CA1048542A - Oxidation of allylacetone to 2,5-hexanedione in a high-boiling hydrocarbon solvent system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An economical process is described for the oxidation of allylacetone to 2,5-hexanedione using palladium chloride catalyst in the presence of copper chloride and oxygen, the reaction being carried out in a heterogeneous solvent system which is a mixture of water and a high-boiling hydrocarbon solvent, specific such solvents being either trichlorobenzene, hexachlorobutadiene, triethylbenzene, diphenyl ether or .alpha.-chloronaphthalene. High yields of product are obtained in minimum reaction times and with minimal losses of the expensive palladium catalyst.

Description

OXIDATION OF ALLYLACEIONE TO 2.,5-HEXANEDIONE
IN A HIGH-BOILING HYDROCARBON SOLVENT SYSTEM

BACKGROUND OF THE INVENTION
1. Field of the Invention This invention relates to an economical, com~ercial process ~or the preparation o~ 2,5-hexanedione, i.e., acetonyl-acetone~ from allylacetone and, more particularly, relates to the oxidation of allylacetone using palladium chloride as a catalyst in the presence of copper chloride and oxygen, whereby high yields of 2,5-hexanedione are prepared with only small losses of the palladium catalyst.

2. DescriPtion of the Prior Art 2,5-Hexanedione or acetonylacetone is important as an organic chemical intermediate. Preparation of this compound through various synthesis routes has been reported in the prior art. For example, Adams et al in J. Am. Chem. Soc., Vol. 72, p 4368 (1950), describe the synthesis of 2,5-hexanedione by condensing propylene oxide with acetoacetic esters to produce alpha-aceto-gamma-valerolactone, which, in turn, is reacted with dilute hydrochloric acid and converted into 5-hydroxy-2-hexanone. To obtain 2,5-hexanedione, the hydroxy-hexanedione product is then oxidized together with sodium dichromate and sulfuric acid. Also, Shenk in Ber., Vol. 77, p 661 (1944), describes the preparation of 2,5-hexanedione by oxidizing 2,5-dimethylfuran to ~-hexene-2,5-dione, which product is then hydrogenated to produce 2,5-hexanedione. Still ~urther, in U.S. Patent 2,525,672, Heilbron et al describe the preparation of 2,5-hexanedione by ~irst reacting 1-bromo-2,3~epoxy-butane with monosodium acetylide in liquid ammonia, and then reacting the 3-hexene-5-yn-2-ol product obtained with mercury sul~ate in sulfuric acid.

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However, ~or one or more reasons, all of these prior art processes are disadvantageous for preparing 2,5-hexanedione conveniently and economically. The process of Adams et al is a multistage reaction which provides only low yields of product.
The Shenk process, besides providing only moderate yields of product, utilizes 2,5-dimethylfuran which is obtained only with difficulty. The Heilbron et al process likewise utilizes reactants which are difficult to handle and only moderate product yields are realized.
More recently, in Kogyo Kakaku Zasshi, 71, (6), p 945-6 (1968), as well as in Japanese Patent Publication No.
1972-11411, Takamori Konaka and Sadao Yamamoto have described a l-step process for producing good commercial yields of 2,5-hexanedione from allylacetone in a mixed solvent system composed of water in either benzene or dimethylformamide, and employing palladium chloride as the oxidation catalyst in the presence of ; prescribed amounts of cupric chloride and oxygen. The process is carried out at temperatures of 60-80 C for time periods ranging generally from 3 to 12 hours but typically from 7 to 12 hours, after which the 2,5-hexanedione product is reported to be easily isolated from the reaction mixture and purified.
However, from practice of this process, substantial quantities of undesirable byproducts oftentimes may be formed and losses of the expensive palladium catalyst are found to be substantial.

SUMMARY OF THE INVENTION
It has now been found that by conducting the oxidation process in a manner similar to that described in the aforesaid Japanese patent publication~ but with the use of a mixed solvent system composed of water and any one of `` 1~4~542 certain high-boiling hydrocarbon solvents, commercially attractive yields of 2,5-hexanedione, i.e., at least about 70% of the theoretical yields can be obtained in short reaction times and with minimal losses of the palladium catalyst.
Specific suitable high-boiling sol~ents are trichloro-benzene, hexachlorobutadiene, triethylbenzene, diphenyl ether, and ~-chloronaphthalene.
Thus, in accordance with the present teachings an improved process is provided for selectively oxidizing allylacetone to 2,5-hexanedione. The process comprises reacting at a temperature of from 30 to 80C and for a period of time of 0.5 to 3.0 hours, allylacetone with palladium chloride in the presence of copper chloride and oxygen in a heterogeneous solvent system composed of water in combination with a high-boiling hydrocarbon which is selected from trichlorobenzene, hexachlorobutadiene, triethyl-benzene, dephenyl ether or ~-chloronaphthalene.
In the process, palladium chloride is util~zed as the oxidation catalyst in the presence of copper chloride and oxygen as reoxidizing agents for the palladium. Depending upon the particular solvent system employed, palladium losses typically will range from about 3 to 23¢ per pound of the hexanedione product, based on a palladium chloride cost of ~333/pound.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The oxidation of allylacetone (or ALA) to 2,5-hexanedione (or HDO) with the secondary oxidation-reduction reactions occurring in the process of this invention broadly may be represented by the following equations:

(1) Oxidation of ALA to HDO:
O O O

2 2 2 3 2 2 CH3CCH2CH2CCH3+ Pd + 2HCl (2) Regeneration of palladium to catalytic palladium chloride:

Pd + 2CuC12-------- PdC1 ~ 2CuCl .. , ,~ . , ~,.i ~
, ~C~48542

(3) Reoxidation of cuprous ion to cupric ion:

2CuCl + 1/2 2 + 2HCl~ 2CuC12 + H20 As illustrated in Equation (1) above, the palladium chloride catalyst is reduced to palladium metal during the oxidation of -4a-_.~
~ .,, 1[14t~54;~

the ALA. The metal is rapidly regenerated ~or reuse again as catalytic palladium chloride by the oxidizing action of the cupric chloride, as set forth in Equation (2). In turn, the cuprous chloride formed from the palladium reoxidation step is reoxidized to cupric chloride in the presence of oxygen and hydrochloric acid (Equation ~).
The ALA which is oxidized in the process of this invention is a commercially-available compound which may be synthesized by various methods. For example, it may be yn-thesized by reacting allyl alcohol and acetone in the presence of an acid-acting catalyst as set forth in U.S. Patent 3,114,772, issued December 17, 1963. Neither the AIA reactant per se nor any particular synthesis method therefor constitute a part of the present invention.
The purity of the ALA is not highly critical for obtaining the desired product yields. In general, however, it is desirable to employ ALA which is at least and preferably more than 93~ pure.
The heterogeneous solvent system employed in the process of this invention is a mixture of water and any one of certain high-boiling hydrocarbons immiscible therewith.
Specifically, a suitable such hydrocarbon may be trichloro-benzene, hexachlorobutadiene, triethylbenzene, diphenyl ether or ~-chloronaphthalene. In general, from 1 to 3 parts of hydrocarbon, by volume, is employed per each part of water in the mixed solvent system.
All of the hydrocarbons have boiling points substantially above 200 C at atmospheric pressure. The 2,5-hexanedione product has a boiling point of about 190 C
at atmospheric pressure. Thus the product may be isolated by . ~ ~ . , ' . - . . . :

fractional distillation procedures.
It is to be noted that use of the aforesaid mixed solvent systems has a further advantage in that any traces of palladium catalyst which may be contained in the solvent phase will remain in the solvent upon distillation of the product. Such amounts of palladium can be reutilized in the process by recycling and reuse of the solvent in a further reaction.
As described previously, palladium chloride is employed herein as the oxidation catalyst in the presence of copper chloride and oxygen as reoxidizing agents for the palladium. From 3 to 150 moles of copper chloride per each mole of palladium chloride generally may be used. More advantageous are ratios of 10-100 moles of copper chloride per mole of palladium chloride, with use of 12-50 moles of copper chloride/mole of palladium chloride being especially advantageous and presently preferred.
The copper chloride requirement, in turn, may be supplied by using either cupric chloride (CuC12) alone, or a mixture thereof with cuprous chloride (CuCl). Use of the mixed copper salts has been found to be advantageous for attaining optimum reaction rates. In such instances, the proportion of CuCl employed typically will be less than 50 weight percent of the mixture.
In addition to the aforedescribed copper chloride/
pallad~um chloride ratios (Cu/Pd), it has also been found desirable to employ copper chloride in a sufficient amount to provide a copper to allylacetone mole ratio (Cu/ALA) ranging generally from 0.1 to 10:1, and preferably ~rom 0.5 to 5.0:1, at least in the initial stages of the oxidation reaction.

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As shown in the Equation (1) above, hydrochloric acid is produced as a byproduct in the initial oxidation reaction. It has been customary in prior art practice to incorporate additional acid into the reaction mixture to supply sufficient amounts of H+ and Cl- ions for most efficient reoxidation of the palladium. In the process of this invention, however, no additional hydrochloric acid needs to be incorporated into the reaction mixture. The PH
of the mixture can be easily maintained between 1.0 to 3.0, depending upon the amount of oxygen in the system. It is to be noted that greater yields o~ product usually will be obtained at a faster rate if no acid is used.
The oxygen required may be introduced into the reaction in finely dispersed form at a prescribed rate or the reaction alternatively may be run under oxygen pressure.
~or example, a satisfactory rate of oxygen feed at atmospheric pressure typically is a minimum of about 2000 cc/min/liter of aqueous oxidant solution. Particularly advantageous oxygen feed rates are 4000-10,000 cc/min/liter of aqueous oxidant solution. In pressurized reactions, a satis~actory minimum oXygen feed rate is about 50 cc/min/liter of oxidant solution.
The process of this invention generally may be carried out within a temperature range of 30-80 C, regardless of the particular high-boiling hydrocarbon in the solvent sys-tem. However, a reaction temperature of 40-50 C is pre~erable when employing diphenyl ether, triethylbenzene, or ~-chloro-naphthalene; a temperature of 40-70 C is particularly advantageous for those processes employing trichlorobenzene or hexachlorobutadiene.

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The reaction times may range generally from 30 minutes to 3 hours, with reaction times of 1~-2 hours being presently preferred.
The particular method for carrying out the process is not especially critical, the process being conveniently effected with different charging procedures, degrees of agitation, rates of heating, and the like. As an example of the various charging techniques applied, the quantity o~ ALA
to be used may, in one instance, be charged entirely at the start of ~he reaction. Alternatively, only a portion of the ALA requirement may be charged initially and the remainder introduced incrementally during the reaction. In still another method, all of the ALA requirement may be fed at a prescribed rate throughout the reaction.
Upon completion of the reaction and cooling of the reaction mixture, whichever operating procedure is employed~
the aqueous oxidant layer and the product-containing solvent layer will separate. When employing any solvent other than triethylbenzene, the solvent layer, being heavier than the aqueous layer, can be easily drawn off from the bottom of the reactor. The aqueous layer may be extracted several times with solvent to recover small quantities of product therein.
The stripped aqueous layer, cont~ining the palladium catalyst, can be recycled and used in a further reaction. The product can be reclaimed from the solvent by simple distillation.
The amount of palladium lost in the reaction, i.e., that amount contained in the product stream which normally is lost provided the stripped solvent is not recycled for a fur-ther reaction, can be determined easily by analysis. Per ~o reaction, the palladium loss typically is in the range of 1~48542 3-23~/pound of product, based on a palladium chloride cost of ~3~/pound. With recycling and reuse of the stripped solvent, however, the average palladium losses may be still further reduced.
After separation, the catalyst-containing aqueous layer can be recycled to the reactor along with fresh ALA and solvent. Further, the solvent layer stripped of product may likewise be recycled. Thus, the process may be repeated in a semicontinuous manner.
It is to be noted that if semicontinuous operation is desirable, the aqueous oxidant solution need not be stripped of product prior to recycling as small amounts of HD0 in the inltial reaction mixture appear to accelerate the reaction rate and improve product yields. It is also to be noted that even if present in the reaction mixture at the start of oxida-tion, the HD0 will not react further to more complex derivatives, e.g., triketones, furans, etc., nor will it form chlorinated byproducts.
In order that those skilled in the art may more completely understand the present invention and the preferred methods by which it may be carried out, the following specific examples are given.

To a 500-cc creased flask equipped with a thermom-eter, agitator, condenser, bottom take off, and oxygen sparger is added 25.4 g (0.190 mole) cupric chloride, 4.1 g (0.040 mole) cuprous chloride, o.80 g (0.0045 mole) palladium chloride, 100 cc water, 10 cc HD0, 150 cc trichlorobenzene, and 20 cc (0.171 mole) of allylacetone (ALA). The reaction mixture : : - . . ~ . . .

1~)48542 contains a Cu/Pd mole ratio of 51:1 and a Cu/ALA mole ratio of 1.34:1.
Agitation is started, oxygen feed (400 cc/min) is ; begun, and the reaction is heated to 70 C. The reaction is continued for 1.1 hours at which time the reaction mixture is sampled and analyzed by vapor phase chromatography. About 95 of the ALA is found to be reacted.
After cooling to room temperature, agitation is stopped. The aqueous oxidant layer and the trichlorobenzene-HDO layer separate with the organic layer settling on the bottom. A quantitative analysis by vapor phase chromatography of the product layer shows the yield of HD0 to be approximately 74~ of theoretical.
The trichlorobenzene-HD0 solution is analyzed to determine the palladium content which is found to be 720 micrograms (~g). There is calculated to be 1200 ~g of palladium chloride present in the product and unavailable for recycllng to the reaction. At a palladium chloride cost of ~333/pound, this quantity of palladium represents a loss of about 3¢/pound of HD0.
HDO can be recovered by distillation from the trichlorobenzene at reduced pressure.

The experiment of Example 1 is repeated with recycling of the aqueous oxidant solution. Through 24 cycles, the average yield of HD0 is 80~ and the average palladium loss is ll~/pound o~ product.

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Using the equipment and general procedure as outlined in Example 1, an additional oxidation reaction is carried out with recycling of the aqueous oxidant solution, using trichlorobenzene and 20.0 g (0.149 mole) cupric chlo-ride, 10 g (0.100 mole) cuprous chloride, and 1.6 g (o.oo9 mole) palladium chloride. The Cu/Pd mole ratio is 17:1 and the Cu/ALA mole ratio is o.91:1. The reaction is conducted for 1.5-2.5 hours at 50 C.
Throu~h 24 cycles, the average yield of HD0 is 830/o and the average palladium loss is 23~/pound of product.

Uslng the equipment as outlined in Example 1, the process of this invention is conducted through 20 runs, employing hexachlorobutadiene (HCBD) as the high-boiling hydrocarbon solvent and recycling the aqueous solution to the reactor without exhaustive extraction of the HD0 after each cycle. The aqueous solution is composed of 25.4 g (0.190 mole) cupric chloride, 4.0 g (0.040 mole) cuprous chloride, o.80 g (0.0045 mole) palladium chloride, 100 cc water, and 25 cc HD0.
The Cu/Pd ratio is 51:1. Twenty cc of ALA (0.159 mole) assaying 930~ and 250 cc HCBD are used in each cycle. The Cu/ALA ratio is 1.45:1. The reaction is conducted at 70 C -~
for 0.8-1.0 hour.
At the end of each cycle, the reaction mixture is cooled to 30 C. The HCBD-product layer is drained from the bottom of the reactor and analyzed for ALA, HD0, and palladium.
A fresh charge of ALA and solvent is added to the reactor and the solution is heated to reaction temperature.

1~48S4Z
VPC analysis of the HCBD-product solution from - each cycle shows the average ALA conversion to be 97~ with a selectivity of 74~. The yield of HD0 product is 72yo. The average palladium loss is less than 4~/pound of product.

Another experiment analogous to Example 4 is conducted with recycling of the aqueous oxidant solution without exhaustive extraction of HD0 after each cycle. In this run, the aqueous solution contains the same amounts of ;
cupric and cuprous chlorides, while 2.4 g (0.01~5 mole) palladium chloride is used, whereby the Cu/Pd mole ratio is approximately 17:1. The Cu/ALA mole ratio is 1.34:1 as 20 cc of 100~ ALA is employed (0.171 mole). The reactions are conducted at 40 C for approximately 1.3 hours.
VPC analysis of the HCBD-product solutions from 10 cycles shows the average ALA conversion to be 86~ with a selectivity to HD0 of 86~. The average palladium loss is found to be 18 ~g/g of HD0. Based on a palladium chloride cost of ~33/pound, the average palladium loss from this series of reactions is about 12~/pound of HD0.

Allylacetone is oxidized as described in Example 1, employing q-chloronaphthalene as the hydrocarbon solvent. The quantities of cupric chloride, cuprous chloride, and palladium chloride (0.190 mole, 0.040 mole, and 0.009 mole, respectively) provide a Cu/Pd mole ratio of approximately 25:1. The Cu/ALA
ratio is 1.~4:1.
This reaction is conducted at 50 C for about 2.5 hours, after which the reaction mixture is cooled, separating 1~)4854Z :
into 2 layers. The solvent-product layer is drawn from the bottom of the reactor and analyzed for unreacted ALA, HD0, and palladium. ALA conversion is found to be 8g% with selectivity of 87~ to HD0. The palladium loss is about 15¢/pound of product.

An oxidation process analogous to Example 4 is carried out using diphenyl ether as the high-boiling solvent component and the same quantities of the copper and palladium chlorides as set forth in Example 6. This reaction is con-; ducted at 50 C for 1.5 hours.
Upon completion of the reaction, the reaction mixture is cooled and the solvent-product layer is drawn off. A fresh charge of ALA and diphenyl ether is added to the reactor and ; the solution is heated to 50 C.
VPC analysis of the diphenyl ether-product solutions from 10 cycles shows that the average ALA conversion from the process is 91~, and the selectivity to HD0 is 87~. The average palladium loss is 17¢/pound of product.

; 20 EXAMPLE 8 The process of this invention is conducted according to the procedure of Example 4, using triethylbenzene as the hydrocarbon component of the solvent system. The Cu/Pd and Cu/ALA mole ratios are as set forth in Example 6. This reaction is conducted at 50 C for about 45 minutes.
Upon completion of the reaction, the triethyl-benzene-product layer is siphoned from the aqueous layer. A
fresh charge of ALA and solvent is added to the reactor and the reaction repeated at 50 C.

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VPC analysis of the product solutions from 7 cycles indicates an average ALA conversion of 90~ with a selectivity to HDO of 77~. The average palladlum lost is 7¢/pound of product.
This experiment is repeated for 4 cycles at a temperature of 40 C for 1.8 hours. The average ALA conversion and HDO selectivity from these runs are 86~ and 89~, respect-ively. The average palladium loss is about 21~/pound of product.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is provided are defined as follows:

1. An improved process for selectively oxidizing allylacetone to 2,5-hexanedione which comprises reacting at a temperature of 30-80°C for a time period of 0.5-3.0 hours allylacetone with palladium chloride in the presence of copper chloride and oxygen in a heterogeneous solvent system composed of water in combination with a high-boiling hydrocarbon which is trichlorobenzene, hexachlorobutadiene, triethylbenzene, diphenyl ether, or .alpha.-chloronaphthalene.

2. The process of Claim 1 wherein the high-boiling hydrocarbon is trichlorobenzene.

3. The process of Claim 1 wherein the high-boiling hydrocarbon is hexachlorobutadiene.

4. The process of Claim 1 wherein from 3 to 150 moles of copper chloride are employed per mole of palladium chloride.

5. The process of Claim 1 which is conducted in a semicontinuous manner by recycling of the aqueous catalyst solution.