Organoaluminum compounds regioselectivity

The regiochemistry of Al-H addition to unsymmetrically substituted alkynes can be significantly altered by the presence of a catalyst. This was first shown by Eisch and Foxton in the nickel-catalyzed hydroalumination of several disubstituted acetylenes [26, 32]. For example, the product of the uncatalyzed reaction of 1-phenyl-propyne (75) with BujAlH was exclusively ds-[3-methylstyrene (76). Quenching the intermediate organoaluminum compounds with DjO revealed a regioselectivity of 82 18. In the nickel-catalyzed reaction, cis-P-methylstyrene was also the major product (66%), but it was accompanied by 22% of n-propylbenzene (78) and 6% of (E,E)-2,3-dimethyl-l,4-diphenyl-l,3-butadiene (77). The selectivity of Al-H addition was again studied by deuterolytic workup a ratio of 76a 76b = 56 44 was found in this case. Hydroalumination of other unsymmetrical alkynes also showed a decrease in the regioselectivity in the presence of a nickel catalyst (Scheme 2-22). 

Reaction with epoxy aiochols-3 Epoxides formed from (E)-allylic alcohols react regioselectively with organoaluminum compounds to provide 1,2-diols with inversion at C j in high yield. 

This type of reaction attracted broad interest when it was discovered that high regioselectivity can also be effected with organoaluminum compounds and other nucleophiles in the presence of Lewis acids and that by employing chiral cyclic acetals (from optically active 1,2- or 1,3-diols) diastereoselective transformations can be realized. - Such reactions are synthetically very valuable when considering that the overall process represents an enantioselective Michael addition, where the chiral auxiliary can be recycled (Scheme 39). ... 

Propargylic lithium alanates or lithium borates react with allylic halides or with carbonyl compounds in a regioselective manner to furnish 1,1 -disubstituted allenes (Scheme The reaction between alkyl halides and aluminum metal is the basis of the oldest method for the synthesis of organoaluminum compounds. For example, propargylic bromides react with aluminum in ether giving organoaluminum compounds that on treatment with acetals yield solely a-allenic ethers (equation 1)." However, the reaction of simple alkyl halides with aluminum metal requires a long reaction time. 

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