Free‐radical 4‐nitrophenylation of thieno[2,3‐b]pyridine. Part 3: Consideration of mechanistic and selectivity factors involved in the substitution process

A 1:1 geometrically oriented encounter complex between thieno[2,3‐b]pyridine (1) and 4‐nitrophenyldia‐zoacetate (2) is proposed to account for the dominant formation (ca. 64%) of the 2‐isomer in the mixture of 4‐nitrophenyl‐l isomers obtained previously. A mechanism involving one‐electron transfer from 1 to 2 plus fragmentation of 2· into 4‐nitrophenyl free radical, N₂, and acetate ion is invoked. Formation of other isomers is discussed. It is noted that there is a close correlation between orientational rules plus mechanisms of reaction for numerous free‐radical substitutions (S_R) with S_N reactions of alkyllithiums on furan, thiophene, N‐alkylpyrroles, pyridine, and their condensed aromatic molecules, including 1, as substrates. Also isomeric selectivities for S_E, S_N, and S_R substitutions into 1 were shown to be qualitatively consistent with one another. While S_E reactions occur largely at position 3 and then at 2, S_N and S_R reactions occur either at 2 or 6. Selectivity for positions 4 or 5 is small or zero.