Nitrous Oxide Reduction-Coupled Alkene-Alkene Coupling Catalyzed By Metalloporphyrin

Metalloporphyrins are of great interest as versatile catalysts, e.g. for oxidation, cyclopropanation and aziridination and so on. These catalysts have been extensively investigated mainly as model compounds for active site of peroxidases and cytochrome P450, i.e., natural oxidation catalysts. Molecular oxygen, peroxides and other various oxidants have been examined for the oxidants.¹ We also have shown that the oxidation of alkanes, alkenes, and aromatics and N-carbonyl cyclic amines is efficiently catalyzed by ruthenium porphyrins with heteroaromatic N-oxides as oxidants.² We therefore considered other molecules having N-O bond could be oxidant for metalloporphyrin catalysts. Nitrous oxide (N₂O) is a physico-chemically CO₂-like stable molecule. Huge amounts of N₂O are generated as a by-product of industrial processes. N₂O itself is a potent contributor to global-warming and an ozone-depleting substance. Here we report N₂O reduction-coupled alkene coupling reaction catalyzed by metalloporphyrin.³

    a-Methylstyrene (1a) dimerized to afford 2,3-dimethyl-2,3-diphenylbutane (2a) mainly with NaBH₄ in the presence of iron meso-tetraphenylporphyrin chloride (Fe^III(TPP)Cl) (1 mol%) under N₂O. Addition of OH^- or CH₃O^- enhanced the reaction, resulting complete consumption of 1a  (Table). Mn(TPP)Cl, Co(TPP)Cl and Rh(TMP)CH₃ also had catalytic activity that was lower than that of Fe(TPP)Cl. The use of 0.05 mol% of Fe(TPP)Cl in the reaction of 1a afforded 2a in 69% yield, i.e., the turnover number reached 1380 or more. The coupling product 2 is considered formed by radical-radical coupling. Methyl methacrylate 1d afforded an isolable head-to-tail coupling product 2d and an inseparable mixture of oligomers, probably due to radical chain reaction. Measurement of the FT-IR of gas phase over the reaction solution showed N₂O was completely consumed under the reaction conditions. UV-Vis spectra and EPR spectra of reaction solution under Ar instead of N₂O clearly indicated the formation of Fe(I)porphyrin intermediate. Bubbling of N₂O into the solution at 20˚C caused the EPR signal to disappear. These results indicate that Fe(I) (TPP) reduces N₂O to N₂ and O^2-, regenerating Fe(III)(TPP). Thus, the reaction enables both N₂O consumption and synthesis of coupling products, some of which would be useful for organic synthesis. We proposed a reaction mechanism (Scheme).

    Since N₂O may become the dominant ozone-depleting substance emitted in the 21st century, the reaction system described here might prove useful as a green process to decrease emissions of N₂O, perhaps in conjunction with large-scale synthetic applications.

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References

(1) (a) B. Meunier, A. Robert, G. Pratviel and J. Bernadou, in The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic Press, San Diego, 2000, ch. 31, vol. 4, pp. 119–187; (b) J. T. Groves, K. Shalyaev and J. Lee, in The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic Press, San Diego, 2000, ch. 27, vol. 4, pp. 17–40.

(2) R. Ito, N. Umezawa and T. Higuchi, J. Am. Chem. Soc., 2005, 127,834–835, and references therein.

(3) S. Saito, H. Ohtake, N. Umezawa, Y. Kobayashi, N. Kato, M. Hirobe, T. Higuchi, Chem. Commun. 2013, 49, 8979-8981.