Magnesium oxide, aldol

The base-catalyzed reaction of acetaldehyde with excess formaldehyde [50-00-0] is the commercial route to pentaerythritol [115-77-5]. The aldol condensation of three moles of formaldehyde with one mole of acetaldehyde is foUowed by a crossed Cannizzaro reaction between pentaerythrose, the intermediate product, and formaldehyde to give pentaerythritol (57). The process proceeds to completion without isolation of the intermediate. Pentaerythrose [3818-32-4] has also been made by condensing acetaldehyde and formaldehyde at 45°C using magnesium oxide as a catalyst (58). The vapor-phase reaction of acetaldehyde and formaldehyde at 475°C over a catalyst composed of lanthanum oxide on siHca gel gives acrolein [107-02-8] (59). 

Zhang, G., Hattori, H. and Tanabe, K. Aldol addition of acetone, catalyzed by solid base catalysts magnesium oxide, calcium oxide, strontium oxide, barium oxide, lanthanum-(III) oxide and zirconium oxide, Appl. Catal., 1988, 36, 189-97. 

It has previously been reported that hydrotalcite catalyzes the aldol condensation of acetone (25). Polyoxometalates are known to dehydrate alcohols due to their acidic nature (IS ). In order to compare the relative basicity of polyoxometalate-pillared hydrotalcites to that of hydrotalcite itself, a variety of hydrotalcites were screened for 2-propanol conversion (Table II). This reaction is known to give propylene when the catalyst contains acidic sites (such as alumina) and acetone when the catalyst contains basic sites (such as magnesium oxide). 

A mixture of formalin and ethanol was passed at 240—320 C over various metal oxides supported on silica gel and metal phosphates. The main products were acrolein, acetaldehyde, methanol, and carbon dioxide. Acidic catalysts such as V-P oxides promoted the dehydration of ethanol to ethene. The best catalytic performances for acrolein formation are obtained with nickel phosphate and silica-supported tungsten, zinc, nickel, and magnesium oxides. With a catalyst with a P/Ni atomic ratio of 2/3, the yields of acrolein reach 52 and 65 mol% on ethanol basis with HCHO/ethanol molar ratios of 2 and 3, respectively. Acetaldehyde and methanol are formed by a hydrogen transfer reaction from ethanol to formaldehyde. Then acrolein is formed by an aldol condensation of formaldehyde with the produced acetaldehyde [40],... 

Zhang, G Hatton, H Tanabe, K. Aldol condensation of acetone acetone d6 over magnesium oxide and lanthanum oxide. Applied Catalysis, 1988 40. 183-190,... 

In this direction, Choudary et al. reported the direct asymmetric aldol reaction of aldehydes and ketones to afford the optically active [3-hydroxy carbonyl compounds in good yields and moderate ee s catalyzed by nanocrystalline magnesium oxide. 

Various magnesium oxide crystals were initially screened in the achiral aldol reaction between p-nitrobenzaldehyde and acetone at room temperature, and NAP-MgO was found to be superior compared to CP-MgO and CM-MgO (Table 5.8). Based on the results of the achiral aldol reaction, the work was further extended to the direct... 

A magnesium enolate of 99 is susceptible to aldol condensation with 4-pentenal, and the crude product can be directly protected to give its ethyl carbonate 100. a-Hydroxylation of the carbonyl group yields the hydroxyl carbonate 101. Reduction of the carbonyl group generates a triol, and this compound can be simultaneously converted to carbonate 102. Swern oxidation of 102 gives ketone 103, which can be rearranged25 to produce lactone product 104 (Scheme 7-32). 

Other chiral magnesium enolates derived from amides are known to react with aldehydes. For example, the aldol-type reaction of magnesium enolate of —)-trans-2-N,N-diethylacetamide-l,3-dithiolanes-5 -oxide with isobutyraldehyde affords a single diastere-omer in 82%. The relative stereochemistry of the adduct originates from a rigid transition state 87 where the oxygen atoms of the enolate and the aldehyde are coordinated to the magnesium atom. ... 

When aldehydes, with or without a hydrogen, are treated with aluminum ethoxide, one molecule is oxidized and another reduced, as in 9-69, but here they are found as the ester. The process is called the Tishchenko reaction. Crossed Tishchenko reactions are also possible. With more strongly basic alkoxides, such as magnesium or sodium alkoxides, aldehydes with an a hydrogen give the aldol reaction. Like 9-69, this reaction has a mechanism that involves hydride transfer.751 The Tishchenko reaction can also be catalyzed752 by ruthenium complexes.753 by boric acid,754 and, for aromatic aldehydes, by disodium tetracarbonylferrate Na2Fe(CO)4,755 OS I, 104. 

Oxidation of the diol (1) to o-diacetylbenzene is conducted in a solution buffered with magnesium nitrate, apparently to precipitate hydroxide ion and so inhibit aldolization of the product (Goldschmidt ). A mixture of the diol and buffer in OH... 

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