98736-79-9Relevant academic research and scientific papers
Mechanism of the Chromium-Catalyzed Epoxidation of Olefins. Role of Oxochromium(V) Cations
Samsel, E. G.,Srinivasan, K.,Kochi, J. K.
, p. 7606 - 7617 (2007/10/02)
The catalytic epoxidation of various olefins with iodosylbenzene is efficiently carried out by a series of chromium(III) cations CrIII(salen)+ (I) which are promoted by pyridine N-oxide (pyO) and related oxygen donors as the cocatalyst.Analysis of the catalytic rate profile and products establish the oxochromium(V) derivative O=Cr(salen)+ (II) and its donor adduct O=Cr(salen)(pyO)+ (III) as the reactive intermediates in the catalytic cycle.The successful isolation as well as the complete spectral analysis and structural characterization by X-ray crystallography of both II and III reveal the basis for oxygen activation in the O=CrV functionality.The mechanism of oxygen atom transfer involves the rate-limiting attack on the olefin by the electrophilic oxochromium(V) cation.The observation of benzaldehyde as a byproduct derived from the pyO-promoted C=C cleavage of styrene provides a method for unequivocally proving the existence of a transient intermediate during oxygen atom transfer.Thus the rate of olefin oxidation is found to be completely independent of the product-forming steps leading to epoxide and benzaldehyde, as they are modulated by added pyO.Steric effects, isotopic 18O tracers, stereochemistry, skeletal rearrangement, and substituent effects all provide mechanistic probes for the structure of the metastable intermediate.
Synthesis and molecular structure of oxochromium(V) cations. Coordination with donor ligands
Srinivasan,Kochi
, p. 4671 - 4679 (2008/10/08)
Synthesis of various oxochromium(V) cations ligated with salen [N,N′-ethylenebis(salicylideneaminato)], i.e. O=Cr(salen)+, is readily achieved from the corresponding chromium(III) complex, (salen)Cr+, by oxygen atom transfer with either iodosylbenzene or m-chloroperbenzoic acid. X-ray crystallographic analysis of O=Cr(salen)+ indicates that the the 5-coordinate Cr atom is situated 0.53 ? above the salen (mean) plane and describes a square-pyramidal configuration with the oxo ligand occupying the apical position. Isotopic 18O-substitution leads to a shift in the O=Cr stretching frequency from 1004 to 965 cm-1 in accord with theoretical predictions. Similarly the magnetic susceptibility and the well-resolved isotropic ESR spectra reliably reflect the d1 electron configuration of the oxochromium(V) species in acetonitrile solutions. Oxochromium(V) and various donor ligands such as pyridine N-oxide, triphenylphosphine oxide, and water form 1:1 association complexes, the formation constants K of which vary from 10-2 to 103 M-1, depending on the donor ligand and the substituent groups located on the salen periphery. X-ray crystallographic determination of the pyridine N-oxide adduct O=Cr(salen)(Opy)+ indicates that the donor ligand fills the apical position in O=Cr(salen)+ to complete the octahedral coordination about Cr. Isotopic 18O-tracer studies of the formation of oxochromium(V) by oxygen atom transfer to the chromium(III) complex are described. From X-ray crystallography, the lattice parameters for the oxochromium(V) complex IIc are as follows: space group, P21/n, monoclinic; cell constants, a = 16.233 (2) ?, b = 6.439 (1) ?, c = 19.523 (4) ?, β = 94.44 (1)°, V = 2034.5 ?3; Z = 4. The lattice parameters of the pyridine N-oxide adduct of the oxochromium(V) complex IIf are as follows: space group, P43212, tetragonal; cell constants, a = 11.938 (1) ?, c = 43.366 (9) ?, V =6174 ?3; Z = 8.
