Amination of Benzylic CH Bonds by Arylazides Catalyzed by Co II –Porphyrin Complexes: A Synthetic and Mechanistic Study

Co II –porphyrin complexes catalyze the reaction of aromatic azides (ArN 3 ) with hydrocarbons that contain a benzylic group (ArR 1 R 2 CH) to give the corresponding amines (ArR 1 R 2 CNHAr). When at least one of the R substituents is hydrogen, the catalytic reaction proceeds further to give the imine ArRCNAr in good yields. The reaction mechanism has been investigated. The reaction proceeds through a reversible coordination of the arylazide to the Co II –porphyrin complex. This unstable adduct can either react with the hydrocarbon in the rate‐determining step or decompose by a unimolecular mechanism to afford a putative “nitrene” complex, which reacts with more azide, but not with the hydrocarbon, to afford the byproduct diaryldiazene. The kinetics of the catalytic reaction have been investigated for a range of azides and substituted toluenes. Arylazides with electron‐withdrawing substituents react at a faster rate and a good correlation is found between the log( k ) and the Taft parameters. On the other hand, an excellent correlation between the logarithm of the rate for substituted toluenes relative to that of toluene and a radical parameter ( σ $\rm{_{JJ}^{{\bf \cdot }}}$ ) alone was found, with no significant contribution by polar parameters. An explanation has been proposed for this anomalous effect and for the very high isotopic effect ( k H / k D =14) found.