|Ph.D Student||Pinchuk Daniel|
|Subject||Chemistry of Metallosilanes (Silenyl Anions) and Silenyl|
|Department||Department of Chemistry||Supervisors||? 18? Yitzhak Apeloig|
|Dr. Dmitry Bravo-Zhivotovs|
The main goal of this research is the synthesis and study of the chemistry of silenyl anions, aiming to broaden the availability and versatility of R2C=SiRLi reagents and to study their chemistry.
1. Synthesis, isolation and characterization of silenyllithiums 2a and 2b
Reaction of bromoacylsilane 1 with tBu2MeSiLi (3.5 eq.) in a 4:1 hexane:THF solvent mixture at -78°C to r.t. yields the solvent separated ion pair (SSIP) 2a. Removal of the solvent and addition of benzene converts 2a into the corresponding contact ion pair (CIP) 2b with two THF molecules coordinated to the lithium atom. 2a and 2b were characterized by UV-Vis and NMR spectroscopy, and by X-ray crystallography. Having the X-ray structures of both 2a (dissociated ion pair) and 2b (CIP), is unique also in organic chemistry.
2. Generation and electron paramagnetic spectroscopy of the first silenyl radicals (R2C=SiR•). Experiment and theory
Oxidation of silenyl lithium 2b with a variety of oxidation reagents yields silenyl radical 3a, which is characterized by an unusually large hyperfine coupling constant in its EPR spectrum. The large hfccs [(a(29Sia)= 137.5-149.6 G] points to a large s-character of the singly occupied orbital and it is in agreement with the calculated strongly bent structure (C=Si-R of 140°- 144°). This is the first observation of a silenyl radical - an analog of the vinyl radical.
3. Reactions of silenyl lithium 2b and main group organometalics. Transmetallation vs. addition
Reaction of silenyl lithium 2b with 2 eq. of (tBu2MeSi)2Zn in toluene produces in quantitative yield a yellow solution of a mixture of 5 and ate complex 6 (Scheme 3, reaction a). Reaction of 2b with 2 eq. of the smaller (tBuMe2Si)2Zn in THF yields the first 1,2-zinclithiomethylsilane 7. 7, having both a silicon-metal and a carbon-metal bond, is an attractive synthon for "one flask" multi-step reactions.
4. Reaction of silenyl lithium 2b with 1-adamantanecarbaldehyde. Synthesis of a siloxirane
Reaction of silenyl lithium 2b with 1 equivalent of 1-adamantanecarbaldehyde at r.t. yields, after workup with H2O/hexane and purification of the reaction mixture by flash chromatography, a quantitative overall yield of siloxiranes 9a-9d.
5. Reaction of silenyl lithium 2b with carbon monoxide. Synthesis of the first lithium 1-silaallenolate
Exposure of silenyl lithium 2b in toluene to CO (1 atm. at r.t.) yields a violet solution of lithium 1-silaallenolate 10 in nearly quantitative yields. 10 was characterized by UV-Vis and NMR spectroscopy, and by X-ray crystallography. 10 is an attractive potential building block for new organosilicon compounds.