Mild and Selective C(CO)–C(α) Bond Activation of Ketones with Rhodium(III) Porphyrin β-Hydroxyethyl

Abstract

Rhodium(III) porphyrin β-hydroxyethyl, Rh III (ttp)CH 2 CH 2 OH (ttp = 5,10,15,20-tetratolylporphyrinato dianion), was found to serve as a precursor of the highly reactive Rh III (ttp)OH for the C(CO)−C(α) bond activation (CCA) of ketones under mild and aerobic conditions of 25−50°C. ■ INTRODUCTIONCarbon−carbon bond activation (CCA) has attracted much research interest in recent decades, as it is a fundamentally important step in organic syntheses and industrial applications. 1−4 Selective CCA is challenging, as the carbon−carbon bonds in organic compounds are kinetically less accessible than the surrounding carbon−hydrogen bonds. In the past, examples of selective CCA of organic compounds were achieved with the utilization of low-valent transition-metal complexes via oxidative addition. 2−4 In contrast, CCA by high-valent transition-metal complexes was less reported, since electronically less accessible high-valent transition-metal intermediates from oxidative addition would be involved. 5 We recently reported that high-valent rhodium(III) and iridium(III) porphyrins cleave the C(CO)−C(α) bonds of ketones with the assistance of water at 200°C. 6,7 Initially, rhodium(III) and iridium(III) porphyrins catalyze the aldol condensation of ketones, giving water as a byproduct. The water that forms hydrolyzes the α-carbon−hydrogen bond activation (α-CHA) product of ketones with rhodium(III) and iridium(III) porphyrin complexes (M III (ttp)CHRCOR′, M = Rh, Ir) to generate Rh III (ttp)OH and Ir III (ttp)OH, respectively (Scheme 1). Rh III (ttp)OH and Ir III (ttp)OH were thus proposed intermediates in cleaving the C(CO)−C(α) bonds of ketones through σ-bond metathesis and oxidization of the alkyl fragments. 6,7 In the metalloporphyrin-based CCA of ketones, some limitations exist. 6,7 (1) High reaction temperature is required for the generation of M III (ttp)OH (M = Rh, Ir). (2) The reaction generally requires over 10 days. (3) Solvent-free conditions limit the use of high-boiling substrates. (4) The extensive and competitive aldol condensation of ketones, catalyzed by Lewis acidic metalloporphyrins, converts a large amount of starting material into undesired products. 6−8 Hence, the aldol-condensable diethyl ketone gives a lower yield of CCA product with rhodium(III) porphyrins in a slow reaction (eq 1), 6 and cyclohexanone is unsuccessful. 9 (5) The proposed intermediate Rh III (ttp)OH is highly reactive but extremely thermally unstable toward decomposition to form [Rh II (ttp)] 2 and unknown rhodium porphyrin complexes. 10 Hence, a mild method to generate Rh III (ttp)OH would be desirable for the facile CCA of ketones.Indeed, mild and selective cleavage of the C(CO)−C(α) bond of isopropyl ketones was recently achieved at 50°C from the hydrolysis of Rh III (ttp)Me to give Rh III (ttp)OH. 6 However, the ketone substrate is limited to non-aldol-condensable isopropyl ketones, since their aldol reactions are reversible without extensive undesirable consumption of ketones.Rh III (ttp)CH 2 CH 2 OH (Figure 1) has been used as...

A Rhodium‐Mediated Contraction of Benzene to Cyclopentadiene: Transformations of Rhodium(III) m‐Benziporphyrin

“…Alternatively, efficient activation over the course of the reaction was considered, including the bond cleavage of esters, aldehydes, or ketones added or formed in solution. Reported carbon–carbon or carbon–hydrogen bond activation triggered by rhodium(III) or iridium(III) porphyrins provided grounds for this hypothesis …”

“…Initially two hypotheses were considered. [24,25] First, 13 CO-labelled rhodium complex 3-13 CO was used as the substrate.T he regular 1 HNMR and 13 CNMR spectroscopic patterns for the formation of compound 5 excluded the incorporation of a 13 Ccenter ( 13 CO) into the CHCHO fragment. [23] Alternatively, efficient activation over the course of the reaction was considered, including the bond cleavage of esters,aldehydes, or ketones added or formed in solution.…”

From para‐Benziporphyrin to Rhodium(III) 21‐Carbaporphyrins: Imprinting Rh⋅⋅⋅η2‐CC, Rh⋅⋅⋅η2‐CO, and Rh⋅⋅⋅η2‐CH Coordination Motifs

“…The choiceo fm etal cationi ss trongly motivated by the extensive interest in reactions catalyzedb yr hodium(III)p orphyrins and in particular by their peculiar capability to contribute to processesi nw hich the activation of the single bond is of critical importance, whichi nvolves the formation of the CÀH, CÀC, and C=Ob onds unders ynthetic and catalytic conditions. [22][23][24][25][26][27][28][29][30][31][32][33][34] An impact on organorhodium chemistry,i mposed by the formation of carbametalacycles,c an also be noted. Such structuralm otivesa fford the stabilization of ap eculiar rhodium-carbon bonding situation, outstandingly reflected in the rhodiumo rganometallic chemistry in the surroundings of pincer-typeligands.…”

A Rhodium‐Mediated Contraction of Benzene to Cyclopentadiene: Transformations of Rhodium(III) m‐Benziporphyrin

“…Reported carboncarbon or carbon-hydrogen bond activation triggered by rhodium(III) or iridium(III) porphyrins provided grounds for this hypothesis. [24,25] First, 13 CO-labelled rhodium complex 3-13 CO was used as the substrate.T he regular 1 HNMR and 13 CNMR spectroscopic patterns for the formation of compound 5 excluded the incorporation of a 13 Ccenter ( 13 CO) into the CHCHO fragment. Under analogous conditions but in the presence of esters (ethyl acetate,ethyl 13 C(2)-acetate,[D 5 ]ethyl acetate, methyl acetate,e thyl propanoate), propanone,o re thanol, 3-CO could not be converted into 5 or into asimilar structure containing the rhodacyclopropane unit.…”