Triphenylphosphine-Carbon Tetrachloride dehydrations

The use of the triphenylphosphine-carbon tetrachloride adduct for dehydration reactions appears to be a very simple way of synthesizing nitriles from amides, carbodi-imides from ureas, and isocyanides from monosubstituted formamides. All of these reactions involve the simultaneous addition of triphenylphosphine, carbon tetrachloride, and tri-ethylamine to the compound to be dehydrated. The elimination of the elements of water is stepwise. An adduct, e.g. (46), is first formed, chloroform being eliminated, which decomposes to produce hydrogen chloride and the dehydrated product. 

The urea (289) was dehydrated in the presence of triphenylphosphine, carbon tetrachloride and triethylamine to give the carbodi-imide (290) and addition of 2-aminothiazole precipitated timegadine (291). This has a powerful anti-inflammatory action, but no hypotension or other biological activity could be detected Scheme 5.66.) [358]. 

DEHYDRATION Alumina. Phosphorus pentoxide. Rhodium(Ill) chloride-Triphenyl-phosphine. Triphenylphosphine-Carbon tetrachloride. 

The synthesis of ureas by reaction of ammonia or aliphatic primary and secondary amines with carbon monoxide at atmospheric pressure under selenium catalysis is noted. The intermediate salt can be oxidized to the sym-urea, or an exchange can be effected with a second amine, leading to unsym-ureas, as shown in Scheme 34. Ureas are readily dehydrated to carbodi-imides by the triphenylphosphine-carbon tetrachloride-triethylamine combination... 

The construction of a 4-pyrone is essentially the construction of a 1,3,5-tricarbonyl compound since such compounds easily form cyclic hemiacetals then requiring only dehydration. Strong acid has usually been used for this purpose, but where stereochemically sensitive centres are close, the reagent from triphenylphosphine and carbon tetrachloride has been employed. ... 

Triphenylphosphine and its derivatives provide another method of dehydration under relatively mild conditions. Benzamide reacts with triphenylphosphine and carbon tetrachloride in tetrahydrofuran (THF) to form probably the chloro-imine (7) which on warming at 45—55° with more of the phosphine, loses hydrogen chloride [29] an attempt to use pyridine as solvent and to facilitate the removal of the hydrogen chloride, gave a very low yield of nitrile. Under optimum conditions the yield was 83%. 

Poly(styryldiphenylphosphine) (8) in carbon tetrachloride converts alcohols to alkyl chlorides up to 20 times faster than does triphenylphosphine, as shown in equation (5). The phosphine oxide by-product is easily separated by filtration, but it cannot be recycled due to concurrent formation of a dichloromethylidinephosphorane. Yields are about 90% from primary and 40-70% from secondary alcohols. Reagent (8) in carbon tetrachloride also dehydrates primary carboxamides and oximes to amines, and secondary carboxamides and oximes to imidoyl chlorides. ... 

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