Aluminum alkoxides, aldehydes from, with alcohols

The aldehyde or ketone, when treated with aluminum triisopropoxide in isopropanol as solvent, reacts via a six-membered cyclic transition state 4. The aluminum center of the Lewis-acidic reagent coordinates to the carbonyl oxygen, enhancing the polar character of the carbonyl group, and thus facilitating the hydride transfer from the isopropyl group to the carbonyl carbon center. The intermediate mixed aluminum alkoxide 5 presumably reacts with the solvent isopropanol to yield the product alcohol 3 and regenerated aluminum triisopropoxide 2 the latter thus acts as a catalyst in the overall process ... 

One of the chemoselective and mild reactions for the reduction of aldehydes and ketones to primary and secondary alcohols, respectively, is the Meerwein-Ponndorf-Verley (MPV) reduction. The lifeblood reagent in this reaction is aluminum isopropoxide in isopropyl alcohol. In MPV reaction mechanism, after coordination of carbonyl oxygen to the aluminum center, the critical step is the hydride transfer from the a-position of the isopropoxide ligand to the carbonyl carbon atom through a six-mem-bered ring transition state, 37. Then in the next step, an aluminum adduct is formed by the coordination of reduced carbonyl and oxidized alcohol (supplied from the reaction solvent) to aluminum atom. The last step is the exchange of produced alcohol with solvent and detachment of oxidized alcohol which is drastically slow. This requires nearly stoichiometric quantities of aluminum alkoxide as catalyst to prevent reverse Oppenauer oxidation reaction and also to increase the time of reaction to reach complete conversion. Therefore, accelerating this reaction with the use of similar catalysts is always the subject of interest for some researchers. 

Ooi and Maruoka found that aluminum alkoxide (83) prepared from aluminum complex (82) and tertiary propargyl alcohol reacts with highly electrophilic aldehydes, such as chloral and pentafluorobenzaldehyde, to give alkynyl transfer products in good yield (Scheme 6.65) [84]. Bisphenol structure of ligands on aluminum center is the most important feature to obtain the alkynylated products, and thus, in reactions with (84) or (85) instead of (83) dramatical decrease in the yield of alkynyl product was observed. Same group also reported that cyanide transfer reaction with acetone cyanohydrin as a cyanide donor catalyzed by complex (82) [84]. 

The reaction of complex hydrides with carbonyl compounds can be exemplified by the reduction of an aldehyde with lithium aluminum hydride. The reduction is assumed to involve a hydride transfer from a nucleophile -tetrahydroaluminate ion onto the carbonyl carbon as a place of the lowest electron density. The alkoxide ion thus generated complexes the remaining aluminum hydride and forms an alkoxytrihydroaluminate ion. This intermediate reacts with a second molecule of the aldehyde and forms a dialkoxy-dihydroaluminate ion which reacts with the third molecule of the aldehyde and forms a trialkoxyhydroaluminate ion. Finally the fourth molecule of the aldehyde converts the aluminate to the ultimate stage of tetraalkoxyaluminate ion that on contact with water liberates four molecules of an alcohol, aluminum hydroxide and lithium hydroxide. Four molecules of water are needed to hydrolyze the tetraalkoxyaluminate. The individual intermediates really exist and can also be prepared by a reaction of lithium aluminum hydride... 

The organozinc intermediate thus formed reacts with aldehydes as Grig-nard reagents do to form alcohols. In the presence of aluminum chloride, elimination of chlorine and fluorine from the vicinal carbons of the dichlorotrifluoroethyl group generates halogenated allylic alkoxides that are protonated to allylic alcohols, in the present case J, 2-chloro-3,3-difluoro-1 -phenylpropen-2-ol [114]. 

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