Experimental Studies

The experimental studies performed in Dr. Wiest's laboratories involve three different aspects:

1. Synthesis of isotopically labeled compounds

The kinetic isotope effect studies require the synthesis of regio- and occasionally stereoselectively labeled starting materials. Because of the limited availability of stable isotopes and the necessity for pure isotopomers, these synthesis are particularly challenging. Besides the various methods known in the literature for the synthesis of isotopically labeled compounds, the use of metallorganic methylene equivalents such as the Simmons-Smith and the Tebbe-Grubbs reactions, are investigated.

2. Measurement of kinetic isotope effects

The substitution of an atom in a molecule by an isotope of the same atom leads to subtle changes in vibronic structure of the molecule. This leads to slightly different activation energies (see picture), which can be measured by reaction of a mixture of labeled and unlabelled starting materials and determination of the isotope distribution in the product by high resolution NMR or mass spectrometry. The kinetic isotope effects obtained by these experiments can be interpreted if the vibronic force constants are known. Because the direct measurement of the vibronic spectra of radical ions is not generally possible, the force constants are compute by quantum mechanical methods. Alternatively, the potential use of electrochemical techniques for the determination of the kinetic isotope effects is explored.

Simplified potential hypersurfaces for two isotopomers showing different activation energies

3. New electron transfer catalyzed reactions

The concept of electron transfer catalysis can be generally applied to orbital controlled reactions (including symmetry forbidden pericyclic reactions). Therefore, the applicability of electron transfer catalysis to new reactions is tested. This is especially promising for reactions with a high activation barrier such as the ene-reaction, aromatic Claisen rearrangements a well as certain 1,3-dipolar cycloadditions and electrocyclic reactions. The catalysis of these reactions by photoinduced or thermal electron transfer is studied.