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

Treatment of 4,4‐dichloro‐1,1‐diphenyl‐2‐azabuta‐1,3‐diene [Cl 2 C=C(H)‐N=CPh 2 ] ( 1 ) with excess sodium isopropylthiolate or sodium thiophenolate in DMF yielded the 2‐azabutadiene derivatives (RS) 2 C=C(H)–N=CPh 2 ( 2 ) ( 2a R = i Pr; 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‐4 H ‐1,4‐thiazine‐6‐carboxylate ( 5 ). The reaction is initiated by substitution of the two vinyl‐bound chloro substituents to give {EtO(O=)CCH 2 S} 2 C=C(H)–N=CPh 2 ( 2c ) as intermediate. A mechanism that accounts for the subsequent cyclisation reaction is proposed. The 2‐azabutadiene derivative (PhO) 2 C=C(H)–N=CPh 2 ( 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)