The best way of ensuring anti-addition of hydrogen across any triple bond is to treat the alkyne with sodium in liquid ammonia.

The sodium donates an electron to the LUMO of the triple bond (one of the two orthogonal π* orbitals). The resulting radical anion can pick up a proton from the ammonia solution to give a vinyl radical. A second electron, supplied again by the sodium, gives an anion that can adopt the more stable trans geometry. A final proton quench by a second molecule of ammonia or by an added proton source (t-butanol is often used, as in the Birch reduction) forms the E alkene.

An alternative, and more widely used, method is to reduce alkynes with LiAlH4 or the related reducing agent known as RedAl. This reaction works only if there is a hydroxy or an ether functional group near to the alkyne because it relies on delivery of the reducing agent to the triple bond through complexation to this oxygen atom.

Making alkenes by addition to alkynes offers two distinct advantages. Firstly, the starting materials can often be made straightforwardly by alkylation of alkynyl anions. Secondly, the same alkyne can be used to make either E or Z alkene. In some early work on sphingo sine (a constituent of cell membranes), some Swiss chemists needed to make both E and Z isomers of the naturally occurring compound. This was an easy task once they had made the alkyne.

Earlier in this chapter you were introduced to the significance of geometrically pure vinyl silanes, which can act as precursors to other alkenes. Controlled reduction of alkynyl silanes gives the corresponding vinyl silanes, with the method of reduction dictating the stereo chemistry. Lindlar hydrogenation adds a molecule of hydrogen across the alkyne in a cis fashion to produce the Z-vinyl silane. RedAl reduction of a propargylic alcohol leads instead to the E isomer.

The mechanism of the aluminium hydride reductions with LiAlH4 or RedAl involve a trans hydroalumination helped by coordination of Al to the triple bond and external nucleophilic attack. The regioselectivity of the hydroalumination is again determined by silicon: the electrophilic Al attacks the alkyne on the carbon bearing the silyl group (the ipso carbon).

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مواضيع ذات صلة

Predominantly E alkenes can be formed by stereoselective elimination reactions

Addition of nucleophiles to alkynes

Using light to make Z alkenes from E alkenes

Equilibration of alkenes

Exploiting cyclic compounds

Lewis acids promote couplings via oxonium ions

Allyl silanes are more reactive than vinyl silanes but also react through β-silyl cations

Allyl silanes as nucleophiles

Vinyl silanes offer a regio- and stereoselective route to alkenes

Aryl silanes undergo ipso substitution with electrophiles

Silicon stabilizes a positive charge on the β carbon

Alkynyl silanes are used for protection and activation