A trans-folded alkene system: Synthesis, structure and isomerization of 2,2′-diaryl-1,1′-bi(1,2-dihydrophthalazinylidenes)

A series of 2,2′-diaryl-1,1′-bi(1,2-dihydrophthalazinylidenes) have been synthesized from 1-hydroxy-2-aryl-1,2-dihydrophthalazines. An X-ray crystal structure showed relief of strain in these new ylidenes by a trans-folding of the alkene linkage. The mechanism of their formation was probed with additives such as sulphur, N-phenylmaleimide, and dimethyl acetylenedicarboxylate which interacted with intermediates in the reaction and gave further interesting new ylidene structures. The mechanism of trans-cis isomerization of the biphthalazinylidenes was studied using 270 MHz ¹H NMR spectroscopy to measure the kinetics and activation parameters for the isomerization. X-Ray crystal structures of the following compounds are reported: 2,2′(p-bromophenyl)-trans-1,1′-bi-[1,2- dihydrophthalazinylidene (3d); 3- [2-(p-bromophenyl) -1,2-dihydrophthalazin-l -yl] -4- [2- (p-bromophenyl)-1,2-dihydrophthalazin-1 -ylidene]-1-phenyl-3,4- dihydropyrrole-2,5-dione, (10d); dimethyl 3-[2-phenyl-1,2-dihydrophthazin-1- ylidene)-2-oxobutanedioate (11), and dimethyl 2-acetoxy-3-(2-phenyl-1,2- dihydrophthazin-1-yl)but-2-ene-1,4-dioate (12).