METHOD FOR THE PREPARATION OF KETONES

Abstract: Method for the preparation of ketones, by means of oxy­ dation of the corresponding secondary alcohols, characterized in that said alcohols are catalytically oxidized by H₂O₂ inside a two-phase system comprising: a) an aqueous phase, containing hydrogen peroxide as the oxi­ dizing agent; b) an organic phase, containing a secondary alcohol and op­ tionally a solvent immiscible with said aqueous phase, as well as a peroxidic catalyst having the general formula Q₃XW₄O₂₄, wherein X represents an atom of phosphorus or of arsenic and wherein Q represents a quaternary cation, con­ taining hydrocarbon groups having a total of from 20 to 70 C atoms.

Description: BACKGROUND OF THE INVENTION The oxidation of secondary alcohols to ketones is one of the basic reactions of the organic synthesis; as an example, mentioning the oxidation of isoborneol or of borneol for obtai­ ning camphor will be sufficient. Many known processes are based on the oxidation of the corresponding secondary alcohols; such processes show, however, heavy drawbacks. Some (stoichiometri­ cal) processes require the use of considerable amounts of ex­ pensive oxidizing agents, or of agents which create heavy envi­ ronmental problems for their recovery or disposal at the reac­ tion end; as an example, we may mention, besides HNO₃, oxalyl chloride, N-halosuccinimides, and the pentavalent vanadium, he­ xavalent chromium and heptavalent manganese compounds. Other (catalytic) processes, although more preferable than the first ones, from the environmental point of view, are not completely satisfactory, because of the long reaction times (see, e.g., the PdCl₂-/NaOAc/O₂ system, or the benzyltrimethylammonium tetrabromo-oxomolybdate/tert.butyl hydroperoxide system), or because of the particularly expensive (PtO₂ or RuO₄), or toxic (OsO₄) catalysts used. Among the catalytic methods, examples are known of oxidation with hydrogen peroxide, the use of which could offer, in principle, undoubted advantages, thanks to their limited cost and to the absence of a reduction product to be di­ sposed of. But also these methods show a poor practical interest, in that: - They require extremely long reaction times (from 1 to 7 days) the catalytic activity being very low (Trost: Israel J. Chem. 1984, 24 , 134); or - It is necessary to work with H₂O₂ at 90%, with evident safety problems (Mares: J. Org. Chem., 1973, 44 , 921). The Applicants have now found that it is possible to oxi­ dize secondary alcohols to ketones by means of a simple and cheap process, free from the drawbacks to be faced in case of the known processes, by using low-concentration aqueous H₂O₂ and resorting to a suitable catalytic system. DISCLOSURE OF THE INVENTION In its broadest aspect, the invention consists of a ca­ talytic method for the preparation of ketones having formula (I): by means of oxidation of the corresponding secondary alcohols having formula (II): wherein R and R₁, equal to or different from each other, are alkyl or aryl-alkyl groups having up to 20 C atoms, aryl or al­ kyl-aryl groups having from 6 to 12 C atoms or cycloalkyl groups having from 3 to 12 C atoms; obviously R and R₁ can be bound to each other, so as to form an alkylic cycle containing from 5 to 12 C atoms, or a polycyclic system, containing preferably up to 40 C atoms. The process is characterized in that said alcohols (II) are oxidized to the corresponding ketones (I) by reacting them with H₂O₂, under stirring at 60-95°C, inside a biphasic system comprising: a) an aqueous phase, containing hydrogen peroxide as the oxidi­ zing agent; b) an organic phase, consisting of a secondary alcohol and op­ tionally of a solvent immiscible with said aqueous phase, as well as of a peroxidic catalyst having the general formula Q₃XW₄O₂₄ (III), wherein X represents an atom of phosphorus or of arsenic, and wherein Q represents a quaternary cation (R₂R₃R₄R₅M)⁺, wherein M is selected from nitrogen and phos­ phorus and wherein R₂, R₃, R₄ and R₅, equal to or different from each other, are selected from hydrogen and hydrocarbon groups, so as to have from 1 to 4 hydrocarbon groups contai­ ning a total of from 20 to 70 C atoms. By such a method, the desired ketones are obtained with optimum yields and in a very pure form; an illustrative, but not limitative example of the alcohols which can be oxidized ac­ cording to the invention is represented by: hexane-2-ol; octane-2-ol; decane-2-ol; cyclohexanol; 2,6-dimethyl-cyclohexanol; menthol; 1-phenyl-ethanol; borneol; isoborneol; 6-hydroxy-heptanoic acid; benzhydrol; 2-ethyl-1,3-hexanediol; dihydrocholesterol; 1,2,3,4,5,6,7,8-octahydro-2-hydroxy-naphthalene. Oxidation catalyst (III) is consisting of a peroxidic complex, containing tungsten, phosphorus (or As), and a suffi­ ciently lipophilic quaternary cation, that can be obtained ac­ cording to usual techniques. According to a preferred but not limitative form, it is better to use, as a catalyst, a compound having formula (III), wherein X represents phosphorus, wherein M (in the quaternary cation Q) represents nitrogen and wherein R₂, R₃, R₄,R₅ are hy­ drocarbon groups containing a total of from 25 to 40 C atoms, such as, e.g., methyltrioctylammonium, dimethyldihexadecylam­ monium, dimethyldioctadecylammonium, or mixtures thereof; in particular, as the catalysts, the compounds having formula: are preferred. The catalysts having formula (IV) and (V) can be pre­ pared, e.g., by reacting tungstic acid (or an alkali-metal tungstate), phosphoric acid (or an alkali-metal phosphate) and hydrogen peroxide, inside an acidic aqueous phase, with a qua­ ternary salt, selected from the group consisting of methyltri­ octylammonium chloride (known on the market under the trade name ALIQUAT 336) and dimethyl[dioctadecyl (75%) + dihexadecyl (25%)]ammonium chloride (known on the market under the trade name ARQUAD 2HT), contained in an organic phase immiscible with the aqueous phase. The reaction between the inorganic reactants can be carried out at from 20 to 80°C; then the quaternary salt dissolve in a solvent (preferably in 1,2-dichloroethane) is added, preferably at room temperature, and stirring of the bi­ phasic mixture is continued for 15-30 minutes. The acidic aqueous phase has preferably a pH lower than 2; to the purp­ ose of obtaining such a range of values, pH is adjusted, if necessary, with a mineral acid (e.g., H₂SO₄ or HCl). In ge­ neral, the molar ratios between the reactants must be the fol­ lowing: per each mol of P, 4 mol of W and up to 2 mol of qua­ ternary salt; as to H₂O₂, from 2.5 to 6 mol of H₂O₂ per mol of W are enough. After the separation of the phases, by evapora­ tion of the organic phase, the compound (IV) or the compound (V) is obtained, respectively, in the oil or in the solid form. The oxidation reaction is carried out according to the double­ -phase technique, and the organic phase contains the alcohol, the catalyst, and, optionally, a solvent immiscible with the aqueous phase; said immiscible solvent can be chlorinated hy­ drocarbons (e.g., 1,2-dichloroethane, trichloroethanes, tetra­ chloroethylene) or optionally substituted aromatic hydrocarbons (e. g., benzene, toluene or xylenes). Usually, the reaction can be carried out under vigorous stirring, at temperatures from 60° to 95°C and under atmospheric pressure, what does not exclude, obviously, that the reaction may be carried out under a superatmospheric pressure. The reaction time (according to the used catalyst and to its amount, to the operating tempera­ ture, to the nature and to the concentration of the alcohol in the organic phase) is generally from 10 minutes to 2 hours; the catalyst is preferably used in a H₂O₂:catalyst molar ratio from 200:1 to 300:1. It is finally recommended to work with an H₂O₂:­ alcohol molar ratio from 1:1 to 2:1, and preferably from 1.2:1 to 1.5:1. When a suitable solvent is used, the alcohol concen­ tration in the organic phase is preferably higher than 70% by weight. The concentration of H₂O₂, in the aqueous phase, is not crytical, and can be from 1 to 70%, and preferably from 10 to 40% by weight. At the reaction end, after the separation of the phases, the ketone (present in the organic phase) is isola­ ted by distillation or by column chromatography, according to usual techniques. The method of the invention can be performed by means of usual equipment and techniques; the catalyst is sufficiently stable and can be therefore prepared and sto­ red until its use. The invention is now disclosed in greater detail in the following Examples, given for purely illustrative and non-limitative purposes. The concentration of hydrogen peroxide and of phosphoric acid is expressed in the Examples as grams per 100 cm³ of solution. EXAMPLE 1 Part A Preparation of catalyst (C₂₅H₅₄N)₃PW₄O₂₄ (IV) To a 4-neck 100-cm³ flask, equipped with blade stirrer and dropping funnel, 3.30 g of Na₂WO₄.2H₂O, dissolved in 20 cm³ of H₂O, 1.5 cm³ of 40% H₃PO₄ and 3 cm³ of H₂SO₄ at 30% by weight were charged at room temperature. Two cm³ of 40% H₂O₂ were then added. Forty cm³ of 1,2-dichloroethane, containing 1.6 g of methyltrioctylammoniumchloride (known in the trade as ALIQUAT 336), were then added, under vigorous stirring, over a 3-minute time. The stirring was maintained for 20 minutes at room temperature, and at the end the reation mixture was de­ canted and the phases were separated. The organic (lower) phase was evaporated in vacuo, thus obtaining 2.82 g of a viscous oil. The analytical results are in agreement with the indicated formula, as shown hereinbelow: - Active oxygen found = 5.67% (determined by means of the ad­ dition of a known excess of Na₂SO₃ in a basic medium, and iodometric back-titration in acidic medium). - Theoretical active oxygen = 5.68% (compound for 8 active oxygen atoms). Part B Oxidation of 2-octanol To a 2-neck 50-cm³ flask, provided with magnetic stirrer, termomether and reflux condenser, 6.4 cm³ of 40% H₂O₂ (about 75 mmol), 0.56 g (about 0.25 mmol) of catalyst (IV), and 6.5 g (50 mmol) of 2-octanol were charged. The biphasic mixture was heated under vigorous stirring, at 90°C and was kept at this temperature for 1.5 hours. A conversion of H₂O₂ > 98% was ob­ tained (as determined by iodometric titration of the aqueous phase). At the end, the phases were separated. The aqueous phase was extracted with ethyl ether and the extract was added to the organic phase. The solvent was evaporated off and the residue was eluted over a silica gel column, using an ether/n-­ hexane (1:1) mixture as the eluent. 5.9 g (46.1 mmol) of 2­ -octanone were obtained with a purity > 99%, as determined by gas-liquid chromatography (GLC). The yield, with respect to the used alcohol, was 92%. EXAMPLE 2 Example 1 was repeated, replacing 2-octanol by 1-phenyl­ ethanol (6.1 g; 50 mmol), and reducing the reaction time to 10 minutes; 5.4 g (45 mmol) of acetophenone were obtained (GLC purity: 98%),what corresponds to a 90% yield. EXAMPLE 3 Example 1 was repeated, replacing 2-octanol by (-)-men­ thol (7.8 g; 50 mmol), increasing the reaction time to 2 hours, and eluting the residue of the organic phase over a column of alumina (instead of silica); 6.78 g (44 mmol) of (-)-menthone were obtained (GLC purity > 99%), what corresponds to a 88% yield. EXAMPLE 4 Example 1 repeated, replacing 2-octanol by 2-ethyl-­ 1,3-hexanediol (7.3 g; 50 mmol) and reducing the reaction time to 1 hour; 5.48 g (38.1 mmol) of 3-hydroxymethyl-4-heptanone were obtained (GLC purity > 99%), which corresponds to a yield of 76%. EXAMPLE 5 Example 1 was repeated, replacing 2-octanol by (±)-iso­ borneol (7.7 g; 50 mmol), adding 2 cm³ of tetrachloroethylene and reducing the reaction time to 45 minutes. 7.22 g (47.5 mmol) of (±)-camphor were obtained (GLC purity > 99%), what cor­ responds to a 95% yield. EXAMPLE 6 Example 3 was repeated, replacing (-)-menthol by 2,6­ -dimethylcyclohexanol (6.4 g; 50 mmol), using 5.1 cm³ (60 mmol) of 40% of H₂O₂, adding 4 g of anhydrous MgSO₄ and reducing the reaction time to 30 minutes. 5.61 g (44.5 mmol) of 2,6-dimethyl­ -cyclohexanone were obtained (GLC purity > 99%), what corresponds to a 89% yield. EXAMPLE 7 Part A Preparation of catalyst (C₃₇H₇₈N)₃PW₄O₂₄ (V) By operating as described in Example 1, but replacing ALIQUAT 336 with 3.10 g of dimethyl[dioctadecyl (75%) + dihexa­ decyl (25%)]ammonium chloride (known in the trade as ARQUAD 2HT), 3.6 g of a white solid were obtained. The analytical values are in agreement with the indicated formula, - Active oxygen found = 4.60% ( determined as in Example 1); - Theoretical active oxygen = 4.63% (computed for 8 active oxygen atoms). Part B Oxidation of benzhydrol Example 3 was repeated, replacing catalyst (IV) by 0.69 g (0.25 mmol), of catalyst (V), replacing 2-octanol with 9.20 g (50 mmol) of benzhydrol, and reducing the reaction time to 1 hour. 9.04 g (49.7 mmol) of benzophenone were thus obtained (GCL purity > 99%), what corresponds to a 99% yield. EXAMPLE 8 Example 5 was repeated, replacing (±)-isoborneol by (-)­ -borneol (7.7 g; 50 mmol); 6.99 g (46 mmol) of (-)-camphor were obtained (GLC purity 97.8%), what corresponds to a 92% yield. EXAMPLE 9 To a 2-neck 50-cm³ flask, provided with magnetic stirrer, termomether and reflux condenser, 3.2 cm³ of 40% H₂O₂ (37.6 mmol), 10 cm³ of tetrachloroethylene, 0.28 g (about 0.125 mmol) of ca­ talyst (IV), and 9.9 g of 90% dihydrocholesterol (25 mmol) were charged. The biphasic mixture was heated at 90°C under vigorous stirring and was kept at this temperature for 45 minu­ tes. A conversion of H₂O₂ > 98% was obtained (as determined by iodometric titration of the aqueous phase). At the end, the phases were separated. The solvent of the organic phase was evaporated off and the residue was eluted over a silica gel column, using methylene chloride as the eluent. 8.80 g (22.8 mmol) were obtained of cholestan-3-one (melting point: 129-130°C, after crystallization from ethanol). The yield, with respect to the used alcohol, was 91%. Data and results of all of the tests are reported in the following table:

Claims: 1. A catalytic method for the preparation of ketones having formula (I): by means of oxidation of the corresponding secondary alcohols having formula (II): wherein R and R₁, equal to or different from each other, are alkyl or aryl-alkyl groups (having up to 20 C atoms), aryl or alkyl-aryl groups (of from 6 to 12 C atoms) or cycloalkyl groups having from 3 to 12 C atoms; and wherein R and R₁ can be bound to each other, so as to form an alkylic cycle containing from 5 to 12 C atoms, or a polycyclic sy­ stem, characterized in that said alcohols are oxidized (by reacting them with H₂O₂ under vigorous stirring), prefera­ bly at 60-95°C, inside a liquid system comprising: a) an aqueous phase, containing, as the oxidizing agent, hydrogen peroxide; b) an organic phase, containing a secondary alcohol and optionally a solvent immiscible with said aqueous phase, as well as a peroxidic catalyst having the general for­ mula Q₃XW₄O₂₄ (III), wherein X represents an atom of phosphorus or of arsenic, Q represents a quaternary cation (R₂R₃R₄R₅M) ⁺, M being selected between nitrogen and phosphorus, and wherein R₂, R₃, R₄ and R₅, equal to or different from each other, are selected from hydro­ gen and hydrocarbon groups, so as to have from1 to 4 hydrocarbon groups containing a total of from 20 to 70 C atoms. 2. A method according to claim 1, wherein the catalyst (III) is selected from the compounds of formula: (C₂₅H₅₄N)₃ PW₄O₂₄ and (C₃₇H₇₈N)₃ P W₄O₂₄. 3. A method according to claim 1, characterized in that in ca­ talyst (III), X represents phosphorus, that in the quaterna­ ry cation M represents nitrogen, and that the radicals R₂, R₃, R₄ and R₅ are hydrocarbon groups containing a total of from 25 to 40 C atoms. 4. A method according to claim 3, wherein the quaternary cation is selected from methyltrioctylammonium, dimethyldioctadecyl­ ammonium and dimethyl- or dihexadecylammonium or mixtures thereof. 5. A method according to one or more of the claims 1-4, wherein the alcohol is selected from octan-2-ol, 1-phenyl-ethanol, (-)-menthol, 2-ethyl-1,3-hexanediol, (-)-borneol, isoborneol, 2,6-dimethylcyclohexanol, benzhydrol, cyclohexanol, dihydro­ cholesterol and 1,2,3,4,5,6,7,8-octahydro-2-hydroxy-naph­ thalene. 6. A method according to one or more of the claims 1-5, wherein the oxidation temperature is from 60 to 95°C, the H₂O₂:alco­ hol molar ratio being from 1:1 to 2:1 and the H₂O₂:catalyst molar ratio being from 200:1 to 300:1. 7. A method according to one or more of the claims 1-6, wherein the concentration of H₂O₂ in the aqueous phase is from 1 to 70% by weight. 8. A method according to claim 7, wherein said hydrogen peroxide concentration is from 10 to 40% by weight. 9. A method according to one or more of the claims 1-8, wherein the used solvent, immiscible with water, is selected from the chlorinated aliphatic hydrocarbons and the optionally substituted aromatic hydrocarbons.

Inventors: GAMBARO, MARIO, RICCI, MARCO, DR., VENTURELLO, CARLO, DR.
Applicants: CONSIGLIO NAZIONALE DELLE RICERCHE, ISTITUTO GUIDO DONEGANI S.P.A.
Application year: 1987
Main classification: C07C45
IPC codes: B01J31, C07C49, C07C67, C07B61, C07C45, C07J9