Alkoxide ion reactions

The cosurfactant in this approach cannot be a primary or secondary alcohol otherwise alkoxide ion reactions will dominate at high pH. 

The oxidative behaviour of glycolaldehyde towards hexacyanoferrate(III) in alkaline media has been investigated and a mechanism proposed, which involves an intermediate alkoxide ion. Reactions of tetranitromethane with the luminol and luminol-peroxide radical anions have been shown to contribute substantially to the tetranitromethane reduction in luminol oxidation with hexacyanoferrate(III) in aerated aqueous alkali solutions. The retarding effect of crown ethers on the oxidation of triethylamine by hexacyanoferrate(III) ion has been noted. The influence of ionic strength on the rate constant of oxidation of ascorbic acid by hexacyanofer-rate(III) in acidic media has been investigated. The oxidations of CH2=CHX (where X = CN, CONH2, and C02 ) by alkaline hexacyanoferrate(III) to diols have been studied. ... 

In general, thiazoles and benzothiazoles are resistant to reaction with hydroxide and alkoxide ions. Reactions of thiazoles and thiazolium salts with oxygen nucleophiles have been discussed in detail in CHEC(1984) and CHEC-11(1996) <1996GHEC-II(3)373>. 

We have already seen how the acidity of alcohols resembles the acidity of water. Here two general approaches are presented for the removal of a proton from an alcohol to form an alkoxide ion reaction with strong bases (such as [(CH3)2CH]2N- or hydride) and reaction with active metals (especially alkali metals). Alkoxides are readily available species whose reactions will be explored at several places in this chapter. 

Recent spectroscopic studies point to the effect of alkyl-group substituents in stabilizing negative charge centers. To see if such effects could be observed for alkoxide ion reactions, CH3O" ions were reacted with... 

Recall from Section 8 13 that the major pathway for reaction of alkoxide ions with secondary alkyl halides IS E2 not Sn2... 

Alkanethiolate ions (RS ) are weaker bases than alkoxide ions (RO ) and undergo synthetically useful 8 2 reactions even with secondary alkyl halides... 

The Williamson ether synthesis (Sec tion 16 6) An alkoxide ion displaces a halide or similar leaving group in an Sn2 reaction The alkyl halide cannot be one that is prone to elimination and so this reaction is limited to methyl and primary alkyl halides There is no limitation on the alkoxide ion that can be used... 

The reaction between an alkoxide ion and an aryl halide can be used to prepare alkyl aryl ethers only when the aryl halide is one that reacts rapidly by the addition-elim mation mechanism of nucleophilic aromatic substitution (Section 23 6)... 

Weak acid (Section 1 16) An acid that is weaker than 1130" Weak base (Section 1 16) A base that is weaker than HO Williamson ether synthesis (Section 16 6) Method for the preparation of ethers involving an Sfj2 reaction between an alkoxide ion and a primary alkyl halide... 

Silane reacts with methanol at room temperature to produce methoxymonosilanes such as Si(OCH2)4 [78-10-4] HSi(OCH2)3, and H2Si(OCH3)2 [5314-52-3] but not H SiOCH [2171 -96-2] (23). The reaction is catalyzed by copper metal. In the presence of alkoxide ions, SiH reacts with various alcohols, except CH OH, to produce tetraalkoxysHanes and hydrogen (24). 

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4 -chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4 -sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

Solvent for Base-Catalyzed Reactions. The abihty of hydroxide or alkoxide ions to remove protons is enhanced by DMSO instead of water or alcohols (91). The equiUbrium change is also accompanied by a rate increase of 10 or more (92). Thus, reactions in which proton removal is rate-determining are favorably accompHshed in DMSO. These include olefin isomerizations, elimination reactions to produce olefins, racemizations, and H—D exchange reactions. 

The 3-substituents in 3-nitro- and 3-phenylsulfonyl-2-isoxazolines were displaced by a variety of nucleophiles including thiolate, cyanide and azide ions, ammonia, hydride ions and alkoxides. The reaction is pictured as an addition-elimination sequence (Scheme 54) (72MI41605, 79JA1319, 78JOC2020). 

This variation from the ester hydrolysis mechanism also reflects the poorer leaving ability of amide ions as compared to alkoxide ions. The evidence for the involvement of the dianion comes from kinetic studies and from solvent isotope effects, which suggest that a rate-limiting proton transfer is involved. The reaction is also higher than first-order in hydroxide ion under these circumstances, which is consistent with the dianion mechanism. 

A competing reaction in any Birch reduction is reaction of the alkali metal with the proton donor. The more acidic the proton donor, the more rapid IS the rate of this side reaction. Alcohols possess the optimum degree of acidity (pKa ca. 16-19) for use in Birch reductions and react sufficiently slowly with alkali metals in ammonia so that efficient reductions are possible with them. Eastham has studied the kinetics of reaction of ethanol with lithium and sodium in ammonia and found that the reaction is initially rapid, but it slows up markedly as the concentration of alkoxide ion in the mixture... 

The fluonne atom of monofluoroaromatk systems activated toward the S Ar reaction by electron-withdrawmg groups is readily replaced by alkoxide ion [7, 2, 3, 4, 5] (equations 1 and 2),... 

The reaction of chlorodifluorotnethane with alkoxide ions generated in low concentration from halide ions and epoxides [28, 29] is an interesting, higher temperature method that gives good to excellent yields of gem-difluorocyclo-propanes from just moderately nucleophilic olefins (equation 9)... 

These reactions are usually performed in water or alcohols as solvents, and the alkoxide ion intennediate is rapidly transformed to an alcohol by proton transfer. 

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