Lithium aluminum hydride unsaturated carbonyl compounds

The reduction of methyl 2-siloxycyclopropanecarboxylates can also be started at the ester function when lithium aluminum hydride in ether is the reagent. The resulting alcohols undergo the wellknown cyclopropylcarbinyl/homoallyl rearrangement upon treatment with acid to provide P/y-unsaturated carbonyl compounds 117. These are synthesized isomerically pure and in good yields in a number of cases, if the two-phase-system 2N hydrochloric acid/pentane is employed 78). Otherwise the very easy isomerization to the conjugated a,p-unsaturated compounds 118 occurs to some extend, which can intentionally be completed by base catalysis. 

For the more reactive nucleophiles, where addition is essentially irreversible, whether 1,2-addition or 1,4-addition occurs depends on the relative rates of addition to the two electrophilic sites, the carbonyl carbon and the /3-carbon. Lithium aluminum hydride usually gives predominantly 1,2-addition and provides a useful way to reduce the carbonyl group of an a,/3-unsaturated compound. Sodium borohydride, on the other hand, often gives a mixture of 1,2-addition and the completely reduced product, where 1,4-addition followed by 1,2-addition has occurred. Thus, the reaction of 2-cyclohexenone with lithium... 

Fukuzawa, S., Fujinami, T., Yamauchi, S., Sakai, S. 1,2-Regioselective reduction of a,P-unsaturated carbonyl compounds with lithium aluminum hydride in the presence of lanthanoid salts. J. Chem. Soc., Perkin Trans. 11986, 1929-1932. 

In connection with hydride ion transfers, it will be observed that addition should occur at a positive center. Since olefin additions are usually initiated by attack at a negative center, the Meerwein-Ponndorf-Verlcy reduction should occur preferentially at the carbonyl group of unsaturated carbonyl compounds. This is actually the case both in this reaction and in reductions carried out with lithium aluminum hydride.42... 

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