Process for the selective oxidation of aromatic compounds

Oxidation is the most important type of reaction in organic synthesis. Numerous base chemicals and fine chemicals result from oxidative processes which frequently make use of atmospheric oxygen, hydrogen peroxide and organic peroxides as oxidizing agents. In many cases efficient, selective reactions only occur if these oxidizing agents are used in the presence of catalysts. These catalysts are generally metal oxides, for example V₂ O₅, CrO₃, MoO₃, WO₃, OsO₄ and RuO₄. Oxometallates such as chromate and permanganate are also occasionally found as oxidizing agents. The said catalysts display high efficiency in epoxidation, hydroxylation and carboxylation reactions on olefins (H. A. Jergensen, Chem. Rev. 1989, Vol. 89, pp. 431-458).

However, use of such systems for the catalytic oxidation of aromatic compounds is subject to many restrictions. Insufficient activity (WO₃) on the one hand and deficient selectivity on the other hand (CrO₃ /H₂ SO₄) in addition to the fact that safety with respect to ecology, health or pharmacology is frequently not ensured (e.g. in the case of CrO₃ or OsO₄) have hindered hitherto the industrial use of such catalysts.

Other processes established in oxidation chemistry, which, e.g. employ electrochemical oxidation, cerium (IV) salts, manganese (III) sulfate or peracids or peroxides (t-BuOOH) in the presence of molybdenum complexes as oxidizing agents, in the oxidation of simple or condensed aromatics or derivatives thereof prove to be highly complex, expensive, frequently burdened with high salt loadings through the stoichiometric use required (cerium (IV) salts, manganese (III) sulfate) and generally also unspecific (R. P. Kreh et al., Org. Chem., 1989, 54, 1526-1531; M. Hudlicky, Oxidations in Organic Chemistry, ACS Monograph 186, Washington/DC, 1990, S. 92-98; T. A. Gorodetskaya et al., U.S.S.R. Patent 1 121 255, 1984; Chem. Abstr., 1985, 102, 203754; W. Adam et al., Synthesis, 1993, 280-282, J. Skarzewski, Tetrahedron, 1984, 40, 4997-5000; S. Yamaguchi et al., Bull. Chem. Soc. Jpn., 1986, 59, 2881-2884; M. Periasamy, M. V. Bhatt, Tetrahedron Lett. 1978, 4561-4562; Y. Asakawa et al., 1988, J. Org. Chem., 53, 5453-5457; W. Chen, Chem. Abstr, 1987, 17, 58620).

Work by Buchler et al. (DE-A-3731689, DE-A-3731690) has shown that rhenium complexes epoxidize olefins but not aromatics.

EP-A-380085 discloses organorhenium compounds which are used as catalysts for the oxidation of olefins in the presence of hydrogen peroxide. It was established here that other rhenium compounds, namely Re₂ O₇, ReO₃, ReO₂ and (CH₃)₃ SnOReO₃ do not display any catalytic activities in the target oxidation of olefins.

The German Application (P 4402333.2) which has an earlier priority but which had not been published at the date of filing of the present application dicloses organorhenium oxides which can be effectively used as selective catalysts for the oxidation of a multiplicity of aromatic compounds to give quinoid systems in the presence of peroxide-containing reagents.

It is a disadvantage that the catalyst must be prepared from simpler rhenium compounds before use which implies taking up a certain amount of time and additional costs.

The object is therefore to find a storable, active catalyst system which is accessible as easily as possible, inexpensive and can be handled easily which achieves the desired selectivity in the oxidation of aromatics.

The invention relates to the use of compounds of the formula

Rea Ob Xc Ld                           (I)

in which

X=F, Cl, Br, I or OH,

L=Lewis base,

a=an integer 1, 2 or 3

b=zero or an integer from 2 to 9,

c=zero or an integer from 1 to 9,

d=zero or an integer from 1 to 6,

and the sum of a, b and c is such that it is appropriate to the zerovalency or trivalency to heptovalency of rhenium with the proviso that if b is not equal to zero, b≧2·a, as catalysts for the oxidation of electron-rich aromatic compounds and derivatives thereof.

Examples of the Lewis base L are pyridine, bipyridine, t-butylpyridine, amines, in particular secondary and tertiary amines such as triethylamine and quinuclidine, H₂ O and polyethers such as diglyme.

Preferably, compounds are used in which b is an integer from 2 to 9, c is zero or 1 and X, L, a and d have the abovementioned meanings.

Particularly preferably, the rhenium oxides ReO₂, ReO₃ and Re₂ O₇, rhenium trioxofluoride (FReO₃) and rhenium trioxochloride (ClReO₃) and their corresponding Lewis-base L-adducts are used.

Very particular preference is given to ReO₃ and Re₂ O₇.

The invention further relates to a process for the oxidation of electron-rich aromatic compounds which comprises oxidizing in an organic solvent electron-rich C₆ -C₂₂ -aryl compounds and derivatives thereof in the presence of a catalyst of the formula Re_a O_b X_c L_d (I), in which X, L, a, b, c and d have the abovementioned meanings, and of a peroxide-containing compound.

Suitable aryl compounds for the process according to the invention are electron-rich aromatic compounds or condensed aromatic systems having 6 to 22 carbon atoms, preferably having 6 to 14 carbon atoms, which if appropriate can be monosubstituted or polysubstituted, identically or differently, by an electron-donor group. Typical suitable electron-donor groups are hydroxyl, C₁ -C₃ -alkoxy, N-acylamino, N-acylamino-C₁ -C₃ -alkyl, acyloxy- and C₁ -C₃ -alkyl.

Examples of such aryl compounds are xylenes, disubstituted, trisubstituted or tetrasubstituted C₁ -C₃ -alkylbenzenes or C₁ -C₃ -alkoxybenzenes, naphthalene and monosubstituted to hexasubstituted C₁ -C₃ -alkyl or C₁ -C₃ -alkoxy derivatives thereof, anthracene and C₁ -C₃ -alkyl or C₁ -C₃ -alkoxy derivatives thereof, phenanthrene and higher condensed aromatics, phenol, hydroquinone, resorcinol pyrocatechol and pyrogallol, but also biphenyl.

Preferred aryl compounds are naphthalene and anthracene and derivatives thereof, particular preference is given to naphthalene and derivatives thereof, in particular 2-methylnaphthalene.

By means of the process according to the invention, the aryl compounds are generally oxidized to the corresponding quinoid systems. From 2-methylnaphthalene, for example, is obtained 2-methyl-1,4-naphthoquinone, the parent substance of the vitamin K series.

In the case of higher-substituted (trisubstituted and above) aryl compounds in which the formation of a quinoid system is not possible, by means of the process according to the invention the corresponding hydroxyl compound is prepared. Typical examples of such higher-substituted aryl compounds are 1,2,3,5,8-pentamethylnaphthalene; 1,2,3-trimethylbenzene, mesitylene and 1,3,5-trimethoxybenezene. From these starting materials are obtained, for example, by the process according to the invention the following hydroxyl compounds: 4-hydroxy-1,2,3,5,8-pentamethylnaphthalene, 1-hydroxy-3,4,5-trimethylbenzene, 1-hydroxy-2,4,6-trimethylbenzene and 1-hydroxy-2,4,6-trimethoxybenzene.

In accordance with the process according to the invention, the aromatic compound to be oxidized is dissolved in an organic solvent and the catalyst is added. The concentration of the dissolved aromatic compounds is 0.1 mol in 10-1000 ml, preferably 0.1 mol in 25-250 ml, particularly preferably 0.1 mol in 50-200 ml of solvent. Suitable organic solvents are for example glacial acetic acid, THF, tert-butanol or tert-butyl methyl ether, preferably glacial acetic acid or THF. The catalyst can be used in an amount of 0.01-10.0 mol %, preferably 0.1-2.0 mol %. To this solution is added the peroxide-containing compound (5-90% by weight) in a molar ratio of 1:1 to 20:1, based on the aromatic compounds to be oxidized.

The reaction is substantially pH-independent; preferably, the reaction is carried out at pH≦7.

The reaction mixture is stirred at a temperature of 10°-100° C., preferably 20°-70° C., until reaction is complete. The reaction mixture is then worked up in a manner usual for those skilled in the art, i.e. for example neutralized, extracted and dried. The crude oxidation product can be further purified, for example by high-vacuum distillation or by recrystallization.

The rhenium compounds of the formula I are commercially available as commercial grade compounds (ReO₂, ReO₃, Re₂ O₇) or may easily be prepared (G. Brauer, Handbuch der Praparativen Anorganischen Chemie, Handbook of Preparative Inorganic Chemistry!, 3rd Edition, Enke-Verlag, Stuttgart 1981). However, their suitability as oxidation catalyst of aromatics is novel and was in no way to be expected (EP-A-380085). Their particular advantage is that they are generally commercially available, nonvolatile and substantially pH-independent and may be separated off in a simple manner from the oxidation system. The compounds of the formula I in combination with peroxide-containing compounds, such as hydrogen peroxide, inorganic peroxides, e.g. alkali metal peroxides, in particular sodium peroxide, and percarboxylic acids and salts thereof such as m-chloroperbenzoic acid, peracetic acid and magnesium monoperoxonaphthalate, are highly active catalysts for the oxidations according to the invention.

Particularly preferably, the compounds of the formula I are used in combination with hydrogen peroxide, the concentration of which can be in the range of 5-90%, preferably 60-85%. Commercial grade Perhydrol (=30% strength H₂ O₂) can also be used.

EXAMPLES

General procedural instructions for the rhenium-catalyzed oxidation of aromatic compounds.

The substrates to be oxidized were dissolved in glacial acetic acid, THF or mixtures thereof and the catalyst was added. Finally, hydrogen peroxide was added as oxidizing agent. The reaction mixture was stirred at 20°, 40° or 70° C. (see Table) until reaction was complete.

Work-up:

The reaction solution was neutralized using a saturated sodium hydrogen carbonate solution. The aqueous mother liquor was extracted three times using methylene chloride; the combined extracts were dried over MgSO₄. The solvent was then removed in vacuo. After removal of the methylene chloride, generally yellow, solid oxidation products were obtained. The examples carried out in accordance with the above procedural instructions are to be taken from Table 1.

                                  TABLE 1__________________________________________________________________________Oxidation examples according to the general proceduralinstructions with associated reaction conditionsAryl compound            Catalyst (0.2 mmol                       t   T          YieldNo.  (10 mmol in each case)            in each case)                        h!  °C.!                               Solvent                                      (% by weight)                                              Product__________________________________________________________________________1    2,3-dimethylnaph-            ReO3.sup.γ)                       4   20  AcOH/THFa)                                      30      2,3-dimethylnaph-thalene                                       thoquinone2    2,3-dimethylnaph-            Re2 O7.bipyridine.sup.α)                       4   20  AcOH/THFa)                                      43      2,3-dimethylnaph-thalene                                       thoquinone3    2,3-dimethylnaph-            FReO3.bipyridine.sup.α)                       4   20  AcOH/THFa)                                      11      2,3-dimethylnaph-thalene                                       thoquinone4    2,3-dimethylnaph-            ClReO3.bipyridine.sup.α)                       4   20  AcOH/THFa)                                      74      2,3-dimethylnaph-thalene                                       thoquinone5    2,3-dimethylnaph-            BrReO3.bipyridine.sup.α)                       4   20  AcOH/THFa)                                      5       2,3-dimethylnaph-thalene                                       thoquinone6    2,3-dimethylnaph-            C6 H14 N3 ReO3 ReO4.sup.α)            3          4   20  AcOH/THFa)                                      11      2,3-dimethylnaph-thalene                                       thoquinone7    2,3-dimethylnaph-            ReO3.sup.α)                       24  20  THFb)                                      43      2,3-dimethylnaph-thalene                                       thoquinone8    2,3-dimethylnaph-            Re2 O7.sup.α)                       24  20  THFb)                                      51      2,3-dimethylnaph-thalene                                       thoquinone9    2,3-dimethylnaph-            ClReO3.sup.α)                       4   20  THFb)                                      23      2,3-dimethylnaph-thalene                                       thoquinone10   2,3-dimethylnaph-            ClReO3.bipyridine.sup.α)                       5   20  THFb)                                      36      2,3-dimethylnaph-thalene                                       thoquinone11   Mesitylene  ReO3.sup.α)                       4   20  AcOHc)                                      69      4-hydroxymesitylene12   mesitylene  Re2 O7.sup.α)                       4   20  AcOHc)                                      14      4-hydroxymesitylene                                      37      1,3-dihydroxy-                                              mesitylene13   mesitylene  Re2 O7.sup.γ)                       2   40  AcOHc)                                      73      4-hydroxymesitylene14   mesitylene  Re2 O7.sup.ε)                       1   20  AcOHc)                                      38      4-hydroxymesitylene15   1,2,3-trimethyl-            Re2 O7.sup.β)                       0.17                           70  AcOHc)                                      34      3,4,5-trimethyl-benzene                                       phenol16   1,3,5-trimethoxy-            Re2 O7.sup.θ)                       1   20  AcOHc)                                      20      2,4,6-trimethoxy-benzene                                       phenol17   2-methylnaphthalene            Re2 O7.sup.δ)                       4   20  AcOHc)                                      57      2-methylnaphthoquinone                                              118   2-methylnaphthalene            Re2 O7.sup.η)                       4   20  AcOHc)                                      33      2-methylnaphthoquinone                                              219   2-methylnaphthalene            Re2 O7.sup.η)                       4   40  AcOHc)                                      57      2-methylnaphthoquinone                                              .20   2-methylnaphthalene            Re2 O7.sup.δ)                       4   40  AcOHc)                                      70      2-methylnaphthoquinone__________________________________________________________________________ C6 H14 N3 = 1,3,7triazacyclononane Solvent: a) 22 ml AcOH + THF (THF quantity sufficient for complete dissolutio of the substrate after addition of H2 O2) b) 22 ml THF c) 22 ml AcOH (= glacial acetic acid); 85% H2 O2 ; molar ratio of aryl compound/H2 O2 = α) 1:20, β) 1:15, γ) 1:10, δ) 1:7, ε) 1:5, η) 1:3, θ) 1:1