t-Butoxymagnesium Bromide


[19065-60-2]  · C4H9BrMgO  · t-Butoxymagnesium Bromide  · (MW 177.32)

(associated alkoxide base)

Preparative Method: the reagent is prepared in situ by reacting anhydrous t-BuOH with 1.0 equiv EtMgBr in Et2O or THF.1,2 The crystalline solid, which has a dimeric structure, is obtained by the reaction of MeMgBr with acetone in Et2O.3

Handling, Storage, and Precautions: use the same precautions that are recommended for Potassium t-Butoxide, i.e. avoid contact of solutions with eyes, skin, and clothing. Conduct all reactions under an inert atmosphere.

Oxidation of Alcohols.

Bromomagnesium alkoxides of a variety of alcohols including saturated primary alcohols (eq 1) react with N-Chlorosuccinimide in the presence of t-BuOMgBr to give the corresponding carbonyl compounds in good yields.2 A very low yield of the aldehyde is produced when the unsaturated alcohol, 7-phenyl-4-hepten-1-ol, is reacted similarly (eq 2). Although the substitution of m-Chloroperbenzoic Acid or Diacetatobis(tri-o-tolylphosphine)palladium(II) for NCS leads to a significant improvement in the yield of the unsaturated aldehyde, best results are obtained if 1,1-(Azodicarbonyl)dipiperidine is used as the oxidizing agent (eq 2).2 This method gives high yields of oxidation products when applied to a variety of other alcohols as well. The combination of t-BuOMgBr and 1,1-(azodicarbonyl)dipiperidine is capable of oxidizing keto alcohols to diketones without prior formation of the halomagnesium alkoxide of the substrate (eq 3).2

Rearrangement Reactions.

Cis- and trans-2,4-diphenylthietane dioxides rearrange to the corresponding cis- and trans-3,5-diphenyl-1,2-oxathiolane (2,3)-cis-2-oxides (eqs 4 and 5, respectively) upon reaction with t-BuOMgBr.1 These reactions are stereospecific with respect to the phenyl groups, but stereoselective with respect to the oxygen atoms on sulfur. When EtMgBr is used as the base, both of these sulfones undergo stereoselective rearrangement to the trans-1,2-diphenylcyclopropanesulfinate anion. The influence of t-BuOMgBr on the course of these reactions is attributed to its unique dimeric structure.4

The protected aldopyranose 2,3,4,6-tetra-O-benzyl-D-glycopyranose is readily converted into the protected ketopyranose 1,3,4,5-tetra-O-benzyl-L-sorbopyranose by an internal hydride transfer process in the presence of 5.0 equiv of t-BuOMgBr in CH2Cl2 (eq 6).5 This rearrangement proceeds poorly in THF and not at all in PhH or MeCN. Bromomagnesium alkoxides of other alcohols, e.g. i-PrOH, 2,4,6-trimethylphenol, and 4-chlorophenol, in CH2Cl2 were also reasonably effective for the reaction.5

1. Dodson, R. M.; Hammen, P. D.; Davis, R. A. JOC 1971, 36, 2693.
2. Narasaka, K.; Morikawa, A.; Saigo, K.; Mukaiyama, T. BCJ 1977, 50, 2773.
3. Moseley, P. T.; Shearer, H. M. M. CC 1968, 279.
4. Dodson, R. M.; Hammen, P. D.; Fan, J. Y. JOC 1971, 36, 2703.
5. Casiraghi, G.; Rassu, G.; Cornia, M.; Nicotra, F. CC 1989, 1242.

Drury Caine

The University of Alabama, Tuscaloosa, AL, USA

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