[14937-45-2] · C28H60BrP · Tri-n-butylhexadecylphosphonium Bromide · (MW 507.75)
Alternate Name: HDBP.
Physical Data: mp 56-58 °C.
Solubility: insol H2O.
Form Supplied in: white solid; widely available.
Tributylhexadecylphosphonium bromide is used as a typical phase-transfer catalyst in two-phase reactions carried out without or with a base. HDBP is thermally stable,6 but is much less stable under basic conditions than corresponding quaternary ammonium salts.7 Because of its high lipophilicity, HDBP cannot be readily removed from nonpolar solvents. A method of removal of HDBP from the crude products consists in stirring of the reaction mixture with an ion exchange resin (H-form) at room temperature.8 It appears that HDBP does not offer any advantages over typical phase-transfer catalysts, i.e. tetraalkylammonium salts, being much more expensive; nevertheless, it has been applied in many reactions.
Haloalkanes and sulfonate esters are reduced by Sodium Borohydride to hydrocarbons (eq 1);9 CO2R, CONR2, NO2, and CN groups are not affected. By using NaBD4 in D2O, deuterio derivatives can be obtained. The reactions can also be catalyzed by tetraoctylammonium chloride. Alkyl-, aryl-, and arylsulfonyl azides are reduced to the corresponding amines or sulfonamides (eq 2).10 Using this reaction, alkyl bromides can be converted into amines in a one-pot phase-transfer reaction. The halide first reacts with Sodium Azide/H2O in the presence of HDBP; the aqueous phase is then replaced by an aqueous solution of NaBH4.10
HDBP has been used in the reduction of disulfides and N-tosylsulfilimines to thiols and sulfides, respectively, by aminoiminomethanesulfinic acid (eq 3),11 and in the debromination of vic-dibromoalkanes by Sodium Iodide and an excess of Sodium Thiosulfate12 or Sodium Sulfide13 (eq 4); the latter process is also catalyzed by quaternary ammonium salts.14
N-Alkylation of potassium phthalimide with alkyl halides or methanesulfonates has been achieved in a solid-liquid two-phase reaction (eq 5).15 18-Crown-6 is an equally efficient catalyst.16 HDBP catalyzes the addition of hydrohalic acids to alkenes (eq 6),17 and the conversion of water-insoluble n-alkanols into alkyl chlorides with aq HCl (eq 7);18 other catalysts can also be used.19
The cleavage of dialkyl and aryl alkyl ethers with concd. HBr is accelerated by HDBP.20 Dialkyl ethers are converted into alkyl bromides (eq 8), whereas alkyl aryl ethers form phenols and alkyl bromides. Other catalysts have also been used.
The addition of Chloramine-T to sulfides produces sulfilimines (eq 9);20 other catalysts have also been used.21 The selective oxidation of sulfides to sulfoxides by Sodium Periodate (eq 10)22 has been achieved using HDBP. HDBP has been used in the conversion of alkyl halides or methanesulfonates into alkyl fluorides with aq Potassium Fluoride (eq 11).23 Secondary alkyl halides undergo b-elimination in competition with substitution.
Polish Academy of Sciences, Warsaw, Poland
Warsaw Technical University, Poland