Synthetic and mechanistic investigations of ruthenium olefin metathesis catalysts

Abstract

The ruthenium-based catalysts (PCy[subscript 3])[subscript 2](Cl)[subscript 2]Ru=CHPh (1) and (IMesH[subscript 2])(PCy[subscript 3])(Cl)[subscript 2]Ru=CHPh (2) [IMesH[subscript 2] = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene] show high olefin metathesis activity in the presence of most common functional groups and have been widely used in synthetic chemistry. This thesis describes mechanistic, structural, and synthetic studies aimed at understanding the reactivity of these complexes, and at developing new olefin metathesis catalysts with superior properties. Chapter 2 details the effects of ligand variation on the mechanism and activity of ruthenium-based olefin metathesis catalysts. A series of ruthenium complexes of the general formula (L)(PR[subscript 3])(X)[subscript 2]Ru=CHR[superscript 1] were prepared, and the influence of the ancillary ligands L,X, R, and R[superscript 1] on the rates of phosphine dissociation and initiation as well as on the overall catalytic activity was examined. Chapter 3 describes the synthesis of a series of ruthenium benzylidenes containing N-heterocyclic carbene ligands. The new complexes, of the general formula (IMesH[subscript 2])(X)[subscript m](L)[subscript n]Ru=CHPh, were prepared using a variety of synthetic methods, and the bis-pyridine adduct (IMesH[subscript 2])(Cl)[subscript 2](C[subscript 5]H[subscript 5]N)[subscript 2]Ru=CHPh served as a particularly valuable synthon in these systems. Several of these compounds were characterized by X-ray crystallography, and the barriers to benzylidene and N-heterocyclic carbene rotation were determined using [superscript 1]H NMR spectroscopy. Chapter 4 describes the preparation of a series of four-coordinate ruthenium benzylidenes that serve as analogues of the 14-electron olefin metathesis intermediate (L)(Cl)[subscript 2]Ru=CHPh. These coordinatively unsaturated species have the general formula (L)(OR)[subscript 2]Ru=CHPh, and are stabilized by sterically bulky and π-donating alkoxide ligands, such as tert-butoxide, hexafluoro-tert-butoxide, and perfluoro-tert-butoxide. The new compounds were characterized by X-ray crystallography, and their reactivity with incoming ligands, including substituted alcohols, phenols, carboxylates, and pyridine, was investigated. In addition, their olefin metathesis activities were examined in the presence and absence of HCl co-catalyst. Chapters 5 and 6 describe the synthesis and characterization of neutral and cationic tris(pyrazolyl)borate ruthenium complexes. The new complexes were characterized by NMR spectroscopy and X-ray crystallography, and their olefin metathesis activities were explored.