Trimethylsilyl migration

When there is no adjacent, reactive C—H bond into which the carbene could insert, insertion into a silicon-silicon bond may occur, resulting in a l, 2-trimethylsilyl migration from silicon to carbon. This has been proposed on the basis of the known structures of several compounds.87 A sequence of further steps, each of which has been separately observed, is required to explain the structures of the final products. 

There is one tolane addition to a silole that does not yield a 7-silanorbornadiene. Silole (138) is trapped as the rearranged silole (139) formed by a 1,5-trimethylsilyl migration. It gives the only reported example of a 1-silanorbornadiene (140) and can be trapped by ketones and alkenes as well (Scheme 215) (79JA2733). 

The final product of the deprotonation depends strongly on the deprotonation reagent and/or the reaction conditions. Thus, in THF and using MeLi (or NaH) as base, an anionotropic l,3-Si,0-trimethylsilyl migration occurs in the alkoxymethylsilane 191 with formation of the silyl anion 192 instead of elimination of silanolate. Therefore, after hydrolytic work-up only trimethylsiloxy(bis(trimethylsilyl)silyl)alkanes 193 were obtained (equation 48).108,117... 

A totally different course was observed with type I anions (R= aromatic or aliphatic X=OSiMe.) the expected trimethylsilyl substituted enol ethers were not obtained. Instead, after a 1,4-0.0-trimethylsilyl migration and splitting off of (RO)2P-O benzoins or acyloines were obtained. 

A more pronounced effect of this type occurs in the addition of MeOH to the transient (l-sila)hexatriene 21a, the product of photochemical [l,3]-trimethylsilyl migration in pentamethylphenyldisilane (20a equation 12)82,83. Photolysis of 20a in the presence of methanol in acetonitrile solution leads to the formation of three regioisomeric addition... 

Similarly, when silene 2a is generated in presence of excess tris(trimethylsilyl)silyllithium, the lithium silanide is added across the silicon-carbon double bond to give an organolithiiun intermediate, which undergoes a rearrangement, a l,3-Si,C-trimethylsilyl migration, resulting in formation of a lithium silanide, which is trapped with chlorotrimethylsilane to yield the polysilane 7. The H-silane 8 is obtained as the protonation product after usual hydrolytic work up (Eq. 4-5). 

The reaction of 161 with lithium metal involves a reduction, a trimethylsilyl migration, and formation of a boron-carbon bond to afford 162. Subsequent reaction with trimethylsilyl chloride gives 1,2-diborolane 146 (Scheme 14) <2002AGE1526, 2004EJI3063>. 

Products derived from the two silenes (163) and (164), formed by competing 1,3-trimethylsilyl migrations, were obtained on irradiation of l-(3,4-dihydro-2H-6-pyranyl)-1-phenyltetramethyl-disilane (165)Separate studies have shown that silenes are intermediates in the photorearrangement of analogous benzenoid disilanes. 1,4-Bis(pentamethyldisilanyl)naphthalene (166), however, behaves in a different fashion, and in the absence of a trapping agent is converted on irradiation into the trisubstituted naphthalene (167) In the presence of methanol, the adduct (168) is also obtained the proposed pathway is outlined in Sotaeme 8. 

Fig, 2. Variable temperature proton nuclear magnetic resonance spectra of lithium tris(trimethylsilyl)hydrazide (6) in hexane reveals the temperature-dependent intramolecular trimethylsilyl migration. Temperatures are recorded in degrees centigrade. 

Fig. 3. Proton nuclear magnetic resonance spectra of compound 6 in diethyl ether indicate the existence of a dimer monomer equilibrium as well as independent temperature dependence of trimethylsilyl migration in both dimer and monomer forms.

These results may be interpreted in the following way (22) At room temperature the species exists mainly as dimer (or ion quadruplet) with rapid migration of MeaSi groups on the NMR time scale. As the sample is cooled to -20°C, trimethylsilyl migration becomes slow so that two peaks are observed for the dimer. The related compound LiN(SiMc3)2 shows similar monomer-dimer equilibria in solution in THF and hydrocarbons (72). Below —20°C, monomer (ion pair) is observable in equilibrium with dimer. Between -20°C and -60°C silyl migration is rapid in the monomer but slow in the dimer (compared to the proton spin relaxation time). Finally at -80°C anionic rearrangement for the monomer also becomes slow, so that two pairs of 2 1 peaks are observed. 

Keywords silanes, silylcarbenium ions, rearrangements, l,2-5i,C-trimethylsilyl migration, hypersilyl alcohols... 

Scheme 1-280. Predominant inversion upon benzyl migration and almost exclusive retention upon trimethylsilyl migration to the lithiated centers of 361 and 362.

Wang Z-X, Chai Z-Y, Li Y-X (2005) Reaction of aryl azides with tris(trimethylsilyl) silyllithium synthesis of tmeda or THF adducts of [Li N(Ar)Si(SiMe3)3 ] and 1,4-trimethylsilyl migration from oxygen to nitrogen. J Organomet Chem 690 4252... 

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