Enyne metathesis is a synthetically useful catalytic process for the synthesis of 1,3-dienes (eq 1). Despite considerable recent use of the ruthenium carbene-mediated reaction in synthetic applications,^{1, 2} mechanistic study of the reaction is lacking. The analogy with alkene metathesis has helped rationalize some of the data, but there are several shortcomings and the current mechanistic model lacks predictive power. Shortcomings include knowing whether a [Ru]=CHR or [Ru]=CH2 is the true catalyst; which step is rate-determining; why there is 1:1 E/Z selectivity in some cases, but high E-selectivity in others; and how electronic effects influence reaction rate. The Diver group has improved the scope of the intermolecular (cross) enyne metathesis^3,4 and developed new applications.^5,6 More recently, the Diver group, in collaboration with Professor Keister's group, has developed a kinetic method to study enyne metathesis reaction rates. This method uses IR spectroscopy to follow the disappearance of the alkyne C-H absorbtion. The method is simple, fast and broadly applicable for evaluating functional group effects in the alkyne or alkene reactants.

The undergraduate student will be involved in evaluating functional group effects on reaction rate of intramolecular, ring-closing enyne metathesis. Critical questions include the effect of chelation and whether electron-withdrawing groups retard the reaction rate. The experience will utilize both organic synthesis and analytical methods for the determination of reaction rates. The required 1,6-enynes will be synthesized by standard approaches (Scheme, at right). In panel b, a more demanding synthesis is planned that may be suited to an undergraduate with a career interest in organic synthesis. Using the IR method, the student will analyze 'their molecules' and learn how subtle changes in structure affect reactivity. This project will provide a strong foundation for graduate work by conjoining synthesis, catalysis, mechanism and physical organic study in an area of topical interest.

References

1. Diver, S. T. and A. J. Giessert (2004). Synthesis: 466-471.
2. Diver, S. T. and A. J. Giessert (2004). Chem. Rev. 104: 1317-1382.
3. Giessert, A. J., N. Brazis and S. T. Diver (2003). Org. Lett. 5: 3819-3822.
4. Smulik, J. A., A. J. Giessert and S. T. Diver (2002). Tetrahedron Lett. 43: 209-211.
5. Kulkarni, A. A. and S. T. Diver (2003). Org. Lett. 5: 3463-3466.
6. Kulkarni, A. A. and S. T. Diver (2004). J. Am. Chem. Soc. 126: ASAP