Steric effects in nucleophilic substitution of five-membered rings. Influence of the nature of the leaving group on secondary steric effects

Kinetic data from the substitution of piperidine into some 2-substituted 3,5-dinitro-4-methyl-(IIa–d) and 3,5-dinitro-thiophens (Ia–d) show the occurrence of a small secondary steric effect (k(Id)/k(IId)= 8) only when the leaving group is SO2Ph. This effect seems to depend on the interaction of the groups at C-2, –3, and –4 of the thiophen ring, as the results obtained for other thiophens indicate.