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An alkenyl aluminate has an anionic aluminium-containing unit attached to an alkene carbon. Intramolecular oxygen coordination can close a five-membered ring in directed propargylic alcohol reductions.

The free propargylic OH first reacts with an aluminium hydride, releasing hydrogen and forming an O–Al bond. Coordination of the nearby alkyne to this aluminium center organizes a five-membered O–C–C–C–Al unit. Hydride addition from the opposite side of the activated alkyne gives a cyclic alkenyl aluminate: aluminium bonds to the distal alkyne carbon, while hydride is delivered to the carbon nearer $\mathrm{C}{\mathrm{H}}_2\mathrm{O}$.

The key step is directed trans hydroalumination. The new C–H and C–Al bonds are anti. Aqueous workup protonolyzes C–Al with retention of its alkene geometry and hydrolyzes the O–Al attachment. Replacing that C–Al bond by C–H therefore produces the E allylic alcohol, with $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ and the rest of the chain on opposite alkene sides. A generic syn hydroalumination drawing followed by simple protonation would predict the wrong geometry.

The free OH provides the directing tether; the silyl-protected oxygen is not equivalently available. Primary electrophile-trapping studies of Red-Al-treated propargylic alcohols support the alkenyl-alanate description.^[ref]

A propargylic alcohol has its hydroxyl-bearing carbon adjacent to a carbon–carbon triple bond. Its alkoxide can coordinate a metal hydride and direct alkyne reduction.

Red-Al is sodium bis(2-methoxyethoxy)aluminium hydride. Suitable propargylic alcohols undergo oxygen-directed trans hydroalumination to cyclic alkenyl aluminate species; aqueous workup gives E allylic alcohols.

Organic Letters 8, 3761–3764. Red-Al converts propargylic alcohols to alkenyl alanates that can be intercepted by electrophiles; this supports a directed trans-hydroalumination intermediate.