Journal of the Chemical Society, Dalton Transactions p. 1997 - 2009 (1996)
Update date:2022-08-03
Topics:
Brooks, Wendell V. F.
Brownridge, Scott
Passmore, Jack
Schriver, Melbourne J.
Sun, Xiaoping
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.
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