Enantioselective desymmetrization of meso‐epoxides with anilines catalyzed by polymeric and monomeric Ti(IV) salen complexes

Abstract

The active catalysts for the enantioselective ring opening (ARO) of meso-stilbene oxide, cis-butene oxide, cyclohexene oxide, cyclopentene oxide, and cyclooctene oxide with various substituted anilines were generated in situ by the reaction of Ti(O(i)Pr)(4) with poly-[(R,R)-N,N'-bis-{3-(1,1-dimethylethyl)-5-methylene salicylidene} cyclohexane-1,2-diamine]-1 and (1R,2R)-N,N'-bis[3,5-di(tert-butyl)salicylidene] cyclohexane-1,2-diamine-2. These catalysts in the presence of nonracemic imine as an additive provided β-amino alcohol in excellent yield (99%) and chiral purity (enantiomeric excess (ee) up to 99%) for the ARO of meso-stilbene oxide with aniline. The same protocol was less effective for the ARO of cyclic epoxides; however, when triphenylphosphine was used as an additive, there was a significant improvement in catalyst performance for the ARO of cyclohexene oxide (yield, 85-90%; ee, 63-67%). Both in situ generated polymeric and monomeric catalysts performed in a similar manner except that the polymeric catalyst Ti(IV)-1 was more active and recycled several times with retention of enantioselectivity when compared with the monomeric catalyst Ti(IV)-2, which was nonrecyclable.

An Evolutionary Model Encompassing Substrate Specificity and Reactivity of Type I Polyketide Synthase Thioesterases

“…The results are given in Figure 3. In our initial screening of Al III catalysts, we have used monomer,17 dimer,18a polymer,18b salalen (half reduced salen),18c salan (reduced salen),18d and amino alcohol based ligands 18e. We observed that the chiral monomeric Al III salen complex is an effective catalyst in terms of reactivity and enantioinduction (Figure 3).…”

Asymmetric Ring-Opening of Epoxides Catalyzed by Metal–Salen Complexes

“…Kureshy and coworkers [55,56] investigated several different approaches to achieve the enantioselective synthesis of vicinal amino alcohols by the ARO of meso-epoxides. One such approach is the use of in situ generated monomeric and polymeric Ti(IV)salen complexes from ligands 32 and 33 (Scheme 9) [55]. In order to achieve an efficient asymmetric catalysis, high catalyst loading (20 mol%) and the addition of additives was required.…”

Mechanochemical synthesis of unsymmetrical salens for the preparation of Co–salen complexes and their evaluation as catalysts for the synthesis of α-aryloxy alcohols via asymmetric phenolic kinetic resolution of terminal epoxides

“…Chiral metal–salen complexes were designed for catalyzing reaction processes that resulted in good yield, high regioselective and enantioselective control for the asymmetric ring opening of terminal epoxides. Various metals have been explored to optimize the catalytic properties of chiral metal–salens, such as Cr [ 7 ], Co [ 8 ], Fe [ 9 ], Ti [ 10 ], Al [ 11 ], Y [ 12 ], and Mn [ 13 ] and investigated with numerous nucleophiles to afford chiral molecules. In addition to the variation of metals, salen ligands have also been studied with regard to conformational differences, for instance, oligosalen [ 14 ], macrocyclic oligosalen [ 15 ], and polymeric salen [ 16 ].…”