Dehalogenation of Perchloroketones with Triphenylphosphine or Trialkyl or Triaryl Phosphites

Abstract

Perchlorodivinyl ketone (21, b.p. 69 'C/O.3 mm, n'," = 1.5661, d20 = 1.6963121, is readily obtained (yield 91.2 %) by treatment of perchloro-3-pentanone ( I ) , b.p. 94-95 "C10.6 mm, ng = 1.5682, with triphenylphosphine in ether at 5 to 35°C. The triphenylphosphine dichloride formed is advantageously converted into triphenylphosphine oxide by addition of water before isolation of the ketone ( 2 ) . Trimethyl phosphite and ( I ) afford dimethyl phosphorochloridate (yield 100 %) as well as the ketone (2) (yield 95.3 %). With both reagents there is a common intermediate whose retention time in gas chromatography lies between those of (1) and ( 2 ) ; this compound is most likely perchloro(ethy1 vinyl ketone). $C12 cc1By analogy with the mechanism of dehalogenationtzl of cc,@-dibromo compounds with trialkyl phosphites [3,41 and triphenylphosphine [51, we assume that the phosphine (or phosphite) first removes the chlorine atom cc to the carbonyl group, this being followed by loss of a P-chlorine atom as an anion. Perchlorobutanone (3) reacts equally smoothly with triphenylphosphine in ethereal solution and, after addition of water, gives 1,1,3,4,4-pentachloro-3-buten-2-one (yield 82.1 %; b.p. 89OC/9 mm, n'," = 1.5440, d20 = 1.6628; ref.[61: b.p. 99-100 "C, n'," = 1.5442). Trichloromethyl trichlorovinyl ketone (4) is formed in the first step of this reaction. A second mole of triphenylphosphine then reacts with (4) to give an enol phosphonium salt (S), which is extremely sensitive to solvolysis and with water decomposes to 1,1,3,4,4-pentachloro-3-buten-2-one (6), triphenylphosphine oxide, and HCl. The ketone (4) is identical with a sample prepared by a different route"]. Trialkyl phosphites react with (3) by an analogous first step, which leads to (4), alkyl chloride, and dialkyl phosphorochloridate; then a second mole of trialkyl phosphite brings about a Perkow reaction. OP(CSH5)S ClzHC-6-CCl=CC1_' " = A -c c 1 = C c l 2 C P An excess of triphenyl phosphite has no influence, so that the product (4) can be distilled continuously when the mixture is heated in an oil bath.[ (3b), respectively, which can be used for aminomethylations [21. Preparation of N-Functional