INTRAMOLECULAR CYCLIZATIONS AND RING-CLEAVAGE REACTIONS OF 5-MEMBERED HETEROARYLNITRENES AND THEIR PRECURSORS

The formal generation of nitrenes at the alpha-position of aromatic five-membered heterocyclic systems almost invariably results in the opening of the original heteroaromatic ring leading to the formation of a nitrilic hetero-1,3-diene product. Similarly, the formal generation of nitrenes at the beta-position of certain azole systems gives rise to ring-cleavage resulting in the formation of two fragments containing two or three of the original ring atoms. However, it is in general by no means clear whether the above ring-cleavage reactions actually proceed from initially formed nitrenes or directly from their precursors. On the other hand, normal arylnitrene behaviour can be exhibited by nitrenes generated at the beta-position of the thiophene, furan, and selenophene rings. In fact, these latter beta-nitrenes, in the presence of a suitable ortho-substituent, appear to undergo intramolecular cyclizations in a fashion comparable to that of the similarly substituted phenylnitrenes. In this article we survey and discuss the known instances of the title reactions of five-membered heteroarylnitrenes and their precursors which have appeared in the literature up to the end of 1990.