Ketones with hydride transfer reagents

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 ... 

Little is known of the details of these processes. The most extensive investigations have been done with NaBH4. Studiesof the kinetics31,32,48,95 show that the reaction is first order in ketone and in hydride, that transfer of the first hydride is the slowest step, and that the alkoxyborohydrides formed in in the first step react very quickly.69 The kinetics are compatible with direct reaction between ketone and hydride or with reversible formation of a complex between the reactants folllowed by a slow hydride transfer. Reduction rates do not depend on pH,90,95 provided the solution is sufficiently alkaline for the reagent to be stable. 

The Oppenauer oxidation of alcohols (by a ketone in excess, with an aluminium alkoxide as catalyst) proceeds by hydride transfer through a cyclic transition state (7) in which a molecule of the alcohol and a molecule of the reagent ketone (e.g. acetone) are simultaneously coordinated to one aluminium atom [39]. The reaction actually establishes an equilibrium... 

Like S,S-Dimethyl-N-(p-toluenesulfonyl)sulfoximine and (dimethylamino)dimethyloxosulfonium tetrafluoroborate, the 7Y-tosylsulfilimine (1) reacts as a methylene transfer reagent, converting aldehydes and ketones to epoxides (eq 3). Thus (1) is heated at 80-90 °C for 0.5 h in DMSO in the presence of Sodium Hydride, and the resulting anion is allowed to react with carbonyl compounds to give 1-mono- and 1,1-disubstituted oxiranes in 46-56% yields. ... 

The DMB reagents are used in conjunction with a catalytic amount of the homochi-ral 2,5-dimethylborolanyl mesylate (DMB-OSO2CH3 = DMB-OMs). Hydride transfer occurs at the face of the mesylate-ketone coordination complex that exhibits the least steric interactions. The availability of both forms of enantiopure DMB allows the preparation of enantioenriched (/ )- or (5)-alcohols in a predictable and controlled manner. 

Ketones can be reduced to secondary alcohols by H and a catalyst or a complex metal hydride or by hydrogen transfer reagents. Deoxygenation with a strong base at 150°C (302°F) can form olefin. Ketones react with thiols to form thioacetals. Reaction with anhydrous hydrazine may be explosive. 

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