1.2- Silicon shift

Murai and coworkers reported on operationally simple aldol reactions with lithium enolates generated from carbonylation of silylmethyl lithium species [57]. Upon 1,2-silicon shift, a-silyl acyllithium species can be stereo-selectively converted to (E) lithium enolates that undergo addition to aldehydes to give /3-hydroxy acylsilanes (Scheme 14). 

Scheme 14 1,2-Silicon shift toward lithium enolates...

These lithiooxiranes can be trapped by various electrophiles with retention of the configuration. The addition to aldehydes occurs with a low diastereoselectivity [but this can be enhanced by adding ClTi(OPr-/)3]. The reaction with enones occurs in a 1,2 fashion only. Intramolecular 1,4-silicon shift has also been reported. The reaction of the enantiomerically pure TMS-stabilized lithiooxirane 189 (Scheme 80) with an aldehyde has been used in a total synthesis of (-l-)-cerulenine. It must be noted that protodesi-lylation using TBAF (tetrabutylammonium fluoride) occurs with conservation of the oxirane stereochemistry. 

In the presence of a catalytic amount of palladium(O), silylated vinyloxiranes (310) have been found to rearrange into a-silylated-/I, y-unsaturated aldehydes (312), not only with complete chirality transfer, but also with total retention of the double bond stereochemistry. A mechanism involving a [l,2]-silicon shift from carbon to carbon via a 7r-allylic palladium complex (311) has been invoked358 for the transformation. New... 

A further efficient preparation of these silyl enol ethers proceeds through an intramolecular 1,2-silicon shift in an a-silyl acyl lithium substrate (8), prepared from an a-lithiosilane (Scheme 23)". This method appears very simple to carry out, and produces the silyl enol ethers in good yields with high isomeric purity (usually, E isomer > 90% of mixture). 

Nakajima has shown that a-cyclopropyl acyl silane (23) results from reaction of 1-trimethylsilyl cyclopropyl lithium with dichloromethyl methyl ether at low temperature in THF solution, in a reaction said to involve a carbene intermediate and a 1,2-silicon shift (Scheme 56)147. 

As already illustrated in equations 3 and 4, steric interactions prevent the generally preferred 1,3-silicon shift in favour of a 1,2-shift allowing introduction of two silyl groups in neighbouring positions (equation 4). It should also be noted that the analogous 1,2-shift cannot be excluded in step (b) for the rearrangement processes shown in Scheme 12. 

Acylsilanes. Carbonylation of trimethylsilylmethyllithium (1) in ether at 15° followed by quenching with ClSilCH,), results in the trimethylsilyl enolate (2) of acetotri-methylsilane. The reaction evidently involves insertion of CO to give an acyllithium (a), which undergoes a 1,2-silicon shift to give the lithium enolate (b) of an acylsilane. 

The reaction of a 1,1-dibromocyclopropane with an alkyllithium generally leads to 1,1-elimination producing a cyclopropylidene, which normally rearranges to an allene, but may be trapped by other carbenic reactions. When there is a silyl substituent on C2, the carbene is apparently trapped by a 1,2-silicon shift giving a silyl-substituted cyclopropene. ... 

A mechanism is proposed to accommodate the observed stereoselectivity. The mechanism includes a first silicon shift to an acetylenic bond and a carbene-type zwit-terionic rhodium complex (7) as the key intermediate, which undergoes isomerization from a higher energy form (Z-complex, 6) to a lower energy form ( -complex, 8) followed by reductive elimination to cis-isomer (3) as the kinetic product. 

Debromination. Selective dehromination of 1,1-dihromoalkenes affords predominantly (98 2) the ( )-isomers. gem-Dihromocyclopropanes are dehrominated. A preparation of 1-trialkylsilylcyclopropenes hy this method takes advantage of a 1,2-silicon shift." ... 

Two useful syntheses of cyclopropyl acyl silanes have been reported (Scheme 62). In a process that involves 1,2-silicon shift, cyclopropyl acyl silanes (211) may be prepared in moderate yield. Lewis or protic acid treatment of (211) leads to either acyclic... 

Dienes. The reaction of Fischer carbene complexes with allenes gives 1,3-dienes into which the nonmetal moiety is incorporated. The transformation is a formal ene-type process. 1-Alkoxy-1,3-dienes arise when the carbene complexes and propargylsilanes are heated together. A 1,2-silicon shift accompanies the carbene insertion into the C-H bond. 

The silane (144) undergoes a C to O silicon shift on irradiation, the resulting oxacarbene (145) being trapped by maleic ester as the cyclopropanol derivative (146). 

Thermolysis of the silane (154) results in extrusion of phenylmethoxycarbene, which can be trapped by diphenylacetylene as the cyclopropenyl ether (155). Interestingly, the silyl ether (156) is formed with 95% retention of configuration by a C to O silicon shift. ... 

Seiser, T. Cramer, N. Rhodium(I)-Catalyzed 1,4-Silicon Shift of Unactivated Silanes from Aryl to Alkyl Enantioselective Synthesis of Indanol Derivatives. Angew. Chem., Int. Ed. 2010,49,10163-10167. 

Scheme 13 Enantioselective -carbon elimination induced by rhodium(I)-catalyzed 1,4-silicon shift of trialkyl silanes...

Seiser T, Cramer N (2010) Rhodium(I)-catalyzed 1,4-silicon shift of unactivated silanes from aryl to alkyl arantioselective synthesis of indanol derivatives. Angew Chem Int Ed 49(52) 10163-10167. doi 10.1002/anie.201005399... 

While 576 represents the expected elimination product, 577 is the result of an 1,2-silicon shift. 

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