PALLADIUM CATALYSED CROSS COUPLING REACTIONS
To prepare a diarylalkynefrom an
Transition metal catalysis; preparative vacuum sublimation for the isolation of the product.
Transition metal mediated reactions are becoming increasingly important in organic synthesis as
organic chemists become familiar with the myriad of organometallic mechanisms involved in these
Palladium is by far the most developed and used as far as application to organic synthesis is
concerned2and indeed homogeneous palladium catalysts are used in the commercial production of
acetaldehyde, cinnamates and other products. Palladium undergoes many reactions with organic
systems and, apart from the well known hydrogenation and oxidation (Wacker and other2)
processes, it acts as a catalyst for many key C—C bond forming reactions.
Three areas are particularly important: the reaction ofπ-allyl-(η3-allyl)palladium cations with
nucleophiles, the cross coupling of organometallic reagents (such as organotin, organoboron and
organozinc reagents) with organic halides, and the reaction of organic halides (usually aryl/alkenyl
halides) with olefins - the Heck reaction. The first of these was extensively developed by Trost,3
the second by Kumada4and later by Stille,5Suzuki6and others and the last by Heck himself.7The
catalytically active component in these reactions is frequently a Pd(0) species. However, the added
material is often a Pd(II) salt as in these cases the Pd(0) is generated in situby reduction with the
organometal reagent, solvent or some other species in the reaction mixture.
A full account of this fascinating and widely used area of chemistry, including the generally
accepted mechanisms, can be culled from the references cited (1–7). It is important to note that the
ready oxidative addition of the aryl halides and triflates to the Pd(0) species reverses the
conventional conceptsof reactivity in aryl systems: in nucleophilic displacement processes, as
normally encountered, the alkyl halides are the more reactive. In the palladium catalysed reactions,
the alkyl halides are the unreactive species.
In this experiment, 4-nitrobromobenzene is cross coupled with phenylacetylene to give the mixed
diarylalkyne. The product is conveniently purified by vacuum sublimation, a clean and much