C-H versus C-O Addition: A DFT Study of the Catalytic Cleavage of the β-O-4 Ether Linkage in Lignin by Iridium and Cobalt Pincer Complexes

The potential energy surfaces of the reactions involved in the catalytic cleavage of 2-phenoxy-1-phenylethanol, a model of the beta-O-4 linkage in lignin, by ((PCP)-P-ipr)-Ir, ((PCOP)-P-ipr)-Ir, ((PCP)-P-ipr)-Co and ((PCOP)-P-ipr)-Co complexes have been studied using the M06/6-311G**/LANL2TZ level of theory. Both iridium and cobalt are found to be active towards the cleavage of the beta-O-4 linkage, with rate constants of 44.7 s(-1) and 5.1 x 10(6) s(-1), respectively. The iridium catalysts prefer the 'initial C-H addition' pathway, showing a kinetic preference of 16.8 kcal mol(-1) over the 'direct C-O insertion' pathway, while the cobalt catalysts prefer the 'direct C-O insertion' route which is kinetically favored by 15.7 kcal mol(-1) over the 'initial C-H addition' pathway. A two-state reactivity occurs along the preferred pathway for the cobalt-catalyzed reaction.

Automated summary

The potential energy surfaces of the reactions involved in the catalytic cleavage of a model compound of the beta-O-4 linkage in lignin have been studied using different complexes of iridium and cobalt catalysts. Both iridium and cobalt catalysts are active towards the cleavage of the beta-O-4 linkage, with iridium preferring the 'initial C-H addition' pathway and cobalt preferring the 'direct C-O insertion' pathway. The cobalt-catalyzed reaction goes through a two-state reactivity along the preferred pathway.