Theoretical studies of intramolecular rearrangements of siloxycarbenes

This thesis describes a thorough theoretical investigation of the mechanisms of intramolecular rearrangements of siloxycarbenes by means of contemporary ab initio molecular orbital theory and density-functional theory calculations combined with the quantum theory of atoms in molecules (AIM), the electron localization function (ELF) and natural bond orbital (NBO) analysis. New approaches for probing the mechanisms of organic reactions using these well-established electronic-structure methods are described. The thesis is divided into five chapters. Chapter 1 addresses the importance of understanding siloxycarbene reactivity, and provides a concise description of the electronic structure of carbenes, a brief review of singlet carbene rearrangements and an outline of the main objectives of this work. Chapter 2 describes an investigation of the mechanisms of intramolecular rearrangements of methoxy(siloxy)carbene, with emphasis on 1,2-silyl migration and decarbonylation. It is shown that 1,2-silyl migration and decarbonylation are concerted rearrangements involving intramolecular front-side nucleophilic attack by the carbene lone pair at silicon and by the methoxy oxygen at silicon, respectively. It is worthy to note that both of these rearrangement mechanisms are symmetry-forbidden according to Woodward-Hoffmann rules and similar rearrangements involving intramolecular front-side nucleophilic substitution are quite rare. Also noteworthy is the finding based on AIM analysis that the changes in molecular structure associated with 1,2-silyl migration occur via a bifurcation mechanism, while those associated with 1,2-methyl migration, which involves an anion-like shift to the "vacant" carbene p orbital, occur via a conflict mechanism. Chapter 3 deals with the effects of substituents and the role of hyperconjugation in siloxycarbene rearrangements. It is found that electron-withdrawing substituents in methoxy(substituted-siloxy)carbenes generally lower the barriers for 1,2-silyl migration and decarbonylation, consistent with the mechanisms proposed in Chapter 2. The ease of intramolecular rearrangements of siloxycarbenes is also shown to be related to the extent of the pertinent hyperconjugative interactions in the parent carbene conformers. Chapter 4 describes a study of the mechanisms of intramolecular rearrangements of 3-sila-2-oxacyclohexylidene, a cyclic siloxycarbene. Chapter 5 summarizes the key findings of this work and provides some perspective on these findings. It also provides insight into how the approaches described in this work may be extended to other systems. (Abstract shortened by UMI.