Hypophosphorous acid as a precursor to novel phosphine ligands for catalysis

An industrially relevant new route to functionalised phosphorus ligands for use in catalysis is under development.

Industrially organophosphorus compounds are important for a wide range of applications including as pharmaceuticals, pesticides, plasticisers, flame-retardants, corrosion inhibitors, in catalysis and as synthetic intermediates. Incorporation of multiply functionalized groups into organophosphorus compounds can be problematic; and thus relatively few are commercially available. Furthermore, the synthetic routes currently employed originate from phosphorus trichloride and use aggressive reagents (organo-lithium or magnesium reagents) which mean that these procedures are not very amenable to the synthesis of multifunctionalised phosphines.

The advantages of employing hypophosphorous acid as a starting material for the bulk synthesis of organophosphorus chemicals has been demonstrated in the synthesis of highly functionalised aryl and alkyl H-phosphinic acids (Montchamp, J.-L. Phosphorus, Sulfur, Silicon. 2013, 188, 66-75). We have recently developed the use of these H-phosphinic acids as starting materials for a new high yield synthesis of functionalized aryl- and alkyl-phosphines. Our process uses milder reagents than those used in previous syntheses. Initial tests indicate that the route can successfully be applied, on a preparative scale, to some sensitive functionalised substituents.

The process has been successfully optimised, but further functional group tolerance needs to be explored. In addition to testing a wide range of functional groups, we will tune the process to make it applicable to functionalised alkyl substrates. Other advantages of our method are that the starting materials are cheap and readily available, and products are easily separated from the reaction mixture. Our H-phosphinic acid process therefore appears attractive from a bulk industrial scale perspective since it results in a greener process with less waste co-products generated. With a small library of variously multifunctionalised phosphines in hand, extensions of the proposed project can be envisaged. The products comprising the library would be used as starting materials for a range of further elaborated phosphorus compounds of potential interest to fine chemical producers. Families of such chemicals to be targeted are phosphine, phosphonite and phosphonate ligands for application in catalysis.

Ultimately, there is scope for the synthesis of enantiopure for use in asymmetric catalysis. Applications in enantioselective catalytic synthesis of drug candidates is an important area (e.g. Nobel Prizes, Chemistry, 2001 and 2010). In additional collaborative work we have synthesized phosphinic acids and phosphine oxides, functionalised with donor groups. The coordination of these compounds to lanthanides and transition metals can be investigated.