Report
Teaser, summary, work performed and final results
Periodic Reporting for period 2 - YlideLigands (Tailoring Ylidic Compounds as Ligands for Organometallic Chemistry)
Teaser
Lewis bases are a fundamental class of compounds that are of utmost importance in almost any chemical transformation in large-scale chemical processes as well as in nature. According to the HSAB concept, the nature of Lewis bases determines important properties such as the...
Summary
Lewis bases are a fundamental class of compounds that are of utmost importance in almost any chemical transformation in large-scale chemical processes as well as in nature. According to the HSAB concept, the nature of Lewis bases determines important properties such as the stability or solubility of compounds or the selectivity of reactions. Thus, Lewis bases are used far beyond simple acid-base pairs. In coordination chemistry they act as efficient σ-donor ligands, which crucially affect the electronics of the metal and thus its properties and reactivity. Additionally, bulky Lewis bases as part of Frustrated Lewis Pairs are applicable in bond activation reactions including catalytic transformations. Typical Lewis bases are neutral donor compounds with a free pair of electrons, such as amines or phosphines. In contrast, carbon-centered Lewis bases such as carbenes have long been neglected due to their usually high reactivity and sensitivity. Yet, the last decades have revealed a revolution in this context. Carbenes have proven to be particularly powerful reagents not only as ligands in metal complexes, but also in organocatalysis and bond activation chemistry. The development of new potent donor bases is however important and necessary to broaden the library of substituents and ligands, which are needed to further manipulate and tune the properties and reactivities of compounds and metal complexes and to open new possibilities for chemical synthesis and catalysis.
This project takes aim at the development and application of novel ylidic, carbon-centred Lewis bases. By means of a smart molecular design, systems with unusual electronic properties and donor capacities will be prepared and their reactivity towards main group element compounds and transition metal complexes will be explored. For example, applications of neutral bisylidic compounds as well as anionic metalated ylides will be studied. Employing experimental and computational methods a fundamental understanding of the electronic structure depending on the actual ligand design and its influence on the donor capacity will be provided. This will allow the manipulation and tailoring of the properties and reactivities of the bases and thus open applications such as their use in bond activation reactions, as strong donors for the stabilization of reactive compounds as well as their application as electronically flexible ligands in catalytically active metal complexes.
Work performed
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.
Final results
With the application of ylide moieties as functional groups in main group chemistry we could certainly introduce a new and powerful tool for tailoring and manipulating electronic properties beyond the usual substitution effects of classical functional groups. This methodology was already successfully applied in the stabilization of reactive species and the design of a novel class of phosphine ligands with ideal properties for the generation of high-performance homogenous catalysts. The application of ylide-substituents in both directions will further be explored in the second part of this project. Thereby, we expect to set new landmarks in homogenous catalysis and to isolate and apply compounds with so far unprecedented properties and reactivities.
Website & more info
More info: https://www.ruhr-uni-bochum.de/ac2/research/ylide.html.en.