Roman V. Ottenbacher

Highly Efficient, Regioselective, and Stereospecific Oxidation of Aliphatic C–H Groups with H2O2, Catalyzed by Aminopyridine Manganese Complexes

Aminopyridine manganese complexes [LMn(II)(OTf)(2)] having a similar coordination topology catalyze the oxidation of unactivated aliphatic C-H groups with H(2)O(2), demonstrating excellent efficiency (up to TON = 970), site selectivity, and stereospecificity (up to >99%).

Nonheme Manganese-Catalyzed Asymmetric Oxidation. A Lewis Acid Activation versus Oxygen Rebound Mechanism: Evidence for the “Third Oxidant”

The catalytic properties of a series of chiral nonheme aminopyridinylmanganese(II) complexes [LMn(II)(OTf)(2)] were investigated. The above complexes were found to efficiently catalyze enantioselective olefin oxidation to the corresponding epoxides with different oxidants (peroxycarboxylic acids, alkyl hydroperoxides, iodosylarenes, etc.) with high conversions and selectivities (up to 100%) and enantiomeric excesses (up to 79%). The effect of the ligand structure on the catalytic performance was probed. Epoxidation enantioselectivities were found to be strongly dependent on the structure of the oxidants (performic, peracetic, and m-chloroperbenzoic acids; tert-butyl and cumyl hydroperoxides; iodosylbenzene and iodosylmesitylene), thus bearing evidence that the terminal oxidant molecule is incorporated in the structure of the oxygen-transferring intermediates. High-valence electron-paramagnetic-resonance-active manganese complexes [LMn(IV)═O](2+) and [LMn(IV)(μ-O)(2)Mn(III)L](3+) were detected upon interaction of the starting catalyst with the oxidants. The high-valence complexes did not epoxidize styrene and could themselves only contribute to minor olefin oxidation sideways. However, the oxomanganese(IV) species were found to perform the Lewis acid activation of the acyl and alkyl hydroperoxides or iodosylarenes to form the new type of oxidant [oxomanganese(IV) complex with a terminal oxidant], with the latter accounting for the predominant enantioselective epoxidation pathway in the nonheme manganese-catalyzed olefin epoxidations.

Highly Enantioselective C-H Oxidation of Arylalkanes with H2O2 in the Presence of Chiral Mn Aminopyridine Complexes

Bioinspired chiral Mn‐aminopyridine complexes [(S,S)‐LMnII(OTf)2] and [(R,R)‐LMnII(OTf)2] (where (S,S)‐L=(2S,2′S)‐1,1′‐bis((3‐methyl‐4‐(2,2,2‐trifluoroethoxy)pyridin‐2‐yl)methyl)‐2,2′‐bipyrrolidine, and (R,R)‐L=(2R,2′R)‐1,1′‐bis((3‐methyl‐4‐(2,2,2‐trifluoroethoxy)pyridin‐2‐yl)methyl)‐2,2′‐bipyrrolidine) have been shown to efficiently catalyze the benzylic C−H oxidation of arylalkanes with hydrogen peroxide in the presence of carboxylic acid additives, affording enantiomerically enriched 1‐arylalkanols and the corresponding ketones. Optically pure additive N‐Boc‐(L)‐proline, in combination with [(R,R)‐LMnII(OTf)2] complex, affords 1‐arylalkanols in up to 86 % ee, which is the highest reported enantioselectivity for direct benzylic hydroxylations with H2O2 in the presence of transition‐metal catalysts. Oxidative kinetic resolution only slightly contributes to the increase of the observed enantiomeric excess over the reaction course. The observed kH/kD values (3.5–3.6 for the oxidation of ethylbenzene/d10‐ethylbenzene) and competitive oxidation data are consistent with either a hydrogen‐atom transfer/oxygen rebound or hydride transfer/oxygen rebound asymmetric hydroxylation mechanism.

Chiral Manganese Aminopyridine Complexes: the Versatile Catalysts of Chemo- and Stereoselective Oxidations with H2O2

In the last decade, manganese(II) complexes with N-donor tetradentate aminopyridine ligands emerged as efficient catalysts of enantioselective epoxidation of olefins and direct selective oxidation of C-H groups in complex organic molecules, with environmentally benign oxidant hydrogen peroxide. In this personal account, we summarize the progress of these catalysts with regard to ligands design, structure-reactivity correlations, evaluation of the substrate scope, as well as mechanistic studies, shedding light on the nature of active sites and the mechanisms of selective oxygenations. Several practically promising catalytic syntheses with the aid of Mn aminopyridine catalysts are exemplified.

Highly Efficient Aromatic C−H Oxidation with H2O2 in the Presence of Iron Complexes of the PDP Family

The catalytic activity of a series of iron complexes of the PDP family (PDP=N,N′‐bis(2‐pyridylmethyl)‐2,2′‐bipyrrolidine) in the oxidation of aromatic substrates with H2O2 has been studied. In the presence of acetic acid, these complexes efficiently catalyze the oxidation of benzene and alkylbenzenes with high selectivity for oxygen incorporation into the aromatic ring (up to 93 %), performing up to 84 catalytic turnovers. The parent complex, [(PDP)(OTf)2], has demonstrated the highest catalytic efficiency and aromatic oxidation selectivity. The yield of products of oxidation of different substrates increases in line with increasing number of electron‐donating alkyl groups of the substrates: halogenbenzenes

Enantioselective Benzylic Hydroxylation of Arylalkanes with H2O2 in Fluorinated Alcohols in the Presence of Chiral Mn Aminopyridine Complexes

A series of chiral bioinspired Mn‐aminopyridine complexes of the type [L*MnII(OTf)2] (where L* is 2,2′‐bipyrrolidine derived ligand, bearing trifluoroalkoxy and alkyl substituents) have been tested as catalysts in benzylic C−H hydroxylation of arylalkanes with H2O2 in fluorinated ethanols media. In 2,2,2‐trifuoroethanol, the yield of the target ethylbenzene oxidation product, chiral 1‐phenylethanol, reaches 45 %, which is much better than in the common solvent CH3CN (5‐6 %). The selectivity for 1‐phenylethanol formation increases in the following order: CH3CN<2‐fluoroethanol<2,2‐difluoroethanol<2,2,2‐trifuoroethanol, while 2,2‐difluoroethanol ensures the highest asymmetric induction in this series, affording chiral benzylic alcohols with up to 89 % ee. In trifluoroethanol, the observed primary kH/kD value of 2.3 has been measured for the oxidation of 1‐phenylethanol/α‐D‐1‐phenylethanol, which is similar to that in CH3CN (2.2). At the same time, depending on the solvent, CH3CN or 2,2,2‐trifuoroethanol, the oxidations of 1‐phenylethanol demonstrates drastically different linear free‐energy relationships; possible effect of the hydrogen‐bond donor (HBD) nature of CF3CH2OH is discussed in this context. Noticeably, it has been shown that by switching the absolute chirality ((S,S)− or (R,R)−) of the catalyst, the oxidation of complex substrate of natural origin, estrone acetate, can be diverted to predominant formation of either the tertiary C9‐alcohol or of the C6‐ketone, respectively.