Aryl Ether Cleavage by Group 9 and 10 Transition Metals: Stoichiometric Studies of Selectivity and Mechanism

The reactivity of terphenyl diphosphines bearing aryl methyl ether or aryl-aryl ether moieties with M-0 (M = Ni, Pd, Pt), M'(I) (M' = Co, Rh, Ir), or M-II centers was investigated to gain mechanistic insight into intramolecular aryl ether bond cleavage in structurally related metal complexes. Rh-I converts the aryl methyl ether moiety to an aryl C-H bond. This is similar to reactivity previously observed at Ni-0 that involves C-O oxidative addition, beta-H elimination liberating CH2O, reductive elimination of an aryl C-H bond, and decarbonylation of CH2O. Ir-I leads to unselective aryl and alkyl C-O bond activation. In the presence of excess CO, Rh-I and Ir-I display a shift in selectivity and reactivity and cleave the alkyl C-O bond. Co-I does not perform C-O cleavage. Alkyl C-O bond activation was observed with M-II-halide complexes with loss of MeCl via a Lewis acid-base mechanism. Pd-0 and Pt-0 cleave selectively the O-Me bond via oxidative addition. With a diaryl ether moiety, Pd-0 and Pt-0 are found to be capable of performing aryl C-O bond activation. Various levels of interactions between the central arene and the metal center were observed, and these were correlated with trends in bond activation. Overall, selective cleavage of the stronger aryl ether C-O bond was observed only with Ni-0 and Rh-I. Pd-0 and Pt-0 can perform aryl ether C-O cleavage, but if available, they will cleave the weaker O-Me bond. This study provides insight into the relative reactivity of group 9 and 10 metal centers with aryl ether bonds and suggests future directions for designing systems for metal-catalyzed cleavage of ether C-O bonds in synthetic methodology as well as lignin deoxygenation.