Sila elimination

The above mechanism involves a-opening of the epoxysilane, followed by a 1,2-elimination of a /3-hydroxysilanc (Peterson olefination, Chapter 10). However, it has recently been shown that aj8-dihydroxysilanes, particularly t-butyldimethylsilyl species, undergo an acid-catalysed sila-pinacol rearrangement to produce /J-aldehydo- and /i-kctosilancs (5) ... 

DMSO or other sulfoxides react with trimethylchlorosilanes (TCS) 14 or trimefhylsilyl bromide 16, via 789, to give the Sila-Pummerer product 1275. Rearrangement of 789 and further reaction with TCS 14 affords, with elimination of HMDSO 7 and via 1276 and 1277, methanesulfenyl chloride 1278, which is also accessible by chlorination of dimethyldisulfide, by treatment of DMSO with Me2SiCl2 48, with formation of silicon oil 56, or by reaction of DMSO with oxalyl chloride, whereupon CO and CO2 is evolved (cf also Section 8.2.2). On heating equimolar amounts of primary or secondary alcohols with DMSO and TCS 14 in benzene, formaldehyde acetals are formed in 76-96% yield [67]. Thus reaction of -butanol with DMSO and TCS 14 gives, via intermediate 1275 and the mixed acetal 1279, formaldehyde di-n-butyl acetal 1280 in 81% yield and methyl mercaptan (Scheme 8.26). Most importantly, use of DMSO-Dg furnishes acetals in which the 0,0 -methylene group is deuter-ated. Benzyl alcohol, however, affords, under these reaction conditions, 93% diben-zyl ether 1817 and no acetal [67]. 

An unexpected elimination of cyclopentadienide anion results on reaction of sila-cyclohexadienes with an extremely hindered aryllithium. ... 

Three different routes to the key compounds for the sila-Peterson elimination, the a-alkoxydisilanes 157, are described in the literature, namely A, reaction of silyllithium reagents with ketones or aldehydes B, addition of carbon nucleophiles to acylsilanes C, deprotonation of the polysilylcarbinols. In addition, method D, which already starts with the reaction of 2-siloxysilenes with organometallic reagents, leads to the same products. The silenes of the Apeloig-Ishikawa-Oehme type synthesized so far are summarized in Table 4. 

When the l-diazo-2-silyl moiety is incorporated into cycles then endocyclic silicon-carbon double bonds should be formed. Most interesting in this connection is the decomposition of l-diazo-2-sila-3,5-cyclohexadiene 331, because the initial Si=C product should be silabenzene 332170. The outcome of the nitrogen elimination depends on the conditions used. The products isolated are 333 (equation 81), 335 via bicyclic 334 (equation 82) and 338, formed via silafulvenes and 336 (equation 83). 

Enantiomerically pure (.E)-l,l,3,3,6,6-hexamethyl-l-sila-4-cycloheptene 25, the smallest nonbridged (it)-cycloalk-ene which can be isolated in a pure form at room temperature, was synthesized by the Corey-Winter elimination of hexamethyl-l-sila-fra r-4,5-cycloheptanethiocarbonate 24 with l,3-dimethyl-2-phenyl-l,3,2-diazaphospholidine (Equation 7) C1997AGE159, 1999TA3483>. 

In 1998, Langlois et al. completed the third synthesis of manzamine C using a strategy based on Sila-Cope elimination (Scheme 8.9) [80]. In this approach, the key intermediate, piperidine derivative 146, was prepared in six steps from 2-methylpyr-idine 143. Oxidation of 146 afforded a mixture of diastereomeric N-oxides 147 and 148. Sila-Cope elimination of N-oxide 147 led to compound 149, while Cope elimination of N-oxide 148 led to compound 150. Oxidative cleavage of the N-O bond in 149 followed by treatment with N-chlorosucdnimide provided the chlor-oamine 153. Elimination, hydrolysis, ditosylation, and finally basic treatment of the ditosylate intermediate afforded the cyclic sulfonamide 124, which is the direct synthetic precursor of manzamine C [73]. 

The iron complex 73 is prepared from the germolyl anion and iron(ll) chloride <20020M1734>. This complex 73 can be further lithiated with MeLi to 74 and elimination of Li[Si(SiMc3)3] gives the l,l -sila-[l]-l,l -digermaferro-cenophane 75 <20020M1734>. 

Following a single oral dose of 20 mg to 3 subjects, peak plasma concentrations of 0.010, 0.013, and 0.024 pg/ml of debrisoquine were reported at 1.5 to 3.5 hours peak concentrations of the 4-hydroxy metabolite averaging 0.035 ig/ml were attained in 1.5 to 2.5 hours. The 4-hydroxy metabolite was eliminated more rapidly than debrisoquine and was not detectable after 24 hours (J. H. Silas et al., Br. J. din. Pharmac., 1978,5, 27-34). 

On treatment of the vinyl-fiinctionalized hypervalent silane (2-Me2NCH2C6H4)(CH=CH2)Si(H)2 (1) with sulfur in chloroform at 25 °C the formation of the l-thia-2-sila-1,3-diene 9 is observed. It appears that 9 is extremely sensitive to moisture By elimination of H2S oligomeric 3 is formed [6]. 

The formation of heterobutadiene 9 can be explained by an insertion-elimination reaction Insertion of sulfur into the silicon-hydrogen bond yields in the first step (2-Me2NCH2C6H4)(CH=CH2)Si(SH)2 as intermediate. This species eliminates H2S, producing the 1-thia-2-sila-1,3-diene 9, which is one of the rare examples possessing a silicon-sulfiir double-bond unit... 

Although TsiSiMe2Cl is not susceptible to direct substitution by sodium methoxide in MeOH, it does nevertheless react, but the major product is (Me3Si)2CHSiMe2OMe148. The most likely mechanism for this reaction involves an intermediate sila-alkene, as shown in Scheme 5. It is interesting to note that this mechanism is directly analogous to the E2 elimination mechanism at carbon. A similar reaction has also been observed for TsiSi(CH=CH2)2Cl with sodium methoxide131. 

---