Donor‐Stabilized Phosphenium Adducts as New Efficient and Immobilizing Ligands in Palladium‐Catalyzed Alkynylation and Platinum‐Catalyzed Hydrogenation in Ionic Liquids

Abstract

Abstract:The straightforward synthesis of a new donor-stabilized phosphenium ligand 3d by addition of bromodifurylphosphine to 1,3-dimethylimidazolium-2-carboxylate 1 is described. The obtained ligand exhibits a very strong p-acceptor character, comparable to that of triphenyl phosphite [PA C H T U N G T R E N N U N G (OPh) 3 ] or of tris-halogenophosphines, with a n CO (A 1 ) at 2087 cm À1for its nickel tricarbonyl complex. This ligand, as well as the related 3a which was obtained from chloroA C H T U N G T R E N N U N G diphenylphosphine, were tested in palladiumcatalyzed aryl alkynylation and in the platinum-catalyzed selective hydrogenation of chloronitrobenzenes, both in an ionic liquid phase. In C À C bond cross-coupling we observed that the increase of the p-acceptor character in ligand 3d, due to the introduction of an additional electron-withdrawing group, provides a very efficient catalyst in the alkynylation reaction of aryl bromides with phenylacetylene, including the deactivated 4-bromoanisole or the sterically hindered 2-bromonaphthalene. The catalytic activity decreases with recycling due to the sensitiveness of ligands to protonation in the ionic phase.Conversely, a multiple recycling of the metal/ligand system in non-acidic media was achieved from platinum-catalyzed hydrogenation of m-chloronitrobenzene. The catalytic results obtained by employing the complex of platiA C H T U N G T R E N N U N G num(II) chloride with 3a [transPtCl 2 (3a) 2 ] in comparison with the non-ionic related trans-tris(triphenylphosphine)platinum dichloride [trans-PtCl 2 A C H T U N G T R E N N U N G (PPh 3 ) 2 ] complex clearly indicate that the simultaneous existence of a strong p-acceptor character and a positive charge within the ligand 3a significantly increases the life-time of the platinum catalyst. The selectivity of the reaction is also improved by decreasing the undesirable formation of dehalogenation products. This cationic platinum complex trans-PtCl 2 (3a) 2 is the first example of a highly selective catalyst for hydrogenation of chloronitroarenes immobilized in an ionic liquid phase. The system was recycled six times without noticeable metal leaching in the organic phase, and no loss of activity.

Steric, Electronic, and Secondary Effects on the Coordination Chemistry of Ionic Phosphine Ligands and the Catalytic Behavior of Their Metal Complexes

“…These ligands have proven to be highly effective in Pd-catalyzed aryl alkynylation reactions as well as in Pt-catalyzed hydrogenation reactions in ionic liquids. [58] Furthermore, these ligands display remarkable coordination properties (see section on ligand substitution reactions later).…”

“…[53] Andrieu et al reported on cationic imidazolium-2-phosphines 19 a-d, which can be described as phosphenium cations stabilized by a N-heterocyclic carbene donor base. [57,58] The lone pair on the phosphorus atom is not delocalised into the imidazolium fragment and thus remains available for coordination to a metal center. By measurement of the CO stretching frequency in the corresponding complexes [Ni(CO) 3 L] (L = 19 a-d), it was established that this remarkable class of phosphine ligands act as very weakly s-donat- ing and/or very strongly p-accepting ligands (Table 2, entries 39-42).…”

The Recent Development of Phosphine Ligands Derived from 2-Phosphino-Substituted Heterocycles and Their Applications in Palladium-Catalyzed Cross-Coupling Reactions

“…38 Such ligands L20a-c 39 and L20d could be obtained directly from 1,3-dimethylimidazolium-2-carboxylate (Schemes 31 and 32, respectively). …”

Recent Advances in Imidazoliumyl‐Substituted Phosphorus Compounds

“…Another approacht oi midazoliumyl-substituted P III compounds relies on the use of the imidazolium 2-carboxylate 83 as NHC precursor. [68,69] The reaction of 83 with one equivalent of R 2 PCl (R = Ph, iPr,C y) affords the chloride salts of cations 84 a-c + under release of CO 2 (Scheme 24). The anion is exchanged to PF 6 À by treatmentw ithK [PF 6 ].…”