Are MnIV 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

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.