Structure and Bonding in 1,3-Disilabicyclo[1.1.0]butanes and Related Species

概要

General Introduction
The tetrahedral geometry around tetra-coordinated atomic center with σ bond can be rationalized as the maximization of the overlap between two larger lobes of the bond-forming sp3 hybrid atomic orbitals. However in some instances, the σ bond between the two bonding center occurs between two atoms with a geometry inverted from the typical tetrahedral geometry (Chart 1.). These bonds are called inverted σ bonds and have been rationalized as bonds formed from the overlap between the smaller lobes of hybrid atomic orbitals. The properties of these inverted σ bonds are not well understood and the mechanism of their stabilization is still under heavy exploration. Among the compounds with an inverted σ bond, one of the model compounds used to explore the bonding situation is [1.1.1]propellane 1 which contain an inverted C–C σ bond as the central bond. Unfortunately, the generalization of the theories regarding inverted σ bond from carbon to other heavier group 14 elements has not been fully realized despite reports of compounds containing inverted σ bond involving heavier group 14 elements. This doctoral thesis focuses on the elucidating the chemistry of inverted Si–Si σ bond found in isolable 1,3-disilabicyclo[1.1.0]butane 2 and related species from both experimental and theoretical perspectives. In chapter 1, the reaction of 2 with aldehydes was explored. The mechanism of the reaction is further elucidated using both experimental and computational approach. In chapter 2, the bonding situation of the inverted Si– Si σ bond in silicon analogues of [1.1.1]propellane 3 are examined under various theoretical frameworks. The results of the inverted Si–Si σ bonds are contrast with the results of inverted C–C σ bond in 1. In chapter 3, the interaction between the bridgehead silicon atoms in silicon analogues of bicyclo[1.1.0]butane 4 and 5 are probed using valence bond theory. An emphasis is put upon the bond stretch isomerization of 4 to elucidate the relationship between the structural parameters and bonding situation. In chapter 4, thermal unimolecular isomerization reactions between 4, 2,3- disila-1,3-butadiene 6, and 1,2-disilacyclobutene 7 are explored in detail using highly accurate ab initio calculation.

Chapter 1. Reactions and Mechanisms of 1,3-Disilabicyclo[1.1.0]butane with Aldehydes
The isolable 1,3-disilabicyclo[1.1.0]butane 2 possesses a Si–Si bond between bridgehead silicon atoms with unique inverted bond geometry. The inverted bond has been shown to exhibit unusual reactivity. For instance, compound 2 undergoes addition reaction with aryl ketones across the inverted Si–Si bond to form bis-silylated products. This reactivity differs greatly from a normal Si– Si bond which does not interact with carbonyl groups. In this chapter, the bis-silylation of 2 with a carbonyl compound is expanded with the reaction with aldehydes along with experimental and computational mechanistic study.

Compound 2 reacted with various aryl aldehydes in benzene at room temperature to form bis- silylated adducts 8 (Scheme 1). Substituents on para position of the benzene ring have minimal effects on the reactions. The reactions with α,β-unsaturated aldehydes under the same reaction

condition also proceed smoothly to provide only 1,2 adduct. The reaction also proceeded with alkyl aldehyde, though the reaction time and temperature must be increased. The mechanism of the reaction is explored with cyclopropanecarboxaldehyde-based radical clock probe. The reaction gave ring opened product 9 suggesting a stepwise mechanism with a radical intermediate (Scheme 2). This result is supported by computational study of the reaction mechanism which located a singlet biradical structure as an intermediate in the process.

Chapter 2. Theoretical examination of inverted Si–Si σ bond in silicon analogues of [1.1.1]propellanes
The inverted C–C σ bond in 1 has received attention from various research groups exploring the strained hydrocarbon systems. Even though the nature of the inverted bond has not been fully elucidated, its properties and reactivity are very well-documented in the literatures. On the other hand, the understanding of an inverted Si–Si σ bond is still under development. This chapter attempts to leverage the understanding of the inverted C–C σ bond in 1 and extends them to the inverted Si–Si σ bond in 3 which shares the same skeletal structure.

Stepwise hydrogenation reaction across the inverted Si–Si σ bond in 3 provide estimated bond dissociation energy of 100-169 kJ mol-1, about half the strength of normal Si–Si bond in disilane. Wiberg bond index analysis of 3 also displays a fractional bond order between the two hemispheroidal silicon centers. However, quantum theory of atoms in molecules (QTAIM), electron localization function (ELF), and natural bond orbital analyses all failed to recognize existence of a chemical bond between the two silicon atoms. These results deviated significantly from the results observed in 1 where QTAIM and ELF recognize a weak bonding interaction between the two bridgehead carbon atoms. Valence bond theory analysis of the bond energy reveals that the inverted σ bond is classified as a charge shift (CS) bond, a type of chemical bond where the stabilization is derived from the resonance between covalent and ionic structures (Scheme 3.). The results pointed to a very weak bond with a CS nature between the two bridgehead silicon atoms in 3.

Chapter 3. Analysis of the Bonding Situation in Silicon Analogues of Bicyclo[1.1.0]butanes and Their Bond Stretch Isomerism
1,3-Disilabicyclo[1.1.0]butane 4 and tetrasilabicyclo[1.1.0]butane 5 exhibit bond stretch isomerism with respect to the changing bridgehead Si–Si bond length. In both compounds, the geometry around the bridgehead silicon atoms differ between the two isomers with the long bond (lb) isomer having a hemispheroidal geometry and short bond (sb) isomer having a normal or planar geometry. In this chapter, the bonding nature of bridgehead Si–Si bonds, were theoretically investigated in detail using valence bond theory. The bond type of the bridgehead bond in the sb and lb isomers can be classified as covalent and CS bonds, respectively, indicating that the bonding nature is switchable in one molecule (Scheme 4.). The detailed examination of the bond stretch isomerization in 4 demonstrated that the bonding nature is mainly influenced by the geometry inversion rather than the bond elongation.

Chapter 4. Theoretical Study on 2,3-Disilabutadiene Unimolecular Isomerization Reactions
The unimolecular isomerization reactions of conjugated dienes are very well known reactions and provide a powerful synthetic method in organic chemistry. However, the chemistry of heavier group 14 elements conjugated systems was shown to be different from those of carbon systems and warranted more detailed exploration. In this chapter, the unimolecular isomerization reactions of 1,3-disilabicyclo[1.1.0]butane 4, 2,3-disila-1,3-butadiene 6, and 1,2-disilacyclobutene 7 were explored and the electronic structure of the structures along the process are examined in detailed (Scheme 5.).

The isomerization between 6 and 7 is shown to be an electrocyclic reaction with significant energy barrier and very stereoselective similar to the electrocyclic reaction of the carbon system. On the other hand, the isomerization between 4 and 6 is shown to be a two-step process of silene- silylene isomerization follow by barrierless [2+1] cycloaddition reaction. The biradical character and non-dynamic electron correlation effects are demonstrated to be not significant in most process, except for the isomerization between the trans isomer and long bond isomer of 4. This mechanism is very similar to a related unimolecular isomerization of tetrasilabicyclo[1.1.0]butane 5.