Nucleophilic substitution in five-membered rings. Influence of steric interactions in the reaction area on activation by a nitro-group

The reactivity of some 2-L-3-nitro-4-R-5-X-thiophens (L = Br or SO2Ph; R = H or Me; X = H or NO2) with various nucleophiles (sodium methoxide and benzenethiolate, aliphatic, cyclic, and aromatic amines) has been measured in methanol. The results obtained have been interpreted in the light of the electronic and steric effects of the nucleophiles. The KH/KMe values calculated confirm the data previously obtained with piperidine, i.e., absence of a secondary steric effect (s.s.e). and presence of a small s.s.e. for bromine and phenylsulphonyl, respectively, as leaving groups. The special behaviour of N-methylaniline is discussed.