Dibromomethane acidity

With special techniques for the activation of the metal—e.g. for removal of the oxide layer, and the preparation of finely dispersed metal—the scope of the Refor-matsky reaction has been broadened, and yields have been markedly improved." The attempted activation of zinc by treatment with iodine or dibromomethane, or washing with dilute hydrochloric acid prior to use, often is only moderately successful. Much more effective is the use of special alloys—e.g. zinc-copper couple, or the reduction of zinc halides using potassium (the so-called Rieke procedure ) or potassium graphite. The application of ultrasound has also been reported. ... 

Several modifications of the Simmons-Smith procedure have been developed in which an electrophile or Lewis acid is included. Inclusion of acetyl chloride accelerates the reaction and permits the use of dibromomethane.174 Titanium tetrachloride has similar effects in the reactions of unfunctionalized alkenes.175 Reactivity can be enhanced by inclusion of a small amount of trimethylsilyl chloride.176 The Simmons-Smith reaction has also been found to be sensitive to the purity of the zinc used. Electrolytically prepared zinc is much more reactive than zinc prepared by metallurgic smelting, and this has been traced to small amounts of lead in the latter material. 

COMPOUND NAME CHLORODIFLUOROMETHANE DICHLOROFLUOROMETHANE CHLOROFORM HYDROGEN CYANIDE DIBROMOMETHANE DICHLOROMETHANE FORMALDEHYDE FORMIC ACID METHYL BROMIDE METHYL CHLORIDE METHYL FLUORIDE METHYL IODIDE NITROMETHANE METHANE METHANOL METHYL MERCAPTAN METHYL AMINE METHYL HYDRAZINE METHYL SILANE... 

Another titanium-based reagent for the methylenation of carbonyl compounds is that prepared from dibromomethane/zinc/titanium tetrachloride and related systems (Scheme 14.25) [48]. These systems transform a wide variety of carboxylic acid derivatives to terminal olefins in the same way as titanocene-methylidene does. 

Employment of Zn—Cu couple is representative of another approach. For example, reaction of perfluoroalkyl iodides with carbonates gives fluorocarboxylic esters (equation 107)132. Similarly, reaction of perfluoroalkyl iodides with Zn—Cu couple and C02 or S02 in DMSO affords the perfluorocarboxylic acids and sulfonyl chlorides, respectively133. A double methylene inserted product is formed when dibromomethane is used as a substrate (equation 108)134. 

Thiocarbamoylation of malonic acid derivatives with phenyl isothiocyanate gives the sodium salts 193 which are not isolated. Alkylation of these salts in situ using dibromomethane or bromochloromethane yields the 1,3-thiazetidine derivatives 40 (Scheme 63) <2005MI499>. 

The difficulty in obtaining this monomer in the pure state arises from the fact that the known methods of preparation involve the simultaneous formation of considerable amounts of the isomers of spiropentane which are difficult to remove. The method adopted as giving the most satisfactory yields is that of Applequist, Fanta, and Henrickson (I, 2). The spiropentane is prepared by reaction of zinc dust with pentaerythrityl tetrabromide in alcohol in presence of the sodium salt of ethylenediamine-tetraacetic acid as complexing agent. The yield of hydrocarbon (spiropentane plus various ethylenic compounds) is of the order of 84%. The spiropentane is obtained in the pure state by treating the mixture with bromine in dibromomethane. The yield of pure spiropentane was found to be 62%. 

These Lewis acids require a two solvent medium for optimum utility. The perfluoro aliphatic component constitutes the first which dissolves the catalyst (1). The second component is preferably consists of a halocarbon such as dichloro- and dibromomethane mono- and dichlorobenzene chlorotoluene and bromobenzene. Toluene has also been found to be effective. Interfacial agents have also been used and are discussed (2). 

With regard to the preparation of the Zn/Cu couple, several methods have been developed Dibromomethane may be used instead of diiodomethane when a specifically prepared Zn/Cu couple is applied . The use of Zn/Ag couple often gives better results . Ultrasound irradiation of the reaction mixture has been shown to facilitate the reaction . The cyclopropanation of alkenes with diiodomethane and diethylzinc can be carried out in hydrocarbon solvents and is particularly suitable with easily polymerizable olefins such as vinyl ethers . It has been reported that molecular oxygen remarkably promotes the reaction of diiodomethane with diethylzinc and substantially increases the yield of the adducts . A convenient modification which avoids the handling of pyrophoric diethylzinc has been reported . In reaction of olefins which are sensitive to the unavoidably produced zinc iodide (the Lewis acid), the addition of one equivalent of dimethoxyethane (DME) to the solvent has been recommended . Zinc iodide is then precipitated as the 1 1 DME complex as it is formed. Zinc salts, which often complicate the workup of the reaction mixture, can also be removed as precipitates by the addition of pyridine prior to the workup . ... 

Cyclopropanation of alkenes carrying electron-withdrawing groups with dihalo-carbenes does not take place under the normal reaction conditions unless the dihalocar-bene is generated from trihalomethylphenylmercury compounds. By this procedure acrylonitrile was converted into l-cyano-2, 2-dichlorocyclopropane in 78% yield and other acrylic derivatives are transformed into dichloromethyl cyclopropane carboxylates (186) (equation 46). Treatment of electron-deficient olefins with dibromomethane in the presence of Ni(0) complex/Zinc/Lewis acid seems to be very effective for cyclopropanation. ... 

A reaction that appears to be mechanistically similar to the Tebbe reaction was developed by Oshima in 1978. Diiodomethane or dibromomethane in the presence of zinc is treated with a Lewis acid to form, presumably, a divalent complex (72), which reacts with aldehydes and ketones to produce the corresponding methylene derivative (73 Scheme 18). This reagent complements the reactivity of the Tebbe reagent, in that the zinc methylenation is not reactive towards esters or lactones. Because it is an electrophilic reagent, it is suitable for the methylenation of enolizable ketones and aldehydes. 

Di(2-ethylhexyl) phthalate Chloromethane Methyl ethyl ketone 2-Hexanone Fluoranthene Benzo [a] anthracene Benzo [b] fluoranthene Benzo [k] fluoranthene Benzo [a] pyrene Benzo [g,h,i]perylene Dibromomethane Bromochloromethane Monochloroacetic acid Dichloroacetic acid Trichloroacetic acid... 

On the other hand, diacetone D-glucose was transformed to (-)- and (+)-sesbanimide A via the open chain derivative 3-0-benzyl-D-glucose diethyldithioacetal 17, which was prepared by hydrolysis of 3-C -benzyl-l,2 5,6-di-C -isopropylidene-D-glucofuranose (16) and subsequent treatment with ethanethiol in the presence of acid (Scheme 3)4 Acetonation of 17 under kinetic control followed by methylenation with dibromomethane afforded the key intermediate 1,3-dioxane 18. Deprotection of the dithioacetal unit in 18 followed by... 

Benzodioxins.—A synthesis of 5- and 8-hydroxy-l,3-benzodioxin-4-ones from 2,6- and 2,3-dihydroxybenzoic acid, dibromomethane, and benzaldehyde diacetate has been described. AA -Carbonyldi-imidazole reacts with 2-hydroxybenzamide to give the 1,3-benzodioxin in 72% yield (Scheme 9). Amides which lack intramolecular hydrogen-bonding react differently. ... 

Dihydro-l,3-dithiepins (218) have usually been obtained from (Z)-l,4-dichloro-2-butene (214) via the isothiouronium salt (215) (Scheme 36). For the preparation of the 2-unsubstituted 4,7-dihydro-l,3-dithiepin, the isothiouronium salt can be reacted directly with dibromomethane and potassium hydroxide in methanol <76TL1251,780PP133), but isolation of the dithiol (217) and acid-catalyzed reaction with an aldehyde or ketone in a second step is preferred for the preparation of 2-substituted 4,7-dihydro-1,3-dithiepins <93JCS(P2)87l>. 

There is no such compound as methanediol, so the polymer cannot be made by the usual reaction between diacid and diol, and is instead synthesised from an alkali metal salt of terephthalic acid and dibromomethane. Trials with this polymer [83] showed that it rapidly thermally degraded at the melt processing temperature. It was suggested that this initially involved loss of formaldehyde and formation of anhydride structures, swiftly followed by crosslinking and rapid discoloration. 

Interaction of a ribonucleoside with dibromomethane under phase-transfer conditions gave 23 - -methylene acetals. In the case of the uridine derivative, glycosyl cleavage occurred in acid before complete deprotection, and, since purine nucleosides are more labile, methylene acetals are unlikely to be of any practical... 

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