Dechlorination of pyridines in acidic, zinc-containing mediums

Details Dechlorination of pyridines in acidic, zinc-containing mediums Dechlorination of pyridines in acidic, zinc-containing mediums

1999-07-15

2001-02-06

Rotman, Alan L. (Department: 1612)

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

C546S346000

Reexamination Certificate

active

06184384

ABSTRACT:

BACKGROUND
The present invention resides generally in the field of chlorinated pyridines. More particularly, the present invention relates in one preferred aspect to processes for preparing 3,5-dichloropyridine by dechlorinating 2,3,5,6-tetrachloropyridine in an acidic medium such as that provided by an alkanoic acid, especially acetic acid.
As further background, polychlorinated pyridine derivatives are important intermediates in the preparation of pesticides. Consequently, much effort has been made at both the academic and industrial levels to find improved, economically-practicable processes for their preparation.
3,5-Dichloropyridine is one such intermediate. Proposed processes for its production have been widely diverse and have included both the selective chlorination of intermediate compounds, the selective dechlorination of higher-chlorinated pyridines, and other routes.
For example, JP011211268 (1989) describes the preparation of 3,5-dichloropyridine by dechlorinating chloropyridine compounds under catalytic hydrogenation conditions in the presence of palladium on carbon. JP 03200769 (1991) describes a process for oxidizing hydrazinopyridine derivatives to give chloropyridine compounds.
Despite work toward convenient and effective routes to 3,5-dichloropyridine, there remain needs for improved processes for the practicable, commercial-scale production this compound. The present invention is addressed to these needs.
SUMMARY OF THE INVENTION
A feature of the present invention is the discovery that 3,5-dichloropyridine can be selectively prepared by the dechlorination of higher-chlorinated pyridines in an acidic medium in the presence of zinc.
Accordingly, in one preferred embodiment, the invention provides a process for preparing 3,5-dichloropyridine which comprises dechlorinating a tri-, tetra- or pentachloropyridine compound having chlorines at the 3- and 5-positions, in an acidic medium including zinc, so as to selectively form 3,5-dichloropyridine. In a more preferred form, the invention provides for the preparation of 3,5-dichloropyridine by dechlorinating 2,3,5,6-tetrachloropyridine to selectively form 3,5-dichloropyridine in a medium including an alkanoic acid and zinc.
In a particularly preferred embodiment, the invention provides a process for selectively preparing 3,5-dichloropyridine by dechlorinating 2,3,5,6-tetrachloropyridine in acetic acid in-the presence of zinc.
Dechlorination processes as described herein can optionally be conducted in the presence of a quaternary ammonium catalyst, for example one having a cation of the formula:
wherein:
R
4
, R
5
, and R
6
, which may be the same as one another or may differ, are H; C
1
to C
6
alkyl, and wherein two of R
4
, R
5
and R
6
may together as alkyl groups form a ring; C
1
to C
6
alkenyl; alkyl-phenyl, wherein the alkyl is C
1
to C
6
and the phenyl is optionally substituted with C
1
to C
6
alkyl, C
1
to C
6
alkenyl, halogen, hydroxyl, or C
1
to C
6
alkoxy; or phenyl, optionally substituted with C
1
to C
6
alkyl, C
1
to C
6
alkenyl, halogen, hydroxyl, or C
1
to C
6
alkoxy; and
R
7
is C
1
to C
6
alkyl; C
1
to C
6
alkenyl; alkyl-phenyl, wherein the alkyl is C
1
to C
6
and the phenyl is optionally substituted with C
1
to C
6
alkyl, C
1
to C
6
alkenyl, halogen, hydroxyl, or C
1
to C
6
alkoxy; or phenyl, optionally substituted with C
1
to C
6
alkyl, C
1
to C
6
alkenyl, halogen, hydroxyl, or C
1
to C
6
alkoxy.
It has been found that the use of such catalysts speeds the reaction, while not eliminating the selectivity toward the desired 3,5-dichloropyridine product.
The present invention provides improved processes for preparing 3,5-dichloropyridines by the dechlorination of higher-chlorinated pyridine derivatives. The preferred processes are highly selective for the 3,5-dichloro- products, and can be conducted so as to achieve high conversion of the higher-chlorinated pyridine starting material. Additional preferred embodiments of the invention as well as their features and advantages will be apparent from the description that follows.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to certain of its embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and modifications and applications of the principles of the invention as described herein being contemplated as would normally occur to one skilled in the art to which the invention pertains.
As disclosed above, the present invention features processes for preparing a 3,5-dichloropyridine compound by the dechlorination of a higher-chlorinated pyridine compound in an acidic medium in the presence of zinc. In particular aspects of the invention, provided are processes for selectively preparing 3,5-dichloropyridine by dechlorinating 2,3,5,6-tetrachloropyridine in a medium containing an alkanoic acid, especially acetic acid, and zinc.
In a general sense, the starting materials for processes of the invention will be encompassed by the formula:
wherein R
1
, R
2
, and R
3
, are H, chloro, or a non-interfering substituent such as a C
1
to C
20
hydrocarbon, e.g. alkyl, with the proviso that at least one of R
1
, R
2
, and R
3
is chloro. Illustrative starting materials thus include, for example, 2,3,4,5,6-pentachloropyridine, tetrachloropyridine compounds such as 2,3,5,6-tetrachloropyridine, optionally substituted at the 4-position with a non-interfering substituent as disclosed above, especially lower (C
1
to C
6
) alkyl, and 2,3,4,5-tetrachloropyridine, optionally substituted at the 6-position with a non-interfering substituent as disclosed above, especially lower alkyl.
An especially preferred feature of the invention involves the discovery that 2,3,5,6-tetrachloropyridine can be selectively converted to 3,5-dichloropyridine in good yield by dechlorination in the presence of zinc in an acidic medium. The preferred starting material, 2,3,5,6-tetrachloropyridine
can be obtained commercially or can be prepared using procedures well known to the art and literature. For example, suitable processes by which 2,3,5,6-tetrachloropyridine can be made are disclosed in U.S. Pat. No. 5,591,857 issued Jan. 7, 1997 and the background literature discussed therein. As will be appreciated, this and other polychlorinated pyridine starting materials may be obtained from these or other known sources or chemical routes without departing from the present invention.
The selected chlorinated pyridine starting material is dechlorinated in the presence of zinc as an electron donor to form a 3,5-dichloropyridine compound. It is preferred that the zinc be used in particulate form to provide increased surface area for the reaction. Zinc chips or zinc dust may be used. As to amounts, it is preferred that at least 0.5 gram atoms of zinc be used per gram atom of chlorine to be removed. Typically, about 0.5 gram atoms to 3 gram atoms of zinc will be used per gram atom of chlorine to be removed, more preferably about 1 to 3 gram atoms of zinc per gram atom of chlorine. Thus, as an example, in the case of the dechlorination of 2,3,5,6-tetrachloropyridine to 3,5-trichloropyridine, it will be preferred to use about 1 to about 6 gram atoms of zinc per mole of 2,3,5,6-tetrachloropyridine. Most preferably in this case, about 2 to about 4 gram atoms of zinc are used per mole of 2,3,5,6-tetrachloropyridine.
As disclosed above, the inventive processes can optionally be conducted in the presence of a catalyst, such as a compound having both hydrophobic and hydrophilic character and traditionally used as a phase transfer catalyst. Preferred such compounds will demonstrate the capacity to increase the rate of reaction to form the desired dechlorinated pyridine derivative and assist in reducing the undesired agglomeration of the zinc particles. Preferred catalyst compounds include organic quaternary ammonium compounds. For example, the p

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