REACTIVITY OF SIGMA-BISHOMOAROMATIC HETEROCYCLES - ELECTROCYCLIC REACTIONS AND CYCLOADDITIONS OF SYN-BISHOMOFURAN AND ANTI-BISHOMOFURAN AND ANTI-BISHOMOTHIOPHENE

The title compounds syn-4a,b and anti-4a,b, synthesized by the CuCl-catalyzed reaction of homofuran 1a or homothiophene 1b with diazomethane, are very different in their thermal stability. syn-4a isomerizes to 2,3-dihydrooxepine (7a) readily at 130-degrees-C whereas the rearrangement anti-4a --> 7a requires a temperature of about 370-degrees-C. A similar difference in thermal reactivity is observed between syn-4b and anti-4b. But syn-4b undergoes in addition to the rearrangement to 2,3-dihydrothiepine (7b) a dimerization (to 12?) on thermolysis in [D6]benzene solution at 140-degrees-C, whereas mainly isomerization to 9b, cis-10b, and 11 (product of the elimination of sulfur from cis-10b) occurs on thermolysis in the gas phase. The formation of 7a and 7b, respectively, is explained by the electrocyclic ring opening of syn-4a or syn-4b leading to the carbonyl ylide 5a or the thiocarbonyl ylide 5b followed by a sigmatropic 1,4 hydrogen shift. To rationalize the additional products in the thermolysis of syn-4b an electrocyclic conrotatory ring closure in 5b leading to the highly strained episulfide trans-10b is assumed to compete with the hydrogen shift. Under the reaction conditions trans-10b is unstable and either dimerizes in solution or isomerizes in the gas phase. The formation of 9 b in the thermolysis of syn-4 b indicates that a two-step pathway may compete with the concerted process. syn-4a,b react with various pi acceptor-substituted olefins to form cycloadducts of the general type 6a,b. The cycloaddition occurs stereospecifically cis with respect to the configuration of the starting olefin and non-stereoselectively with respect to the endo/exo ratio. The kinetic analysis of the reaction of syn-4a with fumaronitrile (FN), N-phenylmaleinimide (NPMI) or dimethyl dicyanofumarate (DCFM) shows unambiguously, that the adducts are formed by a 1,3-dipolar cycloaddition of the intermediate carbonyl ylide Sa rather than a homo-Diels-Alder reaction of syn-4a (in contrast to the cycloaddition of homofuran 1a). A comparison between the enthalpies of reactions syn-4a --> 5a (DELTAH(R) less-than-or-equal-to 22.4 kcal/mol) and syn-4d --> 5d (DELTAH(R) greater-than-or-equal-to 47.5 kcal/mol) shows the carbonyl ylide to be a 1,3-diradical, highly resonance-stabilized by the heteroatom (resonance energy R.E. almost-equal-to - 25 kcal/mol).