Phosphorus pentachloride with alkenes

The addition of phosphorus pentachloride to alkenes and alkynes yields complexes of the type RPClj PClg, which, when treated with sulphur dioxide, will give phosphonyl chlorides (6.197). Hydrolysis of the aluminium trichloride complexes (6.151) and (6.152) will also give phosphonyl or phosphinyl halides (6.198). 

Alkynes can be prepared by elimination reactions under conditions similar to those used to form alkenes. Because an alkyne has two n bonds, two molar equivalents of HX must be eliminated from the starting material. One such suitable reactant is a vicinal dihahde, a compound with halogen atoms on adjacent carbon atoms. We recall that such compounds result from the addition of a halogen to the double bond of an alkene (Section 6.5). A geminal dihahde, which has both halogens on the same carbon atom, can also be used to synthesize alkynes. Geminal dichlorides can be made by reaction of phosphorus pentachloride with an aldehyde or ketone. 

Reactions of Phosphoranes.—The reactions of phosphorus pentachloride (101) with simple organic molecules continue to attract attention, notably in the Russian literature. For example, the preparative uses of alkene-addition reactions of (101) have been examined for a-methylstyrene (102), as outlined in Scheme 7.85... 

A simple method for introducing a triple bond into an organic compound is to treat an appropriate dihalide with a strong base. Since vicinal dihalides (usually the bromide) are readily formed by reaction of bromine with an alkene, and geminal dihalides from aldehydes or ketones with phosphorus pentachloride, the method is a useful general procedure for the preparation of terminal and non-terminal alkynes from readily available starting materials. 

The reactions of phosphorus pentachloride (78) with electron-rich alkenes continue to be prominent in the Russian literature, and a few selected examples are given in Schemes 7 and 8. In the reaction of (78) with 2-methylpropene, the formation of... 

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