Creating novel ligand libraries from air-stable, chiral primary phosphines

Abstract

The fearsome reputation of primary phosphines, many of which are toxic and highly reactive towards atmospheric oxygen, has constrained the use of these versatile compounds in synthetic chemistry. However, a few examples of user-friendly stable primary phosphines have been reported which owe their stability to high steric encumbrance or is as yet unexplained. Recently an electronic stabilisation has allowed for the synthesis of novel MOPtype phosphorus ligands with previously inaccessible architectures that have potential applications in homogeneous asymmetric catalysis; an introduction into the topic is given in Chapter 1. The first air-stable chiral primary phosphines 1a,b were developed in our labs. We subsequently simplified and improved the synthetic approach to afford these and previously unreported synthons on a multigram scale, which is described in Chapter 2. Phosphiranes are highly strained heterocycles with a small sum of bond angles at the phosphorus (Σ°(P): <260). They act as ligands with interesting properties upon metal complexation due to the unusual electronics they possess as a result of the imposed ring strain; this leads to high s-character at the phosphorus and both lowered HOMO and LUMO energy levels compared to their acyclic counterparts. In Chapter 3 we report the synthesis of chiral binaphthyl-phosphirane ligands 14a,b offering high thermal and air stability, as well as the synthesis and solid state structures of their platinum(II) dichloride complexes. Initial findings for the application of the phosphiranes in the palladium catalysed asymmetric hydrosilylation of styrene are discussed. Furthermore, we were able to synthesise MOP-dimethylphosphine, MOP-bis(dimethylamino) phosphine and MOP-dimethylphosphonite ligands in one-pot reactions from 1a,b. Their peculiar structural and electronic parameters, in addition to those of MOP-phosphiranes 14a,b, are discussed in Chapter 4. The coordination chemistry of these compounds was investigated on platinum(II) and palladium(II) metals elucidating their cis/trans influences III and aryl side-on coordination respectively. We also carried out comparative studies in the allylic alkylation of (rac)-(E)-1,3-diphenylallyl acetate and the hydrosilylation of styrene, utilising palladium complexes of those MOP-type ligands as asymmetric catalysts. In Chapter 5 we report the efficient synthesis of novel MOP-phosphonite hybrid ligands 33a,b and 34a,b which incorporate two binaphthyl groups around the single phosphonite P-donor. We present their methallylpalladium complexes, which were studied in detail both in the solid-state and in solution. The palladium catalysed asymmetric hydrosilylation of styrene was again carried out and the results analysed in view of the molecular structure of the ligands. Furthermore, rhodium complexes of the same ligands were investigated, in particular with a view to examining their binding behaviour towards the metal. An unusual aromatic side-on binding mode was revealed by X-ray crystallography and further elucidated in solution by extended NMR experiments. Solution NMR studies also revealed a dynamic behaviour of these complexes, triggered by the hemilabile binding of the ligands towards the metal centre. Finally, we describe the synthesis of novel MOP-phosphonodichalcogenoite and MOP-phosphaalkene ligands in Chapter 6. Their corresponding gold(I) complexes were prepared and representative examples were characterised by X-ray diffraction. For the MOP-phosphonodiselenoite derivatives we also report the characteristic 77Se NMR data.