TEMPO-Mediated Catalysis of the Sterically Hindered Hydrogen Atom Transfer Reaction between (C5Ph5)Cr(CO)3H and a Trityl Radical.

We have demonstrated the ability of TEMPO to catalyze H· transfer from (C5Ph5)Cr(CO)3H to a trityl radical (tris( p- tert-butylphenyl)methyl radical). We have measured the rate constant and activation parameters for the direct reaction, and for each step in the catalytic process: H· transfer from (C5Ph5)Cr(CO)3H to TEMPO and H· transfer from TEMPO-H to the trityl radical. We have compared the measured rate constants with the differences in bond strength, and with the changes in the Global Electrophilicity Index determined with high accuracy for each radical using state of the art quantum chemical methods. We conclude that neither is a major factor in determining the rates of these H· transfer reactions and that the effectiveness of TEMPO as a catalyst is largely the result of its relative lack of steric congestion compared to the trityl radical.