Phosphorus(V) Chloride1

PCl5

[10026-13-8]  · Cl5P  · Phosphorus(V) Chloride  · (MW 208.22)

(chlorination of alcohols,2 carboxylic acids,3 amides,4 aldehydes, ketones, and enols; Bischler-Napieralski synthesis of 3,4-dihydroisoquinolines;5 Beckmann rearrangement of oximes6)

Alternate Name: phosphorus pentachloride.

Physical Data: mp 179-181 °C (sublimes); d 1.6 g cm-3.

Solubility: sol CS2, CCl4. Typical solvents for reactions which use PCl5 are CCl4, CHCl3, CH2Cl2, hexane, decane, benzene, toluene, and diethyl ether.

Form Supplied in: white to pale yellow solid; widely available. In the solid state, PCl5 exists in the ionic form, [PCl4]+ [PCl6]-.

Purification: by vacuum sublimation.7

Handling, Storage, and Precautions: reacts with moisture to liberate hydrochloric acid and phosphoric acids. It is extremely corrosive to the skin, eyes, and mucous membranes. It should be stored in a dry area in containers impervious to moisture and resistant to corrosion. Laboratory quantities may be stored in polyethylene bags placed within glass containers. This reagent should be used in a fume hood.

Chlorides from Alcohols and Phenols.

Alcohols are converted to alkyl chlorides with the formation of 1 equiv of HCl and POCl3 by the action of PCl5.2 The stability of the substrate and product to acidic reaction conditions and the ease by which the product can be separated from the reaction byproducts greatly influences the suitability of PCl5 for this functional group transformation. Depending upon the structure of the alcohol, dehydration to form an alkene8 or rearrangement9 may be significant competing processes. The conversion of the 3-hydroxymethyl cephalosporin to the 3-chloromethyl derivative10 shown in eq 1 illustrates the use of PCl5 for the chlorination of a primary alcohol with a substrate of considerable functional complexity and lability.

In select cases, secondary alcohols undergo chlorination with inversion of configuration upon reaction with PCl5 (eq 2).11

Tertiary alcohols are converted to tertiary chlorides under mild conditions with retention of configuration, presumably through an SN1, tight ion-pair mechanism (eq 3).12 The reactions are complete within minutes of mixing the tertiary alcohol with PCl5 in CHCl3 or ether at 0 °C in the presence of calcium carbonate. Chlorination of 1,1,1-trifluoro-2-(trifluoromethyl)-3-butyn-2-ol with PCl5 leads to 1-chloro-3,3-bis(trifluoromethyl)allene (eq 4).13 This tertiary alcohol is unreactive towards SOCl2, POCl3, PCl3, concd. HCl, and POCl3/pyridine. Phenols substituted with strong electron-withdrawing groups can be converted to aryl chlorides with PCl5.14

A polymer-supported form of PCl5 has been developed by slurrying PCl5 with Amberlite IRA 93 resin in CCl4.15 The resulting resin is effective for the chlorination of primary and secondary alcohols in yields of 70-98%. Some elimination products are observed with secondary alcohols. The reaction may be conducted in hydrocarbon, ether, or halogenated hydrocarbon solvents. The phosphorus byproducts are retained on the resin and separated from the product by filtration. Regeneration of the resin is possible by washing with aqueous acid and base. Acid-sensitive substrates are susceptible to degradation.

Carboxylic Acid Chlorides from Acids.

PCl5 may be employed for the conversion of carboxylic acids to their acid chlorides.16 Typically, the acid is combined with PCl5 either neat17 or in solvents such as ether18 or benzene.19 The use of PCl5 is most convenient when the product can be isolated by direct crystallization20 or distillation21 from the crude reaction mixture. In some cases, aqueous workups may be employed to remove acidic byproducts without significant hydrolysis.22 Because of the difficulties associated with separating the acid chloride from the byproduct POCl3, the use of PCl5 for this functional group transformation has largely been supplanted by SOCl2, COCl2, or (COCl)2,23 particularly in conjunction with catalytic amounts of DMF.24 The polymer-supported form of PCl5,15 described above for the chlorination of alcohols, also converts carboxylic acids to the corresponding acid chlorides in 48-91% yields.

Nitriles from Amides.

Primary amides are dehydrated to afford nitriles upon heating with PCl5.4a-e Due to the severity of the reaction conditions, substrates of limited functional complexity are tolerated. The reactions are generally conducted either neat25 or with POCl326 as a solvent. This method is useful for products which can be isolated by distillation from the crude reaction mixture. Secondary amides, most notably N-alkylbenzamides, when subjected to PCl5 may undergo the von Braun degradation to produce a benzonitrile and an alkyl chloride.27

Chloromethyleneiminium Chlorides and Imidoyl Chlorides from Amides, Oximes, Hydroxamic Acids, and Ureas.

Tertiary amides react with PCl5 to afford chloromethyleneiminium chlorides,4d,g-i whereas secondary amides afford imidoyl chlorides.4d-f,j In each case, POCl3 and HCl are generated as reaction byproducts. The amide chlorides and imidoyl chlorides are frequently used as intermediates for subsequent chemical transformation and are not isolated. When desired, isolation has been accomplished by either distillation or crystallization. In choosing PCl5 as a reagent for this transformation, consideration must be given to the acid lability and reactivity of the substrate. Depending upon the substrate and reaction conditions, the formation of imidoyl chlorides by the action of PCl5 may be accompanied by further reaction to produce a nitrile and alkyl chloride via the von Braun degradation.27 a-Chlorination can be a problem if excess PCl5 is used, and self-condensation can occur at elevated temperatures if an a-CH is present in the imidoyl chloride.4g The presence of a strong electron-withdrawing group a to the amide carbonyl can influence the course of the chlorination of an amide with PCl5, as seen by the formation of the diazadiphosphetidine (eq 5) when N-methyltrifluoroacetamide is reacted with PCl5.28

A highly practical example of the reaction of PCl5 with a tertiary amide is the synthesis of Dimethylchloromethyleneammonium Chloride, the Vilsmeier reagent. This salt is obtained in 88% yield by the careful addition of PCl5 to N,N-Dimethylformamide while allowing the reaction to exotherm to 100 °C followed by cooling to 0 °C, filtration, and washing.29 The Vilsmeier reagent is useful for the chlorination of alcohols and acids.

PCl5 has found considerable application for the removal of amide side chains30 in the industrial scale manufacture of semisynthetic cephalosporins31 and penicillins.32 The intermediate imidoyl chlorides, produced by the action of PCl5 on a secondary amide, are converted to imino ethers upon the introduction of an alcohol to the reaction mixture. The imino ethers undergo further reaction to afford the deprotected C(7)-amino cephalosporin or C(6)-amino penicillin.33

Iminium and imidoyl chlorides have a prominent position as intermediates for the synthesis of heterocyclic compounds. In this regard, PCl5 has found extensive application. Imidoyl chlorides derived from the treatment of N-(2-phenylethyl)carboxamides with PCl5 can undergo Bischler-Napieralski cyclization5 to form 3,4-dihydroisoquinolines (eq 6).34 In addition to PCl5, this cyclization has been effected with P2O5, POCl3, POCl3 in refluxing xylene or decalin, P2O5 in pyridine, and polyphosphoric acid. N,N-Disubstituted oxamides (eq 7)35 and the imines or hydrazones of a-acylamino ketones (eq 8)36 react with PCl5 to form imidoyl chlorides which undergo subsequent intramolecular cyclization to produce imidazoles.

The reaction of oximes with PCl5 can lead to imidoyl chlorides via Beckmann rearrangement.6 The reaction is frequently conducted in ether at or below ambient temperature. Aromatic, hydrocarbon, and halogenated hydrocarbon solvents are also used. The oxime of 4-phenyl-3-methyl-3-buten-2-one undergoes Beckmann rearrangement within 5 to 15 min upon treatment with PCl5 in decalin at 0 °C to give an imidoyl chloride.37 Subsequent treatment with P2O5 at reflux affords 1,3-dimethylisoquinoline (eq 9). 2-(Oximino)indan-1-ones afford 1,3-dichloroisoquinolines when subjected to 1 equiv of PCl5 in POCl3 followed by treatment with anhydrous HCl and then a second equiv of PCl5 (eq 10).38 Treatment of an oxime which contains an a-alkoxy, a-alkylthio, or a-alkylamino group with PCl5 can lead to second-order Beckmann rearrangement,39 as exemplified in eq 11.40

The reaction of alkylbenzohydroxamates with PCl5 in ether offers a general preparative method for the synthesis of O-alkylbenzohydroximoyl chlorides.41 Ketenimines42 can be derived by the treatment of secondary amides bearing a single a-CH with PCl5 followed by dehydrohalogenation with triethylamine.43 Alkoxycarbonylimidoyl chlorides are obtained by the reaction of the esters of N-acylcarbamic acids with PCl5.44 N,N-Dialkylureas in which the alkyl groups are primary react with PCl5 to form 1,3-diaza-2-PV-phosphetidinones, whereas chloroformamidinium chlorides are produced when the alkyl groups are secondary.45 N-Alkyl-N-nitrosamides undergo rearrangement to the corresponding N-alkyloxamides via an intermediate N-nitrosochloroiminium chloride upon reaction with PCl5.46

gem-Dichlorides from Ketones, Aldehydes, and Esters.

Ketones and aldehydes may be converted into gem-dichlorides by the action of PCl5.47 Formate esters48 and esters with strong electron-withdrawing groups a to the carbonyl47 react with PCl5 to give the corresponding gem-dichlorides. Conversions have been effected either neat,49 or with PCl3,50 POCl3,51 or halogenated hydrocarbons52 as solvents. The reaction rate and product distribution are influenced by the solvent employed.52a,b Polar solvents, such as nitromethane and acetonitrile, greatly retard the reaction. The presence of an a-CH group can lead to the formation of vinyl chlorides as frequent byproducts.52a,53 Perhaloacetones are resistant to chlorination with PCl5 under the typical conditions employed for this conversion; however, the gem-dichlorides can be obtained in modest yields upon heating at 275-300 °C in an autoclave.54

Vinyl Chlorides from Enols.

PCl5 reacts with the enolic form of enolizable cyclic55 and acyclic56 carbonyls to form vinyl chlorides. A side-reaction encountered with esters has been formation of the acid chloride, in which case reesterification during the reaction workup affords improved yields (eq 12).55 Hydroxypyridines,57 hydroxyquinolines,58 and hydroxyisoquinolines38 are converted to chloropyridines, chloroquinolines, and chloroisoquinolines by the action of PCl5.

Other Applications.

Alkanes,59 arylalkanes,59a heteroaromatics,60 and alkenes59,61 have been chlorinated with PCl5 to afford chloro alkanes, chloro aromatics and vic-dichlorides. The use of PCl5 as a catalyst for the chlorination of alkanes, cycloalkanes, and arylalkanes has been reported.62 Secondary nitriles are a-chlorinated when subjected to PCl5 either neat63 or in an inert solvent such as CCl4 or CHCl3.64 Anhydrides are converted to diacid chlorides by the action of PCl5.65 Sulfonyl and sulfamoyl chlorides may be prepared by heating the corresponding acid66 or acid salt67 either neat or in an inert solvent with PCl5. PCl5 has been used to deoxygenate N-oxides,68 hydroxylamines,69 and sulfoxides.70 Hydroxylamines in which the a-carbon is tertiary can undergo the Stieglitz rearrangement to afford imines when treated with PCl5.71 Highly fluorinated carbinols bearing an a-CH, which prove resistant to the action of SOCl2, SO2Cl2, HCl, and ZnCl2, are converted to alkenes by the action of PCl5.72 Cyclic acetals of a-keto acids, when reacted with PCl5 in CH2Cl2, afford esters of halohydrins.73 Glyoxal bis-acetals are converted to 1,2-dichloro-1,2-dialkoxyethanes by the action of PCl5.74 This has been developed into a procedure for the synthesis of alkynic diethers.75 Imines have been converted into amides upon heating with PCl5, either neat or in a solvent such as xylene, followed by treatment with water.76 A method has been developed for the synthesis of alkyl chlorides by the reaction of PCl5 with the alkyl salicylates.77 The yields are higher in several cases for this procedure when compared to other methods for the chlorination of alcohols; however, synthesis of the alkyl salicylate is required.

Related Reagents.

Oxalyl Chloride; Phosphorus(III) Chloride; Phosphorus(V) Oxide; Phosphorus Oxychloride; Thionyl Chloride; Triphenylphosphine-Carbon Tetrachloride; Triphenylphosphine Dichloride; Dimethylchloromethyleneammonium Chloride.


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John E. Burks, Jr.

Eli Lilly and Company, Lafayette, IN, USA



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