Alkoxide magnesium

Dialkylaminoethyl acryhc esters are readily prepared by transesterification of the corresponding dialkylaminoethanol (102,103). Catalysts include strong acids and tetraalkyl titanates for higher alkyl esters and titanates, sodium phenoxides, magnesium alkoxides, and dialkyitin oxides, as well as titanium and zirconium chelates, for the preparation of functional esters. Because of loss of catalyst activity during the reaction, incremental or continuous additions may be required to maintain an adequate reaction rate. 

Many metal alkoxides are soluble ia the corresponding alcohols, but magnesium alkoxides are practically insoluble. Only the distillable alkoxides, like those of alumiaum, titanium, and zirconium, are soluble ia weaMy polar solvents. The double alkoxides are soluble ia alcohol K[Li(OC2Hy)2],... 

Calcium Methylate Ethylate and Ethylate. Calcium methoxide [2556-53-8] and ethoxide [2914-17-2], Ca(OCH2)2 and Ca(OC2H )2, are white powders soluble in the corresponding alcohol (max concentration 1%). They are packaged and stored like the magnesium alkoxides. 

In this case, the magnesium alkoxide protects the ketal from cleavage. " ... 

A related class of compounds are the alkyl-magnesium alkoxides these can be prepared by reaction of MgR2 with an alcohol or ketone or by reaction of Mg metal with the appropriate alcohol and alkyl chloride in methylcyclohexane solvent, e.g. ... 

Tertiary magnesium alkoxides 20, bearing a /3-hydrogen, may undergo a dehydration reaction upon acidic workup, and thus yield an alkene 21 ... 

A Grignard reaction begins with an acid-base complexation of Vfg2+ to the carbonyl oxygen atom of the aldehyde or ketone, thereby making the carbonyl group a better electrophile. Nucleophilic addition of R then produces a tetrahedral magnesium alkoxide intermediate, and protonation by addition of water... 

Nucleophilic addition of an alkyl group R- to the aldehyde or ketone produces a teliahedral magnesium alkoxide intermediate. . . ... 

Magnesium alkoxides (formed by ROH- -Me2Mg —>ROMgMe) have been decomposed thermally, by heating at 195-340°C to give the alkene, CEU, and MgO. Syn elimination is found and an Ei mechanism is likely. Similar decomposition of aluminum and zinc alkoxides has also been accomplished. ... 

House and Trafkante suggested that magnesium alkoxides promote deprotonation of ketones in the reaction with Grignard reagents leading to by-products. See Reference 18(a). 

Several strategies have been proposed to improve the regioselectivity of nitrile oxide cycloaddition. Kanemasa and coworkers have reported high-rate acceleration and regioselectivity in nitrile oxide cycloadditions to the magnesium alkoxides of allylic and homoallylic alcohols (Eq. 8.64)."... 

The initial addition product is unstable and loses a magnesium alkoxide to form a ketone which is more reactive toward Grignard reagents than esters. 

The analogous magnesium alkoxide dimer, (273), is more active than (272), although polydispersities are generally higher (ca.1.5), a result of transesterification side reactions.824 An increased level of control may be acquired by the addition of 1 equivalent of PrOH (Mw/Mn... 

Stereoselective cyclization controlled by a substituent remote from the reaction center is often difficult to achieve. However, 1-mediated cyclization of the substrates illustrated in Eq. 9.54 proceeds in a highly stereoselective manner when the hydroxy group is converted to a magnesium alkoxide prior to cyclization [99,100]. The effect of the alkoxide group is much more favorable than that of the corresponding TBS ether. 

Modified MPV-type reductions carried out with chiral magnesium alkoxides and with chiral Grignard reagents have been discussed in detail (1). These reagents differ from the aluminum alkoxides since the Grignard reaction is essentially irreversible. Chiral alkali metal alkoxides have also been used to effect asymmetric reductions (1). 

A recent breakthrough in this field was made by Kanemasa and co-workers (136-138), who outlined a method to overcome the low selectivity of the cycloaddition to allylic alcohols by converting the hydroxyl group to a magnesium alkoxide. Coordination of both reactants to the metal ion accelerates the cycloaddition, affording the isoxazoline product with good to excellent stereoselectivity (136). Two transition states were proposed as pathways to the anti (erythro) and syn... 

Ab initio calculations also confirm that the use of an allyl magnesium alkoxide in place of the alcohol functionality will lead to high or complete stereoselectivity (138). When homoallylic alcohols are used, the Kanemasa protocol afforded the respective isoxazolines with poor stereoselectivity ( 55 45) in the case of terminal aUcenes, but with very high diastereoselectivity (up to 96 4) in the reaction of cis-1,2-disubstituted olefins (136). Extension of this concept to the reaction of a-silyl allyl alcohols also proved feasible and produced the syn (threo) adducts as nearly pure diastereomers (>94 6) (137). Thus, the normal stereoselectivity of the cycloaddition to the Morita-Baylis-Hillman adducts (anti > syn, see above) can be reversed by prior addition of a Grignard reagent (176,177). Both this reversal... 

Since a magnesium alkoxide undergoes rapid ligand exchange with free alcohols even in dichloromethane at ambient temperature, the regiocontrol of nitrile oxide cycloadditions of multisubstituted allylic alcohols can be effectively attained... 

Reaction M PhCNO-MgBrCl and free alcoholic forms of dipolarophiles were used. Reaction N PhCNO and the magnesium alkoxides of dipolarophiles were used. 

The reactions of fused and unfused a-pyrones with magnesium alkoxides, sodium alkoxides, and water as the nucleophile have been studied in depth, particularly with respect to the effects of chelation <1996J(P1)2715>. The annelation reactions of pyranose derivatives, with acetylacetone and ethyl acetoacetate, to give a variety of annelated fused pyrans have been reported, with the products obtained as single diastereoisomers (Scheme 13) <1997LA1019>. 

TABLE 8. Selected H and NMR data for magnesium alkoxide and aryloxide compounds... 

Magnesium alkoxide, simply prepared by dissolving magnesium in alcohol in the presence of a crystal of iodine, can be used to prepare magnesium enolates of bifunctional compounds. 

Thus, a-sulfinyl lithium carbanion of 1-chloroethyl p-tolyl sulfoxide was reacted with 1,4-cyclohexanedione mono ethylene ketal (195) to afford the adduct (196) in quantitative yield. The adduct was treated with ferf-butylmagnesium chloride (magnesium alkoxide was initially formed) followed by isopropylmagnesium chloride to result in the formation of magnesium /3-oxido carbenoid 197. The /3-oxido carbenoid rearrangement then takes place to give one-carbon expanded magnesium enolate 198. Finally, an electrophile was... 

Stiles and coworkers (70) have taken advantage of the effect of chelation to carboxylate compounds having active methylene groups. The reactions of these substances with C02 in the presence of aluminum or magnesium alkoxide readily produce chelate products (Equations 23 and 24). 

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