Room-Temperature Selective Aliphatic Carbon–Carbon Bond Activation and Functionalization of Ethers by Rhodium(II) Porphyrin

Abstract

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).

Photocatalytic Carbon–Carbon σ-Bond Anaerobic Oxidation of Ketones with Water by Rhodium(III) Porphyrins

“…error of k obs (10 5 s −1 ) more probable transition state due to mechanistic simplicity and generality, as the CCA of ethers which bear no enolizable form is also successful. 7 Concerning the CCA step, this type of cis six-or sevencoordinate transition state (TS1) of a metalloporphyrin bearing two cis reacting ligands, while uncommon, is not unprecedented. The X-ray structure of the six-coordinated cis oxo peroxo tungsten porphyrin complex has been reported.…”

User-friendly aerobic reductive alkylation of iridium(iii) porphyrin chloride with potassium hydroxide: scope and mechanism

“…Therefore, the reaction is compatible with air. When the reaction was carried out in THF, a slightly lower 76% yield of 3a was obtained together with some other unidentified coproducts (Table 1, entry 3), likely due to the minor carboncarbon bond activation (CCA) 19 and carbon-hydrogen bond activation (CHA) 20 of THF.…”

Carbon–Carbon Bond Activation by Group 9 Metal Complexes

“…The regio‐selective cleavage of C(α)–C(β) bond in symmetrical dialkyl ether with rhodium porphyrin hydroxide complex has been observed. The Rh III (tmp)OH, generated in situ from Rh II (tmp) 42 and H 2 O, cleaved the weaker C(α)–C(β) bond in (RCH 2 ) 2 O to yield the Rh III (tmp)R 43 and the alkyl formate as the organic co‐product (Table ) via a σ‐bond metathesis mechanism (Scheme ) . The alcohol then underwent condensation with Rh III (tmp)OH to give the intermediate Rh III (tmp)OCH 2 OR′ and H 2 O.…”