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.