|M.Sc Student||Mashin Eivgeni|
|Subject||Synthesis via Si-H activation and Characterization|
of Intramolecular Amino-Stabilized Silyllithium
|Department||Department of Chemistry||Supervisors||? 18? Yitzhak Apeloig|
|Dr. Dmitry Bravo-Zhivotovs|
Silyl anions play a key role, having many applications in silicon, organic and organometallic synthesis, similarly to the role of carbanions in organic synthesis. In particular, silyl anions with bulky substituents are popular reagents for the synthesis of low-coordinate silicon compounds such as silyl radicals, silylenes, silenes, etc., in which the bulky R3Si substituents provide kinetic stabilization. An internal donor group can provide additional thermodynamic stabilization via intramolecular coordination to the inherently reactive low-coordinated silicon compounds. To our best knowledge, intramolecular coordinated metallosilanes are unknown. Progress in this field will open many new opportunities in silicon chemistry and in catalysis and will allow the synthesis of novel organosilicon compounds.
In this work we demonstrated that the Si-H bonds in half-pincer silanes (where the half-pincer ligand is o-phenyl-CH2NMe2) can be activated by organolithium reagents in the presence of tBu2Hg, yielding the corresponding ate-complex silyl mercurates 32 in good yields. These silyl mercurates react with Li metal to yield the desired half-pincer silyllithium reagents. Activation of the Si-H bond using the same synthetic procedure in a more sterically hindered silane (in which two tBuMe2Si substituents are attached to the silicon atom, instead of one), leads to the formation of a zwitterionic silyllithium [Ar(N)(tBuMe2Si)2Si- Li]4 (41). In the solid state 41 forms a Li4 core tetrahedron, which is supported by agostic interactions with the aromatic rings along with intramolecular coordination of the dimethylamino groups.
In contrast, reaction of the pincer silanes Ar(N)2(tBu2MeSi)SiH (25a) and Ar(N)2(tBu)SiH (25b) with organolithium regents leads to facile metalation of the Si-H bond and formation of monomeric intramoleculary coordinated silyllithium compounds Ar(N)2(tBu2MeSi)SiLi (46a) and Ar(N)2(tBu)SiLi (46b) in high yield, in which the lithium atom is strongly coordinated to the two dimethylamino groups maintaining the Si-Li bond. For both compounds X-ray molecular structures were determined.
Tripodal silyl lithium Ar(N)3SiLi (50) was also synthesized in high yield by metalation of the tripodal silane Ar(N)3SiH (26) with tBuMe2SiLi. Based on the 13C NMR of 50 we conclude that the anionic charge is mainly located on the silicon atom without significant charge delocalization to the aryl substituents.