[6779-08-4] · C19H15Cl2P · Dichloromethylenetriphenylphosphorane · (MW 345.21)
(Wittig reagent for the formation of 1,1,-dichloroalkenes from aldehydes and ketones)
Physical Data: yellow powder, mp 115 °C (dec).
Solubility: sol THF and ether; insol hydrocarbons; reacts with acidic protons.
Handling, Storage, and Precautions: the ylide is generally prepared in situ and used immediately, but the solid can be stored under N2 for several days. It reacts with air, water, or other acidic protons.
The dichloromethylene Wittig reagent can be prepared in three different ways:
Dichloromethylenetriphenylphosphorane was first prepared by generation of dichlorocarbene (from Chloroform and Potassium t-Butoxide) in the presence of Triphenylphosphine (eq 1).2 Under these conditions, dichlorocarbene is immediately trapped by the phosphine to give the ylide as a suspension in heptane. This method has been used to transform aliphatic2,3 and aromatic2,4 aldehydes and ketones to the corresponding dichloroalkenes in yields of 50-80%. For best results the t-butanol formed during ylide generation should be removed by vacuum distillation prior to carbonyl condensation.4
The most popular method for synthesis of dichloromethylenetriphenylphosphorane is the reaction of Ph3P with a large excess of CCl4 in the presence of a carbonyl compound.5 The ylide is formed in situ and reacts with the substrate to give dichloroalkenes (eq 2).6 Recent improvements include the use of BrCCl3 in place of carbon tetrachloride7,8 and acetonitrile as solvent.9
Aldehydes,5,10 ketones,5,9 and keto esters9 react under these conditions to give good yields of the R2C=CCl2 products. A useful variation of the reaction used Hexamethylphosphoric Triamide in place of triphenylphosphine,8,11 although under these conditions certain ketones did not react.8 Synthetic applications of the ylide include preparation of steroid derivatives8,10 and insecticide analogs.12,13
The dichloromethyltriphenylphosphonium salt has been prepared from triphenylphosphine and BrCHCl22 or more simply from reaction of 2 equiv Ph3P with CCl4 in the presence of a small amount of water (eq 3).14 Only a few Wittig reactions have been reported for the ylide formed by phosphonium salt deprotonation. Treatment of the salt with Sodium Hydride in THF gave the dichloro ylide, which was then used to convert a 4-substituted cyclohexanone into the dichloromethylene derivative in 57% yield (eq 4).9 When phenyllithium was used to form the ylide, reaction with benzophenone did not afford any of the expected dichloroalkene.2
Alternatives to the Wittig reaction for the formation of dichloroalkenes from carbonyl compounds include the Horner-Emmons reaction of Cl-substituted phosphonate anions,15 and a unique example of the Peterson alkenation reaction16 using the lithium anion of TMS-CHCl2. The above reactions generally use aldehydes or ketones as substrates; Wittig reaction of other carbonyl compounds such as esters with this ylide have not been reported. For an example of alkenation with a lactone, see Chapleur.11
Charles F. Marth
Nalco Chemical Company, Naperville, IL, USA