v. Braun; Weissbach, Chemische Berichte, 1930, vol. 63, p. 3052,3055
作者:v. Braun、Weissbach
DOI:——
日期:——
Are Mn<sup>IV</sup> Species Involved in Mn(Salen)-Catalyzed Jacobsen−Katsuki Epoxidations? A Mechanistic Elucidation of Their Formation and Reaction Modes by EPR Spectroscopy, Mass-Spectral Analysis, and Product Studies: Chlorination versus Oxygen Transfer
作者:Waldemar Adam、Cordula Mock-Knoblauch、Chantu R. Saha-Möller、M. Herderich
DOI:10.1021/ja0005082
日期:2000.10.1
EPR and ESI-MS/MS evidence is presented that in the absence of an olefinic substrate the reaction between the Mn-III(salen) complexes Al (X = Cl) and A2 (X = PF6) and PhIO or NaOCl as oxygen sources leads to paramagnetic Mn-IV(salen) complexes. Depending on the solvent and the counterion, two distinct Mn-IV (salen) complexes intervene. In CH2Cl2, regardless of the counterion, a ClOMnIV(salen) complex (B1) and a HOMnIV(salen) complex (B1') are formed by Cl and H atom abstraction from CH2Cl2, and the latter deprotonates to the neutral OMnIV(salen) complex (B2). In EtOAc as solvent, only the complex B2 is obtained from Al (X = Cl), presumably by inner-sphere electron transfer from the chloride ion. The Mn-IV(salen) complexes display the following reaction modes toward 1,2-dihydronaphthalene (1), styrene (2), and the radical probe 3 as substrates: Complex B1 chlorinates the olefins 1/2 through an electrophilic :pathway to yield the 1,2-dichloro adducts 1a/2a and the chlorohydrins 1b/2b (nucleophilic trapping of the initially formed benzylic cation), while with olefin 3 the ring-opened dichloro product 3a results. Complex B2, however, epoxidizes these olefins through a radical pathway, as evidenced by the formation of isomerized stilbene oxide 4c (cis/trans ratio 36: 64) from cis-stilbene (4). The relevance of these paramagnetic Mn-IV(salen) species in Jacobsen-Katsuki catalytic epoxidations is scrutinized.
Efficient conversion of alkenes to chlorohydrins by a Ru-based artificial enzyme
Artificial enzymes are required to catalyse non-natural reactions. Here, a hybrid catalyst was developed by embedding a novel Ru complex in the transport protein NikA. The protein scaffold activates the...