טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentZborovsky Lieby
SubjectAlkali Metal Substituted Silenes. Preparation, Structure
and Reactivity
DepartmentDepartment of Chemistry
Supervisors Dr. Dmitry Bravo-Zhivotovs
? 18? Yitzhak Apeloig
Full Thesis textFull thesis text - English Version


Abstract

The main target of this work was the preparation of novel anions with a silicon-carbon double bond, i.e., LiR’Si=CR2 via silenolate (R3Si)2Si=C(OLi)R’ precursors. Thus, the first enol form silenolates (R3Si)2Si=C(OLi)1-Ad (R3Si=tBuMe2Si, tBu2MeSi) were obtained by a lithium-bromine exchange reaction of Br(R3Si)2SiC(O)Ad with tBuMe2SiLi in hexane. The structure of the silenolates was characterized by X-ray crystallography indicating its clear doubly-bonded (silene) character. Addition of THF to silenolate (tBu2MeSi)2Si=C(OLi)1-Ad in the presence of excess R3SiLi  (R3Si= tBuMe2Si, tBu2MeSi) resulted in the formation of the first silenyllithiums Li(tBu2MeSi)Si=C(SitBu2Me)1-Ad and Li(tBuMe2Si)Si=C(SitBu2Me)Ad that were characterized by X-ray crystallography and NMR spectroscopy. The suggested mechanism leading to the silenyllithium proceeds via elimination of tBu2MeSiOLi, forming a transient silyne that rearranges to silylidene which reacts with excess silyllithium to give the silenyllithium product. The elimination reaction requires both THF addition and the presence of excess silyllithium.

Aiming to deepen our understanding of the reactions of acylsilanes with silyllithium reagents, we explored the reactions of non-branched acylsilanes (R3Si)C(O)R’ with silyllithium reagents. We demonstrate that the reaction between silyllithium and acylsilanes involves a single electron transfer (SET) leading to the formation of the corresponding acylsilane anion-radical. The stability of this intermediate depends on kinetic (the size of the acylsilane and of the silyllithium) and thermodynamic (the polarity of the solvating media) factors. THF solvent as well as bulky tBu2MeSi substituents (on both the acylsilane and the silyllithium) stabilize the anion-radical, resulting in its isolation in the form of a unique triplet biradical  which was characterized both by X-ray crystallography and EPR spectroscopy. On the other hand, performing the same reaction in hexane using the smaller tBuMe2Si substituent leads to the formation of an α-silyl carbanion.