Pyrroles with oxidising agents

Electron-rich heterocycles can also be coupled with olefins in the presence of a suitable palladium(II) catalyst. The oxidative coupling requires the use of a stoichiometric amount of palladium however, unless a suitable oxidising agent is added to the reaction. In an early example N-sulphonylated pyrrole was reacted with 1,4-naphthoquinone in the presence of an equimolar amount of palladium acetate to give the coupled product in good yield (6.92.).124... 

Simple pyrroles are generally easily attacked by chemical oxidising agents, frequently with complete breakdown. When the ring does survive, maleimide derivatives are the commonest products, even when there was originally a 2- or 5-alkyl substituent. This kind of oxidative degradation played an important part in early porphyrin structure determination, in which chromium trioxide in aque-... 

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