CA1095089A - Cyclohexene-(1)-dione-(3,6)-tetraalkyl diketals, process for preparing them and their use - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This application is directed to a process for the preparation of a hydroquinone dialkyl ether of the formula wherein each R represents a primary or secondary C1-4 alkyl radical in which a diketal of the formula II'

Description

~9S~89 p-Benzoquinone tetramethyl diketal of the formula C 3 ~ OC~3 may be prepared, for example by anodic oxidation of anisol or of hydroquinone dimethyl ether in methanol/KOH according to N.L.
Weinberg and B. Belleau, Tetrahedron 29 (1973), pages 279 to 285.
The compound (I) may be prepared especially suitably by anodic oxidation of benzene, likewise in methanolic solution, this pro-cess, however, requiring special conductive salts (cf. British Patent 836,949)~
The catalytic reduction of (I) with hydrogen in neutral to slightly acid medium with stoichiometric consumption of hydro-gen gives hydroquinone dimethyl ether in good yield (cf. German Offenlegungsschrift 25 47 464), which is a desired intermediate for the preparation of dyestuffs. It is quite natural that the catalytic reduction of (I) to hydroquinone dimethyl ether in neutral to slight acidic medium is advisable only when anisol or, in particular, benzene is uses as the starting compound. Said catalytic reduction, however, involves some difficulties, since frequently an undesired decomposition of (I) occurs. When further investigating the reaction possibilities of (I), in particular during the catalytic hydrogenation, it has been found surprisingly that the catalytic hydrogenation in a basic medium results in a product, which is completely different from that obtained in hy-drogenation in a neutral to slightly acidic medium, namely cyclo-- hexene-(l)-dione-(3,6)-tetramethyldiketal of the formula

- 2 -~s~9 X (II) when hydrogenating until about 1 mol of H2 per mol of diketal of the formula I has been absorbed.
It has moreover been found that the diketal of the formula (II) is capable of reketalizing to form the corresponding cyclohexene~ dione-(3,6)-tetraalkyl diketal, when it is dissol-ved in a higher alkanol than methanol, with substantial exclusion o~ water, at a temperature approaching room temperature and at a siightly acid pH, and that this diketal, as well as tetramethyl diketal, forms the corresponding hydroquinone dialkyl ether by intramolecular rearrangement while 2 mols of alkanol are split off, when the te~perature is increased.
The present invention, conse~uently, provides first cyclohexene-(l)-dione-(3,6)-tetraalkyl diketals of the formula X

RO OR (II') wherein R is a straight chain or granched no- tertiary alkyl radical having of from 1 to 4 carbon atoms, a diketal of the for-mula (II') with R being CH3, namely cyclohexene-(1)-dione-(3,6)~
tetramethy1 diketal, being preferred.

(II) , ~ -l~9S0~9 The cyclohexene-(l) dione-(3,6)-tetraalkyl diketals are prepared in accordance with the invention by reducing p-ben~oqui-none tetramethyl diketal of the formula (I) CH3 ~ OCH3 ~ (I) with about 1 mol, more precisely with of Erom 0.8 to 1.1 mols, preferably of from 0.9 to 1.1 mols and in particular of from 0.95 to 1.0 mol, of H2 per mol of diketal (I) in the presence of a hydrogenation catalyst, in a basic medium, at at temperature of from about -10 to +150C, preferably of from about +10 to +50C, in particular of from about +15 to +40C and by reketalizing the cyclohexene-(1)-dione-(3,6)-tetramethyl diketal optionally by treating it with an alcohol possessing a C2 4 primary or secondary alkyl radical, at a temperature of from about -10 to 35C, prefer-ably of from about 0 to 25C and in particular of from about +5 to 25C in a slightly acid medium with substantial exclusion of water, to yield a compound of the formula II' with R not being CH3.
The reduction is suitably carried out in a solvent of usual purity which is conventional for use for catalytic hydro-genations, for example acetic acid methyl or -ethyl ester, dioxane, tetrahydrofurane and others. A preferred solvent is methanol or the alcohol ROH whose R corresponds to the desired radical R in the final product (II'). The solutions employed for reducing gener-ally contain of form about 10 to 80 % by weight, preferably of from about 20 to 50 % by weight, of the compound (I).

i~9S089 Preferred catalysts are the noble metal catalysts be-longing to group VIII of the periodic table which are conventional for use for catalytic hydrogenations, as such,as well as in the form of their oxides, with or without a carrier material, for example active carbon. Preferred catalysts are palladium and platinum. A suitable catalyst which does not belong to said group is in particular Raney-niciel. The catalysts are employed in usual amounts, preferably in an amount of from about 0.005 to about 0.2 % by weight (metal), calculated on the starting com-pound (I).
The pH of the hydrogenation solution is adjusted at avalue greater than 7 by the addition of a base. A pH in the range of from about 8 to 10, measured with humid pH paper,ispre-ferred. Suitable bases are the conventional inorganic or organic bases, for example alkali metal hydroxides or alcoholates such as NaOH, KOH, NaOCH3, KOCH3, NaOC2H5 and others or nitrogen bases, for example triethylamine, cyclohexylamine, pyridine and others.
They are generally used in an amount of from about 0.0001 to 5 %
by weight, preferably of from about 0.01 to 2 % by weight, in particular of from about 0.1 to 1 ~ by weight, calculated on the substance (I).
The hydrogenation takes place already under a hydrogen pressure of from about 0.2 to about at least 200 bars. A pressure of from about 1 to 100 bars, in particular of from about 10 to 50 bars, is preferred.
The hydrogenation proceeds according to the gross equa-tion `

- ~09~;~89 ~ cat.

(I) (II) and is preformed under said conditions until about 1 mol of hydro-gen per mol of compound (I) is consumed. As hydrogenation devices there may be used apparatuses which are conventional for reactions of this type, for example agitator or shaking autoclaves or adequate glass or enamel vessels provided with a stirrer. Upon completion of the hydrogen absorption, the reaction product is treated in known manner, for example by cooling and expanding the reaction vessel, filtering off the catalyst, distilling off the solvent and by subsequently distilling or crystallizing the compound obtained of the formula II.
The highly selective hydrogenation under the indicated reaction conditions yielding cyclohexene diketal is surprising, especially owing to the fact that in subsequent hydrogenation under the same conditions as above, the cyclohexane stage is ob-tained in smooth manner. The by-products formed depend on the catalyst and on the base employed. Only hydroquinone dimethyl ether and cyclohexane-1,4-dione-tetramethyl diketal are obtained in varying small quantities, these compounds, however, being valuable intermediates.
For the purpose of reketalizing (II) to ~II")~ the latter diketal having higher alcohol xadicals in the ketal groupings, the compound ~II) may be isolated in the manner ~)95089 described, be dissolved in an alcohol R'OH which has a primary or secondary alkyl radical R' with 2 to 4 carbon atoms, preferably a primary alkyl radical, and the solution may be acidified with sub-stantial exclusion of water, that is to say that not more than S about 0.5 % by volume of water should be present. When this alcohol R'OH has already been used as the solvent in the hydrogen-ation, isolation may be dispensed with. After having filtered off the catalyst, the alcoholic solution may be further employed, for example be acidified at a temperature of from about -10 to +35C, preferably of from about O to +25C, in particular of from about +5 to +25C, whereby the reketalization yielding the cyclohexene-(l)-dione(3,6)-tetraalkyl diketal of the formula R'O OR' ~ (II") R'O OR' wherein R' is a primary or secondary alkyl radical of from 2 to 4 carbon atoms, takes place. In order to achieve a reaction as complete as possible, the reaction mixture is suitably kept at said temperature range for a certain period of time, preferably of from 2 to at least 5 minutes. It is particularly advantageous to remove the formed methanol by distillation, at said tempera-tura, under reduced pressure.
The alkylketals formed of the formula II" may be ob-tained in pure form by further distilling off the alcohol R'OH in a neutral, preferably in a basic medium.
For acidifying upon hydrogenating, there is used a mineral or organic acid, for example HCl,H2S04,HC104, p-toluene-sulfonic acid, formic acid, acetic aci~, which should be preferably anhydrous, or acid salts, for example KHS04, HCl and H2S04 being ~' ~'' .- ~ , l~S0~9 used preferably.
In addition to the quantity of acid which is necessary for neutralizing the bases employed during hydrogenation there is required a quantity of acid in the range of from about 0.001 to 0.01 % by weight, calculated on the batch. For the batch there are used for example about 1 to 50 % by weight solutions, pre-ferably about 2 to 10 ~ by weight solutions of (II) in R'OH. The pH of these solutions should be smaller than 7, preferably of from 3 to 4 (measured with humid pH paper).
When acidifying the diketals (II') optionally dissolved in the alcohol ROH and when heating them for a period of time of from about 10 to 60 minutes to a temperature of from about 60 to 120~C, preferably to reflux temperature, they rearrange according to the gross equation 15 RO OR OlR

~ H ~ ~ + 2 ROH

RO OR OR

(II') to form the corresponding hydroquinone dialkyl ether, while 2 mols of alcohol are split off; said ethers can be isolated by distilla-tion upon completion of the reaction and upon subsequent neutrali-zation.
Hydroquinone ethers, which are inter alia desired inter-mediates for the dyestuff preparation, especially for the prepara-tion of yellow pigment dyestuffs, are, consequently, readily accessible by the present invention. A particular advantage of the process according to the present invention resides in the fact that with minimal quantities of salts formed practically no lO~S089 waste waters are obtained, and consequently, the process haslittle pollution effects.
Since p-benzoquinone-tetramethyl diketal (I) used as the starting material is stable to acids and readily rearranges in trimethoxy benzene in a highly exothermal reaction, an exact checking of the pH is required when operating in an acid medium, whereas a reduction in alkaline medium with su~sequent acid-catalyzed splitting off of methanol proceeds more safely and assures a higher yield of hydroquinone dimethyl ether. The cyclohexene-(1)-dione-(3,6)-tetramethyl diketal of the formula (II) obtained on reduction in alkaline medium is a novel compound which is a valuable intermediate to be used in a wide field of application. For example hydroquinone dimethyl ether may be directly prepared therefrom according to the invention, which is an intermediate for the preparation of valuàble yellow pigment dyestuffæ or dyestuff components in photography, in known manner.
Cyclohexene-(l)-dione-(3,6)-tetramethyl diketal may also be fur-ther reduced under the same conditions to cyclohe~ane-(1,4)- ~
dione-tetramethyl diketal, and by acid hydrolysis of the latter cyclohexane-(1,4)-dione is obtained, which is an important starting product for the field of semiconductors. By reaction with malonitrile to yield l,4-bis-(dicyanomethylene)-cyclohexane and subsequently with N-bromosuccinimide tetracyanoquinone dime-thane, a semiconductor component is obtained (cf. J.Am.Chem.Soc.
84 (1962)l 3372).
From the cyclohexene-(l)-dione-(3,6)-tetraalkyldiketals with higher alkyl radicals in the ketal groupings obtainable by reketalization with higher alcohols than methanol from cyclo-hexene-(l)-dione-(3,6)-tetramethyl diketal (II), there may be _ g ~09~i089 prepared in analogous manner the corresponding hyaroquinone dialkyl ethers and cyclohexane-(1,4)-dione-tetraalkyl diketals.
The cyclohexene-(l)-dione-(3,6)-tetraalkyl diketals, too, are novel compounds.
The following examples illustrate the invention. The solvents used in the examples were technically absolute, that is to say, their water content was less than 0.3 % by volume. Inso-far as it was operated under pressure, there were used stirrer autoclaves made from stainless steel.
E X A M P L E 1:
Cyclohexene-(l)-dione-(3,6)-tetramethyl-ketal 377.6 g (1.886 mol) of p-benzoquinone tetramethyl di-ketal were dissolved in 1060 g of methanol. Upon addition of 0.5 g of triethylamine and 1 g of Pd supported by animal charcoal (5 % concentration) hydrogen was injected at room temperature under a pressure of from 50 to 10 bars. After absorption of 42.5 liters of hydrogen (liters measured under normal conditions of temperature and pressure) over a period of 20 minutes, the pressure was released, the catalyst was filtered off, the solvent (methanol) was distilled off under normal pressure and the reaction product was distilled in vacuo (20 mg Hg) at a tempera-ture of from 110 to 150C. According to gas chromatographic - calibration the distillate consisted of 4 % of hydroquinone di-methyl ether, 93 % of cyclohexene-(1)-dione-(3,6)-tetramethyl ketal (II~ and 2 % of tetramethyl ketal of cyclohexanedione-(1,4).
The compound (II) distilled substantially at 20C and could thus be separated in pure form (melting point: 38C).
The yield of II was 343.2 g which corresponded to a yield of 90 % of the theory.

~09S0~39 E X A _ P L E 2:
2~103 mols of p-benzoquinone tetramethyl ketal were dissolved in 1005 g of methanol. Upon addition of 5 g of NaOCH3 and 1 g of freshly prepared Raney nickel hydrogen was injected under a pressure of from 80 to 30 bars at room temperature.
After absorption of 45 liters of hydrogen over a period of to min-utes, the pressure was released, the catalyst was filtered off, the solvent was distilled off under normal pressure and the reac-tion product was subsequently distilled under reduced pressure (20 mm Hg) at a temperature of from 110 to 150C. According to gas chromatographic analysis the distillate consisted of 11 % of hydroquinone dimethyl ether, 80 % of cyclohexene-(1)-dione-(3,6)-tetramethyl ketal (ii) and of 5 ~ of tetramethyl ketal of cyclo-hexanedione-(1,4).
E X A M P L E 3~
200 g of p-benzoquinone tetramethyl ketal were dissolved in 500 g of methanol, 5 g of NaOCH3 and 1 g of Pt/C ~5 % concen- ,r tration) were added and the batch was hydrogenated under a pressure of from 40 to 10 bars until 21 liters of hydrogen were absorbed. Upon pressure release, filtering off of the catalyst and evaporation of the solvent, 203 g of residue were retained, which were distilled. After a first run of 25 g, 173 g of cyclo-hexane-(l)-dione-(3,6)-tetramethyl ketal having a melting point of 38C distilled over at a temperature of from 122 to 130C
under a pressure of 20 mm Hg. The yield, consequently, was 85 %
of the theoretical yield.
E X A M P L E 4:
85 g of p-benzoquinone tetramethyl ketal were dissolved in 180 g of methanol, 1 g of triethylamine and 1 g of Pt/C (5 %

10~ 89 concentration) were added. Hydrogen was injected under 30 bars until 8.4 liters were consumed. During this process the tempera-ture rose to 38C. Upon working up according to ~xample 1, 75 g of cyclohexene~ dione-(3,6)-tetramethyl ketal;could be isolated which corresponded to a yield of 87 ~ of the theory.
E X A M P L E 5:
Hydroquinone dimethyl ether 100 g of cyclohexene-(1)-dione-(3,6)-tetramethyl ketal were acidified at room temperature with 0.01 mol of gaseous HCl.
Thereafter the formed methanol was evaporated until a bottom temperature of 90C was attained, thereafter the mixture was neutralized with NaOCH3 and hydroquinone dimethyl ether was obtained by distillation under 60 bars/130C which corresponded to a 90 ~ yield~

E X A M P L E 6:
Hydroquinone dimethyl ether To 100 g of cyclohexene-(1)-dione-(3,6)-tetramethyl ketal was added at room temperature 0.1 g of p-toluenesulfonic acid and the batch was heated to 90C until no more methanol distilled over. Subsequently the reaction mixture was neutral-ized with NaOCH3 and 72.1 g of hydroquinone dimethyl ether distilled in vacuo at 60 bars/130C.
E X A M P L E 7:

= = .
Hydroquinone diethyl ether 100 g of cyclohexene-(1)-dione-~3,6)-tetramethyl ketal were dissolved in 1500 g o~ ethanol at 20C and the batch was acidified while stirring with 0.003 g of p-toluenesulfonic acid.
After about 10 minutes the batch was refluxed for 30 minutes, neutralized with NaOCH3, the formed methanol and the solvent were substantially evaporated. The crude yield was 65 g. By recrystallization from ethanol were obtained 57 g of pure hydr-quinone diethyl ether having a melting point of 72C which corre-sponded to a yield of 70 ~ of the theory.
E X A M P L E 8:
_ _ Hydroquinone-di-n-butyl ether 100 g of cyclohexene-(1)-dione-(3,6)-tetramethyl ketal were dissolved in 1500 g of n-butanol at 20C and the batch was acidified while stirring with 0.005 g of p-toluenesulfonic acid.
After about 10 minutes the batch was heated for 30 minutes to 90C, subsequently neutralized with NaOCH3, the formed methanol and the solvent were completely evaporated under slightly reduced pressure and subsequently the reaction product was submitted to a fractionating distillation. 65.3 g of hydroquinone dibutyl ether having a melting point of 46C distilled over under a pressure of 5 mm Hg at a temperature of from 125 to 135C, which corresponded to a yield of 60 % of the theory.
B X A M P L E 9:
Hydroquinone-di-n-butyl ether 50 g of p-benzoquinone tetramethyl ketal wexe dissolved in 200 g of n butanol and 1 g of triethylamine and 0.5 g of Pd/C
(5 % concentration) were added. At room tèmperature under a pressure of from 50 to 30 bars hydrogen was thereafter injected until 5.5 liters were consumed, whereby the temperature rose to

3~C. The pH of the solution was adjusted at 3 with HCl at 10C), the solution was stirred for 30 minutes and was subsequently heated to 70C for 5 minutes. Thereafter butanol and formed methanol were withdrawn on the rotation evaporator and 31 g of hydroquinone dibutyl ether distilled over under 5 mm Hg at a boiling temperature of from 130 to 135C. The yield, consequently, IO~S0~39 was 64 % of the theory.
E X A M P L E 10:
Cyclohexene-(l)-dione-(3,6)-tetra-n-butyl diketal 20 g of cyclohexene-(1)-dione-(3,6)-tetramethyl ketal were dissolved in 250 ml of n-butanol at 0C, the solution was acidified with 0.03 g of propionic acid and was stirred for 5 minutes, simultaneously formed methanol being withdrawn in vacuo under a pressure of 8 mm Hg. Thereafter the pH of the reaction product was adjusted at an alkaline value of 10 with sodium methylate and excess butanol was distilled off. 29 g of residue were obtained which consisted of 5 % of hydroquinone-di-n-butyl ether and 80 % of cyclohexene-(1)-dione-(3,6)-tetra-n-butyl diketal according to gas chromatographic analysis. NMR : (cyclo-hexene-(l)-dione=(3,6)-tetra-n-butyl diketal):
= 4.8 (s), 4 H; ~ = 3.75-4.0, 8H, triplet = 0.8 to 1~85, 28H, multlplet -- /~

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the preparation of a hydroquinone dialkyl ether of the formula wherein each R represents a primary or secondary C1-4 alkyl radical in which a diketal of the formula II' II' wherein R is as defined above, is acidified and heated to a temperature of from 60 to 120°C.

2. A process as claimed in claim 1 in which the diketal is dissolved in an alcohol ROH wherein R is as defined in

claim 1.