O->C silyl migration

OL-Silyl ketones, Reaction of a primary a-bromo ketone with LiN[Si(CH3)3]2 (1) followed by chlorotrimethylsilane or chlorotriethylsilane results in a trialkyl-silyloxyvinyl bromide (a), which is not isolated but treated with BuLi (2 equiv.) to effect O-C silyl migration (equation I). 

Acrylic acid a-anion equivalent. O —> C Silyl migration occurs on treatment of the title compound with r-BuLi in THF at —78°. The ensuing lithium enolate can be alkylated. Quenching with aldehydes generates Baylis-Hillman adducts. 

Enol silyl ethers of acylsilanes. On treatment with butyllithium and trimethyl-silyl chloride, these compounds undergo enol silylation, tellurium-lithium exchange, and O — C silyl migration. The lithium enolates are further silylated. 

Accompanying experimental work [8], we report on DFT calculations of the l,2-(o, X -C->o , -P)-silyl migration of pentacarbonyltungsten phosphinidene complexes la (anti-periplanar) and lb ( g -periplanar) to the corresponding phosphaalkene complexes 2a and 2b. The... 

Reactions of O-silyl O-alkyl keteneacetals Silylacetic acids by 0->C-silyl migration... 

Anionic 1,2-silyl migrations from C to O were applied to the [3 + 2] annulations of /J-X-substituted a,/J-unsaturated acylsilanes (X=SPh and SiMe3) 153 with lithium enolates 154,... 

Among such migrations, the Peterson reactions to give olefins have been shown to proceed via intramolecular 1,3-silyl migrations from C to O (equation 129). 

The 1,3-silyl migrations from O to C in lithiated silyl enol ethers 205 afforded a-silylcarbonyl compounds 206 after hydrolysis (equation 134)326 -330. 

A 1 1 mixture of (E)- and (Z)-allyloxysilanes 231 was prepared by the reactions of ( )-trimethylsilylvinyl sulfone 229 with aryllithiums followed by the addition of aliphatic aldehydes and cyclohexanone (equation 147). An anionic 1,4-silyl migration from C to O in the lithium alkoxide 230 was proposed to be involved. On the other hand, lithium alkoxide 232 lacking an aryl group was stable and gave no rearranged product even at room temperature (equation 148)360. 

Anionic 1,4-silyl migrations of O-trialkylsilyl methyl ketoximes (268) were found to give o -trialkylsilyl ketoximes 269 after hydrolysis (equation 167). Intramolecularity of this migration was confirmed by crossover experiments. Upon heating 269 at 100 °C, a reverse migration to 268 took place. The reaction appeared to be limited to silyl ethers of methyl ketoximes having the (E)-configuration413. 

Silyl migrations from O to C were also found during the synthesis of a-halomethylsilanes415 and diastereoselective preparations of (Z)-vinyl-, epoxy- and cyclopropylsilanes389,416. 

An anionic 1,6-silyl migration from C to O (1,6-Brook rearrangement) was observed during the deprotonation of e-silyl alcohol 324, which gave the corresponding silyl ether 325 (equation 199)466. 

A 1,6-silyl migration from C to O was also observed in the reaction of allylepoxysi-lane 326 with t-BuLi in a THF/HMPA (25 1) mixed solvent at low temperature, which afforded 5-silylpentenal 327 (equation 200). It was proposed that an initial proton abstraction from 326 gave w-silyl alkoxide 328, which underwent 1,2-silyl and then 1,6-silyl migrations to afford dienolate 329. Hydrolysis of 329 provided 327 (equation 201). The intramolecular nature of the transformation was suggested because the facile reaction occurred even in very dilute (0.02 lmol-1) and low-temperature (—78°C) conditions467. 

Chemoselectivity is also achievable by transmetallating a vinyl stannane and closing onto an alkyl chloride (161) or (better) bromide (162). Attempted formation of the six-membered ring 163 with the chloride analogue of 162 led to (retro-Brook) silyl migration from O to C.77... 

Experimentally, 1,2-sllyl migrations with formation of double bonds between the migration origin and the cationic or neutral target have been found to take place intramolecularly, e.g., [1], o X -C- o X -C [2], o -C-+o -Si [3], o X -Sl-+o l -Sl [4], and intermolecularly, e.g., [5]. While in the latter three cases the silyl group shifted... 

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