Barium(II) hydroxide as catalyst in organic reactions. 20. Structure-catalytic activity relationship in the Wittig reaction

Abstract

Three barium hydroxide catalysts with different surface-area values and amounts of basic and reducing sites are used a~ 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 (PhsP+CHzR R = H, CH2CH2CHs, C02Et, and COPh) with different pK,, geometrical, and electronic characteristics 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 = COZEt, COPh) that lead to stable ylides. The weak acids (R = H, CH3CH2CHz) 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, = 6.0) is not governed by the cell lattice because this acid can react to any kind of basic site in the solid. The reaction with R = COzEt (pK, = 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, > 8.8 and adequate geometry are necessary to produce the ylide on the solid surface.

The One‐Pot Wittig Reaction: A Facile Synthesis of α,β‐Unsaturated Esters and Nitriles by Using Nanocrystalline Magnesium Oxide

“…We assume that these peaks also could be due to the salt (d = 22.1-22.4) and the ylide (d = 16.1-16.3), which are co-ordinated with the catalyst. [15,16] This is further confirmed by the addition of the aldehyde (4-nitrobenzaldehyde) to the reaction mixture after 24 h, which gave the desired product and the phosphine oxide at d = 29.26 (lit. Figure 9 in the Supporting Information).…”

“…This was confirmed by the observed peak at d = 20.987 (lit. [15] d = 20.5) in the 31 P NMR spectrum shown in Figure 1 of the Supporting Information. In the second experiment, ethyl bromoacetate and triphenylphosphine were stirred in DMF without aldehyde but in the presence of the catalyst, NAP-MgO, after 1 h there is no phosponium ylide peak; instead we observed the phosphonium salt peak at d = 20.88 and another peak at d = 22.23 in 31 P NMR spectrum.…”

“…The reaction was continued and samples were withdrawn periodically after 2 h, 6 h, 9 h, 12 h, 15 h and they showed peaks in the regions d = 17.8-17.9 and 20.8-20.9 corresponding to phosphonium ylide, Ph 3 P = CHCOOEt {lit. [15] : d17.9} and the phosphonium salt, Ph 3 P + CH 2 COOEtBr À (lit. [15] d = 20.5}, respectively (see Figures 3-7 in the Supporting Information).…”

“…[15] : d17.9} and the phosphonium salt, Ph 3 P + CH 2 COOEtBr À (lit. [15] d = 20.5}, respectively (see Figures 3-7 in the Supporting Information). On increasing the reaction time, i.e., 2-15 h there is a decrease in the intensity of the phosphonium salt peak and a simultaneous increase in the intensity of phosphonium ylide peak in the 31 P NMR spectra.…”

Recent Developments on Catalytic Applications of Nano-Crystalline Magnesium Oxide

“…But when the same reaction was performed without aldehyde and in the presence of catalyst, formation of phosphonium salt and phosphonium ylide were observed. To address this issue, authors were conducted variety of experiments using 31 P NMR spectroscopy [22,59,60].…”