Activation of Aldehydic Carbon−Hydrogen Bonds under Aerobic Conditions by Masked Rhodium(III) Porphyrin Cation

Rh III (ttp)CH 2 CH 2 OH activated the aldehydic carbon−hydrogen bonds of functionalized aryl and enolizable aldehydes to give high yields of Rh(ttp)COR at 50 °C under both anaerobic and aerobic conditions. The Rh(ttp)(C 2 H 4 )OH intermediate was proposed to form via β-hydroxy elimination. The reactions exhibited rate and yield enhancement upon the addition of Ph 3 P, suggesting ligand-promoted β-elimination. The nonlinear free energy relationship of the Hammett plot suggested a multistepwise reaction with the rate-determining step (binding or activation) dependent on the electronic effect of para substituents of aryl aldehydes.