Pyridines 4- nitro-, nucleophilic substitution

A recent communication is based on the nucleophilic substitution of N-silylated iminophosphoranes (Scheme 8), where activated chloro- (12) and nitro heterocycles (13) (triazines, pyrazines, and pyridines) are transformed into monosubstituted iminophosphoranes (14). Of special advantage are the mild reaction conditions and the preferential formation of monosubstituted... 

The 4- and 6-positions of pyrrolo[2,3-3]pyridines can be substituted via palladium-catalyzed cross-coupling reactions with the 4- or 6-halo-substituted derivatives (Scheme 3) <2001SL609>. Nucleophilic displacement of the 4-substituent of 6-chloro-4-nitro- and 4,6-dichloro-pyrrolo[2,3-/ ]pyridines takes place with phenols. Protection of the pyrrole nitrogen with a /3-trimethylsilylethoxymethyl (SEM) group affords good yields of the aryl ethers (Equation 3) <2006TL2069>. 

Trudell and co-workers reported the N-arylation with 2-chloro-3-nitropyridine of [l,2,3]triazolines fused onto [4,5-dpyridazine (Scheme 5) and [4,5-,yjpyrimidine (Scheme 6) <2000JHC1597>. Substitution at the 2-position of the pyridine ring was confirmed by X-ray crystallographic analysis of products 8 and 9. Similarly, [l,2,3]triazolo[4,5-,yjpyrimidine gave 10 in low yield, after nucleophilic substitution of the chloro substituent activated by the nitro group in 2-chloronitrobenzene (Scheme 6). 

In direct contrast with electron-deficient heterocycles like pyridines and the diazines, the five-membered systems do not undergo nucleophilic substitutions, except in simations (especially in furan and thiophene chemistry) where halide is situated ortho or para to a nitro group. In the maifipulation of the five-membered heterocycles, extensive use has been made of the various paUadium(0)-catalysed couplings regimes, as illustrated with one example (see 4.2 for a detailed discussion). 

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