The stereoselective elaboration of alkynes into substituted alkenes via vinylic organometallics derived from hydrometalation and carbometalation of the triple bond comprises one of the most powerful tools for olefinic synthesis. The discovery that mono- and disubstituted alkenylaluminum compounds can be directly synthesized via cis hydroalumination of alkynes thus has played an important role in the development of stereoselective syntheses of functionally di- and tri-substituted olefins. Also, the introduction of the zirconium-catalyzed carboalumination has made available alkenylaluminum compounds that can be elaborated into stereodefined trisubstituted olefins, especially those of terpenoid origin. The fact that the hydroalumination of 1-alkynes not only provides alkenylalanes, but also can be controlled through the choice of solvent to afford 1-alkynylalanes and bis- and tris-aluminoalkanes greatly enhances the versatility of alkyne-derived organoaluminum derivatives. However, the chemistry of geminal aluminoalkanes remains to be delineated.
An important feature of alkenyl- and alkynylaluminum compounds is that they exhibit chemical properties that differ distinctly from those of the corresponding organolithium or organomagnesium compounds or of their organoboron congeners. For example, although unsaturated organoaluminum compounds undergo Grignard-like reactions with a variety of organic and inorganic electrophiles, they show some significant differences in reactivity and substrate selectivity as compared to organolithium and Grignard reagents. This results because the trigonal aluminum in organoalanes possesses a rather low intrinsic nucleophilicity. However, conversion of the organoalanes into the corresponding filled-octet aluminate species by treatment with bases markedly increases their carbanion character. Also, transmetalation of alkenylaluminum compounds with transition metals produces stereodefined organometallics with reaction characteristics different from those of their progenitors, thus providing an additional tool for modifying the reactivity of unsaturated organoaluminum compounds.
For simplicity and clarity, organoaluminum compounds are referred to in this chapter as monomers even though they actually may be associated. Except for the 1-alkynylaluminum compounds, we have chosen to include in this chapter only those alkenylaluminum compounds whose preparations involve the direct formation of a carbon–aluminum bond. Those derived by exchange reactions are not covered.