asymmetric synthesis and catalysis
Chiral indenylmetal complexes for asymmetric catalysis
Asymmetric catalysis is one of the most active areas of current research in organic chemistry. Planar-chiral cyclopentadienyl metal complexes feature amongst the most successful asymmetric catalysts, but they are often very difficult to prepare in enantiomerically pure form. We have devised a new type of chiral cyclopentadienyl ligand, incorporating axial chirality (caused by hindered rotation about a C–C single bond), which allows the direct preparation of planar-chiral metal complexes in enantiomerically pure form. We have recently prepared a wide range of axially chiral chelating indene ligands, such as 1, and are currently exploring the preparation of a number of transition metal complexes 2 using these ligands (see Scheme 1). In addition to our ability to prepare these complexes in enantiomerically pure form, we have also observed that the constrained chelation afforded by our ligand design can in some cases generate metal complexes having unusual coordination environments, and therefore potentially unique reactivity (e.g the ruthenium (II) complex shown in Figure 1). Most recently we have prepared a series of rhodium (III) half-sandwich complexes incorporating planar-chiral indenyl ligands with either thioether or sulfoxide pendant donor groups. The complexes are all able to be readily prepared in enantiomerically pure form, and as single diastereoisomers in the case of the sulfoxide-appended ligands (e.g. the complex shown in Figure 2). . In this project, we will be investigating potential applications of these and related complexes (in particular ruthenium (II) complexes) in a range of catalytic asymmetric transformations.
Electron-rich monocyclic triarylalkoxyhydridophosphoranes
In the course of the synthesis of an indenyl-phosphine ligand we isolated, not the expected indenol-phosphine intermediate, but the P(V) closed-chain tautomer 1 (see Scheme 2 and X-ray structure of one of the isomers shown in Figure 3)). There has been only one other report of such an electron-rich monocyclic triarylhydridophosphorane, compound 2, described by Goldfuss et al. in 2001. These authors carried out calculations [ONIOM(B3LYP/6-31G*:UFF)] that indicated that the open-chain P(III) tautomer of 2 was >19 kcal/mol more stable than the closed-chain P(V) tautomer. Accordingly, they proposed that compound 2 was “metastable”. We have carried out calculations without use of the ONIOM method [B3LYP/6-31G(d)] and this indicates that the P(III) open-chain tautomer of 2 is only favoured by 0.25 kcal/mol. We have also synthesised the phosphine 3 (R = Me) and observed a small amount (ca. 10%) of the closed-chain P(V) tautomer, which 31P DNMR studies have shown is in equilibrium with the P(III) form. In this project we will be synthesising a series of compounds 3, with varying R groups, in order to probe both experimentally and theoretically the factors responsible for the position of this equilibrium, as well as examining the metal coordination properties of these hindered alkoxy-phosphine ligands.
For further information, please contact:
School of Chemistry
University of Sydney NSW 2006
Phone: +61 2 9351 4049