymmetric catalysis and chiral separations 19.…”

Synthesis and characterization of ‘calixsalens’: a new class of macrocyclic chiral ligands

“…E-mail: cjablons@morgan.ucs.mun.ca Chiral Schiff base ligands are efficient chiral auxiliaries for a wide range of enantioselective epoxidations. 1 Our search strategy for new asymmetric catalysts, particularly for asymmetric epoxidation of unfunctionalized alkenes, [1][2][3][4] has focussed on development of chiral macrocycles bearing two salen-based metal binding sites 5 which can act co-operatively 6a,7 within an enzyme-like chiral cavity. This approach is based on the expectation that non-covalent interactions, 8 which are the sole determinants of stereochemical outcome for enantioselective reactions of unfunctionalized substrates, 2a,9 can be maximized within a chiral host-guest paradigm.…”