Combined Theoretical and Experimental Investigation of N-Heterocyclic Carbenes as Lewis Base Catalysts and as Ancillary Ligands in Ru-Catalyzed Olefin Metathesis. Mechanistic Investigation of Fluxional Behavior of Ru-Based Olefin Metathesis Catalysts

Abstract

Chapter 1. Through the use of quantum theory of atoms in molecules (QTAIM) the similarities and differences between transition metal complexes ligated by phosphines and N-heterocyclic carbenes (NHC) were elucidated. Among the key findings, the phosphines were identified as stronger charge donors than NHCs; however, the latter class of ligands exhibits a weaker p-accepting character than the former. Furthermore, Tolman electronic parameter (TEP) was determined to be an inadequate gauge for the total electron donating ability of phosphines and NHCs; rather TEP can serve as a measurement of population of dp set of orbitals of a metal center in question. Computational and experimental studies of the mechanism of NHC-catalyzed boron and silicon addition to a,ß-unsaturated carbonyls reactions were carried out. Through the use of radical traps the mechanisms involving homolytic cleavage of B-B or B-Si bonds were ruled out. Computational (DFT) studies of the mechanism identified two pathways: (1) direct activation of diboron or borosilyl reagents through coordination of NHC to the B atom, (2) net oxidative addition of the diboron or borosilyl reagents to the carbon (II) of the NHC. The insights gained from the aforementioned studies were employed to rationalize the observed lack of reactivity of NHC-activated diboron complexes in the presence of aldehydes. Chapter 2. New C(1)-symmetric chiral monodentate N-heterocyclic carbenes were prepared, and corresponding chiral Ru-carbene complexes were synthesized. These complexes were employed to gain empirical understanding of factors that govern stereoselectivity in Ru-catalyzed enantioselective olefin ring-closing metathesis. The data thus obtained was employed to infer that syn-to-NHC reaction pathways are competitive and non-selective. One plausible mechanism, through which syn-to-NHC pathways can be accessed, involves Berry pseudorotations. Through the use of stereogenic-at-Ru complexes diastereomeric Ru-carbenes were isolated (silica gel chromatography) and spectroscopically characterized in solution phase. The diastereomeric Ru-carbenes were found to undergo non-metathesis stereomutations at Ru center, thereby providing additional support for the above hypothesis regarding accessibility of syn-to-NHC olefin metathesis pathways. Non-metathesis stereomutation at Ru was found to be accelerated in the presence of protic additives, suggesting the plausibility of hydrogen bonding between the acidic proton and the X-type ligands on Ru. Occurrence of hydrogen bonding was corroborated through the use of chiral allylic alcohols in Ru-catalyzed diastereoselective ring-opening/cross metathesis, which was developed into a versatile method for highly diastereoselective functionalization of terminal olefins.