Free radical 4‐nitrophenylation of thieno[2,3‐b]pyridine. Part 2: Isolation and structural studies of three samples of mono (4‐nitrophenyl) products; relative yields of the five possible isomers

From the crude mixtures of isomeric 4‐nitrophenylthieno2,3‐b]pyridines ( 3 ) previously reported 1] were isolated three analytically pure samples, viz. the 2‐isomer (yellow needles, mp 258°, 3a ), the 6‐isomer (red prisms, mp 182°, 3e ), and a ternary mixture of the 2‐, 3‐, and 4‐isomers (orange needles, mp 213°, 3a:3b:3c = 1.3:1.0:0.5). The 258° compound was identified as either 3a or 3b by its ¹H nmr spectrum and definitively as the former by its x‐ray crystallographic analysis. The isomeric identities of the 182° and 213° samples were established from their ¹H nmr spectra only. No 5‐isomer ( 3d ) was identified. Semi‐quantitatively, relative isomeric yields fit the pattern 2‐ (64%)>>6‐ (14%)≥3‐ (12%)>4‐ (6%)≥5‐(≤4%). Crystallographic data for 3a are presented.