Mechanistic, Synthetic and Theoretical Studies of High Valent Metallacycles and metal Alkylidenes

Citation

Anslyn, Eric Van (1988) Mechanistic, Synthetic and Theoretical Studies of High Valent Metallacycles and metal Alkylidenes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/m7kp-4g32. https://resolver.caltech.edu/CaltechTHESIS:10192009-094841353

Abstract

The primary focus of this thesis is on the mechanism of olefin metathesis and ring opening metathesis polymerizations. In addition, several reactions of metal alkylidenes and metallacycles which are not traditionally viewed as part of the olefin metathesis reaction are presented. Olefin metathesis involves the 2+2 cycloaddition of metal alkylidenes with olefins and the 2+2 cycloreversion of metallacyclobutanes. These reactions are becoming common place in organometallic reaction mechanisms and join the traditional oxidative additions, reductive eliminations, ligand substitutions, intramolecular insertions, nucleophilic attacks on coordinated ligands and ligand fluxtionalities as the commonly sited organometallic reactions. A current goal of organometallic chemistry is to understand the influence of oxidation state, electron count, ligand sterics, ligand electronics and substituent effects upon each of these reactions.

The knowledge of mechanisms is essential to be able to understand and rationally manipulate chemical processes. The knowledge also allows for the capability to catagorize mechanistic theories as an organizing device for understanding organometallic chemistry as a whole. Organometallic chemistry, however, is not easily catagorized due to the large complexity of bonding types and structures that inorganic chemistry produces. The work in this thesis has utilized some techniques of physical organic chemistry to study mechanisms and reactive intermediates. These techniques include kinetics, substituent effects, isotope effects, stereochemical studies and theoretical calculations.

Organic chemistry has greatly benefited from the advent and subsequent development of the pericyclic theory for the understanding of covalent bonding, frontier orbitals and symmetry. These same notions have met with various levels of success in organometallic chemistry. The success of theoretical studies in organometallic systems very often depends upon the level of electron correlation and the extent to which the exchange integrals are calculated. The theory presented in this thesis utilizes a fully ab initio method with electron correlation. The structure of organometallic complexes is examined as a function of the nodal planes of the individual metal ligand bonds and their influence on the bonding of other ligands within the same complex. In addition, reactivity of the complexes are probed as a function of the symmetry and energy of the bonding and empty orbitals.

In chapter one, data and speculations relating to the mechanism of cleavage of titanocene metallacyclobutanes is presented. The reactive intermediate is postulated to be a titanocene methylidene-olefin adduct. Chapter two further expands upon these mechanistic studies by presenting the kinetics and polydispersities of the ring opening metathesis polymerizations of slightly strained olefins. Chapter three presents work which utilizes ab initio electronic structure theory calculations to determine the energetics of the 2+2 cycloaddition of molybdenum alkylidene and imido complexes with olefins. In chapters 4, 5 and 6, reactivity different than the normal cycloadditions of metal alkylidenes and cycloreversions of metallacycles is examined. In chapter 4, an electron transfer mechanism for the reaction of titanocene methylidene with activated halides is presented. Chapter 5 discusses the reactivity of titanocene methylidene with inorganic carbonyls. The titanocene methylidene does not perform methylene transfer as is seen with organic carbonyls, but instead, the resultant oxametallacycle rearranges to yield a titanocene ketene complex. Finally, in chapter six, ab initio electronic structure theory calculations are again presented. They are used to explore the interconversion of a metallacyclobutadiene to a metallatetrahedrane. The two complexes are found to be energetically similar due to a balance between the strength of σ and π bonds and the role of strain and resonance effects. Each chapter was written as an individual study and thus includes an Abstract, Introduction, Results and Discussion section and a Summary or Conclusion. Thus, this thesis presents work that attempts to add a little more knowledge to the mechanistic and theoretical understanding of organometallic reaction mechanisms.

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