Abstract:
There is provided a process for the preparation of 2,4,5-tribromopyrrole-3-carbonitrile. 2,4,5-Tribromo -pyrrole-3-carbonitrile is useful as a molluscicidal agent.

Description:
BACKGROUND OF THE INVENTION 
     Pyrrole carbonitrile and nitropyrrole compounds are disclosed to be useful as insecticidal, acaricidal and molluscicidal agents in U.S. Pat. Nos. 5,162,308 and 5,204,332. Those patents also refer to 2,4,5-tribromopyrrole-3-carbonitrile, its molluscicidal use and its preparation via bromination of pyrrole-3-carbonitrile. However, pyrrole-3-carbonitrile and its derivatives are difficult to prepare. Literature methods, such as that reported by A. M. van Leusen, et al., Tetrahedron Letters, 5337, (1972), report yields of 10% or less. 
     The preparation of 2,4,5-tribromopyrrole-3-carbonitrile from 2-trihaloacetylpyrrole-4-carbonitrile compounds is described in United States Patent 5,008,403. However, that process is not well suited for use in large scale preparations of 2,4,5-tribromopyrrole-3-carbonitrile. 
     It is therefore an object of the present invention to provide a new and efficient process for preparing 2,4,5-tribromopyrrole-3-carbonitrile. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a process for the preparation of molluscicidal 2,4,5-tribromopyrrole-3-carbonitrile. 
     Surprisingly, it has been found that 2,4,5-tribromopyrrole-3-carbonitrile is prepared by reacting a vinamidinium salt of formula I ##STR1## wherein R, R 1 , R 2  and R 3  are each independently C 1  -C 4  alkyl, or when taken together, R and R 1 , and R 2  and R 3 , may form a 5- or 6-membered ring in which each of RR 1  and R 2  R 3  are represented by the structure --(CH 2 ) 4  --or --(CH 2 )5--; and 
     X -  is an anion; 
     with a glycine ester of formula II ##STR2## wherein R 4  is C 1  -C 4  alkyl; and a first base in the presence of a first solvent to form a mixture comprising 4-cyanopyrrole-2-carboxylic acid and a 4-cyanopyrrole-2-carboxylate of formula III ##STR3## wherein R 4  is as described above, hydrolyzing said mixture with a second base in the presence of a second solvent to form a 4-cyanopyrrole-2-carboxylic acid salt, and reacting said 4-cyanopyrrole-2-carboxylic acid salt with a third base and a brominating agent in the presence of a third solvent to form the desired 2,4,5-tribromo -pyrrole-3-carbonitrile compound. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Advantageously, the present invention provides a process for the preparation of 2,4,5-tribromopyrrole-3-carbonitrile which avoids the use and handling of toxic reagents such as chlorosulfonyl isocyanate. 
     Surprisingly, it has been found that 2,4,5-tribromopyrrole-3-carbonitrile is prepared by reacting a vinamidinium salt of formula I ##STR4## wherein R, R 1 , R 2  and R 3  are each independently C 1  -C 4  alkyl, or when taken together, R and R 1 , and R 2  and R 3 , may form a 5- or 6-membered ring in which each of RR 1  and R 2  R 3  are represented by the structure --(CH 2 ) 4  --or --(CH 2 ) 5  --; and 
     X -  is an anion; 
     with a glycine ester of formula II ##STR5## wherein R 4  is C 1  -C 4  alkyl; and a first base in the presence of a first solvent to form a mixture comprising 4-cyanopyrrole-2-carboxylic acid and a 4-cyanopyrrole-2-carboxylate of formula III ##STR6## wherein R 4  is as described above, hydrolyzing said mixture with a second base in the presence of a second solvent to form a 4-cyanopyrrole-2-carboxylic acid salt, and reacting said 4-cyanopyrrole-2-carboxylic acid salt with a third base and a brominating agent in the presence of a third solvent to form the desired 2,4,5-tribromo -pyrrole-3-carbonitrile compound. 
     A preferred process of the present invention comprises reacting a formula I vinamidinium salt with about 1 to 5 molar equivalents of a formula II glycine ester and about 2 to 6 molar equivalents of a first base in the presence of a first solvent at a temperature range of about 50° C. to 140° C. to form a mixture comprising 4-cyanopyrrole-2-carboxylic acid and a formula III 4-cyanopyrrole-2-carboxylate, hydrolyzing said mixture with about 1 to 8 molar equivalents of a second base in the presence of a second solvent at a temperature range of about 25° C. to 90° C. to form a 4-cyanopyrrole-2-carboxylic acid salt, and reacting said 4-cyanopyrrole-2-carboxylic acid salt with about 4 to 8 molar equivalents of a third base and about 3 to 9 molar equivalents of a brominating agent in the presence of a third solvent at a temperature range of about 0° C. to 25° C. to form the desired 2,4,5-tribromopyrrole-3-carbonitrile compound. The reaction scheme is illustrated in Flow Diagram I. ##STR7## 
     The product, 2,4,5-tribromopyrrole-3-carbonitrile, may be isolated by dilution of the reaction mixture with water, acidification of the diluted reaction mixture, and filtration of the product or extraction of the product with a suitable solvent. Suitable extraction solvents include substantially water-immiscible solvents such as ether, ethyl acetate, toluene, methylene chloride and the like. 
     First bases suitable for use in the process of the present invention are bases such as alkali metal C 1  -C 6  alkoxides and alkaline earth metal C 1  -C 6  alkoxides, with sodium methoxide being preferred. Second and third bases suitable for use in the process of the present invention are bases such as alkali metal hydroxides and alkaline earth metal hydroxides, with sodium hydroxide being preferred. 
     Brominating agents suitable for use in the process of the present invention include bromine, pyridinium bromide perbromide and N-bromosuccinimide, with bromine being preferred. 
     First solvents such as N,N-dimethylformamide, C 1  -C 4  alcohols, N,N-dimethylacetamide, dimethyl sulfoxide and N-methylpyrrolidone may be employed in the process of the present invention. Preferred first solvents include N,N-dimethylformamide and C 1  -C 4  alcohols, with N,N-dimethylformamide being most preferred. Second and third solvents suitable for use in the process of the present invention include water-miscible ethers such as tetrahydrofuran, dioxane, 2-methoxyethyl ether and 1,2-dimethoxyethane, water, and mixtures thereof. Preferred second and third solvents include dioxane, tetrahydrofuran, water, and mixtures thereof. 
     Preferred formula I vinamidinium salts which are suitable for use in the process of the present invention are those wherein 
     R, R 1 , R 2  and R 3  are each independently C 1  -C 4  alkyl; and 
     X -  is selected from the group consisting of BF 4   - , ClO 4   - , HO 2  CCO 2   -  and Cl - . 
     Most preferred formula I vinamidinium salts are those wherein 
     R, R 1 , R 2  and R 3  are methyl; and X -  is HO 2  CCO 2   - . 
     Formula I vinamidinium salts may be prepared according to the procedures of C. Reichardt and W. Kermer, Synthesis, 1970, page 538; and J. Kucera and Z. Arnold, Collection Czechoslov. Chem. Commun., 32, pp. 1704-1711 (1967). Formula I vinamidinium chloride salts may be converted into the corresponding perchlorate, tetrafluoroborate or oxalate salts by conventional, well-known methods. 
    
    
     In order to facilitate a further understanding of the invention, the following examples are presented to illustrate more specific details thereof. The invention is not to be limited thereby except as defined in the claims. 
     EXAMPLE 1 
     Preparation of [2-Cyano- 3-(dimethylamino) -allylidene] dimethylammonium oxalate ##STR8## 
     A solution of oxalyl chloride (13.1 mL, 0.15 mol) in 1,2-dichloroethane is added dropwise to a solution of N,N-dimethylformamide (11.6 mL, 0.15 mol) in 1,2-dichloroethane while maintaining the reaction mixture temperature below 0° C. The reaction mixture is stirred for 45 minutes, treated with a solution of trans-3-dimethylaminoacrylonitrile (16.4 mL, 0.15 mol) in 1,2-dichloroethane, stirred at room temperature for 16 hours, refluxed for two hours and concentrated in vacuo to obtain a semi-solid residue. A solution of the residue in 2-propanol is treated with a solution of oxalic acid dihydrate (18.9 g, 0.15 mol) in 2-propanol and warmed until a clear solution is obtained. The clear solution is cooled to and held at room temperature for several hours, cooled in a freezer for two hours and filtered. The filter cake is washed sequentially with 2-propanol and ether and dried in a vacuum desiccator at 50° to give the title product (28.6 g, 79%). 
     EXAMPLE 2 
     Preparation of Methyl 4-cyanopyrrole-2-carboxylate and 4-Cyanopyrrole-2-carboxylic acid ##STR9## 
     [2-Cyano-3-(dimethylamino)allylidene]-dimethylammonium oxalate (3.6 g, 0.015 mol) is added to a stirred suspension of glycine methyl ester hydrochloride (1.9 g, 0.015 mol) and sodium methoxide (3.2 g, 0.06 mol) in N,N-dimethylformamide which has been cooled in an ice/acetone bath. The reaction mixture is heated to and held at 120° C. for 16 hours, cooled to room temperature and concentrated in vacuo to obtain a semi-solid residue. The residue is partitioned between ethyl acetate and cold, dilute hydrochloric acid. The layers are separated and the aqueous layer is washed with ethyl acetate. The organic layer and washes are combined, washed sequentially with water and brine, dried over anhydrous magnesium sulfate and concentrated in vacuo to give a mixture of the title products as a tan solid (0.85 g). The mixture contains about 70% methyl 4-cyanopyrrole-2-carboxylate and about 30% 4-cyanopyrrole-2-carboxylic acid. 
     EXAMPLE 3 
     Preparation of 2,4,5-Tribromopyrrole-3-carbonitrile ##STR10## 
     Sodium hydroxide (50% in water, 0.4 mL) is added to a solution of the mixture obtained in Example 2 (0.85 g) in dioxane. The reaction mixture is heated at 80° C. for about 12 hours, cooled to about 10° C. and treated with additional sodium hydroxide (50% in water, 1.2 mL). Bromine (0.9 mL, 0.02 mol) is then added portionwise over 90 minutes to the reaction mixture while maintaining the temperature below 20° C. After the addition is complete, the cooling bath is removed and the reaction mixture is stirred for 90 minutes then poured into water. The aqueous mixture is acidified with hydrochloric acid and filtered. The filter cake is washed with water and dried to give the title product as a tan solid (0.72 g).