Henry Giera

`Thiobenzophenone S‐Methylide' (=(Diphenylmethylidenesulfonio)methanide), and C,C Multiple Bonds: Cycloadditions and Dipolarophilic Reactivities

Thiobenzophenone and diazomethane afford thiadiazoline 1 at −78°. By elimination of N2 from 1 at −45° (t1/2ca. 1 h), (diphenylmethylidenesulfonio)methanide (2), which cannot be isolated but is interceptible by dipolarophiles, is set free. The nucleophilic 1,3-dipole 2 undergoes cycloadditions with electrophilic C,C multiple bonds; the structures of 16 cycloadducts were elucidated. One-step and two-step cycloaddition pathways are discussed in the light of the steric course observed for (E)/(Z)-isomeric ethylene derivatives. Competition experiments with pairs of dipolarophiles at −45° and HPLC analysis of the adducts provided relative rate constants of 26 dipolarophiles, involving 2 C≡C, 13 C=C, 9 C=S, and 2 N=N bonds. In accordance with Sustmann`s reactivity model of concerted cycloadditions, 2 shows the highest selectivity of all known 1,3-dipoles, i.e., the largest spread of rate constants (krel=1 for methyl propiolate and 33×106 for TCNE). As a consequence of low LU energies, thiones are very active dipolarophiles, and fluorene-9-thione (krel=79×106) stands at the top.

Cycloadditions of two thiocarbonyl ylides with α,β-unsaturated esters and nitriles: steric course and mechanism ☆

Abstract In the probably concerted cycloadditions of the sterically hindered thiocarbonyl ylide 1 with fumaronitrile, maleonitrile, and dimethyl fumarate, the dipolarophile configuration is retained whereas retention/inversion 99:1 for dimethyl maleate (51 times less reactive than fumarate) signals a small involvement of a two-step pathway. The latter becomes dominant when two acceptor groups stabilize the anionic terminus of a zwitterionic intermediate. Nonstereospecific cycloadditions of 1 with dimethyl 2,3-dicyanofumarate ( 16 , retention/inversion 60:40) and dimethyl 2,3-dicyanomaleate ( 17 , 76:24) were observed. Special conditions were required to avoid a preceding cis, trans isomerization, 16 ⇄ 17 , catalyzed by thiadiazoline 4 , the precursor of 1 . In the case of the related thiocarbonyl ylide 37 , this catalysis could not be suppressed and the same ratio of adducts (55:45) was obtained with 16 and 17 .

Reactions of a sterically hindered tetrasubstituted thiocarbonyl ylide with acceptor-substituted ethylenes; regioselectivity and stereochemistry

Abstract The 1,3-cycloadditions of the tetra-substituted thiocarbonyl ylide 8 , set free by N 2 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, sit at the branching point of two irreversible reactions: cyclization to thiolanes 23 / 24 and fragmentation to cyclopropanes 31 / 32 plus thione 12 . Both reactions are accompanied by some loss of stereochemical purity. Two mechanisms for the cyclopropane formation are discussed: intramolecular nucleophilic substitution in anti -zwitterions 29 / 30 or unassisted heterolysis leading to a tert -carbenium zwitterion as further intermediate.