SYNTHESIS OF HINDERED PHENOLS

Abstract: A process for the production of 2,6-di-tert-butyl-α-­ methoxy-p-cresol and high molecular weight hindered phe­ nols produced therefrom. A process is disclosed for the production of the methyl ether of 2,6-di-tert-butyl-p-cresol in such a manner that allows subsequent production of 1,3,5,-­ trimethyl-2,4,6-tris (3,5,-di-tert-butyl-4-hydroxybenzyl)ben­ zene in the same pot. The process permits substitution of the methanol solvent/reactant with a hydrocarbon or halogen­ ated hydrocarbon solvent to produce the high molecular weight hindered phenol. This aspect of the invention reduces production cost by eliminating expensive centrifugation associated with prior art processes. The process also con­ serves catalyst by recycling it in the excess methanol. The catalyst is a Mannich base such as 2,6-di-tert-butyl-α-­ dimethylamino-p-cresol formed from the reactants 2,6-di-­ tert-butylphenol and formaldehyde with dimethylamine. The invention is an important improvement over prior art processes in that the production of bisphenol coproducts, notably 4,4′-methylenebis(2,6-di-tert-butylphenol) is low.

Description: ANTIOXIDANT SYNTHESIS This invention relates in general to processes for forming high molecular weight hindered phenols and intermediates therefor. This invention relates in particular to an improved catalyst/solvent system for the formation of hindered phenols. The hindered phenols ultimately formed by the process of this invention and earlier processes hae been widely used as antioxidants in food, packaging materials, stock chemicals, plastics, and the like. A process to produce such hindered phenols is set forth in Rocklin et al, U.S. Patent No. 3,026,264. An improvement on the Rocklin et al process is disclosed in my U.S. Patent No. 4,340,767. There exists a need to reduce the processing and materials costs used in the production of antioxidants such as 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4- hydroxybenzyl)-benzene. The prior art processes have used, for example, 3,5-dialkyl-4-hydroxybenzyl alcohol (also called 2,6-dialkyl-a-hydroxy-p-cresol) as an intermediate to produce the high molecular weight hindered phenols. The alcohol intermedia@es must be isolated from their reaction mass as a solid by means of centrifugation which is quite expensive. Also, adequate washing of the solid filter cake has proven difficult and some of the wash solvents my be carried forward into the product causing various problems. The 5,5-di-tert-butyl-4-hydroxybenzyl alcohol is often.used as an intermediate. However this intermediate is insoluble in preferred solvents such as methylene chloride. Thus strict stoichiometry controls are required and undesirable side reactions with sulfuric acid catalyst may occur. Finally, the use of an alcohol intermediate produces significant amounts of "heel" and certain bisphenols which, while usable, tie the subject process to other processes and markets. The substitution of alkyl ethers of intermediate alcohols has been suggested to produce high molecular weight hindered phenols. Howexter, the ethers are not abays available and their production has not been cost effective. The present invention is directed to the reduction of costs in the production of high molecular weight hindered phenols, commonly used as antioxidants. The present invention is a process for the production of aromatic ethers of structure I: EMI3.1 comprising reacting in the presence of a Mannich base catalyst: formaldehyde; a stoichiometric excess of an alcohol of structure II: (II) R5- OH where R5 is alkyl or cycloalkyl; and a substituted phenol of EMI3.2 <tb> structure <SEP> III: <SEP> OF <tb> <SEP> R2 <SEP> 1 <SEP> R1 <tb> <SEP> (III) <SEP> W <tb> <SEP> R4 <SEP> 23 <tb> where R1 and R2 are the same or different and are selected from alkyls, cycloalkyls, aryls, aralkyls, and alkaryls; and R3 and R4 are the same or different and are selected frctt R, alkyls, cycloalkyls, aryls, alkaryls, and aralkyls. The present invention is also a process for the production of high molecular weight hindered phenols, said process comprising the steps of: (a) reacting in the presence of a Mannich base catalyst: (i) formaldehyde, (ii) a stoichiometric excess of an alcohol of structure II: (TI) R5-OH where R5 is alkyl or cycloalkyl, and (iii) a substituted phenol of structure lII: EMI4.1 where R1 and R2 are the same or different and are selected from alkyls, cycloalkyls, aryls, aralkyls, and alkaryls; and R3 and R4 are the same or different and are selected from , alkyls, cycloalkyls, aryls, aralkyls, and alkaryls so as to form an aromatic ether of structure I: EMI4.2 (b) removing the excess unreacted alcohol; (c) adding a hydrocarbon or halogenated hydrocarbon solvent; and (d) reacting the aromatic ether of structure (I) with benzene or an alkylated benzene. The present invention is also a process for the production of 1,3,5-trimethyl-2,4,6-tris(3,5-di-tetri-butyl-4hydroxybenzyl)benzene with conservation of catalyst materials, eiimination of centrifugation of a chemical intermediate, and minimization of by-product bisphenol intermediate, said process comprising the steps of: : (a) reacting 2,6-dt-tert-butylphenol) formaldehyde, and a stoichiometric excess of methanol in the presence of a catalytic portion of 2,6-di-tert butyl-4-dialkylaminomethylphenol so as to form the intermediate 2,6-di-ert-butyl-a-methoxy-E1- cresol, said catalytic portion being formed from a dialkyl secondary amine, whereby some of the secondary amine remains in the unreacted excess methanol and formation of 4,41-methylenebis(2,6 di-tert-butylphenol) is minimized; (b) recovering the unreacted excess methanol containing some of the secondary amine and recycling said methanol to another intermediate formation reaction thereby conserving catalyst; ; (c) dissolving the intermediate 2 2,6-di-tert-butyl a-methoxy-D-cresol in methylene chloride thereby eliminating the need for centrifugation; (d) reacting the dissolved intermediate with mesitylene in the presence of a sulfuric acid or Friedel-Crafts catalyst; and (e) recovering the 1,3,5-trimethyl-2,4,6-tris(3,5- di-tert-buyl-4-hydroxybenzyl)benzene product. The present invention has been demonstrated as a clean process for the production of certain ethers of hindered para cresols as intermediate compounds. The invention is also 2 process to produce high molecular weight hindered phenols formed from the intermediate aromatic ethers and benzene compound to be combined therewith. The aromatic ether compounds of the invention are of the general structure I: EMI6.1 -here R1 and R2 are the same or different and are selected from straight chain or (preferably) branched alkyls, cycloalkyls, aryls, aralkyls, and alkaryls; 23 and R4 are the same or different and are selected from H (preferably), alkyls, cycloalkyls, aryls, alkaryls, and aralkyls; and R5 is alkyl (preferably of one to six carbon atoms) or cycloalkyl of 5-12 carbon atoms. The alcohols of the invention are of the general structure IT: (II) R5-OH where Rs is as defined above. These alcohols include methanol, isopropanol, ethanol, sec-butanol, n-hexanol, cyclopentanol, isoamyl alcohol, n-propanol, n-butanol, and others. Preferred alcohols are the lower alkyl alcohols such as methanol, isopropanol, sec-butanol, and D-hexanol. More preferred are the unbranched lower alkyl alcohols. These include methanol5 n-butanol, ethanol, n-hexanol, and the like. Most preferred are the C1-C4 straight chain alkyl alcohols. Most especially preferred is methanol since methanol reacts quickly and comDletely in the inventive process, is readily available, and carries catalyst over into the recycled excess methanol when used as a solvent/reactant. The cycloalkyls and branched alkyl groups are preferred substituents for R1 and R2. Branched alkyls and cycloalkyls of one to six carbons are more preferred and the -tert-butyl group is the most preferred substituent for R1 and R2 because of its strong hindrance effect for phenols, especially where an antioxidant use is proposed. The substituted phenols of the invention have the general structure III: EMI8.1 where R1, R2, R3, and R4 are as defined above. Examples of the phenols suitable to make the aromatic ether of the invention include: 2 ,6-di-tert-butylphenol; 2,6-diisopropylphenol; 2,6-sec-butylphenol; 2-methyl-6-tert-butylphenol; 2,6- diethylphenol; 2,6-dicyclopentylphenol; 2,6-dicyclohexylphenol; 2-cyclopentyl-6-tert-bylphenol; 2,6-diisopropyl-3-methylphenol; 2,6-di-tert-butyl-3,5-dimethylphenol; 2,3,6-triisopropylphenol; 2,3,6-triethylphenol; and the like. More preferred are the 2,6-di-lower alkylphenols and 2,6-di-lower cycloalkylphenols, otherwise unsubstituted, i.e., where R3 and R4 are both H. These include: 2,6-di-tert-butylphenol: 2,6-dimethylphenol; 2,6-dicyclopentylphenol; 2 ,6-diisopropylphenol; 2-methyl- 6- tert-butylphenol; 2-ethyl-6-sec-butylphenol; and the like. Most preferred are those 2,6-di-lower-alkylphenols otherwise unsubstituted where the lower alkyl substituents are identical such as 2,6-diisopropylphenol; 2,6-di-sec-butylphenol, and 2,6-di-tert-butylphenol. Most highly preferred is the compound 2,6-di-tert-butylphenol. The intermediate ethers of the invention include but are not limited to: 2,6-di-tert-butyl-&alpha;-methoxy-p-cresol; 2,6-di-tert-butyl-&alpha;-ethoxy-p-cresol; 2,6-di-tert-butyl-3,5-dimethyl-&alpha;-methoxy-p-cresol; 2,6-dicyzopentyl-a-methoy--cresol; 2,6-di-tert-amyl-&alpha;-butoxy-p-cresol; 2,6-di-sec-butyl-&alpha;-cyclohexoxy-p-cresol; 2,6-dicyclohexyl-&alpha;-methoxy-p-cresol; 2,6-dimethyl-a-propoxy-D-cresol; 2-methyl-6-tert-butyl-&alpha;-methoxy-p-cresol; 2-ethyl-6-octadecyl-&alpha;-ethoxy-p-cresol; 2-methyl-6-cyclooctyl-&alpha;-methoxy-p-cresol; 2-&alpha;,&alpha;-dimethylbenzyl-6-methyl-&alpha;-methoxy-p-cresol; 2-methyl-6-isopropyl-&alpha;-cyclopentoxy-p-cresol; and others. Most secondary amines are acceptable for the process. The Preferred amines have the structure IV: EMI10.1 wherein R5 and R7 are separately the same or different and are selected from alkyl, cycloalkyl, alkanol, cycloalkanol, aromatic, heterocyclic, or together with the nitrogen to which they are attached form a ring. Representative examples of these amines are dimethylamine, diethylamine, methylethylamine, diisoamylamine, dibenzylamine, methylisobutylamine, diisobutylamine, dicyclohexylamine, methylcyclohexylamine, ethylcyclopentylamine, methylcyclooctylamlne, diethanolamine, methylethanolamine methyl(2-hydroxybutyl)amine, methyl(2-hydroxycyclohexyl)amine, ethyl'4-hydroxycyclohexyl)amine, N-methylaniline, methyl- tolylamine, dibenzylamine, methylbenzylamine, methyl(&alpha;- methylbenzyl)amine, N-(N-[3-aminopropyl]morpholine)-Nmethylamine, piperidine, piperazine, morpholine, and the like. The dialkylamines, dicycloalkylamines, and alkanolamines are more preferred since they are readily available, cheap, and selectively form Mannich base compounds. While long chain alkylamines are usable, their use may require selection of an expensive solvent/reactant to form the ether intermediate. Included among the more preferred amines are dimethylamine, diethylamine, dipropylamine, di-n-butylamine, diisoamylamine, methylethylamine, diisopropylamine, didodecylamine, methylisopropylamine, and the like. Still more preferably Re and R7 are lower alkyl groups containing one to four carbon atoms such as dimethylamine, diethylamine, methylethylamine, diisopropylamine, methylisobutylamine, and the like. The most preferred dialkylamines are dimethylamine and diethylamine, especially dimethylamine. Preferred alkanolamines are those in which the alkanol groups contain about two to four carbon atoms such as diethanolamine, methylethanolamine, di- (2-hydroxypropylamine, di-(2-hydroxybutyl)amine, ethylethanolamine, isobutylethanol amine, and the like. The most preferred alkanol amines are the dialkanolamines especially diethanolamine. Also suitable is methyl-N,N'-diethylethylenediamine. Other usable secondary amines include piperidine, 1,2,3,4-tetrahydroiso- quinoline, 6-methoxy-1,2,3,4-tetrahydroisoquinoline, morpholine, piperazine, #-methylaminopropiophenone, #-acetylethylbenzyl- amine, benzyl-(2-cyclohexanonylmethyl)amine, and 3,4 methylenedioxybenzyl- (2-cyclohexanonylmethyl)amine Dimethylamine is the most preferred of all secondary organic amines because it readily forms catalyst in the preferred solvent, methanol. Mixtures of secondary amines may be used. The invention uses a Mannich base catalyst to produce the alkyl ethers of para-cresols. The Mannich base catalyst is readily formed by the addition of a secondary organic amine to a combination of formaldehyde and a hindered phenol such as 2,6-di-tert-butylphenol or 2,6-di-cyclopentylphenol The catalyst of the invention may be formed in situ with formaldehyde and the substituted phenol by addition of a suitable secondary amine to form a structure V: EMI12.1 where R1, R2, R3, R4, R6, and R7 are as defined above. Such Mannich base compounds can also be used in the process of the invention by their simple addition to the (substituted) benzene and a compound of structure (I), with appropriate catalyst at suitable reaction conditions. Examples of Mannich base compounds of structure ,(y) are: 2,6-di-tert-butyl-dimethylaminomethylphenol; 2,6-diisopropyl-dimethylaminomethylphenol; 2,6-di-tert-butyl-diethylaminomethylphenol; 2-methyl-6-tert-butyl-dimethylaminomethylphenol; 2,6-diethyl-diethanolaminomethylphenol; and the like. The high molecular Weight hindered phenols according to the invention include those formed from the above ethers and a benzene or alkylated benzene. The methyl substituted benzenes such as toluene, xylene isomers, trimetbylbenzene isomers, and durene are preferred. Nesitylene(l,3,5-tri methylbenzene) is most preferred since it may be used to form . the valuable antioxidant 1,3,5-trimethyl-2,4,6-tris(3,5 di-tert-butyl-4-hydroxybenzyl)benzene and similar homologs. Other suitable compounds include 1,3,5-triethylbenzene, 1,4-diisopropylbenzene, 1,3-dimethyl-5-ethylbenzene, 1,2,4,5-tetramethylbenzene, 1,2-dimethyl-4,5-diethylbenzene, 1-n-butyl-3-ethyl-5-methylbenzene and the like. Preferably, - small amount of dimethylamine is added to a 2,6-dialkylphenol, formaldehyde (fiake or para- formaldehyde) and methanol (sufficiently in excess to act as solvent) in situ. Heating promotes catalyst formation thus the amine may merely be added at the start of the reaction sequence so as to form the catalyst for reaction. The preferred catalysts are the 2,6-dialkyl-a-dimeh37lamino-Q- cresols. The compound 2,6-di-tert-butyl-&alpha;-dimethylamino-p- cresol is most preferred. When a 2,6-dialkylphenol is used as the limiting reactant, dimethylamine remains dissolved in the excess methanol or other reactant/solvent and is thus carried over for reuse in subsequent reactions. Bo'h sulfuric acid and Friqdel-Crafts catalysts are suitable according to Ihe invention for formation of high molecular weight hindered phenolic compounds. Sulfuric acid is preferred but suitable Friedel-Crafts catalysts include AlCl3, AlBr3, ?eCl3, and the like. Suitable solvents for reaction of a benzene or alkylbenzene with the alkyl ethers of the invention include various hydrocarbons and, preferably, halogenated hydrocarbons. These include the paraffins, especially C5-C10 compounds such as pentane, isopentane, hexane, cyclohexane, heptane, octane isooctane, and decane as,well as mixture of these. Also included are alkylene halides such as 1,1-dichlorpropane, 1,2-dichlorpropane, 2,2-dichlorpropane, 1,1-dichloroethane, 1,2-dichloroethane, 1,1,1-trichloroethane, n-butylchloride, sec-butylchloride, isobutylchloride, chloroform, carbon tetrachloride, and the like. Methylene chloride is the especially preffered solvent for production of the high molecular weight hindered phenols. The amount of solvent may vary so long as dissolutior, of the ingredients is achieved to the extent required for reaction. A useful range is 50-500 parts solvent per 100 parts reactant rhenol. A preferred range is from, 100-200 parts solvent par 100 parts reactant. Tbe compounds made by this invention have proven antioxidant activity and are, therefore, very useful, The present invention is especially advantageous over prior art processes not only because of its one pot procedure but also because it minimizes the production ofebtspheiol'., coproducts, notably 4,4'-methylenebis(2,6-di-tert-butylphenol). Earlier attempts to prepare the chemical intermediate of the invention used a strong base catalyst such as potassium hydroxide (KOH). This resulted in the conversion of 25 percent or more of the starting hindered phenol to a bisphenol. belle the bisphenol is a usable compound, it is not nearly as valuable as the chemical intermediate of the invention. Furthermore, such prior art processes unnecessarily tied the production of high molecular weight hindered phenols to 2 relatively lower weight phenolic. When-the inventive process is operated with 2,6-di-ert-butylphenol, the conversion to 4-4'-methylenebis(2,6-di-tert-butylphenol) is as lot; as fdve # percent or less with nearly complete conversion to the desired chemical intermediate. Ac-ording to the invention, the formaldehyde, hindered phenol, excess lower alcohol, and small amount out catalyst are heated to an elevated temperature in a sealed reaction chamber thereby raising the pressure. A suitable reaction temperature range is 50 -200 C., preferably 60 -130 . A suitable range of reaction pressure is 0-1,000 psig, preferably 5-100 psig. Mcre preferably the reaction is carried out with methanol at about 60 -100 C. and 5-25 psig. Such conditions tend to limit the formation of compounds of the type 4,4'-methylenebis-(2,6- dialkylphenol). The procedure of the preferred embodiment permits a one pot process whereby excess solvent from formation of the alkyl ether of the phenol reactant is stripped off, the appropiate solvent for reaction with, e.g., mesitylene is added, and the high molecular weight hindered phenol is formed in situ. This obviates the need for severe mechanic separation of the intermediate such as by centrifuge. The preparation of phenolic compounds by the inventive process may be carried out in several fashions and the ingredients may be combined in any order prior to reaction. Preferably, the acid catalyst is added last. Multiple additions of catalyst may be made. The ratio of reactants for preparation of the phenolic may be combined over a broad range, but excess solvent is preferred for better reaction-mixing. A slight excess of the structure (I) ether over stoichiometric is preferable since excess benzene or alkylated benzene may otherwise lead to an undesirable mixture of monosubstituted and polysubstituted benzenes. The preparation of phenolic may be carried out over a broad range of temperatures but -60 to 140 C. is a usable range. From -10 C. to 30 C. is a preffered range and from Co to 10"C. is more preferred. Although superatmospheric and subatmospheric pressures may be used, they are unnecessary or uneconomical where a sufficiently active catalyst is used. Example Preparation of Ether To a 500 ml., 4 neck, round bottom flask equipped with mechanical stirrer, nitrogen inlet, reflux condenser, and side arm dropping funnel was charged the following: 220 ml. methanol; 24.0 grams paraformaldehyde; 3.0 grams 40% dimethylamine in water. The mixture was heated to reflux (about 68 C.) and 2 solution of 103.2 grams 2,6-di-tert-butylphenol in 50 ml. methanol was added dropwise over a period of three hours under nitrogen flush and at reflux. The reaction mixture was transferred to a one-liter Purr pressure reactor equipped with dual pitch blade impellers. The reactor was heated to and' maintained at 80 C. with stirring for four hours, whereupon the pressure in the reactor reached about 15-20 psig. The react on mixture was cooled to about 650C. and transferred to a one-liter orton, creased, three neck, round bottom flask equipped with a mechanical stirrer having a three-inch, half moc impeller a thermometer, and a side dropping funnel distillation head. The reaction mixture was heeted in the flask in a 100 -120 C. oil bath under nitrogen flusn with agitation to strip off solvent. When the reaction mass temperature reached 95 C. the oll bath was removed and an ice bath applied. Analysis confirmed formation of 2,6-di-tert butyl-a-methoxy-p-cresol. PreDaration of Phenolic The cooled Morton flask was set up-for reflux and the following ere added to the reaction mixture: 200 ml. methylene chloride; 18.8 grams mesitylene; and 1.5 ml. acetic acid. The flask was cooled to 5 C. in an ice bath and stirred at 150 rpm. While maintaining the reaction mixture at 3o7o C., about 25 ml. 84% aqueous H2S04 was added dropwise over two hours. The reaction mass was transferred to a separatory funnel and allowed to settle for about 15 minutes. The lower, acid phase was cut off and the remainder of the reaction mass was returned to the one-liter Morton flask. A second portion of 1.5 ml. acetic acid was charged and a second 25 ml. portion of 84% H2SO4 was added dropwise over two hours as above. The same procedure was followed to separate the acid phase. Thereupon the remaining reaction mass was charged to a one-liter resin flask equipped 'with four neck top, bottom stopcock, mechanical agitator, thermometer, nitrogen flush, and distillation/reflux head. To the reaction mixture is then added 360 ml. deionized water and 100 grams soda ash. The reaction mixture was heated by means of the steam jacket to strip off methylene chloride until no more comes off overhead with a pot temperature of at least 850 C. The flask was set up for reflux and 8oo ml. heptane added. The reaction mass was heated to reflux and all solids dissolved whereupon the mass was allowed to settle for about five minutes before cutting off the lower, aqueous phase. The reaction mass was washed twice with two 200 tl. portions of deionized water and a pH of about 5-7 was determined. The hot heptane solution was transferred to a one liter, three neck, round bottom flask equipped with mechanical stirrer, thermometer, and nitrogen flask. The solution was cooled slowly to 5 C. and the resultant slurry was filtered on a buchner funnel. The product was washed with 140 ml. heptane and dried to provide 83.25 grams 1,3,5-trimethyl-2,4,6-tris(3,5 di-tert-butyl-4-hydroxybenzyl)benzene of over 99 percent purity by high pressure liquid chromatography analysis. The above example demonstrates for-st-on of the catalyst in situ but of course the catalyst could be separately added.

Claims: CLAIMS: 1. A process for the production of aromatic ethers of structure I: EMI20.1 comprising reacting in the presence of 2 Mannich base catalyst: formaldehyde: 2 stoichiometric excess cf an alcohol of structure IT: (II) R5-OH where R5 is alkyl or cycloalkyl; and a substituted phenol of structure III: EMI20.2 where R1 and R2 are the same or different and are selected from alkyls, cycloalkyls,aryls, aralkyls, and alkaryls; and R3 and R4 are the same or different and are selected from H, alkyls, cycloalkyls, aryls, alkaryls, and aralkyls. 2. The process as claimed in Claim 1 in which said alcohol is methanol. 3. The process as claimed in Claim 1 in which said substituted phenol of structure (III) has R3and RA both H'and R1 and R2 are alkyl or cycloalkyl. 4. The process as claimed in Claim 3 in which R1 and R2 are tert-butyls. 5. The process as claimed in Claim 1 in which sal Mannich base is formed in situ from a portion of sac substituted phenol and a catalyst-forming portion of a secondary amine. 6. The process as claimed in Claim 5 in which said secondary amine is of structure (IV): EMI21.1 wherein R6 and R7 are separately the same or different and are selected from alkyl cycloalkyl, alkanol, cycfc- alkanol, aromatic, heterocyclic, or together with the nitrogen to which they are attached form a ring. 7. The process as claimed in Claim 5 in which said secondary amine is diethylamine or dimethylamine. 8. The process as claimed in Claim 1 in which said formaldehyde is an aqueous solution, flake, or paraformaldehyde. 9. The process as claimed in Claim 1 in which said Mannich base is of structure V; EMI22.1 wherein R6 and R7 are separately the same or different and are selected from alkyl, cycloalkyl, alkanol, cycloalkanol, aromatic, heterocyclic, or together with the nitrogen to which they are attached form a ring. 10. The process as claimed in Claim 1 in which said Mannich base catalyst is 2,6-dialkyl--dimethylamino- methylphenol. 11. The process as claimed in Claim 1 in which said reaction is carried out at a temperature of 50"-200"C. 12. The process as claimed in Claim 1 carried out at a pressure of 5-100 psig and an elevated temperature.

Inventors: MINA, GEORGE LOUIS
Applicants: ETHYL CORPORATION
Application year: 1983
Main classification: C07C43
IPC codes: C07C43, C07C41, C07C37