text: Hydroalumination of alkynes may be either stereospecifically cis or trans depending on the conditions employed. When a dialkylalane such as di(isobutyl)aluminium hydride (DIBAL-H) is used, the hydrogen and aluminium delivered from the reagent end up cis in the resulting alkenylalane. This stereospecificity can be explained by invoking a concerted addition of the H–Al bond across the triple bond. In the transition state, partial positive charge builds up on the carbon forming a bond to hydrogen; thus, the carbon better able to stabilize a positive charge becomes attached to hydrogen in the product alkenylalane. Hydroaluminations of terminal alkynes typically produce terminal alkenylalanes as a result. Selectivity in hydroaluminations of internal alkynes is typically low, unless an electronic bias exists in the substrate (such as a phenyl ring in conjugation with the alkyne).
  Stereospecific trans hydroalumination is possible through the use of lithium aluminium hydride. The mechanism of this transformation involves the addition of hydride to the carbon less able to stabilize the developing negative charge  (viz., in the β position to an electron-withdrawing group). Coordination of aluminium to the resulting trans vinyl carbanion leads to the observed trans configuration of the product.