Synthesis and Reactivity Studies of Zwitterionic Silenes and 2-Silenolates
Sammanfattning: This thesis describes synthesis and reactivity studies of 2-amino-2-siloxysilenes and 2-silenolates, species that are strongly influenced by reversed Si=C bond polarization, i.e. an Si?-=C?+ polarization as compared to the natural Si?+=C?- polarization. Because of the reversed polarization, the 2-amino-2-siloxysilenes are zwitterions and the 2-silenolates are predominantly described by the resonance structure with the negative charge at Si. Transient zwitterionic 2-amino-2-siloxysilenes are formed thermolytically from carbamylpolysilanes (tris(trimethylsilyl)silylamides) and trapped with 1,3-dienes in nearly quantitative yields. These silenes have structure and reactivity characteristics that differ from earlier studied Si=C bonded compounds. They are thermodynamically stable toward dimerization and react with 1,3-dienes to give exclusively [4+2] cycloadducts. Their reactions with 1,3-dienes proceed in accordance with inverse electron demand (IED) Diels-Alder reactions which is explained by the electron-rich nature of these silenes. The 2-amino-2-siloxysilenes are also less reactive toward alcohols than earlier silenes. Hence, alcohols do not react with 2-amino-2-siloxysilenes but with the silene precursor, the carbamylpolysilanes, leading to alkoxysilanes in high yields. The latter reaction represents a novel base-free synthetic protocol for protection of primary and secondary alcohols with the fluoride resistant but photolabile tris(trimethylsilyl)silyl group.Another class of formally Si=C bonded compounds, metal 2-silenolates, has been formed in high yields using a novel facile method. Reaction of acyl- and carbamylpolysilanes with potassium tert-butoxide in tetrahydrofurane gives potassium 2-silenolates. The potassium 2-silenolates are stable at room temperature, in contrast to earlier lithium 2-silenolates that degrade rapidly at ambient temperature. The first crystallisable complex of a 2-silenolate was formed and characterized by X-ray crystallography. This 2-silenolate has a pyramidal central Si (?Si = 317.8°), and an Si-C single rather than Si=C double bond (r(SiC) = 1.926 Å). The potassium 2-silenolates give exclusively Si alkylated products with alkyl halides and only [4+2] cycloadducts with 1,3-dienes.
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