E-Z Isomerization and Aggregation Phenomena of Dithiafulvenes in CHCl3

ALEJANDRO M. FRACAROLI, ALEJANDRO M. GRANADOS, RITA H. DE ROSSI

JOURNAL OF ORGANIC CHEMISTRY

American Chemical Society (ACS)

Lugar: Washington DC, Estados Unidos; Año: 2009 vol. 74 p. 2114 - 2119

0022-3263

The physicochemical properties of several 1,3-dithiafulvene (DTF) derivatives having a donor and acceptor group in the molecule were studied. The synthesis of these compounds produces selectively the E isomer, but when the compound is dissolved in CHCl3 isomerization to the Z isomer takes place with a rate that depends on the substituents. The interconversion rate is slow on the NMR time scale; therefore, two separated signals are observed, and they are used to measure the rate constant of isomerization. The equilibrium constant is, in all cases, very close to 1, and this is coincident with the fact that theoretical calculations of the energy of the two isomers in the gas phase differ by less than 0.1 kcal/mol. The isomerization reaction is completely reversible, and the E isomer can be obtained in pure form by selective crystallization. The derivatives with thioalkyl groups have a strong tendency to aggregate in CDCl3. The formation of the aggregates is evidenced from the changes in 1D 1H NMR and DOSY spectra as a function of concentration. The compounds are highly delocalized, and this is reflected by the low activation energy for the isomerization.