Triphenylphosphine dichloride explained

Triphenylphosphine dichloride, (C₆H₅)₃PCl₂, is a chlorinating agent widely used in organic chemistry. Applications include the conversion of alcohols and ethers to alkyl chlorides, the cleavage of epoxides to vicinal dichlorides and the chlorination of carboxylic acids to acyl chlorides.

Structure

In polar solvents such as acetonitrile, Ph₃PCl₂ adopts an ionic phosphonium salt structure, [Ph₃PCl⁺]Cl^-,^[1] whereas in non-polar solvents like diethyl ether it exists as a non-solvated trigonal bipyramidal molecule.^[2] Two [Ph₃PCl⁺] species can also adopt an unusual dinuclear ionic structure - both interacting with a Cl^- via long Cl–Cl contacts.^[1]

Synthesis

Triphenylphosphine dichloride is usually prepared fresh by the addition of chlorine to triphenylphosphine.

Ph₃P + Cl₂ → Ph₃PCl₂

Both reagents are typically used in solution to ensure the correct stoichiometry.

Ph₃PCl₂ can also be obtained by the reaction of iodobenzene dichloride (PhICl₂) and triphenylphosphine.^[3]

Alternatively, Ph₃PCl₂ can be obtained by chlorination of triphenylphosphine oxide with, for example, phosphorus trichloride, as in Grignard's original 1931 synthesis.

Notes and References

1. . 1996 . 2521–2522 . 10.1039/CC9960002521 . S. M. Godfrey . C. A. McAuliffe . R. G. Pritchard . J. M. Sheffield . An X-ray crystallorgraphic study of the reagent Ph3PCl2; not charge-transfer, R3P–Cl–Cl, trigonal bipyramidal or [R3PCl]Cl but an unusual dinuclear ionic species, [Ph3PCl+⋯Cl–⋯+CIPPH3]Cl containing long Cl–Cl contacts . 22.
2. Chem. Commun. . 1998 . 921–922 . 10.1039/a800820e . S. M. Godfrey . C. A. McAuliffe . J. M. Sheffield . Structural dependence of the reagent Ph3PCl2 on the nature of the solvent, both in the solid state and in solution; X-ray crystal structure of trigonal bipyramidal Ph3PCl2, the first structurally characterised five-coordinate R3PCl2 compound . 8.
3. Carle, M. S., Shimokura, G. K. and Murphy, G. K. (2016), Iodobenzene Dichloride in the Esterification and Amidation of Carboxylic Acids: In-Situ Synthesis of Ph3PCl2. Eur. J. Org. Chem., 2016: 3930–3933.