Expanding the Scope of Ruthenium-Based Olefin Metathesis Catalysts

The development of well-defined ruthenium alkylidene (PCy₃)₂Cl₂Ru=CHPh brought about a revolution in the area of olefin metathesis. The objective of the work presented here is to expand the scope of ruthenium-based olefin metathesis catalysts such as (PCy₃)₂Cl₂Ru=CHPh through the development of novel synthetic organic methods for ring-closing metathesis as well as through modification of the ligand sphere of the ruthenium complexes. Chapter 2 describes the application of ruthenium alkylidenes to the catalysis of polycyclization reactions. Several acyclic precursors have been synthesized and reacted with (PCy₃)₂Cl₂Ru=CHPh. These precursors vary in topology and contain acetylenic and/or cycloolefinic metathesis relays. The cyclization reactions proceed in good yields to produce polycyclic polyenes. Chapter 3 focuses on the synthesis of racemic and enantiopure targets containing the 6,8-dioxabicycIo [3.2.1]octane skeleton using an intramolecular ruthenium-catalyzed ring-closing metathesis reaction as the key step. The natural product frontalin is synthesized in racemic and enantiopure forms and in excellent yields using this methodology. Chapter 4 outlines the preparation of a novel imidazolylidene-substituted ruthenium-based complex starting from (PCy₃)₂RuCl₂(=CHPh). The N-heterocyclic carbene-substituted olefin metathesis initiator exhibits increased ring-closing metathesis activity at elevated temperature compared to that of the parent complex (PCy₃)₂Cl₂Ru(=CHPh). Di-, tri-, and tetra-substituted cycloolefins are successfully prepared from corresponding diene precursors in moderate to excellent yields. Chapter 5 describes the preparation of a new family of 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene-substituted ruthenium-based complexes. These air and water tolerant systems exhibit an increased ring-closing metathesis activity at elevated temperature when compared to that of the parent complex (PCy₃)₂Cl₂Ru(=CHPh) as well as to the complexes disclosed previously in Chapter 4. In many instances the activity of these new complexes also rivals or exceeds that of the alkoxy-imido molybdenum-based olefin metathesis catalysts. Applications of chiral N-heterocyclic carbene ruthenium complexes to asymmetric ring-closing metathesis are also briefly discussed. Finally, the synthesis of the Schiff base-substituted ruthenium carbene complexes on a solid support is described in Chapter 6. The activities of the supported complexes are compared to those of their unsupported counterparts. The newly prepared systems are found to be highly stable to air, moisture, and temperature, and exhibit increased catalytic activity in acidic media.