L6 Reduction of C-C multiple bonds

Reduction of alkynes to alkenes - 2 methods

Note how the catalyst favours Z - adsorption and the dissolving metal reduction favours E due to sterics.

Stereospecific/selective control of reduction of isolated alkenes

Hydroboration of alkenes selectivity features

Regioselective - B adds to less hindered end - producing the anti-Markovnikov product

Stereospecific - always syn- addition as concerted reaction

What are other things you can do with the boron structure (add H or NH₂

Some alkene structures have similar sized R groups and hence the regioselectivity is poor. How can we improve it

By making very hindered reagents. Notice the increase in selectivity is huge.

Hydroboration of alkynes to form carbonyls, terminal alkenes and aromatic alkenes

Partial reduction of unsubstituted aromatics: Birch reduction

Note the lack of conjugation (makes it less stable) - this is the kinetic product

Intermediate is not basic enough to deprotonate NH₃ so an alcohol is added to provide protons

How does the presence of an EDG / EWG affect the rate of the Birch reduction

EWG - fast as it lowers the LUMO energy

EDG - slow as it raises energy of LUMO pi*

Also moves the position of the -ve charge away form the EDG

Adding an electrophile to the same carbon

The EWG forces the negative charge onto this carbon.

The charge ends up next to the group as the negative charge at the para- position moves as the proton is more acidic on carbon 1 so intramolecular proton transfer occurs.

Using 2 FGs to direct the charge

Both FGs direct the negative charge to the same spot. This reaction has no added alcohol as the proton comes from the carboxylic acid. The final product is formed via a multitude of steps.

Possible outcomes of reduction of this

First one is easily done by LiAlH₄ or catalytic hydrogenation.

Selective reduction of the alkene

Using dissolving metal reduction.

Selective reduction of the carbonyl - Luche reduction

NaBH₄ and an alcohol produces a fairly even mix of 1,2 and 1,4 addition - however the addition of CeCl₃ pushes the major product massively in the direction of the 1,2 addition, leaving the alkene intact