Synthesis and reactivities of a highly strained thiophene with two fused four-membered rings, 1,2,4,5-tetahydrodicyclobuta[b,d]thiophene

Abstract

The successful synthesis of 1,2,4,5-tetrahydrodicyclobuta[b,d]thiophene (1) and its ususually enhanced reactivities because of large ring strain are described. Thus, intramolecular pinacol coupling of diketo sulfide 14, obtainable from 2-bromocyclobutanone and sodium sulfide, with a low-valent titanium reagent affords the thiolane-3,4-diol 15 in 36% yield based on the bromo ketone. After conversion of 15 to the bis-methanesulfonate 16 in 66% yield, the latter was treated with potassium tert-butoxide to give 1 nearly quantitatively as a rather thermally stable, colorless, highly sublimative, nicely crystalline compound. Bromine rapidly adds to 1 to give the tetrabromide 21, revealing its olefinic character. Furthermore, 1 undergoes Diels-Alder reaction with tetracyanoethylene at room temperature to afford an adduct. The Diels-Alder reaction with maleic anhydride at room temperature gives an adduct in the endo-exo isomer ratio 5:1, whereas the reaction with excess maleic anhydride in refluxing benzene produces a bisadduct with loss of sulfur. Similar behaviors of 1 toward N-phenylmaleimide were also observed. Flash vacuum pyrolysis of 1 gave benzo[b]thiophene but not the expected radialene or its superthiophenophane