A comprehensive picture of the one-electron oxidation chemistry of enols, enolates and α-carbonyl radicals: oxidation potentials and characterization of radical intermediates
“…Initially only the trans isomer of 1 is present . Electron transfer between photoexcited Mo(CNAr 3 NC) 3 and trans ‐ 1 produces radical intermediate A − , which can be oxidized to diradical B by Mo(CNAr 3 NC) 3 + in the electronic ground state . B can then either undergo intramolecular reaction to form product 2 or it can revert to substrate 1 , thereby producing a mixture of cis and trans isomers.…”
We report the first homoleptic Mo(0) complex with bidentate isocyanide ligands, which exhibits metal-to-ligand charge transfer ((3) MLCT) luminescence with quantum yields and lifetimes similar to Ru(bpy)3 (2+) (bpy=2,2'-bipyridine). This Mo(0) complex is a very strong photoreductant, which manifests in its capability to reduce acetophenone with essentially diffusion-limited kinetics as shown by time-resolved laser spectroscopy. The application potential of this complex for photoredox catalysis was demonstrated by the rearrangement of an acyl cyclopropane to a 2,3-dihydrofuran, which is a reaction that requires a reduction potential so negative that even the well-known and strongly reducing Ir(2-phenylpyridine)3 photosensitizer cannot catalyze it. Our study thus provides the proof-of-concept for the use of chelating isocyanides to obtain Mo(0) complexes with long-lived (3) MLCT excited states that are applicable to unusually challenging photoredox chemistry.
“…Initially only the trans isomer of 1 is present . Electron transfer between photoexcited Mo(CNAr 3 NC) 3 and trans ‐ 1 produces radical intermediate A − , which can be oxidized to diradical B by Mo(CNAr 3 NC) 3 + in the electronic ground state . B can then either undergo intramolecular reaction to form product 2 or it can revert to substrate 1 , thereby producing a mixture of cis and trans isomers.…”
We report the first homoleptic Mo(0) complex with bidentate isocyanide ligands, which exhibits metal-to-ligand charge transfer ((3) MLCT) luminescence with quantum yields and lifetimes similar to Ru(bpy)3 (2+) (bpy=2,2'-bipyridine). This Mo(0) complex is a very strong photoreductant, which manifests in its capability to reduce acetophenone with essentially diffusion-limited kinetics as shown by time-resolved laser spectroscopy. The application potential of this complex for photoredox catalysis was demonstrated by the rearrangement of an acyl cyclopropane to a 2,3-dihydrofuran, which is a reaction that requires a reduction potential so negative that even the well-known and strongly reducing Ir(2-phenylpyridine)3 photosensitizer cannot catalyze it. Our study thus provides the proof-of-concept for the use of chelating isocyanides to obtain Mo(0) complexes with long-lived (3) MLCT excited states that are applicable to unusually challenging photoredox chemistry.
“…Additional evidence for deprotonation of the EAA‐ or acac‐appended β ‐substituents is given by the appearance of a new oxidation process at E pa =0.11 to 0.20 V in CH 2 Cl 2 solutions containing 0.1 M TBAOH (see Figure ) for CuTPP(EAA)Ph 2 and CuTPP(acac)Ph 2 . This process is assigned to an oxidation of the negatively charged EAA or acac substituents to give their radical forms on the basis of data in the literature for the oxidation of acetylacetonate (acac − ) and related molecules in nonaqueous media. The oxidation of acac − in CH 2 Cl 2 containing 0.1 M TBAOH is located at E pa =0.50 V (Figure a) while a more facile acac − oxidation of CuTPP(acac)Ph 2 occurs at 0.11 V (Figure c) for a scan rate of 0.1 V/s, thus suggesting a strong electron‐donating effect of the porphyrin unit on the deprotonated form of the acac substituent.…”
Section: Resultsmentioning
confidence: 99%
“…It should be noted that the neutral and deprotonated forms of the β ‐EAA or acac substituents in eq 1a and 1b are both electroactive in the absence of the porphyrin, the neutral form of acac being reduced by one electron at −2.0 to −2.2 V in CH 3 CN with the generation of hydrogen and the anionic form (acac − ) being oxidized by one electron at potentials close to 0.0 V vs SCE in CH 3 CN or CH 2 Cl 2 . Thus, an electrochemical characterization of MTPP(EAA − )X 2 and MTPP(acac − )X 2 should provide information on the interaction between the porphyrin π‐ring system and the negatively charged enolate substituent.…”
The electrochemistry of β-substituted tetraphenylporphyrins with a single ethyl acetoacetate (EAA), acetylacetone (acac), or ethyl acetate (EA) group and two antipodal H, Br or Ph groups was investigated in CH 2 Cl 2 containing 0.1 M tetrabutylammonium perchlorate (TBAP) before and after addition of base to solution in the form of tetrabutylammonium hydroxide (TBAOH). The tri-substituted porphyrins in their neutral form are represented as MTPP(EAA)X 2 , MTPP(acac)X 2 and MTPP(EA)X 2 , where TPP is the dianion of tetraphenylporphyrin, M = H 2 , Ni II , Cu II or Zn II and X = H, Br or Ph. The singly reduced porphyrins are relatively stable on the cyclic voltammetry timescale, but this is not the case for the doubly reduced EAA and acac derivatives, which are highly reactive at room temperature, leading to a porphyrin product assigned as containing a deprotonated EAA À or acac À (enolate) substituent. Enolateappended porphyrins were also generated in-situ by the addition of TBAOH, resulting in mono-anionic porphyrins that are harder to reduce by 110-160 mV as compared to the neutral compounds with the same β-substituents, suggesting a high degree of interaction between the deprotonated EAA À or acac À substituent and the porphyrin π-ring system. The chemically or electrochemically generated enolate-appended porphyrin could also be irreversibly oxidized by one electron at peak potentials between 0.10 and 0.30 V; the exact value depending upon solvent and type of β-diketo substituent.
“…Two equivalents of the stronger oxidizing agent tris(1,10-phenanthroline)iron(III) hexafluorophosphate {[Fe III (phen) 3 (PF 6 ) 3 ], E = +1.08 V vs. SCE} afforded α-carbonyl cations as a result of two subsequent one-electron oxidation steps. [13][14][15] The Jahn group reported the oxidation of ester enolates with CuCl 2 or ferrocenium ions and subsequent trapping of the α-ester radicals with 2,2,6,6-1 oxidation of TEMPO into the 2,2,6,6-tetramethylpiperidine-1-oxoammonium ion, which was nucleophilically attacked to yield α-functionalized carbonyl compounds. The reaction time was significantly reduced by the use of the microreactor flow technique.…”
Fast α-oxyamination of stable enolates, silyl enol ethers, and in situ deprotonated dialkyl 2-oxoalkane phosphonates and diphenyl-2-oxoalkyl phosphine oxides was performed in the presence of [Ru(bpy) 3 ] 2+ (bpy = 2,2Ј-bipyridyl) as a photocatalyst, 2,2,6,6-tetramethylpiperidine nitroxide (TEMPO), and visible light. The key step was the light-induced one-electron
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