[7550-35-8] · BrLi · Lithium Bromide · (MW 86.85)
Physical Data: mp 550 °C; bp 1265 °C; d 3.464 g cm-3.
Solubility: 145 g/100 mL H2O (4 °C); 254 g/100 mL H2O (90 °C); 73 g/100 mL EtOH (40 °C); 8 g/100 mL MeOH; sol ether, glycol, pentanol, acetone; slightly sol pyridine.
Form Supplied in: anhyd white solid, or as hydrate.
Purification: dry for 1 h at 120 °C/0.1 mmHg before use; or dry by heating in vacuo at 70 °C (oil bath) for 24 h, then store at 110 °C until use.
Handling, Storage, and Precautions: for best results, dry before use in anhyd reactions.
LiBr has been extensively used as a source of bromide in nucleophilic substitution and addition reactions. Interconversion of halides2 and transformation of alcohols to alkyl bromides via the corresponding sulfonate3 or trifluoroacetate4 have been widely used in organic synthesis. Primary and secondary alcohols have been directly converted to alkyl bromides upon treatment with a mixture of Triphenylphosphine, Diethyl Azodicarboxylate, and LiBr.5
(Z)-3-Bromopropenoates and -propenoic acids have been synthesized stereoselectively by the reaction of LiBr and propiolates or propiolic acid (eq 1).6
In the presence of a Lewis acid, LiBr acts as a nucleophile in the opening of 1,2-oxiranes to produce bromohydrins (eq 2).7 In the absence of an external Lewis acid or nucleophile, epoxides generally give rise to products resulting from ring-contraction reactions (eq 3).
LiBr-mediated decomposition of dioxaphospholanes results in the exclusive formation of the epoxide, whereas the thermal decomposition produces a mixture of products (eq 4).8
Protection of alcohols as their MOM ethers can be achieved using a mixture of Dimethoxymethane, LiBr, and p-Toluenesulfonic Acid.9
Activated a-bromo ketones are smoothly converted into the corresponding silyl enol ethers when treated with a mixture of LiBr/R3N/Chlorotrimethylsilane.10 Aldehydes are converted into the corresponding a,b-unsaturated esters using Triethyl Phosphonoacetate and Triethylamine in the presence of LiBr (eq 5).11,12 Similar conditions were extensively used in the asymmetric cycloaddition and Michael addition reactions of N-lithiated azomethine ylides (eq 6).13
The addition of LiBr and Lithium Iodide was shown to enhance the rate of organozinc formation from primary alkyl chlorides, sulfonates, and phosphonates, and Zinc dust.14 Beneficent effects of LiBr addition have also been reported for the Heck-type coupling reactions15 and for the nickel-catalyzed cross-couplings of alkenyl and a-metalated alkenyl sulfoximines with organozinc reagents.16 The addition of 2 equiv of LiBr significantly enhances the yield of the conjugate addition products in reactions of certain organocopper reagents (eq 7).17
Finally, concentrated solutions of LiBr are also known to alter significantly the solubility and the reactivity of amino acids and peptides in organic solvents.18
André B. Charette
Université de Montréal, Québec, Canada