Catalytic chemistry of furan and thiophene. Ab initio calculations, using the spin-coupled valence bond method, of the interaction of furan and thiophene with a positively charged centre

Abstract

The spin-coupled valence bond (SC-VB) method is summarized and used to determine the electronic structure of furan and thiophene. The resulting wavefunctions are utilized in a study of the interaction of these molecules with an approaching Li⁺ ion. It is found that the SC-VB description of the lone pairs of the heteroatoms provides a coherent explanation of the unusual fact that, in the minimum-energy structure of the thiophene–Li⁺ complex, the Li⁺ ion lies off the C₂ axis (at an angle of 50°), whereas for the furan complex the Li⁺ ion lies on the C₂ axis. It is suggested that the interactions are sufficient to constrain the molecules at a positive centre on the surface of a catalyst without developing such strong bonds that the catalyst would be poisoned. The implications of this description for the catalytic activation of thiophene by transition-metal ions are discussed.