Magnesium‐Lanthanum Mixed Metal Oxide: a Strong Solid Base for the Michael Addition Reaction

Abstract

Michael additions were achieved quantitatively at room temperature with a broad spectrum of acceptors and donors, using a magnesium-lanthanum mixed oxide, a strong solid base. The best solvent for the reaction is dimethylformamide, but good yields were also obtained using toluene as solvent. The results obtained with magnesium-lanthanum are comparable to those reported for super bases. The catalyst can be separated from the reaction mixture and recycled five times without loss of activity with no other treatment.Keywords: basic catalysis; magnesium-lanthanum mixed oxide; Michael additions; recyclability; solvent effectsThe Michael addition is a reaction for C À C bond formation, involving the nucleophilic addition of a carbanion to a,b-unsaturated carbonyl compounds and is therefore base-catalysed. This reaction has found multiple industrial applications for the production of monomers, [1,2] corrosion inhibitors, [3] coatings, [4] and pharmaceuticals.[5] This reaction is industrially catalysed by liquid bases such as alkoxides, tertiary amines, non-ionic bases, [6,7] and superbases such as proazaphosphatranes. [8 -10] A variety of solid bases has also been described including barium hydroxide, [11] KF supported on g-Al 2 O 3 , [12] a-Al 2 O 3 , [13] phosphates, [14,15] on hydrotalcites, [16,17] amines anchored at surfaces, [18] and organic resins.[19]Most of these solids are weak bases which are effective only with a restricted class of electrophilic olefins such as a,b-unsaturated esters or conjugated enones. Recycling of the catalyst has seldom been demonstrated except for KF/natural phosphate which can be recycled after calcination at 973 K, which represents a severe limitation to its practical use. We report here the properties of Mg-La mixed oxides for different Michael additions. This catalyst has been recently used for the transesterification of diethyl carbonate by alcohols.[20] A comparison of substrates with different pK a allows us to estimate the possibilities of the catalyst and also gives an evaluation of the basic strength at the surface.[21] We used here donors with pK a values, measured in DMSO, [22] varying from 8 for thiocresol to 16.4 for diethyl malonate and 17.2 for nitromethane.The Mg-La mixed oxide was obtained by co-precipitation of Mg and La nitrates (0.386 and 0.129 mol, in water 0.5 L, for an atomic ratio Mg/La ¼ 3) at a constant pH ¼ 10 using a mixture of KOH (1 mol) and K 2 CO 3 (0.26 mol) in 0.52 L of distilled water. The samples (about 0.15 g) were activated at 923 K in an air flow, using a temperature ramp of 10 K/min. Just before use, they were reactivated at 773 K in air and not rehydrated.The chemical analyses of the solid catalyst give the composition: La: 39.8%; K: 5.4%; H 2 O: 37%. The Mg/ La ratio in the solid is then 4.26, compared to 3 in the solution, therefore the precipitation of La is not complete at pH 10. The powder XRD pattern of the uncalcined Mg-La mixed oxide contains diffraction lines of a hydrated lanthanum carbonate as well as magnesium and lanth...

Three-Component Synthesis of Pyrimidine and Pyrimidinone Derivatives in the Presence of High-Surface-Area MgO, a Highly Effective Heterogeneous Base Catalyst

“…[4] Several solid bases have been reported as being effective in this respect, such as zeolites, [5] alkali metals supported on alumina (Na=NaOH=g-Al 2 O 3 ), [6] clay minerals, [7] hydrotalcites (HDT), [8] metal oxides such as magnesium oxide (MgO), and mixed metal oxides, for example magnesium-lanthanum mixed oxide. [9] Among these solid bases, MgO recently has been studied the most. [10] MgO, obtained using a novel but simple procedure, was systematically investigated as a heterogeneous base catalyst for reactions taking place in the liquid phase, specifically the Michael addition and the Knoevenagel condensation.…”

Palladium/Magnesium‐Lanthanum Mixed Oxide Catalyst in the Heck Reaction

“…Magnesium-lanthanum mixed oxide (MgLaO) appears to be a stronger base than hydrotalcite or fluorides [31] showing for the Michael addition the same catalytic properties as the proazaphosphatranes described by Kisanga et al [32] In this report, we present the preparation and characterization of a new palladium/magnesium-lanthanum mixed oxide catalyst (Pd/MgLaO) and the examination of the catalytic properties, in the Heck reaction, of this type of catalyst that does not require an inert atmosphere, is thermally stable, can be easily handled and reused.…”

Catalysis in Water: Aldol‐Type Reaction of Aldehydes and Imines with Ethyl Diazoacetate Catalyzed by Highly Basic Magnesium/Lanthanum Mixed Oxide

“…The Mg/La mixed oxide catalyst was prepared by the co-precipitation method described earlier. [10] Chemical analysis of the solid gave the composition La 2 O 3 29.5 %, MgO: 30.1 %, K 2 O: 3.4 %, H 2 O: 37 %. The catalyst is well characterized by XRD, DTA coupled with mass spectrometry, FTIR and calorimetry.…”

“…The catalyst is well characterized by XRD, DTA coupled with mass spectrometry, FTIR and calorimetry. This mixed oxide is macroporous and shows a surface area of 37.6 m 2 g À1 after calcination at 923 K. Recently, we have efficiently performed transesterifications of diethyl carbonate by alcohols [11] and Michael addition reactions [10] by using this highly Scheme 1. …”

“…Various solid base catalysts such as Mg/La mixed oxide, fluorapatiteA [8][9][10], in which water provided better yields. This high activity of a solid base in water as solvent is rather surprising since it is well known that, for most solid bases, their catalytic properties appear only after removal of water.…”