Dehydrogenative electron-withdrawing substituents

We have too little information to understand the reactivity of azomethine ylides toward nonactivated acetylenes since acetylenes bearing no electron-withdrawing substituent(s) have rarely been utilized in cycloaddition trapping of azomethine ylides. To the best of our knowledge, the only example is the reaction of azomethine ylide 7, stabilized by two ester moieties, with acetylene itself in acetone at 125°C, producing pyrrole 210 after dehydrogenation with chloranil (66TL397). 

Where a strongly electron-withdrawing substituent is present at C-l or C-3, the isoindolines can be conveniently dehydrogenated with a high-potential quinone.23 1,3-Dimethoxycarbonylisoindoles have been prepared... 

Firstly, (benzylidenamino)acetaldehyde acetals 58 are formed they are cyclized to give 4-alkoxy>3,4-dihydroisoquinolines 59, which are converted into isoquinolines by loss of ROH. The electrophilic cyclization is favoured by meta- and or /zo-electron-releasing substituents in the arylaldehyde. Electron-withdrawing substituents (especially NO2 groups) favour an alternative ring closure leading to oxazoles 61 with dehydrogenation. 

The reaction of 3-chloro-4-fluorothiophene 1,1-dioxide with simple alkenes proceeded with low regioselectivity thus, a mixture of isomeric cyclohexadienes 84 and 85 was obtained, and when R=CH2Br a considerable amount of aromatic compound 86 was also formed. It is likely that the aromatic products are formed as the result of slow thermal dehydrogenation of initially formed adducts, and the rate of dehydrogenation increases in the presence of electron-withdrawing substituents thus, 86 was obtained as the main product when the alkene was methyl acrylate (Scheme 84) [88]. 

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