Magnesium Bromide

MgBr2
(MgBr2)

[2923-28-6]  · Br2Mg  · Magnesium Bromide  · (MW 184.11) (MgBr2.6H2O)

[13446-53-2]  · H12Br2MgO6  · Magnesium Bromide  · (MW 292.23) (MgBr2.OEt2)

[29858-07-9]  · C4H10Br2MgO  · Magnesium Bromide Diethyl Etherate  · (MW 258.25)

(Lewis acid capable of catalyzing selective nucleophilic additions,1 cycloadditions,2 rearrangements,3 coupling reactions;4 effective brominating agent5)

Physical Data: mp 165 °C (dec) (etherate >300 °C; fp 35 °C).

Solubility: sol alcohol, H2O; etherate sol common organic solvents.

Form Supplied in: white solid, widely available; etherate gray solid.

Preparative Method: the etherate is easily prepared from reacting a slight excess of Magnesium turnings with 1,2-Dibromoethane in anhydrous diethyl ether.13

Handling, Storage, and Precautions: etherate is flammable and moisture sensitive; freshly prepared material is most reactive and anhydrous. Irritant.

Nucleophilic Additions.

MgBr2 has been shown to form discrete bidentate chelates with various species,6 particularly a- and/or a,b-alkoxy carbonyl compounds,7 and thus functions as a diastereofacial control element in many nucleophilic addition reactions. In many cases, its inclusion completely reverses the nonchelation-controlled stereochemistry observed with nonchelating Lewis acids such as Boron Trifluoride Etherate. Highest diastereoselectivity is observed with a-substituted aldehydes (eq 1).8 High selectivities are observed for b-alkoxy aldehydes as well, including cases where three contiguous chiral centers are defined during the reaction (eq 2).9

Nucleophilic additions to a,b-dialkoxy aldehydes via allyl silanes (eq 3)10 or silyl ketene acetals (Mukaiyama reaction) (eq 4)11 exhibit similarly high selectivities. a-Thio aldehydes also react under MgBr2-catalyzed Mukaiyama conditions with efficient stereocontrol (eq 5).1

Rearrangements.

The oxophilic nature of MgBr2 renders it effective in mediating many rearrangements wherein polarization of a C-O bond initiates the process. A classic application involves the conversion of an epoxide to an aldehyde (eq 6).12

b-Lactones, when treated with MgBr2, undergo ionization with further rearrangement to afford either butyrolactones or b,g-unsaturated carboxylic acids (eq 7).3 The reaction course is dependent upon whether the cation resulting from lactone ionization can rearrange to a more or equivalently stable cation, in which case ring expansion is observed. If the b-lactone bears an a-chloro substituent, ring expansion is accompanied by elimination of HCl to afford butenolides (eq 8).13

Cycloadditions.

The MgBr2-mediated cyclocondensation of a Danishefsky-type diene with chiral a-alkoxy aldehydes affords a single diastereomer, which reflects a reacting conformer in which the alkoxy group is syn to the carbonyl, which is then attacked from its less-hindered face (eq 9).2 The cycloaddition of ynamines with cycloalkenones occurred selectively at the carbonyl, while in the absence of MgBr2 reaction at the alkene C=C bonds was observed (eq 10).14

Organometallic Reactions.

MgBr2 often increases the yields of Grignard reactions, as in the synthesis of cyclopropanols from 1,3-dichloroacetone (eq 11).15 It also serves to form Grignard reagents in situ via first lithiation followed by transmetalation with MgBr2. This technique enables the formation of vinyl Grignards from vinyl sulfones (eq 12),16 and of a-silyl Grignard reagents from allyl silanes (eq 13).17 In the latter case, the presence of MgBr2 provided regioselectivity at the a-position; without it, substitution at the g-position was predominant.

Bromination.

Magnesium bromide serves as a source of bromide ion, for displacement of sulfonate esters under mild conditions and in high yield. The reaction is known to proceed with complete inversion at the reacting carbon center when backside attack is possible (eq 14).5 Even bromination of highly congested bridgehead positions is possible via triflate displacement, although the reaction requires high temperatures, long reaction times, and activation via ultrasound (eq 15).18

Epoxides are also converted regiospecifically to bromohydrins by MgBr2.19

Carbonyl Condensations.

Bis- or tris-TMS ketenimines condense with ketones, mediated by MgBr2, to produce an intermediate that loses hexamethylsiloxane, affording 2-alkenenitriles in high yield with high (E) selectivity (eq 16).20 MgBr2 also enables the use of very mild bases like Triethylamine in Horner-Wadsworth-Emmons reactions, enabling the synthesis of unsaturated esters from aldehydes or ketones without the need for strongly basic conditions (eq 17).21


1. Annunziata, R.; Cinquini, M.; Cozzi, F.; Cozzi, P. G.; Consolandi, E. JOC 1992, 57, 456.
2. Danishefsky, S.; Pearson, W. H.; Harvey, D. F.; Maring, C. J.; Springer, J. P. JACS 1985, 107, 1256.
3. (a) Black, T. H.; Hall, J. A.; Sheu, R. G. JOC 1988, 53, 2371. (b) Black, T. H.; Eisenbeis, S. A.; McDermott, T. S.; Maluleka, S. L. T 1990, 46, 2307.
4. Cai, D.; Still, W. C. JOC 1988, 53, 464.
5. Hannesian, S.; Kagotani, M.; Komaglou, K. H 1989, 28, 1115.
6. Keck, G. E.; Castellino, S. JACS 1986, 108, 3847.
7. Chen, X.; Hortelano, E. R.; Eliel, E. L.; Frye, S. V. JACS 1992, 114, 1778.
8. Keck, G. E.; Boden, E. P. TL 1984, 25, 265.
9. Keck, G. E.; Abbott, D. E. TL 1984, 25, 1883.
10. Williams, D. R.; Klingler, F. D. TL 1987, 28, 869.
11. Bernardi, A.; Cardani, S.; Colombo, L.; Poli, G.; Schimperna, G.; Scolastico, C. JOC 1987, 52, 888.
12. Serramedan, D.; Marc, F.; Pereyre, M.; Filliatre, C.; Chabardes, P.; Delmond, B. TL 1992, 33, 4457.
13. Black, T. H.; McDermott, T. S.; Brown, G. A. TL 1991, 32, 6501.
14. Ficini, J.; Krief, A.; Guingant, A.; Desmaele, D. TL 1981, 22, 725.
15. Barluenga, J.; Florez, J.; Yus, M. S 1983, 647.
16. Eisch, J. J.; Galle, J. E. JOC 1979, 44, 3279.
17. Lau, P. W. K.; Chan, T. H. TL 1978, 2383.
18. Martinez, A. G.; Vilar, E. T.; Lopez, J. C.; Alonso, J. M.; Hanack, M.; Subramanian, L. R. S 1991, 353.
19. Ueda, Y.; Maynard, S. C. TL 1988, 29, 5197.
20. Matsuda, I.; Okada, H.; Izumi, Y. BCJ 1983, 56, 528.
21. Rathke, M. W.; Nowak, M. JOC 1985, 50, 2624.

T. Howard Black

Eastern Illinois University, Charleston, IL, USA



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