Lithium, dibromomethyl

Thus, only few reports were disclosed for the [1,3] Brook isomerization Utimoto, Oshima and coworkers have reported that the treatment of tert-butyldimethyl(dibromomethyl)silane 64 with LDA followed by the addition of an excess of benzaldehyde lead to the 1,3-diol monosilyl ether 66 via the intermediacy of lithium carbenoid 65 (equation 24) . The rate of isomerization was dependent on the solvent used and HMPA was found to be the best solvent. ... 

Chiral cyclic boronic esters with (dichloroniethyl)lithium at —100 C form borate complexes4. Borate complexes cart also be formed by generation of (dichloromethyl)lithium from dichloro-methane and lithium diisopropylamide in the presence of a boronic ester at —78 C to — 5 C (Section 1.1.2.1.2.2,)28,19. In situ generation of (dibromomethyl)lilhium is required for preparing a-bromo boronic esters (see Sections 1.1.2.1.1.2. and 1.1.2.1.3.2.). 

Ester enolates replace bromide from a-bromo boronic esters with remarkable diastereoselcctiv-ity. (Dibromomethyl)lithium is generated by addition of lithium diisopropylamide to dibro-momethane in the presence of a boronic ester at — 78 "C to produce an a-bromo boronic ester. Reaction of the a-bromo boronic ester with lithium 1-tert-butoxy-Tpropen-l-olate yields a product that is almost exclusively the threo-isomer (d.r. = 15 1 to 60 1), as shown by conversion to the / -hydroxy carboxylic ester24. It is worth noting the facility with which a-bromo boronic esters racemize in the presence of halide ions72. 

Treatment of dibromomethyl methyldiphenylsilane with lithium tributylmagnesate at —78°C induced very efficient bromine-magnesium exchange to yield the bromomethylsilane upon protonolysis at —78°C (Scheme 13)" ". Warming the reaction mixture in the presence of a copper salt before protonolysis led to smooth migration of one of the butyl groups to afford 1-silylpentyhnetal. 

Bromomethyl ketones.l Reaction of (dibromomethyl)lithium (1) with esters and then with BuLi (excess) generates the enolate (a) of a bromomethyl ketone. Quenching with acidic ethanol gives the bromomethyl ketone (2) in 70-85% yield. Quenching with A O generates bromo enol acetates. 

In the pioneering work by Wilcox and Gaudino, a straightforward route to the carbocyclic analogue of D-fructofuranose, 64, and its 6-phosphate derivative was delineated [14a,b]. As shown in Scheme 9, the first move consisted of Wittig olefination of benzyl-protected arabinose 60 with carboxy-tert-butylmethylene triphenyl phosphorane to deliver unsaturated ester 61, which was then cleverly elaborated into dibromide 62 via a reaction cascade encompassing Swem oxidation of the secondary OH, ester hydrolysis, diastereoselective addition of dibromomethyl lithium, and carboxylic acid methylation. 

The successive reaction of (dibromomethyl)silanes with LDA (hthium diisopropyl-amide) and two equivalents of benzaldehyde gives 1,3-diol monosilyl ethers in good yield (Scheme 10.221) [574]. This tandem reaction would proceed via anionic 1,3-silyl migration of /l-lithioxyalkylsilane intermediate 152 and addition of the resulting lithium carbenoid to benzaldehyde. Thus, internal activation of the silicon-lithium alkoxide promotes nucleophilic addition of a-haloalkylsilanes. Similar tandem reactions of 2-trimethylsilyl-l,3-dithiane with aldehydes [575] and epoxides [576] have been reported. 

A useful one-carbon ring-expansion reaction which has been developed involves treatment of l-(dibromomethyl)-cycloalkan-l-ols with n-butyl-lithium in hexane to give the ring-expanded ketones, usually in excellent yield e.g. l-(dibromomethyl)-cyclododecan-l-ol, which was obtained by treating cyclododecanone with dibromo-methyl-lithium, gave cyclotridecanone in 89% isolated yield. ... 

Dibromoalcohols, which are readily prepared by addition of dibromomethyl-lithium to aldehydes or ketones, undergo reductive elimination on treatment with zinc and acetic acid in refluxing methylene chloride to give vinyl bromides as mixtures of E- and Z-isomers (Scheme 30). Conjugated 1-bromo-dienes or -trienes can be prepared from the appropriate unsaturated carbonyl compounds. 

Deprotonation. (Dibromomethyl)trimethylsilane can be de-protonated with lithium diisopropylamide at —78 °C. Reaction of the intermediate anion with n-butyl iodide (eq 4) furnished the alkylated product in 93% yield. If the anion was allowed to warm to 20 °C in the absence of an electrophile, the bis(trimethyl-silyl)ethylene (eq 5) was produced. ... 

Lithium-Halogen Exchange. Reaction of (dibromomethyl)-trimethylsilane with n-butyllithium at — 110°C results in the formation of trimethylsilylbromomethyllithium. The silane and the -BuLi are added simultaneously in order to suppress side reactions. Treatment of the intermediate lithium reagent with chlorotrimethylsilane (eq 1) or mercury(II) bromide (eq 2) affords bis(trimethylsilyl)bromomethane or bis(trimethylsilyl-bromDmethyl)mercury, respectively. If the lithium reagent is... 

A convenient synthesis of a-hydroxysilanes from the reaction of trialkylboranes and lithium with trimethyl(dibromomethyl)silane followed by hydrogen peroxide oxidation has appeared [reaction (6)]. Phenylchloroboranes have been im-... 

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