Nucleophilic Attack at Halogen

The combination of triphenylphosphine with esters of trihaloacetic acids provides a reagent system for the stereo- and regio-selective conversion of alcohols into alkyl halides.The bromine-triphenylphosphine adduct has been used at low temperatures (-50 C in dichloromethane) for the removal of the tetrahydropyranyl protecting group from tetrahydropyranyl ethers derived from secondary and tertiary alcohols.The reactions of tertiary phosphines (and other trivalent phosphorus compounds) with iodine in aprotic solvents have received further study, a range of species being identified.The first reported study of the reactions of trivalent phosphorus compounds with monopositive astatine has led to the identification of stable complexes with triphenylphosphine, trioctylphosphine, and triethylphosphite. 

Nucleophilic Attack at Halogen. The reactions occurring in the triphenylphosphine-carbon tetrachloride system continue to attract attention. The salt (67), which is the first isolable product from the reactions of the above system, undergoes ready dechlorination on treatment with trisdimethylaminophosphine (TDAP) to form the ylide (68) and the dichlorophosphorane (69). This reaction offers a convenient route to the ylide (68), and enables the course of other reactions occurring in the triphenyl- 

The reaction of the phosphorane (70) with TDAP offers a convenient route to hexaphenylcarbodiphosphorane (73), which it has hitherto been difficult to prepare. Trimethylphosphine and dimethylphenylphosphine react with carbon tetrachloride in dichloromethane solution to form (74) and (75) the dichlorophosphoranes (74) are insoluble in the solvent, and evaporation of the filtrate affords the pure phos-phoranes (75).  

The reaction of tertiary alkyl- or aryl-phosphines with hexachloroethane results in the formation of dichlorophosphoranes (76) and tetrachloroethene. In contrast, the silylphosphines (77) react with equimolar amounts of hexachloroethane to give a 

A full account has now appeared of the reactions of tri-t-butylphosphine with germanium and tin tetrahalides, preliminary details of which were noted in last year s Report. 

The rates of dehalogenation of a-bromo- and a-iodo-m-cyanobenzyl phenyl sulphones (81) in aqueous DMF by series of alkyidiphenyl- and substituted triaryl-phosphines have been studied. The reaction of optically active benzyl(methyl)-phenylphosphine with (81) proceeds with inversion of configuration at phosphorus. 

Nucleophilic Attack at Halogen. Tertiary phosphine-carbontetrahalide adducts continue to be exploited for halogenation or dehydration reactions. Among those described this year is the addition of triphenylphosphine-carbon 

Aziridine can be obtained in good yields by the simultaneous action of triphenylphosphine, carbon tetrachloride, and triethylamine on N-substituted jS-amino-alcohols (40). 

A simple one-step preparation of cyclotriphosphazenes (41) and cyclotetra-phosphazenes (42), which uses condensation of bis(diphenylphosphine)amine 

French workers prefer the use of tris(dimethylamino)phosphine-carbon tetrachloride for reactions of this type. These reagents are used to substitute one hydroxy-group in 1,3-diols. Heating the salt (44) gives the chloride directly, or the phosphine oxide may be displaced by added nucleophiles. Addition of sodium methoxide gives the oxetans (45). The same reagents can be used to activate selectively the primary hydroxy-group of hexoses and hence allow it to be displaced by added nucleophiles.  

In a similar reaction it has been shown that benzoic acid may be converted into its anhydride by addition of tris(dimethylamino)halogeno-phosphonium salts (46). 

Nucleophilic Attack at Halogen. Further studies have been reported of the reactions of diols with the triphenylphosphine-carbon tetrachloride reagent. It has now been applied to 1,2-diols (in the presence of potassium carbonate) to form epoxides and to the trans-6 o (84), the nature of the product depending on the relative amounts of phosphine and diol present. The major product of reactions involving equimolar quantities of phosphine and diol is (85). The cyclodehydration product (86) is formed in only poor yield. In the presence of carboxylic acids, the triphenylphosphine-carbon tetrachloride system causes ring-opening of epoxides with the formation of c -enol esters, the reaction presumably proceeding via nucleophilic attack by the oxirane at an acyloxyphos-phonium intermediate.  

The reaction of triphenylphosphine with carbon tetrabromide in acetonitrile has been studied by conductimetric titration and found to be rapid, leading to the formation of the salt (87), which was isolated from the reaction mixture. Treatment of alcohols with the triphenylphosphine-carbon tetrabromide reagent in the presence of radiolabelled bromide ion gives a rapid, low-temperature procedure for the synthesis of radiolabelled bromoalkanes under neutral conditions.  

Miyakoshi and S. Saito, Nippon Kagaku Kaishi, 1982,703 Chem. Abstr., 1982,97, 5517). 

Hashimoto, I. Furukawa, and T. Yagasaki, Nippon Kagaku Kaishi, 1982, 1512 Chem. 

The products of the reaction between alkyldiphenylphosphines and hexachloro-ethane in the presence of a tertiary amine are the ylide (88) and the a-chloro-alkylphosphine (89), which are interchangeable, constitutional isomers that interconvert via an intramolecular reversible 1,2-chlorine shift from phosphorus to carbon. Application of the triphenylphosphine-hexachloroethane reagent for the self-condensation polymerization of / -aminobenzoic acid has been studied in detail. Related reactions between dicarboxylic acids and diamines, conducted in the presence of pyridine, involve the cationic phosphorane (90) as a key intermediate. Replacement of hexachloroethane by hexabromoethane or 

2 Nucleophilic Attack at Halogen.- Further applications of tertiary phosphine-tetrahalogenomethane and related reagents have been described. The reactions of primary and secondary alcohols with potassium carboxylates in the presence of the triphenylphosphine-tetrachloromethane reagent lead to the formation of esters in good yield. However, application of this procedure 

The reaction of triphenylphosphine with the geminal dichloro compound (68 R = Cl) in the presence of water leads to the formation of (68 R = H). However, in the absence of water, the 

2 Nucleophilic Attack at Halogen. Two groups have reported studies of the adducts of trialkylphosphines with iodine. Both and 1 2  

The reactions of the alkyldiaminophosphines (70) with either tetrachloromethane or bromotrichloromethane below 0°C give rise to the P-halogeno ylides (71), which, above 0°C, undergo 

The first isolable product in the reaction of triphenylphosphine and carbon tetrachloride is the salt (73), which reacts rapidly with further phosphine to give the stable phosphorane (74).62 In contrast, tris-t-butylphosphine reacts with germanium and tin tetrahalides to form the salts (75) 3 compounds of the latter type have long been postulated as arising from the reactions of phosphines with carbon tetrahalides but so far have defied detection. 

Two routes for the reaction of substrate with the triphenylphosphine-carbon tetrachloride reagent are now recognized.61 62 64 Direct interaction (76) of the substrate with the initially formed dipolar associate leads to the formation of chloroform and the intermediate phosphonium salt (77). 

Direct chlorination of the substrate by the dichlorotriphenylphosphorane present in the reaction mixture competes with the above route. The HC1 liberated is taken up by the dichloromethylenetriphenylphosphorane also present to form dichloro-methyltriphenylphosphonium chloride (78), which reacts further with triphenylphosphine with the eventual formation of chloromethyltriphenylphosphonium 

Castro, Y. Chaplew, and B. Gross, Bull. Soc. chim. France, 1975, 875. 

Applications of the combination of polymer-supported triarylphosphines (82) with carbon tetrachloride for the synthesis of peptides76 and acid chlorides,77 involving a simple filtration and evaporation process for product isolation, have been reported. 

Further reports of the reaction of tris(dimethylamino)phosphine and carbon tetrachloride with aldehydes and esters or amides of trichloroacetic acid have appeared. Addition of tertiary phosphines to the imide (35) gave (36) after hydrolysis.  

Dehydration reactions using the tertiary phosphine-carbon tetrachloride adduct have appeared quite regularly in the literature again this year. Among those reported have been the dehydrations of oximes to nitriles, carboxylic acids to anhydrides, and the amides (37) to the cumulenes (38). Further reaction of the dehydration product from treatment of the 

The formation of a peptide bond using tertiary phosphines and a halogenomethane has been studied in more detail. Trisamino-phosphines and carbon tetrachloride are the reagents of choice. Tripeptides are formed in high yield and with high optical purity. The simultaneous action of triphenylphosphine and carbon tetrachloride on the sulphonyl compounds (41), followed by triethylamine, gave iminophosphoranes (42). 

Several debrominations using triphenylphosphine have been reported. Stilbene dibromides and other vicinal dibromides are debrominated stereospecifically in an fl /i-elimination. 9,9-Dibromofluorene and dibromodiphenylmethane (43) have been converted into the corresponding ethylene, and the compounds (44) gave moderate yields of aroyl cyanides when fused with triphenylphosphine.  

---

Nucleophilic Attack at Halogen. This field of activity continues to be dominated by applications of well-known phosphine-positive halogen combinations. Alcohols can be oxidised to the related aldehydes and ketones under mild conditions by the DMSO-Ph3PX2 system, which provides an alternative to the Swern oxidation." The triphenylphosphine-iodine system has been... 

---