Preparation of Dendritic and Non-Dendritic Styryl-Substituted Salens for Cross-Linking Suspension Copolymerization with Styrene and Multiple Use of the Corresponding Mn and Cr Complexes in Enantioselective Epoxidations and Hetero-Diels–Alder Reactions
Following work with TADDOLs and BINOLs, we have now prepared Salen derivatives (2, 3, 14, 15, 18, 19, 20, 21) carrying two to eight styryl groups for cross‐linking copolymerization with styrene. The Salen cores are either derived from (R,R)‐diphenyl ethylene diamine (3, 15, 19, 21) or from (R,R)‐cyclohexane diamine (2, 14, 18, 20). The styryl groups are attached to the salicylic aldehyde moieties, using Suzuki (cf. 1) or Sonogashira cross‐coupling (cf. 11), and/or phenolic etherification (cf. 5, 7) with dendritic styryl‐substituted Fréchet‐type benzylic branch bromides. Subsequent condensation with the diamines provides the chiral Salens. Corresponding Salens lacking the peripheral vinyl groups (cf. 12, 13, 16, 17) were also prepared for comparison of catalytic activities in homogeneous solution with those in polystyrene. Cross‐linking radical suspension copolymerization of styrene and the styryl Salens, following a procedure by Itsuno and Fréchet, gave beads (ca. 400 μm diameter) which were loaded with Mn or Cr (ca. 0.2 mmol of complex per g of polymer), with more than 95 % of the Salen incorporated being actually accessible for complexation (by elemental analysis). The polymer‐bound Mn and Cr complexes were used as catalysts for epoxidations of six phenyl‐substituted olefins (m‐CPBA/NMO; products 22 a–f), and for dihydropyranone formation from the Danishefsky diene and aldehydes (PhCHO, C5H11CHO, C6H11CHO, products 23 a–c). There are several remarkable features of the novel immobilized Salens: i) The dendritic branches do not slow down the catalytic activity of the complexes in solution; ii) the reactions with Salen catalysts incorporated in polystyrene give products of essentially the same enantiopurity as those observed in homogeneous solution with the dendritically substituted or with the original Jacobsen–Katsuki complexes; iii) some Mn‐loaded beads have been stored for a year, without loss of activity; iv) especially the biphenyl‐ and the acetylene‐linked Salen polymers (p‐2, ‐3, ‐20, ‐21, Figure 2, 3) give Mn complexes of excellent performance: after ten uses (without re‐charging with Mn!) there is no loss of enantioselectivity or degree of conversion under the standard conditions.