[6399-81-1] · C18H16BrP · Triphenylphosphine Hydrobromide · (MW 343.21)
Alternate Name: triphenylphosphonium bromide.
Physical Data: mp 196 °C (dec).
Solubility: sol CH2Cl2, CHCl3; slightly sol THF, benzene; insol ether.
Form Supplied in: commercially available white powder.
Analysis of Reagent Purity: acidometric titration.
Purification: recrystallization from CH2Cl2/Et2O, or by washing with warm EtOAc to remove excess Ph3P.
Handling, Storage, and Precautions: corrosive and hygroscopic; keep dry.
Triphenylphosphine hydrobromide (1) is a convenient, crystalline source of anhydrous HBr for a wide variety of applications, including as a gas.1 This reagent has found an important use in the glycosidation of glycals5,6 without interference from allylic (Ferrier) rearrangement (eq 1). There is a distinct preference for the a-anomer, even when the incoming nucleophile is not sterically demanding. Other commonly used acid sources (Hydrogen Chloride, Hydrogen Fluoride, Amberlyst H-15 resin, p-Toluenesulfonic Acid) and Lewis acids (Boron Trifluoride Etherate) catalyze allylic rearrangement.
Tetrahydropyranylation, particularly of tertiary alcohols, can be catalyzed by (1) (eq 2).7 Depending on the substrate, these reactions are complete in about 20 h, and the conditions are quite mild when compared with those needed with catalysts such as Pyridinium p-Toluenesulfonate. Other acid catalysts such as TsOH can lead to elimination or polymerization of sensitive tertiary alcohols.
Treatment of alkyl chlorides with (1) results in the formation of phosphonium salts. Rather than a direct displacement of chloride by phosphorus, this reaction proceeds by initial conversion of RCl to RBr, followed by formation of the phosphonium salt.8 A wide variety of primary alcohols can be converted directly to the phosphonium bromides in high yield by treatment with (1).3 Both aliphatic and allylic alcohols react with (1) in sealed tubes at 160 °C within 2 h (eq 3). Secondary (and presumably tertiary) alcohols suffer dehydration under these conditions. Allylic alcohols (such as the carotenoid precursor (2))9 do not undergo allylic transposition (eq 4), although there have been cases where reactive alkenes have produced phosphonium salts upon heating with (1).10
Under similar conditions (heating in a sealed tube for 4 h at 180 °C), lactones react to form the free acid phosphonium salt in excellent yield (eq 5).3 Reactive alkenes such as enol ethers,4 indene, and a,b-unsaturated carbonyl compounds11 also undergo addition of (1) when other nucleophiles are not present. This reaction has been used to prepare a novel three-carbon Wittig reagent bearing a protected aldehyde function.11
Mark S. Meier
University of Kentucky, Lexington, KY, USA