Process for the preparation of alkyl methyl-3-carbalkoxyethylphosphinates

Process for the preparation of compounds of the formula ##STR1## where R is (C.sub.1 -C.sub.4)alkyl, which comprises reacting a compound of the formula II ##STR2## with an alcoholate of the formula III EQU Met-OR (III) where Met is Na or K and R has the abovementioned meaning, at temperature from -30.degree. C. to +10.degree. C.

The present invention relates to a process for the preparation of compounds 
of the formula I 
##STR3## 
where R is (C.sub.1 -C.sub.4)alkyl, which comprises reacting a compound of 
the formula II 
##STR4## 
with an alcoholate of the formula III 
EQU Met-OR (III) 
where Met is Na or K and R has the abovementioned meaning, at temperatures 
from -30.degree. C. to +10.degree. C. 
R is preferably methyl and ethyl. 
A temperature range from -20.degree. C. to 0.degree. C. is preferred. 
R=(C.sub.1 -C.sub.4) alkyl is methyl, ethyl, n-propyl, iso-propyl, n-butyl, 
sec.-butyl or tert.-butyl. 
The compounds of the formula I are important precursors and intermediates 
in the synthesis of herbicidally active substances (EP-A 30,424). 
It is known to react structurally simple phosphonic acid halides with 
alcoholates to give the corresponding phoshphinic esters. However, the 
yields in these processes are unsatisfactory (&lt;50%; Houben-Weyl, Methoden 
der org. Chemie [Methods in Organic Chemistry], Vol. XII/1 (1963) p. 248), 
in particular when the use of inert solvents is dispensed with. It is 
possible to achieve product yields of about 80% by adding the solvent 
benzene. 
The preparation of phosphinic esters I in accordance with other references 
in the literature also gives yields which must be considered as 
insufficient on an industrial scale. For example, only 32 to 41% of 
product yield are obtained following Chairullin, Sobchuk, Pudovik, Z. 
obsc. Chim. 37 (1967) No. 3, page 710-714, engl. p. 660-669. Using benzene 
as a solvent, only 47% of theory of the desired ester are obtained with 
cold ethanol (Chairullin, Vasjanina, Pudovik, Z. obsc. Chim. 39 (1969) No. 
2, page 341-346). 
According to Chairullin (Doklady Akad. Nauk SSSR 162 (1965) No. 4, page 
827-828), the compounds of the formula I are obtained in 50-58% yield if 
triethylamine is present as an auxiliary base. In the distillation of the 
crude reaction product, high-boiling anhydrides are formed (Z. obsc. Chim. 
36 (1966) No. 3, page 494-498). 
Thus, the alkyl phosphinates I obtained by the process known from the 
literature must be subjected to additional purification operations before 
they are further used in subsequent reactions. Moreover, the by-products 
which are obtained in substantial yields must be worked up and/or disposed 
of, which is to be considered disadvantageous from the economical and 
ecological point of view. 
All these shortcomings mentioned are avoided in the process according to 
the invention, in which the desired products I are obtained in yields of 
.gtoreq.95% of theory and in such a high purity that they can be used 
immediately for subsequent reactions. Moreover, the process according to 
the invention is very simple to operate and is also suitable for 
continuous operation. 
Examples of the alcoholates III to be used according to the invention which 
may be mentioned are sodium isopropylate, potassium tert.-butylate, sodium 
butylate, but in particular sodium methylate and sodium ethylate. 
The compound of the formula II is accessible from methyldichlorophosphane 
and acrylic acid (Chairullin, Sobcuk, Pudovik, Z.obsc.Chim. 37 (1967) No. 
3, page 710-714). The preparation of the alcoholates III (or of the 
corresponding alcoholate solutions) is known to any person skilled in the 
art. 
The compounds II and III are reacted in a temperature range from 
-30.degree. C. to +10.degree. C., but preferably between -20.degree. C. 
and 0.degree. C. 
The quantity of alcoholate required for the preparation of the compound of 
the formula I is at least 2 moles per mole of educt II. Naturally, the 
quantity of alcoholate must be increased if the educt of the formula II 
employed is contaminated by "acidic" secondary components, such as, for 
example, 2-methyl-5-oxo-1,2-oxaphospholane 2-oxide (German 
Offenlegungsschrift 2,531,238), which components, in turn, can react with 
the alcoholate III. 
The alcoholates are employed in the form of an alcoholic solution. It is 
expedient to add additional quantities of the corresponding alcohol to the 
reaction mixture. This alcohol acts as the diluent so that the reaction 
mixture can still be stirred at the low reaction temperatures according to 
the invention. However, additional inert solvents such as, for example, 
benzene, are not required (see Examples 1-3). 
The process according to the invention can be carried out expediently in 
such a manner that the alcoholate solution is initially introduced at 
-20.degree. C. to 0.degree. C., and the dichloride II is metered in at 
this temperature. The stirred reaction mixture is allowed to come to room 
temperature, precipitated salt is filtered off, and the low-boiling 
compounds are distilled off. The crude product which remains is then 
purified in a high vacuum by short-path distillation. In this process, the 
esters of the formula I are obtained in purities of more than 97% (GC). 
The following examples are intended to further illustrate the invention:

EXAMPLE 1 
Methyl methyl-3-carbomethoxyethylphosphinate I 
339 g (1.883 mol) of sodium methylate solution (30 % strength in methanol) 
and 200 ml of methanol are initially introduced at -20.degree. C., and 189 
g of methyl-3-chlorocarbonylethylphosphinyl chloride (88.4% pure) are 
metered in in the course of 45 minutes. Stirring is continued for 30 
minutes at -20.degree. C., and the reaction mixture is then allowed to 
come to room temperature. The sodium chloride (96.5 g) is filtered off 
with suction and washed with a little methanol, and the mother liquor is 
concentrated. 191.3 g of crude product are then obtained, from which 
another 7.8 g of sodium chloride precipitate. The mixture which is 
obtained after refiltration has the following composition (GC): 85.6% of 
product I (98.7% of theory), 14.0% of Na salt of 
methyl-3-carbomethoxyethylphosphinate, which is attributable to the 
2-methyl-5-oxo-1,2-oxaphospholane 2-oxide, contained in the dichloride II 
as an impurity. Distillation over a short-path evaporator at 0.25 mbar 
(jacket temperature 130.degree. C.) gives 159.8 g of product of the 
formula I which, according to GC, is 97% pure. This corresponds to a 
product yield of 97.4% of theory. The reaction product also contains 2.5% 
of methyl-3-carbomethoxyethylphosphinic acid. 
EXAMPLE 2 
Methyl methyl-3-carbomethoxyethylphosphinate I 
353 g of 30% strength sodium methylate solution (1.96 mol) and 200 ml of 
methanol are initially introduced at -10.degree. C., and 189 g of 
methyl-3-chlorocarbonylethylphosphinyl chloride (96.0% pure) are metered 
in the course of 45 minutes. Stirring is briefly continued at -10.degree. 
C., and the mixture is then allowed to come to room temperature. The 
sodium chloride is filtered off with suction and washed with a little 
methanol and the mother liquor is concentrated. After sodium chloride has 
again been filtered off, 180.9 g of crude product are obtained which, 
according to GC, has the following composition: 93.6% of product I (98.0% 
of theory), 4.5% of the Na salt of methyl-3-carbomethoxyethylphosphinic 
acid. This product can be used undistilled for the subsequent reaction in 
accordance with EP-A 30,424. However, most of the amount of monoester can 
be separated off by distillation as described under Example 1. 
EXAMPLE 3 
Ethyl methyl-3-carbethoxyethylphosphinate I 
Sodium ethylate (1.82 mol) is formed from 700 ml of absolute ethanol and 
42.1 g of Na. The substance is cooled to -20.degree. C., and 189 g of 
methyl-3-chlorocarbonylethylphosphinyl chloride (84% pure) are added at 
this temperature in the course of 45 minutes. The mixture is then allowed 
to come to room temperature and stirring is continued for 1 hours. The 
sodium chloride is filtered off with suction, and the low-boiling 
compounds are distilled off. What remains is a residue of 193.2 g which 
contains 85.9% (95% of theory) of ethyl 
methyl-3-carbethoxyethylphosphinate besides 9.3% (11.9% of theory) of 
methyl-3-carbethoxyethylphosphinic acid and 4.8% (6.1% of theory) of ethyl 
methyl-3-carboxyethylphosphinate. Distillation over a short-path 
evaporator at 0.3 mbar (jacket temperature 140.degree. C.) gives 168.5 g 
of product I, which, according to GC (silylated), is 98.3% pure. This 
corresponds to a product yield of 94.8% of theory. 
EXAMPLE 4 
Comparison Example 
Methyl methyl-3-carbomethoxyethylphosphinate I 
2.2 moles of methanol are initially introduced while refluxing. 189 g (84% 
pure) of methyl-3-chlorocarbonylethylphosphinyl chloride are added in the 
course of 1 hour. A vigorous evolution of waste gas, takes place. Stirring 
is continued for 30 minutes. The low-boiling components are then distilled 
off. This gives 172.3 g of crude product which, according to GC, consist 
of 98.6% methyl-3-carbomethoxyethylphosphinic acid. 
EXAMPLE 5 
Comparison Example 
200 ml of methanol and 2.0 moles of triethylamine are initially introduced 
at 0.degree. C. 189 g (84% pure) of methyl-3-chlorocarbonylethylphosphinyl 
chloride of the formula II are added in the course of 1 hour, during which 
process the temperature should remain the same. A precipitate of 
triethylamine hydrochloride is formed. The mixture is concentrated, the 
salt is filtered off, and a crude product which, besides 38% of theory of 
methyl-3-carbomethoxyethylphosphinic acid, contains about 52% of methyl 
methyl-3-carbomethoxyethylphosphinate. Further P-containing components are 
additionally present in the reaction mixture.