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.Carbon-hydrogen bond activation (CHA) is an important Supporting Information Available: NMR spectra for complexes 4, 7, and 11. This material is available free of charge via the Internet at http://pubs.acs.org. OM070135E