Meerwein-Ponndorf reaction

Nucleophilic addition to C=0 (contd.) ammonia derivs., 219 base catalysis, 204, 207, 212, 216, 226 benzoin condensation, 231 bisulphite anion, 207, 213 Cannizzaro reaction, 216 carbanions, 221-234 Claisen ester condensation, 229 Claisen-Schmidt reaction, 226 conjugate, 200, 213 cyanide ion, 212 Dieckmann reaction, 230 electronic effects in, 205, 208, 226 electrons, 217 Grignard reagents, 221, 235 halide ion, 214 hydration, 207 hydride ion, 214 hydrogen bonding in, 204, 209 in carboxylic derivs., 236-244 intermediates in, 50, 219 intramolecular, 217, 232 irreversible, 215, 222 Knoevenagel reaction, 228 Lewis acids in, 204, 222 Meerwein-Ponndorf reaction, 215 MejSiCN, 213 nitroalkanes, 226 Perkin reaction, 227 pH and, 204, 208, 219 protection, 211... 

Obviously, we can also rationalize these facts by saying that RO, Cl, RC02, etc. are better leaving groups than H or R. However, it is always useful to look at a problem from different angles. For example, the present treatment can explain much more easily why the Meerwein-Ponndorf reaction is reversible (see below). 

If the carbon has a donor substituent, we need an electrophile to attack the hydrogen. This provokes a hydride transfer (cf. the Cannizzaro and Meerwein-Ponndorf reactions) ... 

The reaction of 2-acetylphenothiazine with ethyl magnesium bromide led to 2-(but-2-enyl)phenothiazine (156) A-substituted ketones gave ethynyl derivatives (157) on treatment with acetylene. Reduction of 2-acyl derivatives to alcohols has been performed by the Meerwein-Ponndorf reaction and by using LiAlH. - - Polarographic one-electron reduction of the keto group in 10-acetyl-2-()3-piperidinopropionyl)phenothiazine has been reported. ... 

When the Wittig reaction was run directly on (6), a substantial amount of the isomer (10) was also obtained. This product is considered to arise through (9), formed by an intramolecular Meerwein-Ponndorf reaction involving the hindered carbonyl group and the free hydroxyl group. 

Oxidation of an aldehyde group to a carboxyl group has often been used for identification of the compounds. As far as can be judged from the results, such oxidations, using chromium trioxide/acetic acid, bromine-water, - peroxy acids, - or chlorite, have not been accompanied by any important side-reactions. Hypoiodite titration, using the iodine in sodium bicarbonate-sodium carbonate procedure, has sometimes been used, giving almost stoichiometric aldehyde determinations. - - Reduction by the Meerwein-Ponndorf reaction, with borohydride - - ... 

USE Meerwein-Ponndorf reactions alcoholysis and ester exchange synthesis of higher alkoxides. chelates, and acyl-aies formation of aluminum soaps, formulation of paints waterproofing finishes for textiles. 

Reduction of steroidal ketones. Steroidal ketones have been reduced with this substance in the presence of potassium alkoxides. More recent studies indicate that the hydride originates from the alkoxide ion (Meerwein-Ponndorf reaction) and that thiourea dioxide plays only a minor role. 

Metal alkoxides catalyze the Tishchenko condensation of aldehydes (62), the transesterification of carboxyUc esters, the Meerwein-Ponndorf reaction (63), and other enolization and condensation reactions. 

Aluminum alkoxides are easily soluble in hydrocarbons and in chlorinated hydrocarbons, but sparingly soluble in alcohols. They are sensitive to moisture and dry storage is essential. Aluminum alkoxides are used extensively as intermediates, for example, in the Meerwein-Ponndorf reaction (94). 

Manganese(II) iodide, 312-313 Maimich reaction, 194, 231, 339 Marschallc cyclization, 456 Massoialactone, 308 Maytansenoids, 318 Maytansine, 325 Meerwein-Ponndorf reaction, 16 Meldrum s acid, 143, 313-314, 484 p-Menthene-1, 141 Menthol, 535... 

The most likely mechanism of the reaction is shown in Figure 2.1. It involves two half-reactions , with the formation of a 4-keto sugar intermediate. In the first half-reaction this is precisely reversed, except that the direction of addition of hydrogen is such as to reverse configuration at C-4. Each half-reaction shows some formal resemblance to the Meerwein-Ponndorf reduction of ketones to secondary alcohols, in that the second halfreaction is in the same direction as the Meerwein-Ponndorf reaction, while the first is its reverse. In the Meerwein-Ponndorf reaction the hydride ion is the reducing species and is derived from aluminium isopropoxide in 4-epimerase reactions effective H is derived from NADH, which becomes NAD. ... 

This reaction was first reported concurrently by Meerwein and Schmidt and Verley in 1925, and by Ponndorf in 1926, respectively. It is an aluminum alkoxide-catalyzed reduction of carbonyl compounds (ketones and aldehydes) to corresponding alcohols using another alcohol (e.g isopropanol) as the reducing agent or hydride source. Therefore, it is generally known as the Meerwein-Ponndorf-Verley reduction (MPV) or Meerwein-Ponndorf-Verley reaction. Occasionally, it is also referred to as the Meerwein-Ponndorf reduction, Meerwein-Ponndorf reaction, or Meerwein-Schmidt-Ponndorf-Verley reaction. About 12 years later, Oppenauer reported the reversion of this reaction in which alcohols were reversely oxidized into carbonyl compounds. Since then, the interchanges between carbonyl compounds and alcohols in the presence of aluminum alkoxide are generally called the Meerwein-Ponndorf-Oppenauer-Verley reduction or Meerwein-Ponndorf-Verley-Oppenauer reaction." ... 

---