New Heteroaromatic Azo Compounds Based on Pyridine, Isoxazole, and Benzothiazole for Efficient and Highly Selective Amidation and Mono-N-Benzylation of Amines under Mitsunobu Conditions

4,4'-Azopyridine (2c) is used in conjunction with triphenylphosphine for the efficient conversion of carboxylic acids into amides via Mitsunobu reaction with primary and secondary aliphatic and aromatic amines. The highly selective amidation of only primary aromatic amines with new heterogeneous azo compounds based on benzothiazole 2d and isoxazole 2e is also described. These azo compounds 2c-2e can also be applied for selective mono-N-benzylation of primary aromatic amines. The solid side product heteroaromatic hydrazines obtained under the developed Mitsunobu conditions are easily separated by simple filtration and can be reoxidized to azo compounds for further use.