Regioselectivity in Carbonylation of Pentenoic Acid to Synthesize Cyclic Anhydrides

Abstract

Pentenoic acids (PEA) obtained from biomass‐derived γ‐valerolactone (GVL) can serve as a platform for sustainable and atom‐economic synthesis of adipic acid and its derivatives. Herein, the results are reported on the cyclocarbonylation reaction of PEA to synthesize five‐ (5 a), six‐ (6 a), or seven‐membered (7 a) cyclic anhydrides, with the aim to drive the reaction towards 7 a, a potential intermediate to caprolactam. Carbonylation of 4‐PEA yielded up to 73 % cyclic anhydrides, 7 a and 6 a in ratio 79 : 21. However, from 3‐PEA the same palladium‐based catalytic system in the absence of strong acids yielded 61 % 6 a and 5 a (41 : 59), indicating the inability of the catalyst to force isomerization. Isomerization, which is essential to obtain 7 a from a mixture of PEA isomers derived from GVL, does occur in presence of strong acids; however, this led predominantly to formation of smaller rings (6 a and 5 a) from 4‐PEA. Catalytic systems containing ligands with bulky tert‐butyl substituents yielded 6 a and trace amounts of 7 a. Although 7 a can be obtained from 4‐PEA in reasonable selectivity, there still remains a challenge to selectively synthesize 7 a from a mixture of PEA isomers.