Preparation and characterization of 1,3,2-dithiazolidine and 1,4-dithia-7-azabicyclo[2.2.1]heptane cations, and a mechanistic study of the cycloaddition reactions of alkenes with SNS

Abstract

The cation SNS⁺(as the AsF₆^– salt) underwent quantitative concerted symmetry-allowed cycloaddition reactions with alkenes [C₂H₄, trans- and cis-MeHCCHMe, H₂CCMe₂, MeHCCH₂, Me₂CCMe₂ and norbornene (bicyclo[2.2.1]hept-2-ene)] to give 1,3,2-dithiazolidine cations 1, which in a second quantitative concerted symmetry-allowed cycloaddition reaction with another alkene molecule gave 1,4-dithia-7-azabicyclo[2.2.1]heptane cations 2(with the exception of Me₂CCMe₂). The cycloadducts were characterized by elemental analyses and IR and NMR (¹H, ¹³C, ¹⁴N) spectroscopies. The vibrational spectra were assigned with the aid of frequencies obtained by ab initio(RHF/6–31G*) calculations. When alkene = C₂H₄ the calculated geometry of 2 was in good agreement with that obtained from its crystal structure reported previously; that of 1 correlates well with the experimental data (IR, Fourier-transform Raman, NMR). Kinetic studies showed that the rate constants of the first cycloaddition of SNS⁺ to C₂H₄ are comparable with those of nitrile and alkyne cycloadditions, indicating that the cycloaddition proceeds via the interaction of the highest occupied molecular orbital of the alkene and the lowest unoccupied one of SNS⁺ as was previously observed for various nitriles and alkynes. The second cycloaddition leads to stereospecific 2, except for H₂CCHMe. Contrary to the prediction of a simple frontier molecular model, the rate of the second cycloaddition was faster than the first for C₂H₄, cis-MeHCCHMe, and H₂CCMe₂ and strongly dependent on the steric activity of the alkene. It is proposed that the second cycloaddition likely occurs via a concerted and synchronous pathway.