Two isomeric fourteen-membered bis-dithioacetals derived from (Z)- and (E)-but-2-ene-1,4-dithiols

Abstract

Z)-But-2-ene-1,4-dithiol was found to undergo isomerization into the E isomer. Condensation of (Z)-and (E)-but-2-ene-1,4-dithiols with acetaldehyde gave isomeric fourteen-membered bis-dithioacetals whose structure was determined by X-ray analysis.The formation of monomeric and dimeric cyclic dithio acetals, as well as of polymeric products, is determined by the nature of initial α,ω-dithiol and carbonyl compound [1], and macrocyclic compounds with short hydrocarbon bridges between sulfur atoms are difficultly accessible [2]. While studying seven-membered unsaturated dithioacetals [3], we found that the condensation of but-2-ene-1,4-dithiol with acetaldehyde, apart from previously reported 2-methyl-1,3-dithiacyclohept-5-ene (I) [4], gives two minor products II and III which were isolated in low yields (2 and 3%, respectively; Scheme 1). The 1 H and 13 C NMR spectra of compounds I-III, recorded at low temperature, are fairly similar; the spectra indicated the presence of CH 2 CH=CHCH 2 and CH 3 CH fragments in their molecules. According to the X-ray diffraction data, compounds II and III are fourteen-membered bis-dithioacetals [5]. The formation of macrocyclic compound II might be expected [1], while the presence in the reaction mixture of its isomer III with E configuration at the double bond is difficult to predict and therefore special comment is necessary.Obviously, bis-dithioacetal III could be formed only as a result of condensation of (E)-but-2-ene-1,4-dithiol with acetaldehyde. Stereochemical 1 H NMR analysis [6] showed that the source of (E)-but-2-ene-1,4-dithiol is the corresponding Z isomer (which undergoes isomerization on storage as neat liquid) rather than (Z)-but-2-ene-1,4-diol, (Z)-1,4-dichlorobut-2-ene, and the corresponding dithioacetate [7].Molecule II is oriented along the 2 1 axis, so that only a half of the molecule is independent (Fig. 1). The bond lengths and bond angles therein are within the corresponding standard values. According to [8], the heteroring conformation should be represented as [2525] with equatorial orientation of the methyl groups. Comparison of the molecular structures of compound II and isomeric tetrathiadiene with sulfur atoms at the double bonds [9] shows their essential similarity. An exception is the transannular distance between the sp 2 -carbon atoms, which is longer by 0.48 Å in molecule II.Analysis of intermolecular interactions revealed neither shortened contacts nor hydrogen bonds in the crystalline structure of cis-tetrathiadiene II. However,