PREPARATION OF 3,7-DIMETHYLOCTA-1,7-DIENE

Abstract: Isoprene is selectively hydrodimerized to 3,7-dimethylocta-1,7-diene in the presence of lithium formate, sodium formate, ammonium formate or calcium formate or mixtures thereof, of a catalyst comprising palladium complexed with a tertiary or organophosphorus ligand and of water.

Description: PREPARATION OF 3,7-DIMETHYLOCTA-1,7-DIENE The invention relates to a process for the preparation of 3,7-dimethylocta-1,7-diene by hydrodimerization of isoprene in the presence of a reducing agent, a catalyst comprising palladium complexed with a tertiary organophosphorus ligand and, optionally, of a solvent. 3,7-Dimethylocta-1,7-diene is a chemical intermediate, for example to aroma chemicals. In the process referred to in the beginning, as described in U.S. patent specification 3,732,328 formic acid is used as the reducing agent. This known process, however, allows 3,7dimethylocta-1,7-diene with a low selectivity, 3,6-dimethylocta 1,7-diene primarily being formed. The selectivity, expressed as a percentage, to a certain compound, is defined as: b x 100 , b wherein "a" is the amount of isoprene converted to this certain compound and "b" is the total amount of converted isoprene. The Applicants have now found that the use of certain reducing agents allows a high selectivity to 3,7-dimethylocta1,7-diene, a mixture of dimethylocta-1,7-dienes having greater than fifty mol. per cent of the 3,7-dimethyl-1,7-diene isomer being formed. Accordingly, the invention provides a process for the preparation of 3,7-dimethylocta-1,7-diene by hydrodimerization of isoprene in the presence of a reducing agent, a catalyst comprising palladium complexed with a tertiary organophosphorus ligand and, optionally, of a solvent, characterized in that lithium formate, sodium formate, ammonium formate or calcium formate or a mixture thereof is used as the reducing agent in the presence of water. Water must be present in the reaction mixture in greater than trace amounts. Preferred molar amounts of water are at least equal to the molar amounts of formate salt. Typically, the water will range from 1 to 5 mol. of water per mol. of formate salt. Solvents are not essential to the process of this invention, but a good organic solvent can promote the rate of reaction by a factor of two or more. Non-polar solvents, such as paraffinic, cycloparaffinic or aromatic solvents are useful in the process of this invention. The solvent can be a paraffin or cycloparaffin containing 5 to 16 carbon atoms per molecule, such as hexane, dodecane, pentadecane, cyclohexane and methylcyclohexane. Suitable solvents also include aromatic hydrocarbons, such as benzene, lower alkyl-substituted aromatic hydrocarbons, such as toluene, m-, p- and o- xylene, halogenated aromatic hydrocarbons including chloro-, bromo- and iodo-substituted, for example chlorobenzene. Halogenated lower aliphatic compounds, such as chloroform, methylene chloride and carbon tetrachloride may be used; in particular chloroform is preferred. Further useful solvents are amine solvents. A wide range of amines are useful, provided that they are liquid under reaction conditions. Tertiary amines are preferred to primary and secondary amines. Suitable amine solvents include alkyl amines, cycloalkylamines, arylamines and heterocyclic amines, such as morpholine, pyridine, piperazine and piperidine. Examples of these classes of amines are the lower alkylamines containing 2 to 6 carbon atoms in each alkyl group, such as triethylamine; mono-cyclohexylamine, and N-alkyl-cyclohexyl amines containing up to 12 carbon atoms per molecule; aniline and N-alkylanilines containing up to 12 carbon atoms per molecule and N-alkylmorpholines containing up to 12 carbon atoms per molecule. Further useful solvents are those of moderate coordinating ability, and which include nitriles, such as lower alkyl nitriles, hydrocarbon aromatic nitriles including acetonitrile and benzonitrile, amides including benzamide, acetamide, mono- and di-substituted amides where the substituent is preferably lower alkyl. Suitable substituted amides include N-methyl acetamide, X,N-dimethyl-acetamide and dimethyloformamide. Dialkyl sulphoxides, such as dimethyl sulphoxide and sulphones, such as sulfolane (tetrahydrothiophene 1,1-dioxide) and alkyl substituted sulfolane are satisfactory. By dialkyl it is meant that the sulphur and nitrogen atoms are connected to two different carbon atoms. They may be separate alkyl groups or the same, i.e., a ring alkyl group, e.g., tetramethylene sulphoxide and N-methy;,pyrrolidinone. The alkyl moieties have carbon numbers ranging from 1 to 6. Simple ethers, such as the dilower alkyl ethers including dimethyl ether and dimethyl ether function satisfactorily. Hydrocarbon aromatic ethers, such as the lower alkyl phenyl ethers may be also used. In addition, the cyclic diethers, such as 1,4-dioxane are also suitable solvents. Simple lower alkyl esters of lower alkanoic acids, such as ethyl acetate, methyl acetate and methyl butyrate as well as cyclic diesters, such as ethylene carbonate, are also suitable solvents of moderate co-ordinating ability. Ketones, including lower aliphatic ketones, such as ethyl methylketone and hydrocarbon aromatic ketones, such as acetophenone are also satisfactory solvents. Lower mono- and di-alkanols, such as isopropanol and ethylene glycol may be used, if desired. The preferred solvents of moderate co-ordinating ability include nit riles, formamides, such as dimethyloformsmide di-lower alkyl ethers, lower alkyl phenyl ethers, simple lower alkyl esters of lower alkanoic acids, ketones and lower alkanols. The particularly preferred solvents utilized in this invention include benzene, dimethylformamide, chlorobenzene, anisol, N,N-dimethylacetamide, nitromethane, ethyl acetate, isopropanol, benzonitrile, chloroform, ethyl methyl ketone, acetonitrile, diethylether, acetophenone, toluene, ethylene glycol, ethylene carbonate, propylene carbonate and sulfolane. Particularly desired solvents are dimethylformamide, N-methylpyrrolidinone and dimethyl sulphoxide, which are good solvents for the formate salts in question. The preferred organic solvents will have carbon numbers ranging from 1 to 20. Particularly desired solvents are those which give two-phase systems which allow easy product separation, for example nitromethane, ethylene carbonate and propylene carbonate. The amount of solvent added should be sufficient to dissolve the palladium complexed with a tertiary organo-phosphorus ligand. The formate salts are utilized as a source of hydrogen for the hydrodimerization. Sodium formate is preferred. It is desirable that some formate salt be present during the entire course of the reaction. When operating the process batch-wise, this can be accomplished by adding a stoichiometric amount of formate salt initially, 1 mol. of formate salt for every 2 moles of isoprene, or by continuously or periodically adding additional amounts of formate salt. The catalyst used in the process of this invention is palladium or a palladium compound complexed with a tertiary organo-phosphorus ligand. The palladium may be in any of its possible valence states, e.g., 0, +2, etc. with the zero valence preferred. Suitable palladium compounds include the palladium carboxylat e s, particularly palladium c arb oxyl ate s derived from alkanoic acids containing up to six carbon atoms per molecule such as palladium acetate (OAC), complexes, such as palladium acetylacetonate (AcAc), bisbenzonitrile palladium(II) and lithium palladous chloride as well as the palladium halides, nitrates and sulphates, such as palladous chloride and palladium nitrate (Pd(N03)2(0H2) and palladium sulphate. Suitable reduced palladium-phosphine complexes are Pd(R3P)2 or Pd(R3P)3. The palladium is present in the reaction mixture in catalytic amounts; preferably from 1 to 10 6 molar and more preferably from 10 1 to 10 4 molar. The palladium compounds complexed with a tertiary organophosphorus ligand are typically prepared by reacting the tertiary organo-phosphorus ligand with the appropriate palladium compound as, for example, represented by the following equations: EMI5.1 where R3P is a tertiary organo-phosphine or may be made in situ by adding the palladium compound and the phosphine directly to the reactor. Any tertiary organo-phosphorus ligand which can be dissolved in the reaction mixture may be used. Preferred ligands are represented by the formula: (R )aPRb (I) wherein the R groups which may be the same or different, attached to the phosphorus or oxygen atoms, represent hydrocarbyl groups, "a" is an integer from 0 to 3 and "b" is 3-a. Preferably, each R in formula I represents an aryl, alkyl, aralkyl or alkaryl group, each with less than 20 carbon atoms, preferably less than 12 carbon atoms. Suitable examples of groups represented by R are phenyl, p-tolyl, o-tolyl, m-tolyl, m-chlorophenyl, p-anisoyl, m-anisoyl, methyl, ethyl, propyl and butyl groups with methyl, ethyl, n-propyl, n-butyl, phenyl and chlorophenyl groups being preferred. It is also suitable for the organic radical R to contain functional groups or to satisfy more than one of the valences of the phosphorus atom, thereby forming a heterocyclic compound with the phosphorus atom. Preferably R represents aryl, alkyl, aralkyl, alkaryl or a mixture therof having carbon numbers from 1 to 20, preferably 1 to about 12 carbon atoms and need not be the same, e.g., R1R2R3P, (R10)PR2R3 and (R10)(R20)PR3. Preferably R represents an alkyl or aryl group and each R is the same. Alternatively, the formula for the ligand can be expressed as: L(oR5 > (OR5) (OR6 h R'RR"P(OR' where c, d, e, f, g and h individually equals 0 our 1, c + d + e | + f + g + h equals 3 and R is as defined above. In the most preferred tertiary organo-phosphorus ligands at least one R represents a benzyl group or a branched alkyl, arallyl, alkenyl or cycloalkyl.group, each of the latter four groups having from 3 to 10 carbon atoms with branching occurring at a carbon atom no more than two carbon atoms from the phosphorus or oxygen atom. This preferred R provides a steric hindrance to the catalyst complex which enhances selectivity. Illustrative of the preferred R moiety are, for alkyl, isopropyl, sec.-butyl, tert.-butyl, isobutyl, neopentyl, sec.-pentyl, tert.-pentyl, 2-methylbutyl, sec.-hexyl, tert.-hexyl, 2,2-dimethylpropyl; for aralkyl, alpha-methylbenzyl, alpha,alphadimethylbenzyl, alpha-methyl-alpha-ethylbenzyl, phenylethyl, phenylisopropyl, phenyl-tert.-butyl; for alkenyl, allyl, crotyl, methallyl, 1-methylethenyl, 1 -methyl-2-propenyl, 2-methyl-2propenyl, 1,1-dimethyl-2-propenyl, 1-methyl-3-butenyl and, for cycloalkyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and cycloheptyl. Two or more of the ligands of formula I may be used in the same reaction. The mol. ratio of tertiary phosphorus ligand to palladium is suitably at least 1. Preferably, the mol. ratio of ligand to palladium ranges from 1:1 to 20:1 and more preferably from 2:1 to 5:1. The use of the tertiary organo-phosphorus ligands of the invention provides extremely high selectivities to 3,7-dimethylocta-1,7-diene. Alternatively, the palladium or palladium compound and tertiary organo-phosphorus ligand may be bound onto a crosslinked synthetic resin instead of being dissolved in the reaction medium. Suitable cross-linked synthetic resins include cross-linked polystyrene, poly(alpha-alkyl)acrylates, polycarbonates and polyamides. In the generic sense, the bound ligand will have the generic formula Z-P(OR).R., wherein la R is as defined above, "i" is an integer from 0 to 2, "Åa" is 2-i and Z represents a cross-linked synthetic resin. The bound tertiary phosphine may have the general formula: (,6)m (R6)n (RO)i P P; a wherein R, i and j are defined previously, and R6 represents the repeating unit of the synthetic resin and where m is a positive integer, n is O or a positive integer, m+n equals the total number of repeating units in the resin and the percentage of the repeating units substituted with the tertiary phosphine is represented by the formula: x x 100%. m+n The number of repeating units substituted with the tertiary phosphine is not critical. When less than 5% of the repeating units contain a phosphorous substitute, large quantities of the resin must be used to form the bound catalyst. Accordingly, it is desirable to have at least 10% of the repeating units substituted with a tertiary phosphorus. It is preferred, however, that from 20 to 40% of the repeating units contain a phosphorus substituent. The substituent can be introduced into the resin using well-known techniques, such as those described in the Journal of the American Chemical Society, 97 (7) 1749 (1975) and in Ann. N.Y. Academy of Sciences, 239, 76 (1974). In accordance with those techniques, the palladium compound is complexed with the phosphorus-substituted resin by admixing in a solvent for the palladium compound. The catalyst may be pretreated to enhance reactivity by contacting it with a reducing agent at a temperature of from 20 to 900C for from 0.1 to 5 hours. The reducing agent may be gaseous, solid or liquid. Examples of gaseous reducing agents are hydrogen and carbon monoxide. Examples of liquid or solid reducing agents are hydrazine, NaBHk, NaOCH3, (isopropyl)3P, Cu, Na, and Al alkyl. The reduction may be carried out in a separate autoclave or preferably is carried out in the hydrodimerization reactor prior to the introduction of the isoprene. The palladium complexed with a tertiary organo-phosphorus ligand may be dissolved in the solvent used in this invention prior to reduction. The process can be either continuous or batch. The reaction temperature of the process is not critical, however, it is preferred to maintain the reaction between 0 and 1000C, preferably between 20 and 700C. The process is conducted under a sufficient pressure to maintain liquid phase conditions at the reaction temperature. Typically the pressure is autogenous. The invention will now be illustrated by the following Examples. EXAMPLES 1-9 To a 90 millilitre glass-lined autoclave were charged 0.027 mmol. of palladium acetylacetonate, 0.054 mmol. (isopropyl)3 phosphine, 18.5 mmoles of formate, 10 ml of the solvent listed in Table I, 1 ml (55.5 mmol.) of H20 and 2.5 g (37 mmol.) of isoprene. The stirred reactor was heated to various temperatures for 2 hours, cooled and the product was analyzed by gas chromatography. The results are shown in Table I. TABLE I Ex- Formate Solvent React- Con- Selectivity, % ample source a) ion version, temp., % 3,7 DM 3,6 DM. 2,7 DM C . b) 1 Na DMSO 60 76 65 10 25 formate 2 Na formate DMSO 75 89 65 7 28 3 Na DMSO 60 41 73 8 14 formate (20 ml) 4 Na. DMF 75 81 68 7 25 formate 5 Na. DMF 60 65 73 8 18 formate (20 ml) 6 NH4, DMSO 60 80 55 31 14 formate 7 NH4. DMF 60 86 56 33 11 formate 8 Ca- DMF 60 61 63 5 32 formate g Li DMF 60 78 58 24 8 formate a) DM1? = dimethyl formamide DMSO = dimethyl sulphoxide b) 3,6 DM = 3,6-dimethylocta-1,7-diene 3,7 DM = 3,7-dimethylocta-1,7-diene 2,7 DM = 2,7-dimethylofta-1,7-diene Comparative Experiments 1-9 To compare the different isomer mixes that are obtained using formic acid, amine salt, and other formates the following experiments were run: To an 80 millilitre glass-lined autoclave were charged 0.027 mmol. of palladium acetylacetonate, 0.054 inmol. (isopropyl)3phosphine, 18.5 mmol. of formate or formic acid, 10 ml of the solvent listed in Table II, 1 ml (55.5 mmol.) of H20 (for the formates only) and 2.5 g (37 mmol.) of isoprene. The stirred reaction was heated for 2 hours at various temperatures, cooled and the product was analyzed by gas chromatography. The results are shown in Table II. TABLE II Compar- Formate Solvent React- Con- Selectivity, % ative source a) ion versions 3,7 DM 3,6 DM 2,7 D exper- temp., % 3,7 DM 3,6 DM 2,7 DM iment OC 1 Formic DMSO 60 87 34 61 4 acid 2 Formic DMF 60 75 36 60 4 acid 3 Formic DMF 40 98 38 58 4 acid (CH3CH2)3N 4 - Rb DMF 60 95 45 47 8 formate 5 Cs DMF 60 93 50 43 7 formate 6 K DMF 60 87 30 1 69 formate 7 K DMSO 72 84 39 1 60 formate 8 K DMF 72 81 25 1 74 formate 9 K DMF 85 91 11 1 88 formate a) DMF = dimethyl/formamide DMSO = dimethyl sulphoxide b) 3,6 DM = 3,6-dimethylocta-1,7-diene 3,7 DM = 3,7-dimethylocta-1,7-diene 2,7 DM = 2,7-dimethylocta-1,7-diene

Claims: CLAIMS 1. A process for the preparation of 3,7-dimethylocta-1,7- diene by hydrodimerization of isoprene in the presence of a reducing agent, a catalyst comprising palladium complexed with a tertiary organophosphorus ligand and, optionally, of a solvent, characterized in that lithium formate, sodium formate, ammonium formate or calcium formate or a mixture thereof is used as the reducing agent in the presence of water. 2. A process as claimed in claim 1, characterized in that at least 1 mol. of water per mol. of formate is present. 3. A process as claimed in claim 2, characterized in that from 1 to 5 mol. of water per mol. of formate salt is present. 4. A process as claimed in any one of the preceding claims, characterized in that the tertiary organophosphorus ligand has the formula: (R )aPRb (I) - wherein the R groups which may be the same or different, attached to the phosphorus or oxygen atoms, represent hydrocarbyl groups, "a" is an integer from 0 to 3 and "b" is 3-a. 5. A process as claimed in any one of claims 1 to 3, characterized in that the tertiary organophosphorus ligand has the formula: Z-P(OR)iRj (Il) wherein Z represents a cross-linked synthetic resin, the R groups, which may be the same or different, attached to the phosphorus or oxygen atoms, represent hydrocarbyl groups, "i" is an integer from 0 to 2 and "Åa" is 2-i. 6. A process as claimed in claim 5, characterized in that Z represents a cross-linked polystyrene, poly(alpha-alkyl)- acrylate, polycarbonate or polyamide. 7. A process as claimed in any one of claims 4 to 6, characterized in that each R in formulae I and II represents an aryl, alkyl, aralkyl or alkaryl group, each with less than 20 carbon atoms. 8. A process as claimed in any one of claims 4-7, characterized in that at least one R of the ligand represents a sterically hindering group selected from benzyl or branched alkyl, aralkyl, alkenyl and cycloalkyl groups, each of the latter four groups having from 3 to 10 carbon atoms with branching occurring at a carbon atom no more than two carbon atoms from the phosphorus or oxygen atom. 9. A process as claimed in any one of the preceding claims, characterized in that the temperature ranges from OOC to 1000C, the palladium ranges from 1 to about 10-6 molar and the ratio of tertiary organophosphorus ligand to palladium is at least 1. 10. A process as claimed in any one of the preceding claims, characterized in that the solvent is dimethylformamide, N-methylpyrrolidinone or dimethylsulphoxide.

Inventors: NOZAKI, KENZIE
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1980
Main classification: C07C11
IPC codes: C07C11, C07C2, C07C67, C07C1, B01J31, C07B61