[4+2]-Cycloadditionen von 1,2-Dicyanocyclobuten und seine thermische Ringöffnung zum 2,3-Dicyanobutadien-1,3

Facile synthetic routes to 1,2-dicyanocyclobutene ( 3 ), cyclobutene-1,2-dicarboxylic acid ( 56 ) and derivatives thereof are presented, starting from 1,2-dicyanocyclobutane ( 1 ), a commercially available acrylonitrile cyclodimer. The favored mode of 4+2]-cycloadditions of 3 to cyclic dienes with sp³carbon atoms is the endo-addition (above 90% relative yields of adducts with endo-cyclobutane ring). Exo-cycloaddition, however, is preferred by dienes having no sp³-carbon atom (e.g. furane). Cyclisation reactions involving cis-vicinal substituents in 4+ 2]-cycloadducts afford (m.n. 2)-azapropellanes 18 , 74 and 77 . ¹H- and ¹³C-NMR. spectra of the stereoisomeric adducts are discussed in detail. The structures of the furane adducts 14 and 15 were determined by ¹H-NMR. using the shift reagent Eu(dpm)₃. Reactive butadienes 32 , 53 - 55 are obtained in high yield and purity by gasphase thermolysis (380–420°) of the correspondingly substituted cyclobutenes. 2,3-Dicyanobutadiene-l, 3 ( 32 ) gives good yields of 4 + 2]-cycloadducts with strained cycloolefines, moderate yields with vinylethers and non-activated olefins, and no adducts at all with electrophilic dienophiles (8.g. maleic anhydride, fumaronitrile). Thus, reactions of 32 are typical Diels-Alder reactions with ‘inverse electron demand’. Some of these primary 4+2]-cycloadducts ( 38 , 39 and 45 ) were dehydrogenated to new aromatic ortho-dinitriles 46 - 48 .