Ba(OH)2 as Catalyst in Organic Reactions. 20. Structure-Catalytic Activity Relationship in the Wittig Reaction

Article abstract of DOI:10.1021/jo00276a035

Three barium hydroxide catalysts with different surface-area values and amounts of basic and reducing sites are used as catalysts in the Wittig reactions.These catalysts have different microcrystalline structures and therefore different geometrical properties in the active sites, that govern the adsorption of the reagents on the solid surface.Several phosphonium salts (Ph3P⁺CH2R; R = H, CH2CH2CH3, CO2Et, and COPh) with different pK_a, geometrical, and electronic characteristic have been used.The structure-catalytic activity relationship is analyzed.The most active catalyst, C-200, is less active in the Wittig reaction than in the Wittig-Horner reaction.This fact is explained by geometrical reasons.The process takes place in the homogeneous phase with phosphonium salts of a strong acid character (R = CO2Et, COPh) that lead to stable yields.The weak acids (R = H, CH3CH2CH2) lead to reaction products in the heterogeneous solid-liquid conditions.The cell lattice of the solid governs the process that is carried out in the heterogeneous phase.The process with the strongest acid (R = COPh, pK_a = 6.0) is not governed by the cell lattice because this acid can react to any kind of basis site in the solid.The reaction with R = CO2Et (pK_a = 8.8) is carried out in the homogeneous phase because the ylide is stable.However, the formation of the ylide is controlled by the cell lattice because basic sites with pK_a > 8.8 and adequate geometry are necessary to produce the ylide on the solid surface.