Formation of Alkoxides

Alcohols when treated with Group I metals like sodium and potassium result in alkoxides. 

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Higher alcohols (> C3) react comparatively slowly with sodium because of the slight solubility of the sodium alkoxide in the alcohol a large excess (say, 8 mols) is therefore employed. The mixed ether is distilled off, and the process (formation of alkoxide and its reaction with the alkyl halide) may be repeated several times. The excess of alcohol can be recovered. cj/cloAliphatic alcohols form sodio compounds with difficulty if small pieces... 

Another variant of the above-mentioned routes to silenes involved treatment of the carbinols (Me3Si)3SiC(OH)RR, formed from the addition of organometallic reagents R Li to polysilylacylsilanes, with bases such as NaH64 or MeLi,57,64 leading to the formation of alkoxides. These alkoxides spontaneously lost trimethylsilanolate ion, yielding silenes references for these reactions are listed in Table I. 

There would appear to be two distinct modes of reactivity of early transition metal alkyls with O2. When the metal is not in its highest oxidation state, an O2 complex of variable stability may form, and its subsequent reactivity may or may not involve the metal-carbon bond. The formation of remarkable stable 0x0 alkyls is an example of this pathway. In contrast, d°-alkyls react with O2 by a radical chain mechanism that invariable leads to formation of alkoxide complexes labile alkylperoxo ligands are clearly imphcated as intermediates in these reactions. 

While some metal-alkyl complexes react violently with molecular oxygen and others are inert, there are a few well-documented examples of reactions which lead to the formation of alkoxide... 

In (blactones, scission of either the acyl-oxygen bond or the alkyl-oxygen bond may take place leading to the formation of alkoxide- or carboxylate-grow-ing chains [ 115]. Methylene chloride end groups were observed in the ZnCl2-in-... 

Let us now consider the stereostructures C/ent-C of the two enantiomeric Still-Gennari intermediates of Figure 11.15 from another point of view. The simple diastereoselectivity (see Section 11.1.3) with which the phosphonate A and the aldehyde B must combine in order for the alkoxides C and ent-C to be produced is easy to figure out. If we use the formulas as written in the figure, this simple diastereoselectivity can be described as follows the phosphonate ion A and the aldehyde B react with each other in such a way that a back facephosphonate/back faceaidehyde linkaSe (formation of alkoxide C) and a front facephosphonate/front facealdehyde linkage (formation of alkoxide ent-C) take place concurrently. 

A detailed study of the oxidation of alkenes by O on MgO at 300 K indicated a stoichiometry of one alkene reacted for each O ion (114). With all three alkenes, the initial reaction appears to be the abstraction of a hydrogen atom by the O ion in line with the gas-phase data (100). The reaction of ethylene and propylene with O" gave no gaseous products at 25°C, but heating the sample above 450°C gave mainly methane. Reaction of 1-butene with O gives butadiene as the main product on thermal desorption, and the formation of alkoxide ions was proposed as the intermediate step. The reaction of ethylene is assumed to go through the intermediate H2C=C HO which reacts further with surface oxide ions to form carboxylate ions in Eq. (23),... 

In zeolites, this barrier is even higher. As discussed in Section II.B, the lower acid strength and the interaction between the zeolitic oxygen atoms and the hydrocarbon fragments lead to the formation of alkoxides rather than carbenium ions. Thus, extra energy is needed to transform these esters into carbonium ionlike transition states. Quantum-chemical calculations of hydride transfer between C2-C4 adsorbed alkenes and free alkanes on clusters representing zeolitic acid sites led to activation energies of approximately 200 kJ/mol for isobutane/tert-butoxide (29), 230-305 kJ/mol for propane/sec-propoxide, and 240 kJ/mol for isobutane/tert-butoxide (32), 130-150 kJ/mol for ethane/ethene (63), 95-105 kJ/mol for propane/propene, 88-109 kJ/mol for isobutane/isobutylene, and... 

Formation of alkoxide from hydroxide is a reverse reaction of hydrolysis of alkoxide, which proceeds easily at room temperature and is a highly exothermic reaction (therefore Equation 2.2 has a positive reaction enthalpy). However, metal hydroxide is usually solid and has a polymeric M-(OH)-M network, while metal alkoxide usually has oligomeric structure. Therefore the former compound has lesser freedom (lower entropy). Consequently the unfavorable enthalpy term is overcome by the entropy term at high temperatures and equilibrium is attained. 

O-Alkylation of alcohols with alkylating agents is a practical method not only for the synthesis of unsymmetrical ethers (16), but also for protecting hydroxyl groups (17). The alkylation reactions are usually conducted under strongly basic conditions via the formation of alkoxides from alcohols. However, an alternative method performed under neutral conditions would be desirable for the conversion of alcohols that are sensitive to strong bases. 

Fig. 7. TPD traces from alkyl iodides adsorbed on oxygen- (left) and hydroxide- (right) precovered Ni(lOO) surfaces. The bottom traces correspond to the formation of acetaldehyde from ethyl iodide, while those on top display the desorption of acetone from 2-propyl iodide conversion. The enhancing power of OH surface groups towards partial oxidation pathways is indicated by two observations from these data (1) the yield for acetone increases to the point of resembling that seen with 2-propanol and (2) some acetaldehyde is detected as well. It is at the present time unclear if the OH groups favor the formation of alkoxide intermediates or the subsequent P-hydride elimination step.

Undoubtedly, the aggregation becomes tighter and more extensive as the proportion of nonpolar component increases. Such an aggregation favors the isotactic placement but unfortunately enhances the side reactions, resulting in the formation of alkoxides and ring compounds. 

A possible mechanism for the hydrogenation of carboxyl bond over titania supported ruthenium catalyst with the involvement of Metal Support Interaction (MSI) is seen in Scheme 6. The reaction proceeds via the formation of alkoxide surface intermediates. 

The addition of alcohols to hydrocarbon solutions of Mo(NMc2)3 causes quantitative formation of alkoxides via Reaction 2. The nature of the alkoxide depends on the nature of R see below). The ferf-butoxide... 

Review. Bergbreiter and Killough have discussed the various uses of CsK, particularly in comparison with the reactions of the soluble analog sodium naphthalenide. Perhaps the most useful role of CsK is for the rapid formation of alkoxides from alcohols and of stabilized carbanions from carbonyl compounds. In these two reactions, it reacts more readily than potassium itself and can be separated from the products by filtration. The authors conclude that it has only limited value for reduction of alkyl and aryl halides and sulfonates, reactions accomplished more readily by other reagents. They also note that CeK can be regarded as a suitable reagent for reactions in which sodium naphthalenide is useful. 

The reaction of metals with alcohols can be achieved using electrochemical methods. While the overall reaction is the same as in Equation (9), the formation of the metal alkoxides is the result of two separate electrode processes, i.e., the anodic oxidation (and dissolution) of the metal (Equation (10a)) and the cathodic formation of alkoxide ions and hydrogen (Equation (10b)). 

The earliest developed commercial epoxy resins were diglycidyl ethers of 4,4 -isopropylidine-diphenol (Bisphenol A) formed by reacting epichlorohydrin with the diphenol. The reaction sequences involve formations of alkoxide ions, followed by nucleophilic additions to the least hindered carbons... 

In this chapter, we focus on monitoring the formation of alkoxides, as this is an important intermediate for the synthesis of ether and ester containing substances. The conventional approach is to transform deprotonated moieties, followed by classical analytical methods (e.g. thin layer chromatography, HPLC or NMR spectroscopy). There are only hmited descriptions of in-line methods in the literature, therefore an investigation is of high interest. 

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. ... 

FORMATION OF ALKOXIDES FROM TRANSITION METAL IONS WITH HIGH REACTIVITY... 

Tertiary amines like benzyldimethylamine, pyridine, and imidazole have been widely used as a base to initiate the anionic polymerization of PGE and its derivatives as well as for the synthesis of epoxy resins of diglycidyl ether of bisphenol A (DGEBA). Even if initiation occurs with amine alone, the introduction of an alcohol is a common procedure to suppress the observed induction period and increase the polymerization rate. Two initiation mechanisms have been proposed (Scheme 18) (1) direct nucleophilic attack of the amine onto the cyclic monomer to yield the zwitterion (a) and (2) formation of alkoxide (b) via proton transfer in the presence of alcohol. Fast exchange between dormant alcohol and active alkoxide allows chain growth from both initial amine (a) and alcohol (b). Poly(PGE) oligomers whose degree of polymerization does not exceed 5 are obtained. The presence of terminal double bonds indicates significant transfer to the monomer via... 

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