In this project, we aim at the development and application of novel ylidic, carbon-centred Lewis bases. Special focus has so far been set on the class of metalated ylides as well as bisylidic compounds. While simple ylides (compounds of type On(+)-C(-)R2 with On being an onium moiety) have been known for more than one century and applied in many stoichiometric as well as catalytic transformations – above all Wittig-type reactions – their α-metalated congeners, the so-called yldiides (compounds of type [On(+)-C(2-)R]-), have received only little attention, although they may be used as potent σ- as well as π-donor ligands. This lack of investigations is mainly due to the high reactivity of these species, which relates to the two lone pairs of electrons and the high negative charge at the ylidic carbon atom. In the first part of this project, we addressed the isolation of yldiides in order to study their general reactivity and develop further applications.
By means of a careful molecular design, we succeeded in the isolation of a couple of stabilized systems, which are accessible in gram-scale and thus – for the first time – applicable as isolated reagents. Detailed computational studies on the electronics and bonding situation in metalated ylides in comparison to neutral bisylidic compounds confirmed the special properties of these compounds and allowed for a more profound understanding of these ligand systems, which will help to experimentally tailor their properties for further applications. First reactivity studies already confirmed the excellent applicability of metalated ylides for ylide-functionalization. Owing to the strong nucleophilicity the ylide moiety can be transferred to many different main group elements and thus be used for the manipulation of their properties. This strategy allowed us to isolate the first ylide-substituted boron cation, in which we made use of the strong donor capacity of the yldiide to stabilize a highly electron-deficient compound with novel reactivity.
Besides being excellent reagents for the stabilization of reactive compounds, the yldiides also revealed to be ideal for a facile preparation of ylide-functionalized phosphines (YPhos ligands). Phosphines are amongst the most important ligands in homogenous catalysis. The design of new phosphines has been decisive for crucial developments in homogenous catalysis and contributed to many new synthetic methodologies that nowadays allow for the constructions of complex molecules which are often part in pharmaceuticals or agrochemicals. In a sub-project of this ERC-project, we further developed ylide-functionalized phosphines into a class of ligands with unusual and highly tunable electronic and steric properties which turned out to be ideal for a variety of different transition metal catalyzed reactions. For example, we could demonstrate that due to their strong donor properties, YPhos ligands are excellent ligands for gold-catalyzed hydroamination reactions and palladium-catalyzed C-N coupling reactions. Here, the YPhos-based catalysts were able to operate under unusually mild reactions conditions and showed high activities at low catalyst loadings. Thereby, they could also compete with or even surpass the activity of catalysts based on established ligand systems.
Overall, we were so far able to demonstrate that electronic stabilization of ylidic ligands allows for the allocation of carbon bases with unusual but valuable properties, which can be used in quite different directions, such as for fundamental studies on the stabilization of reactive species but also more applied areas such as in homogenous catalysis. In the second part of our research program we will continue these studies to further broaden the applications of our reagents.
