Synthetic and computational studies on organosulfur radical cations and alpha-metalated sulfides

Abstract

The oxidation potentials and electrooxidation mechanism of 3,6-di substituted-1,2-dichalcogenins was investigated by cyclic voltammetry. An EC mechanism was found experimentally and the basis for the chemical step was found computationally to be a change in the planarity of the ring on electron transfer. Photoelectron spectra were obtained of 3,6-disubstituted-1,2-dichalcogenins a different ionizing photon energies. The interpretation of the photoelectron spectra was assisted by computational simulation. A narrow ionization band was found, and was assigned as the sulfur-sulfur σ orbital by computation and comparison of ionization cross-sections. Computational simulation of the excited state determined that this orbital is paired with a low-energy σ* orbital. Electronic transitions to the σ* orbital were found to be common in disulfides, and the low energy of the σ* orbital in 1,2-dichalcogenins causes their unusual color. The computed geometry of the excited state, coupled with ¹H NMR and ⁷⁷Se NMR data, also provided evidence of the limited anti-aromaticity of 1,2-dithiins. A systematic study of the α-deprotonation of dialkyl sulfides was made with Lochmann's/Schlosser's base. The products were analyzed by GC/MS, and the extent of both deprotonation and decomposition was assessed. Mechanisms of decomposition were evaluated. 4-tert-Butylthiane was alpha-deprotonated and stannylated in good yield, in a 38:1 cis:trans ratio. The oxidation potentials of various α-stannylated dialkyl sulfides were analyzed by cyclic voltammetry. The dependence of oxidation and ionization potential on the C-S-C-Sn dihedral angle was investigated computationally, and was found to obey a Karplus-Barfield-type relationship, with an ionization potential minimum near 90°. The computational predictions were borne out in the oxidation potential of cis-2-trimethylstannyl-4-tert-butylthiane (1.17 V), which was found to be slightly lower than the underivatized sulfide but much higher than other alpha-stannylated sulfides due to the 180°C-S-C-Sn dihedral angle. Various computational techniques were used to find evidence of the cyclic interaction of p-type lone pair orbitals in tetrathiatetraasterane. The neutral species and the radical cation were computationally predicted to be possess the same symmetry (D₄(h)) and cyclic interaction of sulfur lone pair orbitals, but oxidation to the dication was predicted to break a carbon-carbon bond.