STEREOSELECTIVE AND STEREOSPECIFIC OXIDATIONS WITH H₂O₂ CATALYZED BY CHIRAL BIPYRROLIDINE-BASED IRON AND MANGANESE COMPLEXES

To date, Fe complexes with polydentate N-donor ligands are regarded as the best models of natural non-heme oxygenases, capable of catalyzing selective oxidations with H₂O₂. In a few cases, olefin epoxidations with H₂O₂ have been reported to proceed stereoselectively on biomimetic Fe catalysts. Similar Mn complexes have recently been reported to catalyze olefin epoxidation with percarboxylic acids and H₂O₂ in a chemo- and enantioselective fashion, substantially surpassing the Katsuki-Jacobsen Mn-salen systems in terms of catalyst efficiency. While the nature of active species in non-heme Fe/H₂O₂ catalyst systems has been extensively studied, very little mechanistic data on related Mn systems is available as yet.

In this work, enantioselective epoxidation of various olefins by catalyst systems [((S,S)-pdp)M^II(OTf)₂]/H₂O₂/RCOOH (where M = Fe (catalyst 1) or Mn (catalyst 2), R = alkyl) has been studied. Both systems have demonstrate similar catalytic behaviors, the manganese catalyst 2 exhibiting a much higher efficiency (1000 vs. 100 TON) and enantioselectivity (up to 93% ee for some substrates, which are the highest values ever reported for aminopyridine manganese catalysts). The enantioselectivity has been found to serve as a useful mechanistic probe (varying depending on the bulkiness of added carboxylic acid) which, in combination with EPR spectroscopy, allows to reliably conclude on the nature of epoxidizing species. The latter have been assigned to similar [((S,S)-pdp)M^V=O(OC(O)R)]²⁺ complexes (where M = Fe or Mn, R = alkyl).

New data on chemo- and stereospecific C-H oxidations catalyzed by [((S,S)-pdp)Mn^II(OTf)₂]/H₂O₂/RCOOH catalyst system will be reported.