Intramolecular homolytic substitution in selenoxides and selenones

G3(MP2)-RAD calculations provide activation energies for intramolecular homolytic substitution in the 4-(alkylselenoxo)butyl and 4-(alkylselendioxo)butyl radicals ranging from 21–39 kJ mol⁻¹, and 143–170 kJ mol⁻¹for the selenoxide and selenone, respectively. Arrhenius data translate into rate constants for ring-closure of 1.5×10⁵−2.5×10⁸s⁻¹(80°) for the selenoxides, and 5.4×10⁻¹⁴−5.1×10⁻¹¹s⁻¹(80°) for the corresponding selenones. NBO analyses show alkyl radicals are electrophilic during homolytic substitution at selenoxide selenium. The dominant orbital interaction in the transition state is worth 2413 kJ mol⁻¹and involves the SOMO and the lone-pair of electrons on selenium. The corresponding selenones are calculated to ring-close through transition states in which alkyl radicals are nucleophilic, but involve weak (SOMO-> σ* and SOMO-> π*) interactions. Consequently, this chemistry is not viable for selenones because of the lack of lone-pairs of electrons on the chalcogen.