Study of (Tetraphenylporphinato)manganese(III)-Catalyzed Epoxidation and Demethylation Using p-Cyano-N,N-dimethylaniline N-Oxide as Oxygen Donor in a Homogeneous System. Kinetics, Radiochemical Ligation Studies, and Reaction Mechanism for a Model of Cytochrome P-450

Article abstract of DOI:10.1021/ja00323a036

Oxygen transfer from p-cyanodimethylaniline (p-CNDMANO) to cyclohexene as well as "intramolecular" oxygen transfer accompained by demethylation to yield p-cyanomonomethylaniline (p-CNMMA) are strongly catalyzed by ligated (tetraphenylporphinato)Mn^III (i.e., XMn^IIITPP).These reactions have been studied in dry, oxygen-free benzonitrile.Radiochemical studies show that H2O (or TOH) is not bound to XMn^IIITPP in aprotic solvents so that the Mn^III moiety is pentacoordinate.Oxygen transfer occurs through the reversible formation of the hexacoordinated species p-CNDMANO*Mn^III(X)TPP.This species decomposes to p-cyanodimethylaniline (p-CNDMA) + O=Mn^V(X)TPP.Reactions of cyclohexene with O=Mn^V(X)TPP yields cyclohexene epoxide and XMn^IIITPP whereas p-CNMMA is formed directly from the p-CNDMANO*Mn^III(X)TPP complex.The rates of product formation are shown to be dependent upon the nature of the ligand (X^- = F^-, Cl^-, Br^-, I^-, OCN^-).In the absence of the axial ligand X^-, the rates of reaction are extremely slow.Thus, the Mn^III C₂-cap-porphyrin (XMn^IIICAPTPP), which can only form an O=Mn^V porphyrin species wherein the Mn moiety is not complexed to X^- as a sixth ligand, shows almost no tendency to act as a catalyst for oxygen transfer.The necessary presence of the axial ligand X^- and the dependence of rate upon X^- requires the structure of the oxygen transfer species to be quivalent to O=Mn^V(X)TPP.A kinetic analysis is presented (Scheme III) which has allowed the determination of the influence of the ligands X^- upon the various rate constants (Table IV) involved in the overall oxidations.By employing p-CNDMANO as oxygen donor, multiple catalytic turnovers without loss of porphyrin have been realized.