GB2123823A - 4-methyl-2,3,5,6- tetrafluorobenzaldiacetate and a process for its preparation - Google Patents

Abstract

4-Methyl-2,3,5,6- tetrafluorobenzaldiacetate, a chemical intermediate useful in the synthesis of pecticidal compounds, is prepared by contacting tetrafluoro-p-xylene with a mixture of chromic oxide, acetic anhydride and sulphuric acid at a temperature of 10 DEG C or less.

Description

SPECIFICATION 4-Methyl-2,3,5,6-tetrafluorobenzaldiacetate and a process for its preparation This invention relates to 4-methyl-2,3,5,6-tetrafluorobenzaldiacetate, a chemical intermediate useful in the synthesis of pesticidal compounds, and a process for its preparation.

According to one aspect of the invention, there is provided a novel compound of the formula (I):

which is 4-methyl-2,3,5,6-tetrafluorobenzaldiacetate.

The compound may be prepared by the selective oxidation of tetrafluoro-p-xylene under specific conditions.

Thus according to another aspect of the invention there is provided a process for the preparation of 4-methyl-2,3,5,6-tetrafluorobenzaldiacetate which comprises contacting tetrafluoro-p-xylene with a mixture of chromic oxide, acetic anhydride and sulphuric acid.

Preferably, the sulphuric acid is added gradually to a mixture of the other ingredients, to which acetic acid may usefully be added, while maintaining its temperature at or below 1 00C, suitably from 5 to 1 00C. Alternatively, the chromic oxide may be added in small portions, with the same temperature constraint to the other ingredients, to which the sulphuric acid has previously been added slowly to prevent charring. The process is more particularly described in the Example which follows later.

A similar process for the preparation of p-nitrobenzaldiacetate from p-nitrotoluene is described in Organic Syntheses, Collective Volume II, pages 441 to 443. Surprisingly, it has been found that oxidation of the tetrafluoroxylene in the process of the present invention is selective, leaving one methyl substituent intact.

The tetrafluoro-p-xylene starting material may be obtained by the dimethylation of 1,2,4,5tetrafluorobenzene with a methyl halide, conveniently methyl chloride, in the presence of a Lewis acid, such as aluminium chloride, using Friedel-Crafts reaction. The reaction is carried out under pressure in an autoclave at a temperature of about 1 500 C.

The preparation of 1 ,2,4,5-tetrafluorobenzene is described in, for example, the Journal of the American Chemical Society, Vol. LXXIII, January-March, 1951; pp. 145 to 149.

The benzaldiacetate is a precursor of 4-methyl-2,3,5,6-tetrafluorobenzyl alcohol which can be used in the preparation of pesticidal esters. The 4-methyl-2,3,5,6-tetrafluorobenzyl alcohol is derived from the benzaldiacetate by hydrolysing the latter by, for example, boiling it under reflux in an aqueous ethanol/sulphuric acid mixture to form 4-methyl-2,3,5,6-tetrafluorobenzaldehyde which is then reduced to the corresponding alcohol with, for instance, zinc and hydrochloric acid.

The invention is illustrated by the following Examples 1 and 2.

Example 1 (a) Preparation of 4methyl-2,3,5,6-tetrafluorobenzaldiacetate A mixture of 2,3,5,6-tetrafluoro-1 ,4-xylene (2.5 g), acetic anhydride (15 g), chromium trioxide (3.94 g) and acetic acid (25 g) was stirred at OOC and sulphuric acid (SG 1.84; 5.82 g) added dropwise keeping the temperature in the range 5--1 OOC.

On completion of the addition, the mixture was stirred at 1 OOC for 30 minutes, then for a further 2 hours at 20"C and then poured into ice/water (500 g).

The aqueous phase was extracted with methylene chloride (3x75 ml) and the combined organic phase washed with saturated sodium bicarbonate solution (50 ml) then with water (2x50 ml).

Removal of the solvent left 4-methyl-2,3,5,6-tetrafluorobenzaldiacetate (2.6 g) as a brown waxy solid.

The material was crystallized from aqueous ethanol to recover 4-methyl-2,3,5,6tetrafluorobenzaldiacetate (0.82) g) m.p. 82-83 0C.

Analysis Infra-red: Amax 1 780 cm-' due to C=O of an ester group.

N.M.R.:

ppm Type Assignment A ca.8 Singlet C-H B 2.5 Triplet -CH3 C 2.2 Singlet -(OCOCH3)2 Integration: 1:3:6 Mass spectrum The spectrum showed a molecular ion at m/e 294 which corresponds to the suggested structure.

(b) Hydrolysis of 4-methyl-2,3,5,6-tetrafluorobenzaldiacetate 4-Methyl-2,3,5,6-tetrafluorobenzaldiacetate (3.9 g) was boiled under reflux for thirteen hours in a mixture of water (20 ml), ethanol (20 ml) and concentrated sulphuric acid (5 ml). Analysis by gas chromatography of the isolated product showed that the starting material had been partially hydrolysed to a product corresponding to 4-methyl-2,3 ,5,6-tetrafl u orobenza Idehyde.

Example 2 The procedure of Example 1 (a) was repeated except that the 25 g of acetic acid were omitted and 20 ml of acetic anhydride were used instead of 1 5 g.

A yield of 0.72 g of 4-methyl-2,3,5,6-tetrafluorobenzaldiacetate was obtained.

Claims (3)

Claims

1. 4-M ethyl-2 ,3 ,5,6-tetrafluorobenzaldiacetate.

2. A process for the preparation of 4-methyl-2,3,5,6-tetrafluorobenzaldiacetate which comprises contacting tetrafluoro-p-xylene with a mixture of chromic oxide, acetic anhydride and sulphuric acid.

3. A process for the preparation of 4-methyl-2,3,5,6-tetrafluorobenzaldiacetate substantially as herein described with reference to Example 1 or 2.