Dimethylphosphinate bridged binuclear Rh(i) catalysts for the alkoxycarbonylation of aromatic C–H bonds

Abstract

A variety of binuclear rhodium(I) complexes featuring two bridging dimethylphosphinate ligands ((CH₃)₂PO₂⁻) have been prepared and tested in the alkoxycarbonylation of aromatic C–H bonds. The complex [Rh(μ-κO,O′-(CH₃)₂PO₂)(cod)]₂ has been prepared by a reaction of [Rh(μ-MeO)(cod)]₂ with 2 equivalents of dimethylphosphinic acid. Binuclear complexes [Rh(μ-κO,O′-(CH₃)₂PO₂)(CO)L]₂ (L = PPh₃, P(OMe)Ph₂ and P(OPh)₃) were obtained by carbonylation of the related mononuclear complexes [Rh(κO-(CH₃)₂PO₂)(cod)(L)], which were prepared in situ by the reaction of [Rh(μ-κO,O′-(CH₃)₂PO₂)(cod)]₂ with 2 equivalents of L. Conversely, if L = IPr, the reaction of [Rh(μ-κO,O′-(CH₃)₂PO₂)(CO)L]₂ with carbon monoxide affords the mononuclear complex [Rh(κO-(CH₃)₂PO₂)(CO)₂IPr]. The subsequent reaction with trimethylamine N-oxide gives the corresponding binuclear complex [Rh(μ-κO,O′-(CH₃)₂PO₂)(CO)(IPr)]₂ by abstraction of one of the carbonyl ligands. Complexes [Rh(μ-κO,O′-(CH₃)₂PO₂)(cod)]₂ and [Rh(κO-(CH₃)₂PO₂)(cod)(L)] (L = IPr, PPh₃, P(OMe)Ph₂, P(OPh)₃) are active precatalysts in the alkoxycarbonylation of C–H bonds, with the ligand system playing a key role in the catalytic activity. The complexes that feature more labile Rh–L bonds give rise to better catalysts, probably due to the more straightforward substitution of L by a second carbonyl ligand, since a more electrophilic carbonyl carbon atom is more susceptible toward aryl migration. In fact, complexes [Rh(μ-κO,O′-(CH₃)₂PO₂)(CO)₂]₂ and [Rh(μ-Cl)(CO)₂]₂, generated in situ from [Rh(μ-κO,O′-(CH₃)₂PO₂)(cod)]₂ and [Rh(μ-Cl)(cod)₂]₂, respectively, are the most active catalysts tested in this work.