Process for preparing 1,2-oxa-phospholanes

#It is already known that 2-chloroformylethylphosphinic acid chlorides which are readily obtainable from alkyldichlorophosphines and carboxylic acids unsaturated in .alpha.,.beta.-position may be cyclized with acetanhydride to form 2,5-dioxo-1,2-oxaphospholanes, whereby acetylchloride is obtained as by-product. According to this process 2-methyl- or 2,4-dimethyl-2,5-dioxo-1,2-oxaphospholanes may be obtained from the corresponding chloroformalkylphosphinic acid chlorides in a yield of 84.3 or 78.6%. Calculated on methyldichlorophosphine reacting with acrylic acid or methacrylic acid to yield the corresponding 2-chloroformylalkylmethylphosphinic acid chlorides, 2-methyl- or 2,4-dimethyl-2,5-dioxo-1,2-oxaphospholanes is obtained in a total yield of 67.7 or 60.7% (CF. V. K. V.K. Chajrullin, I. I. Sobcuk and A. N. Pudovik, Z.obsc.chim. 37, 710 (1967), V. K. chajrullin, R. M. Kondrat'eva and A. N. Pudovik, Z.obsc.chim. 38, 288 (1968). ##STR3##

#This process has the disadvantage that the crude 2,5-dioxo-1,2-oxaphospholanes are contaminated by rather great quantities of acetanhydride, which may only be split off incompletely even under reduced pressure and leads to discolorations of the crude products at temperatures above 130.degree. C. A high vacuum distillation is indispensable for purifying the 2,5-dioxo-1,2-oxa-phospholanes obtained according to said process.

#It has now been found that 2,5-dioxo-1,2-oxaphospholanes of the formula (I) ##STR4## wherein R.sup.1 represents an alkyl radical having up to 18, preferably 1 to 12, especially 1 to 4 carbon atoms, optionally substituted by halogen, especially by chlorine, preferably up to three times, especially one time, a cycloalkyl radical having up to 8 carbon atoms, especially cyclopentyl, cyclohexyl, an alkenyl radical having up to 8 carbon atoms, especially vinyl and allyl, an aryl radical having up to 14 carbon atoms, especially phenyl, which may be substituted by lower alkyl groups having up to 4 carbon atoms, lower alkoxy groups having up to 4 carbon atoms, halogen or amino groups alkylated or dialkylated by lower alkyl groups having up to 4 carbon atoms, preferably up to twice, or an aralkyl radical having up to 15 carbon atoms optionally substituted in an analogous manner as the aryl radical, especially benzyl

#The following examples illustrate the invention.

#585 g of methyldichlorophosphine containing 1.4% of PCl.sub.3 was introduced into a 2 liter flask provided with a stirrer, a thermometer, a reflux condenser, a drip funnel and a delivery nozzle for N.sub.2 and heated to room temperature (576.8 g of CH.sub.3 PCl.sub.2, 4.94 mols). 365 g of a acrylic acid (5.07 mols) were added dropwise to CH.sub.3 PCl.sub.2 through the drip funnel while stirring and passing over a weak N.sub.2 current in such a manner that the reaction was initiated by a slow addition (increase of the temperature of the contents of the flask to about 40.degree. C.) and that the reaction heat was dissipated while cooling the wall of the flask by means of an ice/H.sub.2 O bath and rapidly adding such that a temperature of 60.degree. C. was maintained in the flask. The addition of acrylic acid was terminated after 1 hour and 20 minutes. The reaction product obtained was a colorless molten mass of 2-chloroformylethylmethylphosphinic acid chloride.

#760 g of 2-carboxyethylmethylphosphinic acid were added portionswise to the molten reaction product while stirring and slowly heating to 110.degree. C. whereby a considerable formation of HCl took place. The addition was terminated after 1 and a half hours, the reaction mixture was heated for 2 hours in a water jet vacuum to 150.degree. C. The contents of the flask were then submitted to a distillation in vacuo under 1 torr, 1253 g of colorless 2-methyl-2,5-dioxo-1,2-oxa-phospholane were obtained crystallizing when cooling.