Diaza-l,3-butadienes

A select group of 1,3-diaza-l,3-butadienes has been reported to undergo [4 + 2] cycloadditions, but the lack of extensive efforts with this system reflects the current difficulty encountered in the preparation of stable 1,3-diaza-l,3-butadienes and their reluctance to participate in the Diels-Alder reactions. Successful efforts which have been described include the thermal isomerization of an unsaturated N-silylurea with the in situ generation and subsequent Diels-Alder reaction of a 2-trimethylsily-loxy-l,3-diaza-l,3-butadiene [Eq. (49)],  

The incorporation of the 1,3-diaza-l,3-butadiene system into a reactive heterocumulene system, imidoyl isothiocyanates and imidoyl isocyanates [Eq. (54)], 

Only a select group of 1,4-diaza-l, 3-butadienes has been shown to function as Ati components of [4 + 2] cycloadditions. Dehydroindigo affords Diels-Alder products on reaction with styrene, vinyl aryls, acrylonitrile, methyl acrylate, and methyl propiolate under forcing conditions [Eq. 

Early studies on the dimerization of substituted o-benzoquinone diimines and their reactions with diarylketenes [Eq. (57)]  

A common and competing reaction of a-diimines, 1,4-diaza-1,3-butadienes, with electron-rich or reactive olefins is [2 -l- 2] cycloaddition to afford azetidine or /3-lactam cycloaddition products [Eq. (59)], 

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Diazadienes 314 (R2 = Ph) afforded, in the same way, good yields of bis-trifluoromethyl dihydroquinazolines 335, as reported by Burger and Penninger (Scheme 74). However, N-unsubstituted heterodienes 314 (R2 = H) yielded upon heating with aromatic nitriles symmetrical triazine derivatives 336 through a [4 + 2] cycloaddition (78S524). Muchowski and co-workers have employed the related 2-trichloromethyl-4-dimethyl-amino-1,3-diaza-l, 3-butadienes 337 for synthesizing 2-trichloromethylpyr-imidines 338 in 38-98% yield by cyclization with electron-deficient acetylenes moreover, triazine 339 was obtained in 80% yield in the reaction of... 

New results in the reactions of trifluoroalkyl substituted 1,3-diaza-l,3-butadienes with HSiCU/ NRs suggest, that in the first step, the base acts catalytically and SiCls is added to the heterobutadiene forming an amidinate type structure (Scheme 3). The further reaction depends on the ligands on nitrogen and carbon, an HCl elimination and formation of the pentacycle only being observed in sterically favorable cases. 

The substituents attached to the 1,3-diaza-l,3-butadiene system determine the course of the reaction and usually a [2-1-2] cycloadduct is mainly obtained in the above reaction, while [4-1-2] cycloadducts are obtained with other substituents (see Section 4.1.4.4). The reaction of phenylketene, generated similarly, with the diazabutadiene affords the [4-1-2] cycloadduct. Bulky substituents favor the [2-1-2] cycloaddition Phenylmalonyl chloride 277 is also used in the synthesis of azetidinones. The obtained cycloadducts 278 have the reactive COCl group for further transformations ". ... 

Jimenez et al. studied the asymmetric Diels-Alder reactions of 1-aryl-1,2-diaza-l,3-butadienes 114, heterodienes derived from sugars, with diethyl azodicarboxylate (115), a heterodienophile [85]. The reactions were performed without solvent in a focused microwave reactor for periods of a few hours. The reaction is stereoselective... 

Some work was carried out to investigate the reaction of 1,2-diaza-l,3-butadienes 409 with 2-imidazolidinethione in methanol at room temperature. 5,6-Dihydro-imidazo[2,l-3]thiazol-3-ones 410 were formed in 27-62% yield (Equation 186) <1999T13423>. 

Avalos et al. (223,224) also found that simpler 2-aminothioisomiinchnones react with nitroalkenes to give dihydrothiophenes and other products, work that includes detailed MO calculations that rationalize both the reactivity of the thioisomilnch-nones and the observed regioselectivity (224). These same thioisomtinchnones (324) react with aryl aldehydes to provide p-lactams 325 following fragmentation and subsequent cyclization of the primary cycloadducts (225). Novel 1,2,4-triazines are produced when 324 is exposed to diethyl azodicarboxylate (226), and detailed synthetic and mechanistic studies have been reported for the reactions of 324 with alkynes (227) and chiral 1,2-diaza-l,3-butadienes (228). 

Monodentate bonded 1,4-diaza-l, 3-butadiene ligands (tr-N two electron) 208... 

DIAZA-l,3-BUTADIENE (a-DIIMINE) METAL COMPLEXES 13.5.3.1 Introduction... 

Bridging 1,4-diaza-l,3-butadiene ligands (two electron + two electron)... 

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