Cycloadditions of `Thiocarbonyl Ylides' with Tetracyanoethylene (=Ethenetetracarbonitrile): Interception of Intermediates

摘要: Thiocarbonyl ylides (=sulfonium ylides) belong to the most nucleophilic 1,3-dipoles (high HO energy). In their reactions with tetracyanoethylene (TCNE=ethenetetracarbonitrile; low LU energy), a borderline crossing from concerted mechanism two-step pathway via 1,5-zwitterion was observed. Steric hindrance at one or both termini of 1,3-dipole is an additional requirement. The 3 and 13, set free by N2 elimination dihydro-1,3,4-thiadiazoles, underwent electrocyclization 1,4-H shift. Ylides 13 are bases afforded MeOH adducts different regiochemistry. Whereas TCNE in abs. THF 45° furnished (3+2) cycloadduct 20, content 0.5 – 5 vol-% gave rise seven-membered lactim ether 22 thiolane 20 65: 35 ratio (Scheme 4). Water (0.5 – 1 vol-%) led lactam 24 adduct same ratio. zwitterion 26, assumed be first intermediate, enters competing reactions: irreversible ring closure reversible formation strained, cyclic `ketene imine' 28, which intercepted H2O. gauche-conformation 32 analogous zwitterion, produced tetrasubstituted `thiocarbonyl ylide' (Scheme 5), derivative 35, while anti-conformation 33 thioxo compound 5 cyclopropane 36 intramolecular substitution.