= Solution
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 $\mathrm{CH_2O}$.
\b[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 \b[E allylic alcohol], with $\mathrm{CH_2OH}$ 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.<trans-Hydroalumination Alkylation: One-Pot Synthesis of Trisubstituted Allylic Alcohols by Neil F. Langille and Timothy F. Jamison>[2006, pp. 3761–3764, trans-hydroalumination and trapping of alkenyl alanates]{ref}
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2024/paper-1-osmate-aluminate.png]
{title=Cyclic intermediates in osmium dihydroxylation and directed trans hydroalumination}
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