Cycloadditions of 'thiocarbonyl ylides' with tetracyanoethylene (=ethenetetracarbonitrile): Interception of intermediates

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 the con certed mechanism to a two-step pathway via a 1.5-zwitterion was observed. S teric hindrance at one or both termini of the 1,3-dipole is an additional r equirement. The ylides 3 and 13, set free by N-2 elimination of dihydro-1,3 ,4-thiadiazoles. underwent electrocyclization or 1,4-H shift. Ylides 3 and 13 are bases and afforded MeOH adducts of different regiochemistry. Whereas 3 and TCNE in ab. THF at 45 OH content of 0.5 - 5 vol-% in THF gave rise t o a seven-membered furnished the (3 + 2) cycloadduct 20, a Me lactim ether 22 and thiolane 20 in a 65:35 ratio (Scheme 4). Water (0.5 - I vol-%) in TH F led to lactam 24 and adduct 20 in the same ratio. The zwitterion 26. assu med to lie the first intermediate, enters competing reactions the irreversi ble ring closure to thiolane 20 and the reversible formation of a strained, cyclic seven-membered 'ketene imine' 28, which is intercepted by MeOH or H 2O. The gauche-conformation 32 of an analogous zwitterion, produced from th e tetrasubstituted 'thiocarbonyl ylide' 13 with TCNE (Scheme 5). led to the thiolane derivative 35, while the anti-conformation 33 afforded the thioxo compound 5 and cyclopropane derivative 36 by intramolecular nucleophilic s ubstitution.