Strongly Basic Reagents

Although some studies dealing with the vinylic metallation of ethene homologues by alkyl sodium have been reported [63, 64], this direct metallation is not interesting from a synthetic point of view, because allylic deprotonation occurs to a considerable extent [65]. Interaction between Lbutylethene (ten-fold excess) and BuLi TMEDA in hexane at reflux temperature, followed by quenching with dimethyl disulfide, resulted in a low yield of the expected vinylic sulfide (only the E-isomer was isolated) [9]  

We succeeded in generating vinylpotassium by passing ethene through a solution of BuLw-BuOK TMEDA in hexane at — 20 °C (21). Derivatization reactions with diphenyl disulfide or benzaldehyde (after addition of THF and lithium bromide) gave the expected derivatives in excellent yields. 

Vinylic hydrogen atoms in strained ring systems have an increased acidity [65]. Cyclopropenes can be smoothly metallated with alkyllithium [66] (in the case of the unsubstituted hydrocarbon, addition across the double bond occurs), lithium dialkylamides in THF [9], or with alkali amides in liquid ammonia [16]. Whereas reaction of propene with strong bases results in the exclusive formation of the allylic derivative, the kinetically preferred process with 1-methylcyclopropene is vinylic deprotonation  

Unfortunately, cyclopropenes are not readily available, and several of them are very unstable. These factors may explain why not much research has been carried out with these olefins. 

The thermodynamic acidity, and kinetic preference for vinylic deprotonation decrease with increasing size of the ring, and in the case of cyclohexene allylic deprotonation predominates [65]. Reaction of cylopentene (two-fold excess) and BuLi f-BuOK TMEDA in hexane at — 20 °C gives the vinylic and allylic 

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A more promising procedure for the formation of alkenes from tosylhydrazones is represented by the Shapiro reaction It differs from the Bamford-Stevens reaction by the use of an organolithium compound (e.g. methyl lithium) as a strongly basic reagent ... 

On treatment with a strong base such as sodium hydride or sodium amide, dimethyl sulfoxide yields a proton to form the methylsulfinyl carbanion (dimsyl ion), a strongly basic reagent. Reaction of dimsyl ion with triphenylalkylphosphonium halides provides a convenient route to ylides (see Chapter 11, Section III), and with triphenylmethane the reagent affords a high concentration of triphenylmethyl carbanion. Of immediate interest, however, is the nucleophilic reaction of dimsyl ion with aldehydes, ketones, and particularly esters (//). The reaction of dimsyl ion with nonenolizable ketones and... 

Strongly basic reagents, such as lithium dialkylamides, are required to promote the reaction. The stereochemistry of the ring opening has been investigated by deuterium labeling. A proton cis to the epoxide ring is selectively removed.115... 

Detailed information about the use, handling and disposal of strongly basic reagents and allcali metals is given in Chapter I of Preparative Polar Organometaliic Chemistry, Vol. 1 [1],... 

Exps. 9 and 10 are convincing illustrations of the high lrineiic acidity of the ethynyl proton. In principle, there are four reaction pathways if l-bromo-5-hexyne and a strongly basic reagent are allowed to interact abstraction of the acetylenic proton, Br-metal exchange, displacement of Br by the "nucleophilic" part of the base, and elimination of HBr with formation of HCsC(CH2)2CH=CH2- Only the first process takes place under the conditions of this experiment. The kinetic stability of the intermediate LiOC(CH2)4Br is sufficient to allow for successful functionalizations with a number of reagents. For alkylations with most of the alkyl halides, the polarity of the medium will usually be insufficient. 

Double 1,6-dehydrochlorination of I,6-dichloro-2,4-hexadiyne, analogous to the formation of butadiyne from 1,4-dichloro-2-butyne and strongly basic reagents, was described for the first time by Bohlmann and Jones et al. [147]. The extremely unstable 1,3,5-hexatriyne was obtained in a moderate yield as a dilute solution in an organic solvent. 

Until recently only a few examples of stereoselective alkylation reactions of localized carban-ions which proceed under auxiliary control have been reported. The reason is obviously to be found in the difficulty of generating such carbanions having no additional stabilization and, if generated, in the low nucleophilicity of these strongly basic reagents. 

Methylsulfinyl Carbanion. Strong bases, eg., sodium hydride or sodium amide, react with DMSO producing solutions of methylsulfinyl carbanion, known as the dimsyl ton, which arc synthetically useful. The solutions also provide a strongly basic reagent for generating other carbanions. 

Unlike alcohols, ethers are not acidic and usually do not react with bases. However, exceptionally strong basic reagents, particularly certain alkali-metal alkyls, will react destructively with many ethers ... 

Because NH3 contains hydrogens, it can act as an acid, and because it has an unshared pair of electrons, it can act as a base. We tend to think of it as a base because that is how it reacts with water. However, in the presence of a strong base, NH3 can react as an acid. Because many organic reactions involve strongly basic reagents, we need to be aware of the potential of any hydrogen-containing species to donate a proton. 

Ethers are also commonly used as solvents for reactions involving strongly basic reagents because they do not have acidic hydrogens but are still polar enough to dissolve the reagents. Ethers that are used include diethyl ether, /etrafrydro/iiran (THF),... 

A major use of ethers in the organic laboratory is as solvents for reactions. Ethers are nonpolar enough to dissolve many organic compounds, and the electrons on the oxygen can interact with alkali metal cations to help solubilize salts. In addition, ethers are nonacidic and are not very reactive. For these reasons they are especially useful in reactions involving strongly basic reagents. In addition to diethyl ether, other ethers that are commonly used as solvents are 1.2-dimethoxyethane (DME) and the cyclic ethers tetrahydrofuran (THF) and 1,4-dioxane ... 

Pathway (a) is followed in the case of the more weakly basic reagents (LiCH2CN, LiCH2C02Et, etc.) and pathway (b) with strongly basic reagents. 

Many organic reactions must be carried out in solvents that are stable to strongly basic reagents. Use SpartanView to compare electrostatic potential maps of th possible solvents ethanol, diethyl ether, and acetone (CH3COCH3). Which of t three is least likely to react with a strong base Explain. 

Ventron also offers Sodium borohydride-SWS, a stabilized water solution of average composition 40% sodium hydroxide and 12% sodium borohydride, sp. gr. 1.4. It is stable indefinitely and can be used as such for reduction of carbonyl compounds or it can be diluted with water, methanol, or ethanol. Most aldehydes are reduced so rapidly that condensations promoted by the alkali do not interfere. For reduction of an a,/3-unsaturated aldehyde, however, the strongly basic reagent is unsatisfactory and should be diluted to the desired concentration and neutralized with carbon dioxide. The neutralized reagent should be used immediately, since the borohydride is no longer stable and decomposes at 25° at the rate of 4.5% per hr. 

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