Dual Iron-Catalyzed Selective Aerobic Lactonization of 1,6-Diols

Selectivity control is a very challenging topic in catalytic oxidations of alcohols using oxygen as an oxidant. Of particular interest is the catalytic aerobic lactonization of diols; due to the selectivity issue referring to the two hydroxy groups, the reaction afforded a mixture of isomeric products. Herein, we report an efficient catalytic recipe of easily available earth-abundant metal catalysts Fe(NO3)3·9H2O and FeCl3 together with TEMPO for the highly selective aerobic oxidative lactonization of 1,6-diols, affording a variety of ε-caprolactones. The reaction was conducted under mild conditions with an O2 balloon or under ambient air enjoying high chemo- and regioselectivity for the oxidative lactonization of non-symmetric diols by the subtle recognition of the steric environment of the two hydroxy groups. Furthermore, reduction and selective aerobic oxidative lactonization constitute a formal carbonyl walking over the ε-caprolactones. Mechanistic experiments reveal a process with a hemiacetal as the intermediate. This protocol unlocks an efficient pathway to derive monomers for polycaprolactones.