Aza-enamines-VII

Aldehyde N,N‐Dialkylhydrazones as Neutral Acyl Anion Equivalents: Umpolung of the Imine Reactivity

“…The strong electron-acceptor substituent R = NO 2 in hydrazones 19 increases the contribution of dipolar structure A and so favors umpolung, that is, the aza-enamine reactivity gives compounds 20ЈЈЈa-e (Scheme 4). The degree of the n-π interaction in dependence of the substituents NAlk 2 and R corresponding to canonical structures A and B parallels with the 1 H, [30] 13 C, [13,28] and 15 N [31] NMR shifts of the azomethine function and the λ max values [23,30] of the hydrazones 19.…”

“…The reactions were performed at room temperature or in boiling benzene or CCl 4 and completed in 1-2 h. [14,27] In order to find out more precisely the direction of the reaction, a trial series was carried out under standardized conditions with 4-tosyl isocyanate 22 as the electrophile and benzaldehyde hydrazones 19a-e with substituents R = OMe, H, and NO 2 in the 4-position. [28,30] The results listed in Scheme 4 show that the reactions proceeded remarkably selectively. It is evident that compounds 21 were formed from hydrazones 19 with R = OMe or H and morpholine, piperidine, or dimethylamine as the amino components.…”

“…However, the amidation of benzaldehyde N,N-dialkylhydrazones 19 can proceed in two different directions depending on the NAlk 2 amino ligands and the substituents in the aromatic part. The attack takes place either according to mesomeric structure A at the azomethine carbon atom to deliver glyoxylic acid sulfonamido hydrazones 20 [14,21,23,27] or according to reactivity B at the azomethine nitrogen atom, which leads via an intermediate 1,4-dipolar cycloaddition to 1,3,5-triazinane-2,4-diones 21 [27,28] as outlined in Scheme 4 (see also ref. [29] ).…”

(Substituted Ylidene)Amino‐2‐Oxo‐Indolylidene Thioureas and Bis(2‐Oxo‐Indolylidene)Urea from (Ylidene)Thiocarbonohydrazides and Isatylidene Malononitrile

“…The methine carbon of 5a , 5b , 5c , 5d , 5e , 5f , 5g , 5h , 5i , 5j had to act as an electrophile with sense of an “umpolung” . This behavior is not unexpected, because aldehydehydrazones are known to react as azaenamines . On the other hand, compound 5a , 5b , 5c , 5d , 5e , 5f , 5g , 5h , 5i , 5j is required to form the spiro‐cyclic alternative structures 7 , 8 , 9 .…”

Novel Synthesis of 1,3‐Thiazine and Pyrimidinethione Derivatives from (1‐Aryl ethylidene)hydrazinecarbothioamides and Tetracyanoethylene

“…Because aldehyde hydrazones are known to react as aza‐enamines toward suitable electrophiles, this behavior should consequently also be open to aldehyde thiosemicarbazones because the substituted thiocarbamoyl group is not a strong acceptor and may even be sterically hindered. Further, investigation of the reactivity of (1‐aryl ethylidene)hydrazinecarbothioamides 6a , 6b , 6c toward (TCNE, 2 ) will be discussed in the present investigation.…”