Rhodium(III) porphyrin hydroxide complexes cleave the unstrained C(CO)–C(α) bond of ketones under mild, neutral, water-promoted conditions, but the 2012 discovery reaction is stoichiometric in rhodium and its proposed σ-bond metathesis mechanism has never been definitively established. Work on this exact system since 2012 shows that the same group achieved photocatalytic turnover (up to 30) for α-substituted aliphatic and cyclic ketones with water as oxidant, broadened the stoichiometric scope to room temperature and to aliphatic ketones via in situ Rh–OH generation, and extended the chemistry to esters, amides, nitriles, ethers, and cyclopropanes. However, the detailed transition-state geometry of the C–C cleavage step remains unestablished, the alternative pathways recognized for this porphyrin family — the Rh(II) metalloradical homolytic radical-substitution manifold and stepwise polar/1,2-addition-type routes — have not been rigorously excluded (mechanistic assignments in the same family were still described as ambiguous in 2021), and a genuinely general catalytic variant — covering aryl ketones, which show no catalytic carbon loss, and ideally thermal conditions — is still lacking; an April 2026 concept article still frames Rh porphyrin catalysis as on a 'quest for catalytic turnover'. The problem — establish how Rh(III) porphyrin hydroxide cleaves the ketone C(CO)–C(α) bond, and build that understanding into a general catalytic method that treats this bond as a functional group — remains open.

Background

Selective activation of carbon–carbon bonds that are neither strained nor pinned by a directing group is a long-standing challenge in organometallic chemistry, because unstrained C–C σ-bonds are strong and kinetically inert. A flagship system for unstrained aliphatic C–C activation is the rhodium porphyrin chemistry developed by Kin Shing Chan's group. Here the metal is bound by a porphyrinato dianion ligand — a tetrapyrrole macrocycle, in this work ttp = 5,10,15,20-tetratolylporphyrinato, or in later work tmp = tetrakismesitylporphyrinato — which stabilizes rhodium in oxidation states I, II, and III. Rh(II) porphyrins are metalloradicals (odd-electron, one unpaired electron largely metal-centered) that cleave C(sp3)–C(sp3) bonds of nitroxides, nitriles, nonenolizable ketones, esters, and amides by homolytic radical substitution, giving Rh(III) porphyrin alkyls. In σ-bond metathesis, by contrast, a metal–heteroatom bond (here Rh–OH) and a substrate σ-bond exchange partners through a four-center-like transition state without a change in formal metal oxidation state; this is the mechanism category proposed, but not proven, for the ketone reaction at issue. C(CO)–C(α) bond activation (CCA) means cleavage of the bond between the carbonyl carbon and the adjacent (α) carbon of a ketone. In 2006 the group showed that Rh(II) porphyrin radicals can activate nonenolizable ketones this way; in 2012, Fung, Li, and Chan reported the milder, complementary Rh(III)-hydroxide route that is the subject of this problem: in water at as low as 50 °C, kinetic α-carbon–hydrogen bond activation (α-CHA) of the ketone occurs first, and hydrolysis of that product delivers RhIII(ttp)OH, which then cleaves the ketone C(CO)–C(α) bond — the acyl fragment transfers to rhodium to give RhIII(ttp)(COR) and the alkyl fragment is oxidized to a carbonyl compound. Because the acyl product contains a strong Rh–C bond, the 2012 reaction is stoichiometric in rhodium. The same group subsequently demonstrated catalytic C–C σ-bond hydrogenation of strained hydrocarbons with water (2012) and photocatalytic anaerobic C–C σ-bond oxidation of ketones with water (2013), reviewed the whole program in a 2017 Accounts of Chemical Research article, and mapped the Rh(I)/Rh(II)/Rh(III) speciation equilibria (the dimeric Rh(II)–Rh(II) porphyrin, Rh(III)–OH, Rh(III)–H, Rh(III)–Cl) that interconvert the reactive forms. What has still not been achieved for the ketone C(CO)–C(α) cleavage is a definitive mechanistic description of the C–C-breaking step and a general catalytic implementation.

Problem Statement

For the mild, water-promoted cleavage of the unstrained C(CO)–C(α) bond of ketones by rhodium(III) porphyrin hydroxide discovered by Fung, Li, and Chan (2012), establish, and convert into a general catalytic method: (1) the detailed mechanism of the carbon–carbon cleavage step — a transition-state-level description of the σ-bond metathesis between the Rh–OH bond and the ketone C(CO)–C(α) bond that is proposed for this reaction, discriminated from the alternative pathways recognized for this Rh(III)–OH/Rh(III)–H porphyrin family, namely the Rh(II) porphyrin metalloradical homolytic radical-substitution manifold and stepwise polar/1,2-addition-type routes of the kind that left the closely related Rh(III) porphyrin acylation by carboxylic acids assigned only as 'either σ-bond metathesis or 1,2-addition' (oxidative addition to a formal Rh(V) acyl–alkyl species is a further possibility to be tested, not a prescribed exclusion), with the routes that furnish the active hydroxide in situ — hydrolysis of the kinetic α-CHA product, RhIII(ttp)CH2CH2OH as a precursor, disproportionation of RhII(tmp) in water to RhIII(tmp)(OH), and the Rh(III)–OH/Rh(II) interconversion — treated as established background that the mechanistic account must be consistent with; and (2) catalytic turnover in this C(CO)–C(α) cleavage beyond the known photocatalytic subset — that is, a catalytic protocol whose ketone scope is not confined to α-substituted aliphatic and cyclic ketones under irradiation, in particular achieving catalytic carbon loss for aryl ketones — net catalytic formation of their one-carbon-shorter carbonyl products, which the known photocatalytic system does not deliver for aryl ketones — and ideally under thermal conditions, with turnover not capped by accumulation of the strong Rh–C(acyl) bond that makes the 2012 reaction stoichiometric. Both strands serve one objective: an understood and catalytically useful transformation that treats the ketone C(CO)–C(α) bond as a functional group under mild, neutral conditions.

References

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