|Ph.D Student||Kaushansky Alexander|
|Subject||Towards Silanones via Metal Halosiloxides|
|Department||Department of Chemistry||Supervisors||? 18? Yitzhak Apeloig|
|Dr. Dmitry Bravo-Zhivotovs|
|Full Thesis text|
The research main goals were: (a) the synthesis and isolation of a stable silanone, R2Si=O, using a novel approach via metal-halogen elimination from halosiloxides; and (b) the study of the reactions of silanones. To achieve this the following intermediate goals (I - VII) had to be achieved:
(I) Synthesis of stable precursors (R3Si)2SiX(OH) (X = F, Br) stabilized by bulky tBu2MeSi substituents, which hopefully are sufficiently bulky to allow isolation of the target silanone (R3Si)2Si=O
(II and III) Generation of novel metal halosiloxides (R3Si)2SiX(OM) (X = F, Br; M = Li, K) or (R3Si)2Si=O→MX complexes through metalation of halosilanols (R3Si)2SiX(OH);
(IV and V) Development of unambiguous trap reactions which can distinguish between a (R3Si)2Si=O→MX complex and metal halosiloxides (R3Si)2SiX(OM), and structural analysis and comparison between them;
(VI) Search for suitable combinations of M/X, for elimination of MX from (R3Si)2SiX(OM) for generating the free bis(silyl)silanone (R3Si)2Si=O;
(VII) Quantum mechanical computational study of the unique reactions of silanone (R3Si)2Si=O, to gain better understanding of its geometry, properties and reaction mechanisms.
Reaction of (R3Si)2SiF(OH) with (Me3Si)2NM (M = Li, K) produces (R3Si)2SiX(OM). The X-ray structures and reactions indicate that (R3Si)2SiX(OM) are metal fluorosiloxides.
Bromosilanol (R3Si)2SiBr(OH) reacts with tBuMe2SiLi to produce unidentified, “NMR silent” product X. X reacts with Me3SiCl, HCl and tBuMe2SiLi to produce the expected addition products of these reagents with a silanone. X is unstable and after 5 hours at r.t. precipitates quantitatively, yielding cyclic trimer [(R3Si)2SiBr(OLi)]3.
X-ray crystallography reveals that [(R3Si)2SiBr(OLi)]3 is a trimer of a silanone•LiBr complex. Disappointingly, the trimer does not eliminate LiBr upon heating, and does not react with Me3SiCl and tBuMe2SiLi. However, it adds H2O and HCl. We suggest that [(R3Si)2SiBr(OLi)]3 is unreactive due to strong Li-O interactions within its (LiO)3 core and due to steric protection of the core by the bulky tBu2MeSi groups, allowing reactions only with small molecules.
Reaction of (R3Si)2SiBr(OH) with (Me3Si)2NK yields bromosiloxide (R3Si)2SiBr(OK), which reacts with Me3SiCl, HCl and H2O as expected for addition to a silanone. At r.t. (R3Si)2SiBr(OK) produces cyclic [-(R3Si)2Si-O-]2, the dimerization product of silanone (R3Si)2Si=O.
Reaction of (R3Si)2SiBr(OH) with (Me3Si)2NK in THF produces (HO)(R3Si)2Si-O(CH2)4O-(R3Si)2Si(HO), the product of THF cleavage. Reaction of (R3Si)2SiBr(OH) with 2 equivalents of (Me3Si)2NK in toluene produces Me(R3Si)2Si(OK). Involvement of a transient free silanone is suggested for both reactions. The proposed mechanism is studied by DFT calculations.
In conclusion, a transient silanone (tBu2MeSi)2Si=O (or its complex with MBr) is generated by MBr elimination from bromosiloxides. Hopefully, the use of larger silyl substituents will lead to the isolation of a stable isolable silanone.