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Process for the Selective Reduction of the 4-Halogen in 2,4-Dihaloanilines

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Process for the Selective Reduction of the 4-Halogen in 2,4-Dihaloanilines Europaisches Patentamt European Patent Office © Publication number: 0 507 41 7 A2 Office europeen des brevets EUROPEAN PATENT APPLICATION © Application number: 92200978.2978.2 © int. Ci.5; C07C 209/74, C07C 231/14 @ Date of filing: 03.04.92 © Priority: 04.04.91 US 680712 © Applicant: DOWELANCO 9002 Purdue Road @ Date of publication of application: Indianapolis, Indiana 46268-3030(US) 07.10.92 Bulletin 92/41 @ Inventor: Pews, Garth R. © Designated Contracting States: 4403 Andre Street DE ES FR GB IT NL Midland, Michigan 48640(US) Inventor: Wehmeyer, Richard M. 205 Zinnia Lake Jackson, Texas 77566(US) Inventor: Hunter, James E. 114 Player Court No. 3 Walnut Creek, California 94598(US) © Representative: Smulders, Theodorus A.H.J., Ir. et al Vereenigde Octrooibureaux Nieuwe Parklaan 97 NL-2587 BN 's-Gravenhage(NL) © Process for the selective reduction of the 4-halogen in 2,4-dihaloanilines. © Substituted 2-haloanilines are prepared from the correspondingly substituted 2,4-dichloro- or 2,4- dibromoanilines by selectively reducing a chloro or bromo substituent para to a protected amino group in the presence of the same halogen as an ortho substituent. The selective reductions are accomplished by protecting the amino function of the aniline with two protecting groups, e.g., as the diacetanilide or the succinimide. CM < o Rank Xerox (UK) Business Services EP 0 507 417 A2 The present invention concerns a process for preparing substituted 2-haloanilines from the correspond- ingly substituted 2,4-dichloro- or 2,4-dibromoanilines. More particularly, the present invention concerns the selective reduction of a chloro or bromo substituent para to the protected amino group of an aniline also having the same halogen as an ortho substituent. 2-Halo and 2,6-dihaloanilines are useful as intermediates in the manufacture of a wide variety of chemical products including, for example, dyes, pharmaceuticals and agricultural chemicals. Unfortunately, 2-halo and 2,6-dihaloanilines, optionally substituted in the 3(5)- and/or 6-position, are often not that easy to obtain. Since direct electrophilic halogenation of anilines typically provides little or no selectivity for halogenation at carbons ortho versus para to the amino group, the 4-position must normally be blocked 10 and then deprotected in order to prevent halogenation from occurring there. For example, 2,6-dichloro-3- methylaniline is presently manufactured from the acetanilide of m-toluidine in a multistep process (see O.G. Backeberg et al., J.Chem.Soc, 1943, 78-80; and H.C. Brimelow et al., J.Chem.Soc, 1951, 1208-1212) involving the following reaction sequence: 75 NHAc NHAc CIS03H 20 (ii) (i) 25 HCI/H20 H202 (iii) 30 i) protection of the p-position by sulfonamidation; 35 ii) hydrolysis of the acetanilide; iii) chlorination of the 2- and 6-positions; and iv) deprotection of the p-position. The yields of the protection (i) and chlorination (iii) steps are relatively low and the use of chlorosulfonic acid and ammonia present difficulties with respect to safe handling and waste disposal. 40 Rather than employing a scheme which involves the protection and deprotection of the reactive para position, it would be desirable to have a process in which a chloro or bromo substituent in the para position could be selectively removed from readily available anilines also having the same halogen in at least one of the ortho positions. The present invention concerns a process for preparing 2-haloanilines of formula (I) 45 50 55 wherein X is CI or Br, R is X, F, C1-C4 alkyl, C1-C+ alkoxy, CF3 or C02R2, R1 is H or C1-C4 alkyl, and 2 EP 0 507 417 A2 R2 is H or Ci -C+ alkyl characterized by: (a) contacting an amino-protected 2,4-dichloroaniline or an amino-protected 2,4-dibromoaniline of formula (II) (ID where R5 is CH3 or CH2CH3, or R3 and R+ taken together are 30 40 45 and X, R and R1 are as previously defined, with a hydrogen source in the presence of a palladium catalyst in an inert organic solvent to selectively remove the 4-chloro or 4-bromo substituent; and 50 (b) hydrolyzing the protecting groups from the amino function to produce the desired aniline. By protecting the amino function of the aniline with two protecting groups, e.g., as the diacetanilide or the succinimide, a chloro or bromo substituent para to the amino function can be selectively reduced in the presence of the same halogen substituent ortho to the amino function. Thus the present process for preparing 2-haloanilines unsubstituted in the 4-position avoids the need of protecting and deprotecting the 55 position para to the amino group and allows the use of readily available starting materials which have been completely halogenated in all of the available reactive ortho and para positions. As used herein, the term "halo" refers to CI or Br, and the terms "C1-C4 alkyl" and "C1-C4 alkoxy" refer to straight-chained or branched hydrocarbon groups of up to four carbon atoms, provided that all 3 EP 0 507 417 A2 substituent groups are sterically compatible with each other. The term "sterically compatible" is employed to designate substituent groups which are not affected by steric hindrance as this term is defined in "The Condensed Chemical Dictionary", 7th edition, Reinhold Publishing Co., N.Y. page 893 (1966) which definition is as follows: "steric hindrance": A characteristic of molecular structure in which the molecules 5 have a spatial arrangement of their atoms such that a given reaction with another molecule is prevented or retarded in rate." Sterically compatible may be further defined as reacting compounds having substituents whose physical bulk does not require confinement within volumes insufficient for the exercise of their normal behavior as discussed in "Organic Chemistry" by D.J. Cram and G. Hammond, 2nd edition, McGraw-Hill w book Company, N.Y., page 215 (1964). The preferred "C1-C+ alkyl" and "C1-C+ alkoxy" groups are -CH3, -CH2CH3, -OCH3 and -OCH2CH3. The most preferred group is -CH3. The amino-protected chloroaniline and bromoaniline starting materials of formula II are known com- pounds, eg., see Japanese Patent Publication Nos. 52-025117 or 56-164106, or can be prepared from the is appropriately substituted chloro- and bromoanilines by conventional acylation procedures. For example, diacetanilides can be prepared from the corresponding chloro- or bromoanilines by treatment with excess acetic anhydride and an acid catalyst, such as, for example, methanesulfonic acid. Alternatively, dipropionamides, succinimides and glutarimides can be prepared from the appropriate chloro- or bromoanilines and the corresponding acid chlorides or bromides, e.g., propionyl, succinyl or glutaryl 20 chloride, in the presence of a hydrogen halide acceptor, such as pyridine or triethylamine. In cases where the aniline contains a bulky ortho substituent, e.g., Br, the introduction of two protecting groups on the amine function may prove difficult and the formation of the diacetanilide {R3 and R+ are each -C(0)CH3} is preferred. The preferred starting materials are those in which X is CI or Br, R is CI, Br, CH3, OCH3 or C02R2, R1 is 25 H or CH3, R2 is CH3 or CH2CH3 and R3 and R+ are -C(0)CH3. The most preferred starting materials are those in which X is CI, R is CI or C02R2, R1 is H or CH3, R2 is CH3, and R3 and R* are -C(0)CH3. The appropriately substituted chloroanilines may in turn conveniently be prepared by protecting the amine moiety of commercially available ortho and meta substituted anilines as the hydrochloride salt, followed by treatment with chlorine. Similarly, appropriately substituted bromoanilines may conveniently be 30 prepared by treatment of commercially available ortho and meta substituted anilines with bromine in acetic acid. In the reduction step, the amino-protected 2-halo-4-chloro- or bromoaniline is contacted with a hydrogen source in the presence of a palladium catalyst. During the course of the reaction, the halogen in the 4- position is selectively replaced by hydrogen. 35 The selective reduction is fairly specific to palladium catalysts, and palladium on carbon has generally been found to be more effective than palladium dispersed on other supports. Thus, the preferred catalyst is 1 to 10 percent palladium on carbon. The percent loading of palladium on the carbon support also affects the efficiency of the catalyst depending upon such factors as solvent or hydrogen source. In some instances, 10 percent palladium on carbon functions better than 5 percent or 1 percent palladium on carbon, 40 even though comparisons are based on equal metal content. Generally, 0.01 to 0.20 equivalents of palladium are employed per equivalent of substrate; from 0.02 to 0.10 equivalents are preferred. The reduction can be conducted using hydrogen gas as the hydrogen source. The hydrogen gas can be continuously sparged into the reaction mixture at atmospheric pressure or the reaction mixture can be pressurized with hydrogen gas in a sealed reactor. With hydrogen gas, however, it is often difficult to 45 control the extent of reduction. This is particularly so when excess hydrogen is sparged into the reaction as is often most convenient. Therefore, it is preferable to use other hydrogen sources. Formate salts are the most convenient and preferred hydrogen source for the present application. By the term "formate salts" is meant alkali metal formates, such as sodium formate and potassium formate, ammonium formate and trialkylammonium formates wherein the alkyl groups are straight-chained alkyl 50 groups of 1 to 4 carbons, such as triethylammonium formate. The trialkylammonium formates, being relatively non-hygroscopic, easily prepared and quite soluble in most organic solvents, are the preferred hydrogen source. The trialkylammonium formates can be prepared by stirring an excess of trialkylamine with formic acid in toluene.
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