[13826-95-4] · Br4Li2Ni · Dilithium Tetrabromonickelate(II) · (MW 392.19)
Solubility: sol THF.
Form Supplied in: prepared from commercially available LiBr and NiBr2.
Preparative Method: anhydrous Lithium Bromide (6.96 g, 0.08 mol) and Nickel(II) Bromide (8.72 g, 0.04 mol) are stirred in dry THF (95 mL) at 25 °C. After 48 h, the stirring is stopped and undissolved solids are allowed to settle. The resulting clear dark blue solution is about 0.4M in Li2NiBr4. In reactions, 1.6 equiv of the reagent are typically added per equiv of substrate and allowed to stir at the requisite temperature until completion.1,3
Handling, Storage, and Precautions: the reagent should be generated in a dry, inert atmosphere and used directly.
Dilithium tetrabromonickelate was initially developed as a source of
soft bromide for use in reactions with epoxides.1a It was reasoned that tetrabromonickelate as a source of bromide, along with the presence of a
hard but mild electrophile (Li+), would facilitate ring opening in the presence of other acid sensitive groups. Examples below show that epoxides undergo regioselective ring opening to give bromohydrins in which the bromine adds preferentially to the less-hindered position of the epoxide (eqs 1 and 2).
In certain acid or base sensitive compounds, other methods, such as the use of alkali metal salts in the presence of dipolar aprotic solvents, are clearly not satisfactory, in that the resultant bromide ion is somewhat
hard and can thus function as a base as well as a nucleophile. The consequences of this are seen in eq 3, where cyclization of the product occurs when LiBr in DMF is used, as opposed to clean epoxide opening without secondary cyclization with Li2NiBr4/THF.
In a synthetic route to a-enol aldehydes, reaction of a-cyano-a-hydroxymethyl epoxides with Li2NiBr4, followed by treatment of the mixture of the two intermediate compounds with pyridine/THF, led to the desired aldehyde (eq 4).3
Suitably protected and functionalized chiral bis-aziridines, prepared from D-mannitol, were allowed to react with Li2NiBr4 in THF. Depending on the protecting group, either chiral polysubstituted piperidines or bis-a-bromoamines were formed (eq 5).2
Lawrence B. Fertel
Occidental Chemical Corporation, Grand Island, NY, USA