Magnesium Methoxide


[109-88-6]  · C2H6MgO2  · Magnesium Methoxide  · (MW 86.39)

(weakly basic metal alkoxide; forms stable chelates with enolizable carbonyl compounds1)

Solubility: slightly sol MeOH, EtOH, DMF.

Form Supplied in: commercially available as neat white powder or as 8 wt % solution in MeOH.

Preparative Method: by addition of small batches of Magnesium metal to anhydrous MeOH.1

Handling, Storage, and Precautions: use same precautions as for other metal alkoxides (see Potassium t-Butoxide); avoid contact with eyes and clothing; handle and conduct reactions under an inert atmosphere. For critical experiments, it is recommended that the reagent be freshly prepared. Use in a fume hood.

Preparation of Magnesium Methyl Carbonate.

The reaction of (MeO)2Mg with CO2 in DMF solution leads to the formation of MeOMgOCO2Me (eq 1).2 This reagent is very useful for the carboxylation of a-methylene ketones, lactones, nitro compounds, etc. (see Methyl Magnesium Carbonate (MMC)).2

Carbonyl Condensation Reactions.

Because of the ability of the magnesium cation to form stable chelates with carbonyl compounds, (MeO)2Mg is a useful base for the promotion of a variety of carbonyl condensation reactions. The reagent catalyzes the condensation of triacetic acid lactone methyl ether with aromatic aldehydes to yield styryl monopyrone methyl ethers (eq 2).3 The condensation of 2,2-dimethoxy-3-butanone with benzaldehyde in the presence of (MeO)2Mg leads to a magnesium chelate of the initially formed a-diketone condensation product, which undergoes intramolecular Michael addition to form 4-phenylcyclopentane-1,2-dione (eq 3).4 Dimethyl oxalate reacts with 2,2-dimethoxy-3-butanone under similar conditions to give 2,5-dimethoxy-p-benzoquinone.4 The use of other bases such as sodium alkoxides, tertiary amines, or quaternary ammonium salts in these reactions leads to unidentified products. Treatment of cyclohexenone with dimethyl 2-methoxycarbonylglutarate in the presence of (MeO)2Mg in MeOH results in a Michael addition followed by an aldol condensation to yield a decalin-1,8-dione derivative which exists primarily in its tautomeric enol forms (eq 4).5

A mixture of diastereomers of 6-isopropyl-9-methylbicyclo[4.3.0]nonen-3-one is obtained by a tandem Michael-aldol reaction of an appropriate acyclic unsaturated keto aldehyde using (MeO)2Mg as the base (eq 5).6 The reaction is reasonably stereoselective for the trans-fused ring system, but it is somewhat more stereoselective and the overall yield is considerably higher when Zirconium Tetraisopropoxide is used as the base. The ability of the zirconium cation to form even stronger chelates than the magnesium cation presumably accounts for this.6b A good yield of the cyclization products is also obtained with Sodium Methoxide, but the stereoselectivity is low.

The methyl ester of phenylsulfinylacetic acid condenses with aliphatic aldehydes to give g-hydroxy-a,b-unsaturated esters directly (eq 6).7 The a-phenylsulfinyl-a,b-unsaturated ester, which is obtained by dehydration of the aldol product, undergoes deconjugation of the double bond to the b,g-position. Then, a [2,3]-sigmatropic rearrangement of the sulfoxide group occurs, followed by cleavage of the sulfenate. A two-step process which involves the use of Sodium Hydride/Zinc Chloride catalyst to effect the aldol condensation and dehydration followed by treatment with an amine base yields the same types of products.

The Darzens condensation of methyl chloroacetate with benzaldehyde occurs relatively slowly in the presence of (MeO)2Mg in MeOH.8 These conditions allow the isolation of methyl 2-chloro-3-hydroxy-3-phenylpropanoate and methyl 2-hydroxy-3-methoxy-3-phenylpropanoate, as well as the products of the Cannizzaro reaction of benzaldehyde, in addition to the expected methyl 2,3-epoxy-3-phenylpropanoate (glycidic ester).

Since the magnesium cation has a high affinity to form stable chelates with b-dicarbonyl compounds, the course of cyclizations of polyketonic-polyenolic systems may change significantly in going from alkali metal alkoxide bases to (MeO)2Mg.3b,9,10 For example, dimethyl 2,4-diacetylglutaconate cyclizes to an acyl pyrone in relatively high yield upon treatment with &egt;2.0 equiv of MeONa in MeOH/PhH (eq 7).9a The same pyrone is formed if the diester is treated with 1.0 equiv of (MeO)2Mg, but when &egt;2.0 equiv of this reagent is used, a phenol derivative and a resorcinol derivative are also formed. Apparently, the chelate of the initially formed acyl pyrone undergoes ring opening with the excess base to form the chelate of a glutaconic ester dianion, which undergoes an intramolecular Michael or aldol reaction to form a phenol derivative or an intramolecular Claisen condensation to form a resorcinol derivative.9a Also, bispyrones undergo ring openings to triketo esters followed by aldol cyclizations, sometimes accompanied by decarboxylations to mixtures of resorcinol derivatives upon treatment with KOH in MeOH (eq 8),10 but phloroglucinol derivatives, which apparently arise via Claisen condensation of open chain magnesium bischelates, are obtained in low yields with excess (MeO)2Mg (eq 9).3b,9a

Triacetic acid methyl ester is conveniently prepared by treating dehydroacetic acid with (MeO)2Mg in MeOH (eq 10).11 The ring opening reaction occurs more slowly when MeOLi or MeONa are employed as the bases.

Related Reagents.

Sodium Ethoxide; Sodium Methoxide.

1. FF 1969, 2, 255.
2. (a) Stiles, M.; Finkbeiner, H. L. JACS 1959, 81, 505. (b) Finkbeiner, H. L.; Stiles, M. JACS 1963, 85, 616.
3. (a) Bu'Lock, J. D.; Smith, H. G. JCS 1960, 502. (b) Douglas, J. L.; Money, T. T 1967, 23, 3545.
4. Muxfeldt, H.; Weigele, M.; Van Rheenen, V. JOC 1965, 30, 3573.
5. Schank, K.; Moell, N. CB 1969, 102, 71.
6. (a) Attah-Poku, S. K.; Chau, F.; Yadav, V. K.; Fallis, A. G. JOC 1985, 50, 3418. (b) Stork, G.; Shiner, C. S.; Winkler, J. D. JACS 1982, 104, 310.
7. Cass, Q. B.; Jaxa-Chamiec, A. A.; Sammes, P. C. CC 1981, 1248.
8. Svoboda, J.; Nič, M.; Paleček, J. CCC 1992, 57, 119.
9. (a) Crombie, L.; James, A. W. G. CC 1966, 357. (b) Crombie, L.; Games, D. E.; Knight, M. H. CC 1966, 355. (c) Crombie, L.; Games, D. E.; Knight, M. H. TL 1964, 2313.
10. Money, T.; Comer, F. W.; Webster, G. R. B.; Wright, I. G.; Scott, A. I. T 1967, 23, 3435.
11. Batelaan, J. G. SC 1976, 6, 81.

Drury Caine

The University of Alabama, Tuscaloosa, AL, USA

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