Phosphonium dibromide salt

The reaction of phosphines and alkyl halides presents an alternative way to generate phosphonium electrophiles (Scheme 3.8). In particular, the combination of a phosphine and carbon tetrabromide (the Appel reaction) allows for in situ formation of a phosphonium dibromide salt (48, X = Br). Treatment of a hemiacetal donor 1 with the phosphonium halide 48 initially provides the oxophosphonium intermediate 38 (X = Br). However, the oxophosphonium intermediate 38 can react with bromide ion to form the anomeric bromide intermediate 49 (X = Br) with concomitant generation of phosphine oxide. With the aid of bromide ion catalysis (i.e. reversible, catalytic formation of the more reactive P-anomeric bromide 50) [98], the nucleophile displaces the anomeric bromide to form the desired glycoside product 3. The hydrobromic add by-product is typically buffered by the presence of tetramethyl urea (TMU). 

The wastes from the synthesis of dibromosilane and phosphonium dibromide salt were hazardous wastes. Residue from KCs was treated with isopropanol before subjecting the residue to flammable hazardous waste. 

Carbamoyl ylids, 101, however, could be dehydrated via 102 R = H using triphenyl-phosphine dibromide and triethylamine (154). This reaction leads to an interesting class of push-pull ynamine phosphonium salts 103 (154), 72). 

Cyclopropyltriphenylphosphonium bromide was isolated in high yield from the addition of one equivalent of sodium hydroxide to the phosphonium salt (71). 2,2-Diphenylisophosphindolium salts (72) can be formed directly by the addition of o-xylylene dibromide to a mixture of chlorodiphenylphosphine and calcium carbide. ... 

By Miscellaneous Routes.—The synthesis of cyclic phosphine oxides from a,co-dibromides and tetraphenyldiphosphine has been described (Scheme 5). The intermediate phosphonium salts are converted into the... 

The phosphonium iodides form addition products with the iodides of mercury, bismuth and thallium, and also vdth bismuth chloride. Perhalides, R PI.Xg, are formed when halogens react with the iodides, these products, like the original iodides, having well-defined crystalline forms moist silver oxide removes the iodine, gnung the corresponding hydroxides. Like the arsonium chlorides the tetra-alkylphosphonium chlorides tend to be deliquescent, but they yield crystalline double salts vdth the chlorides of gold, mercury, platinum, bismuth and zinc. When two molecular equivalents of triethylphosphine react with one molecular equivalent of methylene chloride, ethylene dichloride or ethylene dibromide, phosphonium compounds result in which two atoms of phosphorus are present. 

The diphosphinomethanes (70 R = Me, Ph) react with 1,2-xylylene dibromide to give the bis(phosphonium) salts (71 R = Me, Ph) <820M1266>. By stepwise deprotonation of the salts, the mono-, bis- <81CB1428>, and tris-ylides <820M1266> (72)-(74) have been prepared (74) has been used as a ligand for transition metal ions <820M1266,83AG(E)907>. 

Miscellaneous Processes.—Various types of reaction other than those discussed above have been investigated under PTC conditions, and only some recent instances are cited here. A ready debromination of 1,2-dibromides to alkenes (Equation 7) by aqueous iodide uses catalytic quantities of both a phosphonium salt (to transfer iodide ion) and iodide ion itself. The latter species is regenerated continuously by reduction of the halogen products with sodium thiosulphate in the... 

Solid-Liquid Phase Transfer.—Very few examples are known in onium ion chemistry where the anion to be phase transferred is part of a solid phase rather than in aqueous solution. This contrasts with the situation in crown ether chemistry, as will be discussed later in this review. A useful method, which is cheaper than alternatives, for the generation of dichlorocarbene in neutral conditions involves warming solid sodium trichloroacetate in chloroform, in the presence of quaternary salts. Formaldehyde acetals (25) can be synthesized in good yields by the action of solid KOH on alcohols or phenols dissolved in methylene dibromide containing a quaternary ammonium ion, and a phosphonium salt catalyses the alkylation of solid potassium phthalimide in toluene. Solid-liquid phase transfer mediated by polyamines has been reported. ... 

---