102 Furans and Thiophenes
Palladium(0)-catalysed reactions
Boron and tin derivatives are known at - and -positions of furan and thiophene and are extensively used in cross-
couplings and other reactions, but the -boronic acids have a tendency to protodeboronation under strongly basic
conditions. Halides at either position of either heterocycle have also been widely employed in this context. The small
selection of such carbon–carbon bonding reactions shown illustrates the wide applicability of palladium(0)-catalysed
processes in thiophene and furan chemistry.
Oxidation and reduction
Exhaustive reduction and S-oxidation of thiophenes
There are two aspects of thiophene chemistry that are not mirrored by furan or pyrrole. Firstly, Raney nickel and
hydrogen bring about complete reduction, including removal of the sulfur. Thus thiophene can be used as a frame-
work to build up a branched carbon skeleton with a fi nal removal of the sulfur and reduction. For example, Friedel–
Crafts acylation of 2,5-dimethylthiophene (only -positions available) using succinic anhydride, followed by
Wolff–Kischner reductive removal of the resulting ketone and fi nally Raney nickel hydrogenation/ hydrogenolysis,
produces 5-ethyloctanoic acid.