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Article

Ferrocene-Catalyzed Aromatization and Competitive Oxidative Ring Transformations of 1,2-Dihydro-1-Arylpyridazino[4,5-d]Pyridazines

1
Department of Organic Chemistry, Eötvös Loránd University (ELTE), Budapest, H-1117 Budapest, Hungary
2
Hevesy György PhD School of Chemistry, Pázmány P. sétány 1/A, H-1117 Budapest, Hungary
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(8), 742; https://doi.org/10.3390/catal15080742 (registering DOI)
Submission received: 12 June 2025 / Revised: 28 July 2025 / Accepted: 31 July 2025 / Published: 4 August 2025
(This article belongs to the Special Issue Catalysis in Heterocyclic and Organometallic Synthesis, 3rd Edition)

Abstract

This paper presents the expected and unexpected, but typically substituent-dependent, ferrocene-catalyzed DDQ-mediated oxidative transformations of a series of 5,8-bis(methylthio)-1-aryl-1,2-dihydropyridazino[4,5-d]pyridazines and 8-(3,5-dimethyl-1H-pyrazol-1-yl)-5-(methylthio)-1-aryl-1,2-dihydropyridazino[4,5-d]pyridazines. Under noncatalytic conditions the reactions were sluggish, mainly producing a substantial amount of undefined tarry materials; nevertheless, the ferrocene-catalyzed reactions of the 5,8-bis(methylthio)-substituted precursors gave the aromatic products the expected aromatic products in low yields. Their formation was accompanied by ring transformations proceeding via aryne-generating fragmentation/Diels–Alder (DA)/N2-releasing retro Diels–Alder (rDA) sequence to construct arene-fused phthalazines. On the other hand, neither the noncatalytic nor the catalytic reactions of the 8-pyrazolyl-5-methylthio-substituted dihydroaromatics yielded the expected aromatic products. Instead, depending on their substitution pattern, the catalytic reactions of these pyrazolyl-substituted precursors also led to the formation of dearylated arene-fused phthalazines competing with an unprecedented multistep fragmentation sequence terminated by the hydrolysis of cationic intermediates to give 4-(methylthio)pyridazino[4,5-d]pyridazin-1(2H)-one and the corresponding 3,5-dimethyl-1-aryl-1H-pyrazole. When 0.6 equivalents of DDQ were applied in freshly absolutized THF, a representative pyrazolyl-substituted model underwent an oxidative coupling to give a dimer formed by the interaction of the cationic intermediate, and a part of the N-nucleophilic precursor remained intact. A systematic computational study was conducted on these intriguing reactions to support their complex mechanisms proposed on the basis of the structures of the isolated products.
Keywords: pyridazino[4,5-d]pyridazine; DDQ-mediated dehydrogenative aromatization; ferrocene-catalyst; SET; DFT-supported reaction mechanism; oxidative dimerization; aryne-generating fragmentation; Diels–Alder-retro Diels–Alder sequence; NMR structural elucidation pyridazino[4,5-d]pyridazine; DDQ-mediated dehydrogenative aromatization; ferrocene-catalyst; SET; DFT-supported reaction mechanism; oxidative dimerization; aryne-generating fragmentation; Diels–Alder-retro Diels–Alder sequence; NMR structural elucidation
Graphical Abstract

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MDPI and ACS Style

Hutai, D.; Nagy, T.Z.; Emődi, V.; Csámpai, A. Ferrocene-Catalyzed Aromatization and Competitive Oxidative Ring Transformations of 1,2-Dihydro-1-Arylpyridazino[4,5-d]Pyridazines. Catalysts 2025, 15, 742. https://doi.org/10.3390/catal15080742

AMA Style

Hutai D, Nagy TZ, Emődi V, Csámpai A. Ferrocene-Catalyzed Aromatization and Competitive Oxidative Ring Transformations of 1,2-Dihydro-1-Arylpyridazino[4,5-d]Pyridazines. Catalysts. 2025; 15(8):742. https://doi.org/10.3390/catal15080742

Chicago/Turabian Style

Hutai, Dániel, Tibor Zs. Nagy, Veronika Emődi, and Antal Csámpai. 2025. "Ferrocene-Catalyzed Aromatization and Competitive Oxidative Ring Transformations of 1,2-Dihydro-1-Arylpyridazino[4,5-d]Pyridazines" Catalysts 15, no. 8: 742. https://doi.org/10.3390/catal15080742

APA Style

Hutai, D., Nagy, T. Z., Emődi, V., & Csámpai, A. (2025). Ferrocene-Catalyzed Aromatization and Competitive Oxidative Ring Transformations of 1,2-Dihydro-1-Arylpyridazino[4,5-d]Pyridazines. Catalysts, 15(8), 742. https://doi.org/10.3390/catal15080742

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