Reduction of Acetylenes to Olefins

28 July 2008

The classical way of partially reducing a triple bond is hydrogenation using a Lindlar catalyst which gives good selectivity to the Z alkene but, particularly on scale in batch processes) can give considerable over-reduction to the alkane. Whilst transfer reduction, using donors such as formic acid, has been successful in most other reductions, including asymmetric reduction, it has not previously been successful with acetylenes (especially aromatic acetylenes).

It has recently been reported by the group of Elsevier (the latter was a co-author on the excellent 3-volume set “Handbook of Homogeneous Hydrogenation”, Wiley-
VCH, 2007) that palladium N-heterocyclic carbene complexes catalyse the selective reduction of triple bonds to Z-olefins by formic acid-triethylamine complex with little over-reduction (Hauwert P et al, Angew Chem Int Ed, 2008, 47, 3223). Either THF or MeCN were used as solvents with MeCN generally giving higher selectivity, though
reaction times are longer.

The reaction is less successful with electron withdrawing groups attached to the alkyne, when isomerisation of the initially formed Z isomer occurs. With acetylenic ketones, there is, surprisingly no reduction of the carbonyl group. The method of preparation of the catalyst is crucial to the success, needing a slight excess of the carbene in the scheme below. When the Pd-anhydride complex is in excess, the resultant catalyst gives more over-reduction.