Process for stabilizing 4-halopyridines

Abstract

The present invention relates to a process for stabilizing 4-halopyridines of the formula (I)

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to 4-halopyridines. In particular, this invention relates to a process for stabilizing 4-halopyridines.

2. Description of the Art

4-Halopyridines are compounds which are unstable in the course of storage at room temperature and are obtainable synthetically via methods known per se, for example by chlorinating the corresponding 4-hydroxypyridine with PCl₅ or POCl₃ (Wibaut & Broekmann, Recueil des Travaux Clinique des Pays-Bas et de la Belgique 7659, 78, 593-603). Impurities from the preparation process, especially residues of acid, lead to the decomposition of the 4-halopyridines and the formation of oligomeric products. One product formed in the case of 4-chloropyridine is the salt of the N-(4′-pyridyl)-4-chloropyridinium chloride dimer (Wibaut & Broeckmann, Recueil des Travaux Clinique des Pays-Bas et de la Belgique 1961, 80, 309-312).

Den Hertog et al. (Wibaut & Brockmann, Recueil des Travaux Clinique des Pays-Bas et de la Belgique 1951, 70, 105-111) state that 4-nitropyridine can be stabilized by the addition of 10% aqueous sodium bicarbonate solution, in which case 90% of 4-nitropyridine were still present unchanged at 60° C. after 12 hours.

According to Wibaut & Brockmann (Recueil des Travaux Clinique des Pays-Bas et de la Belgique 1961, 80, 309-312), 4-chloropyridine when mixed with primary amines NH₂R (R═CH₃, CH(CH₃)₂, C₁₂H₂₅) or secondary amines NHR′₂ (R′═CH₃, n-C₄H₉), the formation of the corresponding (4-NHR)- and (4-NR′₂)-pyridines respectively is observed, while aliphatic tertiary or mixed aliphatic-aromatic tertiary amines (NEt₃, diethylaniline) did not react with 4-chloropyridine. 4-Chloropyridine forms an N-(4′-pyridyl)pyridinium salt with the aromatic tertiary amine pyridine. 4-Chloropyridine is currently commercially available only as the hydrochloride, in which the changed electronic conditions in the form of pyridinium salt stabilize the molecule.

SUMMARY OF THE INVENTION

The invention relates to a process for stabilizing 4-halopyridines of the formula (I)

where

• X is fluorine, chlorine, bromine or iodine,
• Y is hydrogen, fluorine, chlorine, bromine, iodine, nitro, NH₂, (C₁-C₄)-alkyl, (C₁-C₄)-alkanoylamine, phenyl-(C₁-C₄)-alkanoylamine, phenylcarbonylamine, (C₁-C₄)-alkylamine, di-(C₁-C₄)-alkylamine, phenyl-(C₁-C₄)-alkylamine, and
• n is 1 or 2,which comprises admixing one or more compounds of the formula (I) with one or more secondary and/or tertiary aliphatic or mixed aliphatic-aromatic amines.

The invention further relates to a mixture comprising

• • (a) one or more 4-halo-substituted pyridine derivatives of the formula (I)
    • where
    • X is fluorine, chlorine, bromine or iodine,
    • Y is hydrogen, fluorine, chlorine, bromine, iodine, nitro, NH₂, (C₁-C₄)-alkyl, (C₁-C₄)-alkanoylamine, phenyl-(C₁-C₄)-alkanoylamine, phenylcarbonylamine, (C₁-C₄)-alkylamine, di-(C₁-C₄)-alkylamine, phenyl-(C₁-C₄)-alkylamine, and
    • n is 1 or 2, and
  • (b) one or more secondary amines and/or aliphatic or mixed aliphatic-aromatic tertiary amines,excluding a mixture comprising (a) 4-chloropyridine and (b) dimethylamine, di-(n-butyl)amine or triethylamine.

DETAILED DESCRIPTION OF THE INVENTION

In the process and in the mixture, in each case independently,

• X is preferably Cl,
• Y is preferably hydrogen or fluorine, and
• n is preferably 1.

In the process and in the mixture, preference is given to using in each case one compound of the formula (I) and one amine.

Particular preference is given to a process or to a mixture in which the compound of the formula (I) is described by the formula (II) or (III):

In the abovementioned process for stabilization, or in the abovementioned mixture, all secondary amines known to those skilled in the art, or preferably aliphatic or mixed aliphatic-aromatic tertiary amines, may be used.

A secondary amine is, for example, diisopropylamine, morpholine, pyrrolidine, piperidine, N,N-dicyclohexylamine, preferably diisopropylamine.

A tertiary aliphatic amine is, for example, N-methylmorpholine, ethyldiisopropylamine, N,N,N′,N′-tetramethylethylenediamine (TMEDA), 1,5-diazabicyclo[4.3.0]-5-nonene (DBN); 1,8-diazabicyclo[5.4.0]-7-undecene (DBU), dicyclohexylethylamine, urotropine, preferably TMEDA, N-methylmorpholine and ethyldiisopropylamine.

A tertiary mixed aliphatic-aromatic amine is, for example, dimethylaminopyridine (DMAP), N,N-dimethylaniline, preferably DMAP.

In the process according to the invention, particular preference is given to using tetramethylethylenediamine (TMEDA), N-methylmorpholine and ethyldiisopropylamine.

The compounds of the formula (I) are useful as a starting material in the synthesis of pharmaceutical products. An example is the use of the compound (II) and (III) in the preparation of compounds which are described in the European patent applications EP 287 982 and can be employed for treating pain and depression. It is particularly advantageous when the 4-halopyridine starting material can first be stabilized and stored for a long period of time. Secondly, it is even more advantageous to have the 4-halopyridine free of impurities (for example of decomposition products) which might contaminate the pharmaceutical product. Finally, there should not be any safety risk especially involving any exothermically decomposable materials.

4-Chloro-3-fluoropyridine may be used, for example, in the preparation of the compound N-(3-fluoro-4-pyridinyl)-N-propyl-3-methyl-1H-indol-1-amine which finds use in the treatment of nerve damage. The chlorine substituent is replaced by an amine with addition of a base.

In the process according to the invention for stabilizing halopyridines or the mixture, preference is given to a content of up to 10%, preferably 5%, of amine.

The limited stability and the process according to the invention for increasing the stability is illustrated hereinbelow by way of an example using the compounds 4-chloropyridine and 4-chloro-3-fluoropyridine (4-CIFPy). The decomposition products of 4-CIFPy were investigated more precisely.

To elucidate the decomposition products, the solid which precipitated in the course of storage was filtered off with suction and subsequently separated by chromatography. To this end, the solid was dissolved in triethylamine and separated on silica gel using the eluent 4:1 ethyl acetate/ethanol. This gave two fractions:fraction 1 having R_f=0.83 and fraction 2 with R_f=0.53 in the eluent system mentioned. It was possible to elucidate the two fractions as the following dimerization/isomerization products:

Two batches of 4-CIFPy having a different content of TMEDA were investigated for its storage stability at 20-25° C.

TABLE 1
	TMEDA
Batch No.	content	Time	4-CIFPy content	Purity (GC)
1	2.15%	start of storage	98.8%	97.8%
		+5 months	98.2%	97.9%
2	0.47%	start of storage	98.9%	99.4%
		+3 months	100.9%	99.5%
		+9 months	99.4%	99.3%

Within the range of measurement precision, the content of 4-chloro-3-fluoropyridine did not change over the period observed; the aliphatic tertiary amine TMEDA is thus capable of stabilizing 4-halo-substituted pyridines.

In order to verify the observed effect various other tertiary and secondary amines were further investigated, for e.g., using the amines triethylamine, N-methylmorpholine, ethyldiisopropylamine and diisopropylamine were each added in different mixtures (A: 10 μl to 5 ml; B: 10 μl to 1 ml) to a sample of 4-CIFPy, and the content of decomposition products after storage at room temperature was determined by means of gas chromatography (GC) (blank value: 0.097%):

TABLE 2
Stabilization of 4-CIFPy using various amines
	Period of storage at room temperature [days]
	3	10	17	24	31	59	80
Amine	Amount	Content of decomposition products [%]
NEt3	A	0.11	0.11	0.12	0.13	0.13	0.09	0.11
	B	0.11	0.12	0.12	0.13	0.13	0.09	0.11
N-methyl-	A	0.11	0.12	0.12	0.12	0.13	0.07	0.10
morpholine	B	0.11	0.12	0.12	0.12	0.13	0.15	0.10
Et(i-Pr)2N	A	0.11	0.12	0.12	0.12	0.13	0.16	0.10
	B	0.11	0.12	0.13	0.13	0.13	0.16	0.11
(i-Pr)2NH	A	0.11	0.12	0.11	0.12	0.13	0.16	0.10
	B	0.11	0.12	0.11	0.13	0.13	0.17	0.11
TMEDA	A	0.11	0.11	0.11	0.12	0.13	0.16	0.10
	B	0.11	0.12	0.12	0.16	0.2	0.22	0.12

TABLE 3
Storage of 4-CIFPy (control experiment)
Storage without amine	Period of storage [days]
[° C.]	3	10	17	24	31	59	80
20-25° C.	0.13	0.21	0.34	0.71	1.5	0.72	0.41
0-5° C.	0.11	0.14	0.13	0.17	0.22	0.38	0.17

In the course of storage at 20-25° C. in the control experiment, it was possible to detect solid at day 31; solid was likewise observed in the refrigerator sample (0-5° C.) from day 59. As soon as solid was present in the sample, it was no longer possible to take a homogeneous sample, so that the secondary component is underrepresented in the GC. In the subsequent measurements, lower values than before were thus found.

The variations in the preceding measurements are within the error range of the integration of the gas chromatograph. Even small amounts of amine are sufficient in order to stabilize 4-chloro-3-fluoropyridine. An example which can be mentioned is that of the two different concentrations of TMEDA; in the diluted case A, only 0.13% of TMEDA is detected. In comparison, the concentrated solution B indicates 0.61% of TMEDA. It is evident from the above-described experiments that the addition of a secondary amine and/or a tertiary aliphatic or mixed aliphatic-aromatic amine in the total concentration range of from about 0.05% to about 5.0%, preferably from about 0.1% to about 2.5%, stabilizes a 4-halopyridine derivative of the formula (I), preferably a compound of the formula (II) and (III).

EXAMPLE 1

Characterization of 4-chloro-3-fluoropyridine (III)

	(III)
1H NMR:
H2	8.5 ppm	J(H2-H6) = 0 Hz, J(H2-H5) = 0 Hz	3JFH = 1 Hz
H5	7.4 ppm	J(H5-H6) = 5.3 Hz, J(H5-H2) = 0 Hz	4JFH = 0 Hz
H6	8.3 ppm	J(H6-H2) = 0 Hz, J(H6-H5) = 5.3 Hz	5JFH = 6.0 Hz
13C NMR:
C2	140 ppm	2JFC = 23 Hz
C3	156 ppm	1JFC = 259 Hz
C4	131 ppm	2JFC = 15 Hz
C5	126 ppm	3JFC = 2.3 Hz
C6	146 ppm	4JFC = 5.7 Hz
19F NMR:
F3	131 ppm

EXAMPLE 2

Characterization of the Dimerization/Isomerization Products by NMR Chemical Shifts

	1H NMR	13C NMR
	H4	8.65 ppm, d	C1	164.06 ppm
	H5	8.8 ppm, d	C2	151.76 ppm
	H6	8.61 ppm, t	C3	151.58 ppm
	H8	8.62 ppm, t	C4	147.29 ppm
	H10	7.8 ppm, dd	C5	139.77 ppm
			C6	137.8 ppm
			C7	135.45 ppm
			C8	127.51 ppm
			C9	123.94 ppm
			C10	121.06 ppm

	1H NMR	13C NMR
	H1	8.86 ppm, d	C1	139.56 ppm
	H2	8.63 ppm, d	C2	147.09 ppm
	H3	8.48 ppm, ddd	C3	126.97 ppm
	H4	8.02 ppm, ddd	C4	139.13 ppm
	H5	7.81 ppm, dd	C5	120.4 ppm
	H6	6.53 ppm, dd	C6	118.3 ppm
			C7	135.9 ppm
			C8	153.38 ppm
			C9	150.8 ppm
			C10	168.95 ppm

Claims

1. A process for stabilizing a compound of formula (III)

2. The process as set forth in claim 1, wherein one amine is used.

3. The process as set forth in claim 2, wherein the content of secondary amine, tertiary aliphatic amine, or tertiary mixed aliphatic-aromatic amine is from about 0.05% to about 5.0%.

4. The process as set forth in claim 3, wherein the secondary amine is diisopropylamine, morpholine, pyrrolidine, piperidine or N,N-dicyclohexylamine.

5. The process as set forth in claim 3, wherein the tertiary aliphatic amine is N-methylmorpholine, ethyldiisopropylamine,N,N,N′,N′-tetramethylethylenediamine (TMEDA), 1,5-diazabicyclo[4.3.0]-5-nonene (DBN), 1,8-diazabicyclo[5.4.0]-7-undecene (DBU), dicyclohexylethylamine or urotropine.

6. The process as set forth in claim 3, wherein the tertiary mixed aliphatic-aromatic amine is dimethylaminopyridine (DMAP) or N,N-dimethylaniline.