Reaktivität s̀1‐bishomoaromatischer Heterocyclen: Elektrocyclische Reaktionen und Cycloadditionen von syn ‐ und anti 1‐Bishomofuran und ‐Bishomothiophen

Reactivity of s̀1‐Bishomoaromatic Heterocyles: Electrocyclic Reactions and Cycloadditions of syn ‐ and anti 1‐Bishomofuran and ‐Bishomothiophene Professor William von E. Doering zum 75. Geburtstag gewidmet. 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°C whereas the rearrangement anti ‐ 4a → 7a requires a temperature of about 370°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 [D 6 ]benzene solution at 140°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 complete 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 9b in the thermolysis of syn ‐ 4b indicates that a two‐step pathway may compete with the concerted process. syn ‐ 4a, b react with various π 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 1‐phenylmaleinimide (NPMI) or dimethyl dicyanofumarate (DCFM) shows unambiguously, that the adducts are formed by a 1,3‐dipolar cycloaddition of the intermediate carbonyl ylide 5a 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 (Δ H R ≤ 22.4 kcal/mol) and syn ‐ 4d → 5d (Δ H R ≥ 47.5 kcal/mol) shows the carbonyl ylide to be a 1,3‐diradical, highly resonance‐stabilized by the heteroatom (resonance energy R.E. ≈ –25 kcal/mol).