Free radical 4‐nitrophenylation of thieno[2,3‐b]pyridine. Part 1: General experimental procedure and prediction of isomeric ratios of products

Thieno[2,3‐b]pyridine (1) was warmed at 45 ± 7° with diazotized 4‐nitroaniline ( 2 ) (molar ratio 1:2 = 3.6:1) in buffered (sodium acetate‐acetic acid) aqueous solution until gas evolution ceased. The reaction mixture was separated into these fractions: (a) water‐soluble (discarded), (b) acetone‐soluble (tars), (c) ether‐soluble, and (d) ether‐ and chloroform‐soluble, but acetone‐insoluble (rust‐colored solids, Y , 12% yield as 4‐nitrophenyl‐ 1 isomers, 3 ). Fractional evaporative distillations of (b) and (c) gave recovered 1 (69%) and yellow‐red sublimates (Z, 20% yield as 3 ). Y and Z were handled separately for isolation and identification of isomers [2]. Three general methods for predicting the isomeric ratios of 3 which one should obtain are presented, viz. (1) an amalgamation of reported free‐radical phenylation results for quinoline and benzo[b]thiophene, (2) calculations of Frontier Radical Densities for positions 2‐6 only, and (3) Molecular Orbital calculations for electron densities and superdelocalizabilities for free‐radical attack at positions 1‐7, as modified by a proposed rearrangement of the attacking group from the heteroatomic N and S positions to adjacent alpha positions.