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

✍ Scribed by Leroy H. Klemm; Scott M. Reed

• Publisher: Journal of Heterocyclic Chemistry
• Year: 2001
• Tongue: English
• Weight: 120 KB
• Volume: 38
• Category: Article
• ISSN: 0022-152X
• DOI: 10.1002/jhet.5570380628

✦ Synopsis

A 1:1 geometrically oriented encounter complex between thieno[2,3-b]pyridine (1) and 4-nitrophenyldiazoacetate (2) is proposed to account for the dominant formation (ca. 64%) of the 2-isomer in the mixture of 4-nitrophenyl-1 isomers obtained previously. A mechanism involving one-electron transfer from 1 to 2 plus fragmentation of 2 -

• into 4-nitrophenyl free radical, N 2 , 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, Nalkylpyrroles, 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.