Development of a Scalable Route for a Key Benzothiazole Building Block via a Pd-Catalyzed Migita Coupling with a Nonsmelly Thiol Surrogate

Abstract

2-Methylbenzo[d]thiazole-6-carbonitrile was internally identified as an important building block and therefore, kg amounts of it needed to be urgently supplied. As the desired benzothiazole was only available in small quantities for a high price (∼1000 USD/g), a robust and scalable route needed to be rapidly developed. The key to success was the use of a 2-iodophenyl N-acetamide precursor to construct the 2-methylbenzo[d]thiazole core via a Pd-catalyzed Migita coupling followed by subsequent intramolecular condensation of the intermediate thiophenol onto the N-acetyl group. To avoid the use of malodorous thiols on scale, 2-ethylhexyl 3-mercaptopropionate was used as a nonsmelly, inexpensive thiol surrogate. The Migita coupling was extensively optimized and allowed the reaction to be performed in a dose-controlled manner with regard to the addition of the thiol surrogate at 40 °C, by using only 0.1 mol % Pd2dba3. The subsequent intramolecular cyclization step was promoted by the one-pot addition of DBU. After aqueous workup and crystallization, 2-methylbenzo[d]thiazole-6-carbonitrile was obtained in 81% yield and excellent purity (99.7% a/a) on a 2.0 kg scale.