Highly reactive rhodium(III) porphyrin hydroxides were formed from the ligand substitution of rhodium porphyrin halides in benzene and were rapidly reduced to rhodium(II) porphyrins and hydrogen peroxide. Thus hydroxide acted as the reducing agent. Oxidative addition of rhodium(II) porphyrin with hydrogen peroxide proceeded rapidly at room temperature to give back rhodium(III) porphyrin hydroxides. Rhodium(II) porphyrins and H2O2 therefore were thermally reversible with rhodium porphyrin hydroxides.

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S elective aliphatic carbonÀcarbon bond activation (ACCA) by transition metals under mild conditions has long been a desirable, but very challenging goal. 1 Transition-metal-mediated carbonÀcarbon bond activation of various substrates has been extensively studied. 2À6 A classical example of the cleavage of a C(sp 3 )ÀC(sp 3 ) bond is driven by releasing the ring strain in cubane with [Rh I (CO) 2 Cl] 2 . 7 Other nonaliphatic CCA by transition-metal complexes include nickel(0)-catalyzed C(sp 2 )À C(sp 2 ) bond activation of biphenylene with acetylenes to afford 9,10-disubstituted phenanthrenes developed by Jones et al. 8 In contrast, there is little precedent for the cleavage of aliphatic CÀC bonds with subsequent functionalization into organics.We have reported the ACCA chemistry of Rh II (tmp) (tmp = 5,10,15,20-tetramesitylporphyrinato dianion) with various organic substrates such as nitroxides, 5,9À11 nitriles, 12 and c-octane. 13 These reactions occur at temperatures higher than 70 °C, and the cleaved non-Rh(por)-containing R fragments are not characterized. It is desirable to achieve functionalization of the cleaved alkyls into organic compounds. We now report the room-temperature, selective ACCA of ethers into Rh(tmp)alkyls and esters 14,15 (Scheme 1).Initially, Rh(tmp) 1 was found to cleave the weakest C(R)ÀC(β) bond of n-butyl ether to give Rh(tmp)Pr 2a in only 15% yield in 1 day at 25 °C (eq , Table 1, entry 1). The more electron-rich (PPh 3 )-Rh(tmp) 16,17 improved the yield to 35% (eq , Table 1, entry 2).Encouraged by the promoting effect of base in ACCA of ethers by rhodium(III) porphyrin, 15 we thus examined the effect of KOH on the ACCA. However, the addition of 10 equiv of KOH was not beneficial, likely due to its poor solubility in n-butyl ether at room temperature (eq , Table 1, entry 3). However, further addition of H 2 O to dissolve KOH enhanced the reaction yield to 54% in 1 day (eq , Table 1, entry 4).To further improve the homogeneity of the reaction mixture, Ph 4 PBr (0.1 equiv) was added as the phase transfer catalyst. To our delight, both the rate and yield of the reaction were enhanced significantly. Selective ACCA of n-butyl ethers occurred efficiently at 25 °C within 10 min to give 2a in up to 83% yield (eq , Table 1, entry 5).

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

Base-promoted, selective aliphatic carbon(R)carbon(β) bond activation (CCA) of ethers by (5,10,15,20tetramesitylporphyrinato)rhodium(III) iodide was achieved.Carbon-carbon bond activation (CCA) by late-transition-metal complexes is important and challenging in the field of organometallic chemistry. 1 Most examples involve ring-strained cubane, 2a cyclopropane, 2b cyclobutanone, 2c and biphenylene 2d or chelating substrates of pincer type ligands, 2e,f amine, 2g and cycloalkanone imine 2h with lowvalent group 9 transition-metal complexes.Examples with high-valent group 9 transition-metal complexes are still rarely documented. Bergman and co-workers have reported the activations of aliphatic nitrile bonds by rhodium(III) complexes 3a,b and cyclopropane carboncarbon bonds by iridium(III) complexes, 3c respectively.Additionally, the intermediate valency of rhodium(II) mesotetramesitylporphyrin Rh(tmp) is also known to activate the aliphatic carbon-carbon bonds of unstrained ketone, 4 amide, 5 ester, 5 nitroxide, 6 and nitrile. 7 Recently, we have reported mild carbon-hydrogen bond activation (CHA) of benzaldehyde by using (5,10,15,20tetratolylporphyrinato)(β-hydroxyethyl)rhodium(III) with a rhodium hydroxo porphyrin complex, Rh(ttp)(OH)-