Reactions of a sterically hindered tetrasubstituted thiocarbonyl ylide with acceptor-substituted ethylenes; regioselectivity and stereochemistry
作者:Grzegorz Mloston、Rolf Huisgen、Henry Giera
DOI:10.1016/s0040-4020(02)00384-8
日期:2002.5
The 1,3-cycloadditions of the tetra-substituted thiocarbonylylide 8, set free by N2 extrusion from thiadiazoline 7, with methyl acrylate and acrylonitrile furnish 3′-and 4′-substituted thiolanes, probably by a concerted pathway. In the reactions of 8 with dimethyl 2,3-dicyanofumarate (27) and dimethyl 2,3-dicyanomaleate (28), zwitterionic intermediates, which are capable of conformational rotation
1,3-Dipolar cycloadditions of aliphatic or alicyclic thiocarbonyl ylides 3A-D – sterically hindered at least at one terminus – with thiobenzophenone produce both regioisomeric 1,3-dithiolanes 4 and 5. According to quantum-chemical calculations (preceding paper), a concerted cycloaddition furnishing 2,4-substituted dithiolanes 4 competes with the formation of an intermediate C,C-biradical 9 which cyclizes
preferentially at the carbonylgroup, rather than the thiocarbonyl group of the four-membered ring. However, the minor product reacted with DMC at the thiocarbonyl group to afford a dimethoxythiirane. A product from a corresponding reaction at the carbonylgroup could not be found. A rationale for the apparent reversal of relative reactivities of the carbonyl and thiocarbonyl groups is offered, with supporting
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 concerted mechanism to a two-step pathway via a 1,5-zwitterion was observed. Steric hindrance at one or both termini of the 1,3-dipole is an additional requirement. The ylides 3