Hexamethyl disilazides

Deprotonation of / /-cyclohexylfluoroacetone imines by lithium hexamethyl-disilazide or tertiary butyl lithium at very low temperature allows a regioselective alkylation at the carbon carrying fluorine [773] (equation 98). 

Lithium cyclohexylisopropylamide Lithium diisopropylamide Lithium hexamethyl disilazide Lithium 2,2,6,6-tetramethylpiperidide Methylaluminum I ij-(4-bromo-2,6-di-tert-butylphenoxide) I (s-(2,6-di-t-butyl-4-methylphenoxy)methyl aluminum mefa-Chloroperoxybenzoic acid Methyl... 

Related competitions between a propargylic ether on one side and a propargylic acetal on the other always delivered the product of an isomerization in the direction of the ether. Compound 96 with stoichiometric amounts of potassium hexamethyl-disilazide serves as a recent example [228] (Scheme 1.42) other references describe the same reactivity [229-231],... 

Mukaiyama found that Lewis acids can induce silyl enol ethers to attack carbonyl compounds, producing aldol-like products.22 The reaction proceeds usually at -78 °C without selfcondensation and other Lewis acids such as TiCl4 or SnCI4 are commonly used. The requisite silyl enol ether 27 was prepared by treatment of ketone 13 with lithium hexamethyl disilazide (LiHMDS) and trapping the kinetic enolate with chlorotrimethylsilane. When the silyl enol ether 27 was mixed with aldehyde 14 in the presence of BF3-OEt2 a condensation occurred via transition state 28 to produce the product 29 with loss of chlorotrimethylsilane. The induced stereochemistry in Mukaiyama reactions using methylketones and a, -chiral aldehydes as substrates... 

Sodium hexamethyl disilazide (NaHMDS) is a strong non-nucleophilic base. 

Notwithstanding us having side-stepped the internal hemiacetalisation problem, difficulties were once again encountered when we attempted to methylenate 66 under Wittig or Tebbe conditions. In both cases, starting aldehyde was always recovered untouched from either reaction. Fortunately, when 66 was subjected to Kocienski/Julia olefmation with 5-methanesulfonyl-1-phenyl-1//-tetrazole and potassium hexamethyl-disilazide at low temperature, diene 67 was formed in 77% yield. 

Hexamethylphosphoramide (Me3N)3P=0 Hexamethylphorous triamide (Me3N)3P Isopropyl — CH(CH3)2 Lithium cyclohexylisopropylamide Lithium diisopropylamide LiN(iPr)2 Lithium hexamethyl disilazide LiN(SiMe3)2 Lithium 2,2,6,6-tetramethylpiperidide meta-Chloroperoxybenzoic acid Methyl — CH3... 

Related Reagents. Lithium Amide Lithium Hexamethyl-disilazide Lithium Diethylamide Lithium Piperidide Lithium Pyrrolidide Lithium 2,2,6,6-tetramethylpiperidide Potassium Diisopropylamide. 

Reductive elimination of sugar tosyl hydrazones using sodium hexamethyl-disilazide/LAH provides a route to various homochiral dioxolanes. Thus, D-xylose, D-arabinose and D-xylose derivatives 71-73 were converted to isopro-pylidene acetals 74-76, while D-xylose derivatives 77 and 78 provided 6-membered acetals 79 and 80, respectively. ... 

The preparation and immediate reaction of alkyllithium for deprotonation of various acidic compoimds precisely illustrate this aspect (Fig. 34).i 8 representative reaction is the quantitative formation in a few minutes of 2-lithio-1,3-dithiane by sonication of f-butyl chloride, lithium, and 1,3-dithiane in THF. In situ generated butyllithium was used for the deprotonation of alkynes and phosphonium salts. With secondary amines, the strongly basic lithium amides are produced (p. 368). This procedure avoids the use of organolithium auxiliaries with solvent and temperature adjustments. o The isotopically labeled Li hexamethyl disilazide was obtained by this method.i i... 

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