Heterocyclic Aromatic Non-Kekulé Molecules. Synthesis and Solution-Phase Chemistry of the Singlet Biradicals 3,4-Dimethylenefuran and 3,4-Dimethylenethiophene

Abstract

The title biradicals 1 and 2 are generated in solution by thermal or photochemical decomposition of the corresponding diazenes, 1,4-dihydrofurano[3,4-d]pyridazine (15) and 1,4-dihydrothieno[3,4-d]pyridazine (16), whose syntheses are described. The biradicals both dimerize, but in the presence of trapping agents, both react with electron-deficient alkenes toform furano(thieno)[3,4-d]cyclohexanes (fused adducts). The furan biradical 1, however, gives predominantly a bridged adduct. The cycloadditions are strictly stereospecific and seem to occur by concerted four-center transition states. Competition studies show that the relative reactivities of alkenes toward the thiophene biradical 2 are quantitatively the same whether the biradical is generated from the diazene precursor 16 or from diallenyl sulfide 6. The latter observation confirms earlier proposals in the literature that biradical 2 could be involved in the chemistry of 6. The reactivity order of the biradicals parallels that of cyclopentadiene toward the test series of alkenes, the selectivities in both series showing a near-unit slope in a bilogarithmic plot, despite the factor of 1010 by which the biradical absolute rates exceed those of the diene. © 1989, American Chemical Society. All rights reserved.