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RESEARCH PRODUCT

Reactivity of 4,4-Dichloro-1,1-diphenyl-2-azabutadiene Towards Alkoxides and Thiolates: Synthesis of Functionalised π-Conjugated 2-Azabutadienes and Unexpected 1,4-Thiazine Formation

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Treatment of 4,4-dichloro-1,1-diphenyl-2-azabuta-1,3-diene [Cl2C=C(H)-N=CPh2] (1) with excess sodium isopropylthiolate or sodium thiophenolate in DMF yielded the 2-azabutadiene derivatives (RS)2C=C(H)–N=CPh2 (2) (2a R = iPr; 2b R = Ph). Nucleophilic attack of the sodium salt of ethyl thioglycolate on 1 afforded as the sole product the six-membered heterocyclic compound ethyl 2-ethoxycarbonylmethylthio-5,5-diphenyl-5,6-dihydro-4H-1,4-thiazine-6-carboxylate (5). The reaction is initiated by substitution of the two vinyl-bound chloro substituents to give {EtO(O=)CCH2S}2C=C(H)–N=CPh2 (2c) as intermediate. A mechanism that accounts for the subsequent cyclisation reaction is proposed. The 2-azabutadiene derivative (PhO)2C=C(H)–N=CPh2 (7) was obtained by the reaction of 1 with sodium phenolate. The regioselectivity of the incoming nucleophile is roughly correlated with its hardness/softness in accord with Pearson’s HSAB principle. The molecular structures of 2a,b, 5 and 7 were determined by single-crystal X-ray diffraction. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)