[3167-62-2] · C5H11Cl2O3P · Diethyl Dichloromethylphosphonate · (MW 221.02)
(Horner-Emmons reagent for the synthesis of 1,1-dichloro-1-alkenes; precursor for dichloromethylphosphine)
Solubility: very sol most organic solvents (ether, THF, CH2Cl2, hexane).
Analysis of Reagent Purity: 1H NMR (CDCl3) d (ppm): 1.39 (t, CH3), 4.32 (quint, CH2O), 5.65 (d, 2JHP = 2.0 Hz, CH); 31P NMR (CDCl3) d (ppm): 11.10.
Form Supplied in: not commercially available.
Preparative Methods: obtained on a large scale through the Kinnear and Perren reaction of Phosphorus(III) Chloride and Chloroform in the presence of Aluminum Chloride, and subsequent treatment with a large excess of EtOH (eq 1).3,4
On a laboratory scale, the preferred synthesis employs the commericially available diethyl trichloromethylphosphonate: this is partially reduced by means of n-Butyllithium (eq 2)2 or by a-lithiochloromethylphosphonate in the presence of lithium salts (eq 3).5 The resulting phosphonate carbanion is stabilized at low temperature by lithium salts and can be protonated in acidic medium or used in situ for further reactions.
A few other methods for the reduction of trichloromethylphosphonate have been reported. These include an electrochemical approach (85%)7 and the reaction of trivalent phosphorus reagents, such as Triethyl Phosphite (65%)1 and Hexamethylphosphorous Triamide (72%)8 with trichloromethylphosphonate. Diethyl dichloromethylphosphonate is also obtained by chlorination of the chloromethylphosphonate with CCl4 or of the methylphosphonate with Benzenesulfonyl Chloride in the presence of bases (n-BuLi or Lithium Diisopropylamide).6
Handling, Storage, and Precautions: stable compound; must be stored in the absence of moisture.
This involves a Horner-Emmons reaction between (EtO)2P(O)CCl2Li and various carbonyl compounds. This method is suitable for the preparation of large quantities of 1,1-dichloro-1-alkenes (up to 1 mol) in moderate to good yields (eq 4).2,9,10
The reaction takes place smoothly and cleanly with aldehydes or ketones, even in the case of hindered carbonyl derivatives, such as adamantanone (to give 1) or camphor (to give 2), of enolizable ketones, and of a,b-unsaturated aldehydes and ketones.
It has been applied to the synthesis of chrysanthemic11 and pyrethric esters (e.g. 3), their nitrogen analogs (e.g. 4),12 and prostacyclin analogs (e.g. 5).13
The Horner-Emmons reaction has also been tested on the phosphonate carbanion generated under electrochemical conditions. Yields are limited by side reactions, even in conditions optimized for the purpose of synthesis (see Le Menn et al.7 and references therein). Further treatment of the gem-dichloroalkenes RCH=CCl2 with butyllithium in ether or THF at a suitable temperature affords the corresponding alkynyllithium derivatives, which can be hydrolyzed or alkylated to give terminal and internal alkynes, respectively (eq 5).13-15
H2PCHCl2 has been prepared by reduction of diethyl dichloromethylphosphonate. The best results are obtained with Dichloroalane as reducing agent (eq 6).16 The half-life of the dichloromethylphosphine is 6 h at rt in THF solution.
Angela Marinetti & Philippe Savignac
Ecole Polytechnique, Palaiseau, France