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 M0 (M = Ni,
Pd, Pt), M′I (M′ = Co, Rh, Ir), or MII centers was investigated to gain mechanistic insight into
intramolecular aryl–ether bond cleavage in structurally related
metal complexes. RhI converts the aryl–methyl ether
moiety to an aryl C–H bond. This is similar to reactivity previously
observed at Ni0 that involves C–O oxidative addition,
β-H elimination liberating CH2O, reductive elimination
of an aryl C–H bond, and decarbonylation of CH2O.
IrI leads to unselective aryl and alkyl C–O bond
activation. In the presence of excess CO, RhI and IrI display a shift in selectivity and reactivity and cleave
the alkyl C–O bond. CoI does not perform C–O
cleavage. Alkyl C–O bond activation was observed with MII–halide complexes with loss of MeCl via a Lewis acid–base
mechanism. Pd0 and Pt0 cleave selectively the
O–Me bond via oxidative addition. With a diaryl ether moiety,
Pd0 and Pt0 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
Ni0 and RhI. Pd0 and Pt0 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.