Alkoxide anion transfer

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... 

The data for the four compounds [83]—[86] show a good linear relationship (correlation coefficient r = 0.995) between the (C-)H" C( = 0) distance and the activation energy for hydride transfer reaction of the alkoxide anion (Fig. 16). Here also there is a simple and strong correlation between geometry and reactivity ground state structures closer to the presumed transition state structure give faster reactions. 

The addition of an alcohol to the basic two-phase system increases the apparent extraction of base into the organic phase, but it is generally acknowledged that it is the alkoxide anion which is being transferred [e.g. 43-50]. Optimum conditions for this co-catalytic effect requires the formation of highly lipophilic and highly basic alkoxides anions, either in the aqueous phase or at the interface. 

The generation of the dichloromethane under phase-transfer conditions may be facilitated by the addition of a trace of ethanol. Alkoxide anions, generated under the basic conditions, are more readily transferred across the two-phase interface than are hydroxide ions (see Chapter 1). Although this process may result in the increased solvolysis of the chloroform, it also produces a higher concentration of the carbene in the organic phase and thereby increases the rate of formation of the cyclopropane derivatives from reactive alkenes. 

Table 9.1). The rate of dehydrobromination from the intermediate bromoalkenes follows the pattern 2-bromoalkenes > Z-l-bromoalkenes > E- -bromoalkenes the corresponding chloro derivatives react more slowly. For optimum yield, the reaction temperature should be <100°C to reduce decomposition of the catalyst, and the concentration of base should be kept low to prevent isomerization of the resulting alkynes. [3-Elimination of HBr from 1,2-dibromo-1 -phenylethane can be controlled to yield 1-bromo-l-phenylethene in 83% yield [15]. The addition of alcohols and diols have a co-catalytic effect on the elimination reaction, as the alkoxide anions are transferred more effectively than the hydroxide ions into the organic phase [13]. 

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... 

Following the oxidative addition, in the transmetalation step the anionic form of the heteroatom containing coupling partner (amide, alkoxide) is transferred onto the palladium, which is usually achieved by the combined use of the neutral form of the nucleophile and a suitable base. The choice of the proper base might be crucial for the success of the coupling. The transmetalation, as depicted in Figure 2-3, usually follows a coordination-... 

They proposed a polymerization scheme closely related to other well-known chemical reactions of metal alkoxide with carbonyl compounds (20). In Scheme 2, complex [A] is converted to [B] by hydride ion transfer from the alkoxyl group to the carbon atom of aldehyde (Meerwein-Ponndorf reduction). Addition of one molecule of monomer to the growing chain requires transfer of the alkoxide anion to the carbonyl group to form a new alkoxide [C]. Repetition of these two consecutive processes, i.e., coordination of aldehyde and transfer of the alkoxide anion, constitutes the chain propagation step. 

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 ... 

Generally, most oxygen transfer reactions employing alkyl hydroperoxides require transition metal activation, since the alkoxide anions are relatively poor leaving groups, even poorer than the hydroxide anion. The reasons why they are sometimes employed in preference to hydrogen peroxide are as follows ... 

In case the anion is the polymeric species (anionic polymerization) a carbanion or an alkoxide anion forms the active chain end. Initiation is achieved by direct attack of organometaUic compounds, or by electron transfer from alkali metals, alkali metal complexes, or ionizing radiation. In case the cation is the polymeric species (cationic polymerization) a... 

There are currently two proposed mechanisms for the acyloin ester condensation reaction. In mechanism A the sodium reacts with the ester in a single electron transfer (SET) process to give a radical anion species, which can dimerize to a dialkoxy dianion. Elimination of two alkoxide anions gives a diketone. Further reduction (electron transfer from the sodium metal to the diketone) leads to a new dianion, which upon acidic work-up yields an enediol that tautomerizes to an acyloin. In mechanism B an epoxide intermediate is proposed. ... 

Tishchenko reaction When aldehydes are treated with aluminium ethoxide, one molecule is reduced while the other is oxidised, and the product is the resulting ester.This reaction involves a hydride anion transfer. With more basic alkoxides, aldehydes with an a-hydrogen undergo the aldol reaction. 

Proton transfer can occur from water to the alkoxide anion or to the nitrogen lone pair or to both. There are four common charge types of the tetrahedral intermediate. 

However, it is one of the most valuable and frequently used anions in PTC reactions. It was shown that the quantity of OH ion extracted into the organic phase decreased as its concentration in the aqueous phase increased, and the observed overall activity of OH ion actually increased due to the desiccating effect of the concentrated aqueous solution of OH ion [110]. Addition of a small amount of alcohol to a hydroxide-promoted PTC system usually causes a dramatic increase in reaction rate. One reason is that the alkoxide anions produced are more easily transferred into the organic phase than the highly hydrated OH ion and are at least as basic as OH ions. The other reason is that the solvation of the OH ion with alcohol rather than with water increases its organophilicity [111]. In the PEG-catalyzed dehydrohalogenation of 2-bromo-octane in toluene/KOH(aq) medium, a maximum 126-fold increase in the reaction rate was observed in the presence of methanol [112]. It was observed that the decomposition of various quaternary ammonium cations was retarded by the addition of methanol [113]. 

Each monomer addition to the growing chain requires a transfer of the alkoxide anion to the carbonyl group. This results in a formation of a new alkoxide anion. (A hydride transfer from the alkoxide group to the carbon atom of the aldehyde can take place by the Meerwin-Ponndorf... 

In principle, hydroxide anion is very difficult to transfer from aqueous to organic phases, yet it is one of the most valuable and most commonly used anions in the PTC systems. Addition of small amounts of alcohols to PTC systems requiring hydroxide transfer causes a dramatic increase in rates. Therefore, addition of alcohol enhances the PTC reaction as the cocatalytic effect. For example formation of alkoxide anions, RO, which are more readily transferred than the highly hydrated hydroxide anion, and which can serve as a strong base just as well as OH", and solvation of the hydroxide with alcohol rather than with water, making the hydroxide anion more organophilic and more easily transferred. ... 

The alkoxide anion (R O ) and AIH3 produced above now react together, forming [AhOR lHj]", which then continues to act as a hydride transfer agent ... 

Interpretation of early X-ray and NMR spectroscopic results lead to the hypothesis that the serine hydroxyl is activated via a charge-relay mechanism including proton removal from serine to the buried aspartate anion via the neighbouring histidine (cf. Fig. 4 b). This double proton transfer would yield neutral aspartic acid and an alkoxide anion with enhanced nucleophilicity. Later this was questioned on the basis of more precise NMR and neutron diffraction studies (cf. [241] for references). At variance with earlier quantum chemical calculations [246, 247] predicting the aspartate as the ultimate proton acceptor, we stressed the importance of the electrostatic effect of the environment including the protein dipoles, surrounding water molecules and a counter ion, and concluded that, while the Asp-His couple exists in a neutral form in vacuo, the ion-pair form is stabilized by the environment [239, 241]. These results have been confirmed by recent sophisticated calculations [218]. 

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