C HEMISTRY - Research: Dr. Christine Hahn
Atom Economic Alkene Functionalization by Highly Electrophilic Transition Metal Complexes
| Introduction |
The direct addition of protic nucleophiles H-Y such as HOR, HSR, HNR2, and HPR2 across the C-C double bond is one of the most valuable reactions to functionalize unsaturated hydrocarbons because of its atom economic character. That means all atoms of the starting materials are converted to the product, and no waste product is formed.
Unsaturated hydrocarbons such as simple alkenes are petrochemicals obtained from cracking and reforming of crude oil, and are used as industrial feedstock for production of numerous basic chemicals. The addition of water, ammonia, or other nucleophiles to alkenes is the first key step to produce certain functionalized hydrocarbons (alcohols, ethers, amines, etc.), and can only be realized in the presence of a catalyst. The development of appropriate catalysts for this type of reaction is currently one of the most challenging research fields. The goal is the improvement of catalytic efficiency, which includes the control of chemoselectivity no side products), regioselectivity (in case R1 ? R2), and stereoselectivity (in case R2 ? H) of the reaction, and mild conditions (~ room temperature and normal pressure). The realization of efficient catalytic additions of protic nucleophiles to simple alkenes will have an enormous impact not only for the industrial production of basic chemicals but will also provide a new methodology in organic synthesis. Multiple step synthesis of pharmaceutical and natural products might be shortened by several steps.
| Research |
For the development of suitable catalysts, it is crucial to understand the process of substrate activation. Late transition metals in higher oxidation state are highly electrophilic, which can strongly activate the C-C double bond by π-coordination. The Dewar-Chatt-Duncanson Model provides a detailed picture explaining the activation process.
The proposed general reaction mechanism for the catalytic alkene functionalization includes nucleophilic attack of the protic donor molecule H-Y to the activated alkene. Detailed studies of equilibria with model complexes of palladium(II) and platinum(II) by NMR spectroscopy provided experimental data on thermodynamic stability of individual intermediates.
Those model complexes where synthesized using a tridentate chelate ligand (PNP pincer ligand) for stabilization. Halide abstraction with silver salt in the presence of alkenes gave first examples of dicationic palladium(II) and platinum(II) alkene complexes that could be isolated and fully characterized by NMR spectroscopy and X-Ray crystal structure analysis. Organometallic complexes of this type are air sensitive compounds therefore all synthetic steps are carried out under inert gas.
13C NMR and IR/Raman spectroscopic parameters as well as bond lengths obtained from X-ray structure analysis display clearly the trend of alkene activation by increasing positive complex charge.
Further research objects in this field are the synthesis and spectroscopic characterization of new highly electrophilic transition metal complexes containing Rh, Ir, Pd, Pt, Ag, and Au. Beside the influence of the central metal, specific chelate ligands will be used to investigate steric and electronic effects on alkene activation and their reactivity toward nucleophiles. The emphasis of this project lies on fundamental reactivity studies using NMR spectroscopy to establish general structure-activity relationships with the goal to approach efficient catalytic C-C and C-heteroatom bond formations.
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Updated June 22, 2006 |
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