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

A variety of binuclear rhodium(i) complexes featuring two bridging dimethylphosphinate ligands ((CH)PO) have been prepared and tested in the alkoxycarbonylation of aromatic C-H bonds. The complex [Rh(μ-κO,O'-(CH)PO)(cod)] has been prepared by a reaction of [Rh(μ-MeO)(cod)] with 2 equivalents of dimethylphosphinic acid. Binuclear complexes [Rh(μ-κO,O'-(CH)PO)(CO)L] (L = PPh, P(OMe)Ph and P(OPh)) were obtained by carbonylation of the related mononuclear complexes [Rh(κO-(CH)PO)(cod)(L)], which were prepared in situ by the reaction of [Rh(μ-κO,O'-(CH)PO)(cod)] with 2 equivalents of L. Conversely, if L = IPr, the reaction of [Rh(μ-κO,O'-(CH)PO)(CO)L] with carbon monoxide affords the mononuclear complex [Rh(κO-(CH)PO)(CO)IPr]. The subsequent reaction with trimethylamine N-oxide gives the corresponding binuclear complex [Rh(μ-κO,O'-(CH)PO)(CO)(IPr)] by abstraction of one of the carbonyl ligands. Complexes [Rh(μ-κO,O'-(CH)PO)(cod)] and [Rh(κO-(CH)PO)(cod)(L)] (L = IPr, PPh, P(OMe)Ph, P(OPh)) 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'-(CH)PO)(CO)] and [Rh(μ-Cl)(CO)], generated in situ from [Rh(μ-κO,O'-(CH)PO)(cod)] and [Rh(μ-Cl)(cod)], respectively, are the most active catalysts tested in this work.