Substitution reactions of nitrothiophenes. 6. Disparate mechanisms for substitution reactions at neopentyl carbons bearing 4- and 5-nitrothienyl groups

The reaction of the (5-nitro-2-thienyl)- and (4-nitro-2-thienyl) neopentyl chlorides (9 and 10) with ptoluenesulfmate, azide, and p-toluenethiolate ions proceeds smoothly and in high yield (70-95%) under mild conditions (20 °C, Me2SO) to give the sulfones 14 and 15, the azides 16 and 17, and the sulfides 18 and 19, respectively. The mechanisms of these substitutions are quite different, however. The substitutions in the 5-nitro series take place by the SRn1 mechanism, whereas those in the 4-nitro series take place by the ionic SN (aeae) process, which involves initial attack of a nucleophile at the 5-position of the thiophene ring