Transformations of thiocarbonyls into alkenes via Barton-Kellogg olefination.

Abstract

The transformation of a thiocarbonyl compound into an alkene by stepwise treatment with a diazo compound and triphenylphosphane is known as Barton-Kellogg olefination. As a model reaction, 4,4'-dimethoxythiobenzophenone and diazocyclohexane were used to prepare [bis(4-methoxyphenyl)methylidene]cyclohexane, C₂₁H₂₄O₂. The crystal structure of the latter, as well as that of the intermediate thiirane, 2,2-bis(4-methoxyphenyl)-1-thiaspiro[2.5]octane, C₂₁H₂₄O₂S, have been determined and their molecular conformations and geometries are generally consistent with those of related structures in the literature. Variations in the influence of four substituents on crowded thiirane rings are minimal and the main differences are noted in the presence of bulky tert-butyl substituents. The conformation of the intermediate thiirane is influenced by weak intramolecular C-H...S interactions. A three-dimensional supramolecular structure of the methylene cyclohexane compound results from the combination of three distinct weak C-H...π interactions. Under similar reaction conditions, 5-phenyl-3H-1,2-dithiole-3-thione has been transformed into 3-[bis(4-methoxyphenyl)methylidene]-5-phenyl-3H-1,2-dithiole, C₂₄H₂₀O₂S₂, by treatment with bis(4-methoxyphenyl)diazomethane. The crystal structure of the 1,2-dithiole product reveals a molecule with an all-trans 2,4-hexadiene core, in which the Csp²-Csp² bond lengths display an alternating character that suggests little delocalization of the double bonds. The 1,2-dithiole ring is nearly planar, with just a slight puckering into an envelope form. Two weak C-H...π and one C-H...O interaction link the molecules into thick two-dimensional supramolecular layers.