The Erker Group: Selected Research Developments

The Erker research group has become known for its contributions especially in the fields of Organometallic Chemistry and Catalysis as well as in Main Group Element Chemistry.

1. Organometallic Chemistry and Catalysis.

Professor Erker’s group has made a variety of contributions to the chemistry of organozirconium compounds and its group 4 metallocene neighbours. This includes a variety of syntheses of novel types of organometallic compounds, e.g. the (s-trans-butadiene)metallocenes or isolable planar-tetracoordinate carbon compounds, electronically stabilized by σ-donor/π-acceptor properties of zirconocene substituents. Various aldehyde zirconocene complexes derived from Zr-hydrides and carbon monoxide were prepared, some of which served as molecular Fischer-Tropsch models. The research group was very active in developing homogeneous Ziegler-Natta catalysts that were highly active and at the same time highly selective. The synthesis of new ligand frameworks and the development of novel activation procedures were systematically pursued. Detailed mechanistic studies to determine the general as well as the specific rules of operation of such catalysts represent another important area of the experimental activities of his research group at Münster. This led to 1-alkene polymerization reactions aiming at new materials. A typical example is the research on the formation of elastomeric polyolefins by means of controlling the conformational properties of group 4 metallocenes. A series of new types of "constrained geometry catalysts" have been reported that are useful for polymerization catalysis. The Erker research group developed special metallocene-borate betaine catalysts, which are active single component catalysts that do not need additional external activators. They were designed from the butadiene zirconocene derivatives and strongly electrophilic boranes. These systems made it possible to observe the organometallic Ziegler-Natta polymerization catalyst “at work” and to monitor and characterize the reaction profile of the essential olefin insertion step on relevant metallocene catalyst systems using special analytical methods. 

Some of the organometallic research work opened pathways for developments in Main Group Element Chemistry. The Erker group has significantly contributed to the chemistry of phosphorus ylide reactions with zirconocene complexes. The research group succeeded in preparing of selenobenzophenone by “Staudinger Chalcogenation” by treating the respective phosphorus ylide with elemental selenium. In this context it was shown that the Wittig olefination reaction of thiocarbonyl compounds proceeds by betaine formation, in contrast to the normal Wittig olefination reaction of ketones and aldehydes under salt free conditions.

2. Main Group Element Chemistry: Frustrated Lewis Pairs (FLPs) and More.

In recent years, Erker's research group has focused in particular on the development of the chemistry of FLPs and has played a key role in this. They especially prepared a variety of alkandiyl-bridged intramolecular FLP systems that contained active Lewis acid and Lewis base combinations, often with strongly electrophilic boryl functionalities and nucleophilic phosphanyl or amino groups. These types of main group element combinations show reaction modes that are usually typical for transition metal containing systems and thus efficiently mimic the behaviour of transition metal. Typical is the metal-free FLP small molecule coordination and/ or activation. A most active system of this kind from Erker’s research group is an ethylene bridged bulky phosphane/borane pair, in which the boron Lewis acid and the phosphorus Lewis base have only a weak intramolecular interaction. This and a variety of related intra- or intermolecular main group element Lewis acid/Lewis base combinations activate dihydrogen under very mild reaction conditions. So some of these systems have been used as active metal-free hydrogenation catalysts of functionalized olefins (e.g. enamines, silyl enolethers) and of bulky imines at room temperature, and some modified systems even hydrogenate some conjugated ynones or enones under catalytic metal-free conditions. These systems also cleanly and reversibly add carbon dioxide and some trap sulfur dioxide. The intramolecular P/B FLPs were able to add nitric oxide (NO) and generate novel types of persistent nitroxyl radicals. The intramolecular "Erker FLPs" show a reaction behaviour toward a variety of small molecules (e.g. carbon monoxide or isonitriles) that is reminiscent of typical transition metal coordination chemistry.

The new field of frustrated Lewis pair chemistry is expanding rapidly and brings surprising new reactions over and over again. The Erker research group has found a series of unprecedented new reactions that take place at the frustrated Lewis pair templates or in the FLP regime. This includes the specific reduction chemistry of CO with B-H boranes, the discovery of the phospha-Stork reaction and the recent development of a new borataalkene chemistry. This chemistry is complemented by the latest studies on novel very reactive B-H borane reagents and on reactive B-H containing borenium ions. The former were applied in the multi-component syntheses of boron containing heterocyclic products, the latter opened new pathways to compounds containing B=C double bonds. The research group discovered rare examples of unusual 1,1-hydroboration reactions and of modern variants of the 1,1-carboboration reaction using strongly electrophilic fluoroaryl containing boranes. Such reactions have been successfully utilized for the synthesis of a variety of heterocycles including phospholes and reactive boroles. Quite recently the Erker research group has demonstrated the relationship between borole examples and the unique rare class of the bora[4]pyramidanes, the unconventional three-dimensional square-pyramidal constitutional isomers of the planar conjugated borole frameworks. First specific intermolecular reactions of the non-classical bonding system in such borapyramidanes have been found and investigated, which could indicate a remote relationship between the pyramidane systems with the non-classical carbocations.