75778-52-8Relevant articles and documents
Structural and oxo-transfer reactivity differences of hexacoordinate and pentacoordinate (nitro) (tetraphenylporphinato)cobalt(III) derivatives
Goodwin,Bailey,Pennington,Rasberry,Green,Shasho,Yongsavanh,Echevarria,Tiedeken,Brown,Fromm,Lyerly,Watson,Long,De Nitto
, p. 4217 - 4225 (2008/10/08)
The oxo-transfer catalyst (nitro)(pyridyl)cobalt(III) tetraphenylporphyrin has been reinvestigated by substitution of the distal pyridine ligand with 4-N,N-dimethylaminopyridine and 3,5-dichloropyridine. Differences in their structures and in the reactivity of the compounds toward catalytic secondary oxo transfer were investigated by FT-IR and UV-visible spectroscopy, cyclic voltammetry, X-ray diffraction, semiempirical calculations, and reactions with alkenes in dichloromethane solution. Very modest differences in the hexacoordinate compounds' structures were predicted and observed, but the secondary oxo-transfer reactivity at the nitro ligand varies markedly with the basicity of the pyridine ligand and the position of the coordination equilibrium. Oxo transfer occurs rapidly through the pentacoordinate species (nitro)cobalt(III) tetraphenylporphyrin that is generated by dissociation of the pyridine ligand and therefore is strongly related to the Hammett parameters of these nitrogenous bases. The reactive pentacoordinate species CoTPP(NO2) can be generated in solution by addition of lithium perchlorate to (py)CoTPP(NO2) by Lewis acid-base interactions or more simply by using the weaker Lewis base Cl2py instead of py as the distal ligand. In contrast to pentacoordinate (nitro)iron porphyrins, disproportionation reactions of CoTPP(NO2) compound are not evident. This pentacoordinate derivative, CoTPP(NO2), is reactive enough to stoichiometrically oxidize allyl bromide in minutes. Preliminary catalytic oxidation reaction studies of alkenes also indicate the involvement of both radical and nonradical oxo-transfer steps in the mechanism, suggesting formation of a peroxynitro intermediate in the reaction of the reduced CoTPP(NO) with O2.
Activation of Cobalt-Nitro Complexes by Lewis Acids: Catalytic Oxidation of Alcohols by Molecular Oxygen
Tovrog,Benjamin S.,Diamond, Steven E.,Mares, Frank,Szalkiewicz, Andrew
, p. 3522 - 3526 (2007/10/02)
Lewis acid dramatically enhance the oxidation power of cobalt-nitro complexes.Thus, in the presence of BF3*Et2O or LiPF6, cobalt-nitro complexes such as pyCo(saloph)NO2 or pyCo(TPP)NO2 oxidize primary alcohols to aldehydes and secondary alcohols to ketones.No reaction is observed in the absence of Lewis acid.The effect of Lewis acids is attributed to their association with the nitro ligands, thereby increasing its electrophilicity.The results strongly suggest that the oxidation proceeds via an "ester-like" intermediate, which in a nonradical pathway collapses to the carbonyl product, water, and the corresponding nitrosyl complex.Importantly, it has been found that the reoxidation of the nitrosyl complexes by molecular oxygen is facile in the presence of Lewis acids.This finding facilitated the conversion of the stoichiometric oxidation of alcohols into a catalytic system using molecular oxygen as the oxidant.Initial oxidation rates are rapid.However, the rates decline as the byproduct, water, accumulates in the reaction mixture.
