Alkoxide anions protonation

Base catalyzed nitrile hydrolysis involves nucleophilic addition of hydroxide ion to the polar C N bond to give an imine anion in a process similar to nucleophilic addition to a polar C=0 bond to give an alkoxide anion. Protonation then gives a hydroxy imine, which tautomerizes (Section 8.4) to an amide in a step similar to the tautomerization of an enol to a ketone. The mechanism is shown in Figure 20.4. 

The mechanism of the Feist-Benary reaction involves an aldol reaction followed by an intramolecular 0-alkylation and dehydration to yield the furan product. In the example below, ethyl acetoacetate (9) is deprotonated by the base (B) to yield anion 10 this carbanion reacts with chloroacetaldehyde (8) to furnish aldol adduct 11. Protonation of the alkoxide anion followed by deprotonation of the [i-dicarbonyl in 12 leads to... 

Protonation of the alkoxide anion intermediate gives the neutral alcohol addition product. 

Alcohols can be selectively bound to the same host type if they are combined with an amine and vice versa, considering that a cation and an anion will be formed through a proton transfer. The so-formed alkoxide anion will bind to the boron atom, while the ammonium ion will be complexed by the crown ether (147, Fig. 39). Competition experiments involving benzyl-amine have shown enhanced selectivity for the complexation of alcohols with... 

Initial attack by base on (34) yields the alkoxide anion (36), internal attack by this ROe then yields the epoxide (37) with inversion of configuration at C (these cyclic intermediates can actually be isolated in many cases) this carbon atomf, in turn, undergoes ordinary SN2 attack by eOH, with a second inversion of configuration at C. Finally, this second alkoxide anion (38) abstracts a proton from the solvent to yield the product 1,2-diol (35) with the same configuration as the starting material (34). This apparent retention of configuration has, however, been brought about by two successive inversions. 

Simple examples shown above are the base-catalysed formation of oxygen- and nitrogen-containing ring systems. We have shown base-initiated ionization of the alcohol to an alkoxide anion in epoxide formation the anion is a better nucleophile than the alcohol. For pyrrolidine synthesis, the amino group is sufficiently nucleophilic for reaction to occur, but base is required to remove a proton from the first-formed intermediate. 

The reaction is usually performed with homogeneous basic catalysts such as alkali hydroxides, alkoxides, and tetraalkyl ammonium hydroxide (161,162). The mechanism accepted for this transformation starts with the abstraction by the base catalyst of a proton from the hydroxyl group of the alcohol to generate the alkoxide anion, which reacts with acrylonitrile to form the 3-alkoxypropanenitrile anion. The 3-alkoxypropanenitrile anion abstracts a proton from the catalyst to yield 3-alkoxypropane nitrile. 

Excluding polymerizations with anionic coordination initiators, the polymer molecular weights are low for anionic polymerizations of propylene oxide (<6000) [Clinton and Matlock, 1986 Boileau, 1989 Gagnon, 1986 Ishii and Sakai, 1969 Sepulchre et al., 1979]. Polymerization is severely limited by chain transfer to monomer. This involves proton abstraction from the methyl group attached to the epoxide ring followed by rapid ring cleavage to form the allyl alkoxide anion VII, which isomerizes partially to the enolate anion VIII. Species VII and VIII reinitiate polymerization of propylene oxide as evidenced... 

In base, the ring is cleaved by attack of the nucleophile on the less substituted C to form an alkoxide anion, which is then protonated. Reactivity is attributed to the highly strained three-membered ring, which is readily cleaved. 

Mechanism. Removal of an a-hydrogen from the acetaldehyde by NaOH produces a resonance-stabilized enolate anion. Nucleophilic addition of the enolate to the carbonyl carbon of another acetaldehyde gives an alkoxide tetrahedral intermediate. The resulting alkoxide is protonated by the solvent, water, to give 3-hydroxybutanal and regenerate the hydroxide ion. 

Initiation takes place by rapid reaction of an ammonium salt with the anhydride (Eq. (46)) whereby ammonium carboxylate is formed. In the propagation step, the carboxylate anion opens an epoxy ring and forms an ammonium alcoholate (Eq. (47)). The latter reacts with the anhydride to yield another ester bond, and ammonium carboxylate is recovered (Eq. (48)). Termination occurs through decomposition of the ammonium counter ion, the alkoxide anion abstracting a proton from the quaternary nitrogen with the formation of a deactivated tertiary amine. 

The vertical electron affinity (EA) of acetone is given as —1.51 eV by Jordan and Burrow386. Lifshitz, Wu and Tiernan387 determine—among other compounds—the excitation function and rate constants of the slow proton transfer reactions between acclone-Ih, acetone-Dg and other ketones. The acetone enolate anion has been produced in a CO2 laser induced alkane elimination from alkoxide anions by Brauman and collaborators388-390. These show, e.g. that the methane elimination from t-butoxide anion is a stepwise process ... 

However, a variant (Scheme 2) using mixed carbonates 5 has become very popular. The leaving group decomposes into C02 and the alkoxide anion (6), which is itself a strong enough base to subtract the active proton from the pronucleophiles. This variant can be considered an allylation under neutral conditions, since no external base is added and the required alkoxide is generated in situ and never accumulates. 

At the stage of the betain 11, a reaction with the alcohol involving attachment of a proton (deuterium) to the free electron pair occurs (17). The resulting unsolvated alkoxide anion of the complex now abstracts a proton from the carbon atom bearing the deuterium whereby a betain with conformation 18 is formed. To comply with the stereochemical conditions o the Elcb-transition state, the free... 

Protonation of the resulting alkoxide anion leads to the alcohol illustrated below. This reaction is known as an aldol condensation. 

Treating the aldol adduct, A, with hydrochloric acid protonates the alkoxide anion and then protonates the resulting alcohol as part of a solvolysis reaction. Water then leaves, generating a carbocation. The carbocation then undergoes an El elimination (see Chapter 6) giving the illustrated product. 

In the second phase of this transformation, illustrated below, a six-membered ring is formed through an intramolecular 1,2-addition. Subsequent protonation of the alkoxide anion and elimination of water generates the final product. 

As a nucleophile attacks the carbonyl group, the carbon atom changes hybridization from sp2 to sp3. The electrons of the pi bond are forced out to the oxygen atom to form an alkoxide anion, which protonates to give the product of nucleophilic addition. 

Ketyls behave in a manner that depends on the solvent that they are in. In protic solvents (ethanol, for example), the ketyl becomes protonated and then accepts a second electron from the metal (sodium is usually used in these cases). An alkoxide anion results, which, on addition of acid at the end of the reaction, gives an alcohol. 

The reduction of carbonyl compounds with metal hydride reagents can be viewed as nucleophilic addition of hydride to the carbonyl group. Addition of a hydride anion to an aldehyde or ketone produces an alkoxide anion, which on protonation gives the corresponding alcohol. Aldehydes give 1°-alcohols and ketone gives 2°-alcohols. 

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