The Erker group at Münster works on a variety of topics centered around the general fields of Organometallic Chemistry / Organic Chemistry.
1. Frustrated Lewis pairs: small molecule binding and activation
Frustrated Lewis pair chemistry is currently a most actively pursued research area in the Erker group. Small molecule activation is often thought to be a typical domain of transition metal containing chemistry; however, combinations of active Lewis acids and Lewis bases have recently been shown to give rise to related reaction profiles. We have contributed a series of main group element containing systems that are able to bind and/or activate small molecules by the co-operative action of non-quenched pairs of Lewis acids and bases [“frustrated Lewis pairs” (FLPs)]. FLP chemistry is a rapidly growing emerging field in chemistry. The Erker group has contributed a variety of very active intramolecular FLP systems, many of which split dihydrogen under mild conditions, bind small molecules (carbon dioxide, sulphur dioxide, nitrogen monoxide, olefins, acetylenes, carbonyl compounds, even some transition metal complexes, etc. etc.). Some of our N/B and P/B FLPs serve as active metal-free hydrogenation catalysts. In some cases selective carbon monoxide reduction could be achieved. The new field of frustrated Lewis pair chemistry is expanding rapidly and we find surprising new reactions over and over again.
2. Carbon-carbon bond activation and 1,1-carboboration chemistry
The group has found examples indicating that strong non-activated C-C bonds can be cleaved by reacting them with very electrophilic boron Lewis acid derivatives: we have used the 1,1-carboboration reaction of internal alkynes to cleave non-activated carbon-carbon sigma-bonds and we have found protocols that allow for the insertion of a borylene unit into a C=C double bond in the course of a formal 1,1-carboboration reaction of an alkene. In addition we have developed advanced versions of the 1,1-carboboration reaction using strongly electrophilic boranes. This was found useful for the synthesis of a variety of interesting heterocycles, including new functionalized boroles and phospholes. 1,1-Carboboration is developing more and more into a useful method for preparing new alkenyl borane reagents and Lewis acids.
3. Development of novel catalyst systems
The group had a long standing interest in the development of highly active catalyst systems that at the same time exhibit high selectivities. Homogeneous Ziegler-Natta catalysts often show this specific combination of characteristic features. The research group had developed new ligand frameworks and novel activation procedures. Detailed mechanistic studies to determine the general as well as the specific rules of operation of such important catalysts represent another important area of the experimental activities that had been pursued by the group in Münster.
4. Metal catalysed polymerization reactions
The group had been interested in 1-alkene polymerization reactions leading to new materials. Our studies on the formation of elastomeric polyolefins by means of controlling the conformational properties of group 4 metallocenes are a typical example. The group had also investigated various copolymerization reactions, especially those concerning the formation of functionalized polymers by means of coordination polymerization. We had described a series of new types of "constrained geometry catalysts" in the literature that were useful in copolymerization.
5. Surface chemistry and heterogeneous catalysis
The group developed a series of organic systems that feature self-assembly when deposited on surfaces. In a close collaboration with physicists at Münster we developed a surface chemistry of hydrophobically-modified peptide derivatives and of similar nucleobase derivatives and have analysed their surface aggregates. Experimental investigations on the catalytic application potential of such surface nano-systems resulted in the finding of a new way of dehydrogenative polymerisation of alkanes on a specific gold surface in ultra-high vacuum.
6. Unusual organic and organometallic compounds
Metal-containing substituents are able to stabilize unusually structured organic compounds. We synthesized and isolated a sizable number of stable molecules that contain planar-tetra-coordinate carbon using group 4 bent metallocene substituents. We prepared a variety of stable small ring metalla-cycloallenoid complexes of the group 4 metallocenes, compounds that occupy an intermediate position between the unique “Rosenthal-metallacyclocumulenes” and the “Suzuki-metallacycloalkynes”. We are also increasingly using group 4 metallocene building blocks for generating metal containing frustrated Lewis pairs. These combine favourably typical features of transition metal complexes with those of the metal-free FLPs.