Alkoxide formation

Oligosaccharides were methylated using NaOH in DMSO for sugar alkoxide formation(2). 

A similar in situ approach to alkoxide formation employs the readily accessible tris(amide)-Y(NTMS2)3, (293), and PrOI 1.881 In the absence of alcohol the polymerization of CL is fast but not controlled (Mw/Mn > 3). However, upon addition of alcohol, a controlled living system with polydispersities 1.1-1.2 results. At least 50 equivalents of PrOH may be added (the excess effects rapid chain transfer) with molecular weights in good agreement with theoretical values. Similar results have been reported using Nd(NTMS2)3, (294), and PrOH.882 The reactions of PrOH with (292) and (293) have both been studied by NMR, and in both cases Y5(/r5-0)(0 Pr)13 is not... 

When substrates such as ot-chiral allylic alcohols are used, reactions with achiral nitrile oxides are affected both by alkoxide formation and the use of wellcoordinating cations (136-138). In some cases, hydrogen bonding with the nitrile oxide s oxygen atom can also play an important role (135). 

The cyclopropanation of alkenes using external stoichiometric chiral additives can be divided according to their general mechanistic scheme into two classes. The enantios-elective cyclopropanation of allylic alcohols, in which a pre-association between the corresponding zinc alkoxide and the zinc reagent probably takes place, constitutes the first class. The second class involves the enantioselective cyclopropanation of unfunctionalized alkenes. The latter implies that there will be no association between the reagent and the alkene through alkoxide formation. 

Chloro alkoxide formation is essentially complete at this time and can be conveniently monitored by quenching a small aliquot and subjecting it to GLC analysis. Using a 5(1 m x 0. mm OV-1 capillary column at 110 C and a flow rate of 0.87 mL/m1n H-, earner) the submitters found retention times of 3.2 min for 2-chlorocyclohexanone and 6.7 min and 7.2 min for trans- and cis-1-propynyl-2-chlorocyclohexanols, respect vely. 

The mechanism of this transformation presumably involves palladium alkoxide formation followed by jS-hydride elimination (equation 191). 

The /f-alkoxy ester 111 is formed by nucleophilic substitution of 114 with alkoxide. Formation of 109, the esters 111, and 112 can be regarded as the nucleophilic addition to alkenes promoted by Pd(II). 

Step 1 NaH treatment results in mono-alkoxide formation, which is critical to minimize formation of a bis-silyl ether. In the original procedure, a 96% yield is reported for the formation of the mono-silyl ether. 

The high electron density in the double bond system of ethylenes makes nucleophilic attack unfavorable unless the system is substituted with one or more electron withdrawing groups such as -N02, -CN, -COR. When these substituents are present, attack by alcohols or alkoxide ions occurs at the beta-carbon predominantly. For example, researchers have found (12) that sodium methoxide or sodium ethoxide added rapidly at room temperature to beta-nitrostyrene leads to the alkoxide formation of the derivative (Reaction VIII). This reaction is generally not only for arylnitroalkenes (13) but also for other activated double bonds (14). Another example of alcohol addition to an activated double bond includes the reaction of alcohols with acrylonitrile to produce a cyano-ethylated ether (14A). 

Brook rearrangements may be carried out with either catalytic or stoichiometric base. With catalytic base, the reaction can be considered an equilibrium between 41 and 42. The strength of the Si-0 bond (about 500-520 kJ mol-1) compared with the Si-C bond (about 310-350 kJ mol-1) means that, provided the anion 33 forms reasonably rapidly (some degree of stabilisation is required), Brook rearrangement (alkoxide formation) is favoured over retro-Brook. Organolithiums 33 may be present as intermediates in the catalytic Brook rearrangement, but their reactivity cannot be exploited under these conditions. 

V versus SCE [64], The cathodic limiting reaction is hydrogen evolution, thus forming the acid anion as the coproduct. The apparent electrochemical window of acetic acid is about 4 V [63], whereas that of formic acid is around 1 V [49], For methanol and ethanol, there are reports on limiting cathodic potentials around -2 V versus mercury pool electrode [65], and their accessible electrochemical window is around 2 V. The cathodic limiting reactions are probably hydrogen evolution and an alkoxide formation. 

The dehydrogenation reaction proceeds through the simultaneous elimination of the zeolitic proton and a hydride ion from the alkane molecule, giving rise to a transition state which resembles a carbenium ion plus an almost neutral H2 molecule to be formed. For the linear alkanes, the TS decomposes into an H2 molecule and the carbenium ion correspondent alkoxide. However, for the isobutane molecule the reaction follows a different path, the TS producing isobutene and H2. Most certainly the olefin elimination is flavored to the alkoxide formation due to steric effects as the t-butyl cation approaches the zeolite framework. The same mechanism is expected to be operative for other branched alkanes. 

The alkoxide precvirors are commonly formed as one of a series of homoleptic alkoxides Af(OR) , where n = 1—6. Organic molecules such as alcohols tend to be strong ir electron donors and thus stabilize the highest oxidation state of the metal [36]. The specific synthesis route of an alkoxide is dictated by the electronegativity of the metal. The most common routes for metal alkoxide formation are [37]... 

In the original formulation of his theory, Nef chose the hydroxyl group /3 to the carbonyl group as the site of alkoxide formation with the base, in the initial step of the saccharinic acid rearrangement. This was later amended to the formulation shown above, in order to accommodate the... 

Again, cationic differences are evident. For instance, potassium hydroxide is more effective in methylation (with dimethyl sulfate) than sodium hydroxide. In alkali-cellulose, addition products are formed by the interaction of alkalis with hydroxyl groups, and the tendency for alkoxide formation increases in the order LiOH < NaOH < KOH < RbOH < CsOH < organic quaternary bases. ... 

Table 22.1 lists three examples of cyclic alkenyl carbenium ions that live long enough in zeolites to be detected by NMR [6]. Obviously, alkoxide formation is not favored and the proton affinities of their parent hydrocarbon compounds are so large that they win the competition with the zeolite framework for the proton. 

The present results suggest a ready means for catalytic alcohol formation via carbene-like bihapto formyl and acyl species (e.g., equations (18) and (19)). Precedent exists for the alkoxide formation step of equation (19) (47,48,82,83). Chain growth could occur via the insertion of an unsaturated surface site into a H3C-0M(or R-OM) bond (an oxidative addition) to yield a metal-carbon bond, followed by further carbonylation, as illustrated in equations (20) and (21). There is good precedent for the insertion of metal ions into carbon-oxygen bonds (84,85,86). Hydro-... 

Alkali metal hydroxides, dissolution rate effect, 521-523f Alkoxides, formation of uniform precipitates, 451-464 Aluminum, silicic acid effect on adsorption in food, 612/ 613 Aluminum in biological systems, 604, 605f, 606 Aluminum-modified silica sol, formation, 62, 63/ Aluminum-silicon interactions in biology,... 

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