Process for the synthesis of aryl alkyl monoethers

Details Process for the synthesis of aryl alkyl monoethers Process for the synthesis of aryl alkyl monoethers

2002-02-20

2004-09-21

Richter, Johann (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Oxygen containing

C568S650000

Reexamination Certificate

active

06794547

ABSTRACT:

The present invention relates to a process for the synthesis of aryl alkyl ethers.
It relates more particularly to a process for the improved synthesis of aryl alkyl ethers by O-alkylation of the corresponding phenol compounds.
Aryl alkyl ethers are very useful intermediates, in particular for the preparation of dyes, plant-protection agents and fragrances. Their applications are reported in particular in Ullmann Enzyklopädie der Technischen Chemie, Volume 13, pages 450-453 and Volume 14, pages 760-763. A great many preparation processes have consequently been provided.
Some consist in alkylating phenol derivatives with alkyl halides or alkyl sulphates, as indicated in Pure & Applied Chemistry, Volume 68, No. 2, pages 367-375 (1996). However, these processes exhibit numerous disadvantages. Some reactants, such as dimethyl sulphate, are highly toxic. Furthermore, the acid released during the reaction must be neutralized; in point of fact, some phenols are highly sensitive to the neutralizing agents.
Other processes use an alcohol, for example methanol, as alkylating agent. The reaction is carried out at very high temperatures of greater then 250° C. In the majority of cases, this reaction is not selective and C-alkylation by-products are formed. When it is selective, the degree of conversion is low. Such a process is described in Volume 44 of Catalysis Today, pages 253-258 (1998).
Processes for alkylation by dialkyl carbonates were then envisaged, in particular processes for O-alkylation by dimethyl carbonate. The catalyst used is chosen from tertiary amine salts, diamines, quaternary ammonium salts or tertiary phosphines. Such processes are disclosed, for example, in U.S. Pat. No. 4,192,949. However, the reaction temperature, pressure and duration of these syntheses, carried out in a closed reactor, remain high, which is highly disadvantageous industrially.
In order to prepare aryl alkyl esters under milder conditions, the authors of the article which appeared in Synthesis, Volume 5, pages 382-383 (1986), recommend the use of potassium carbonate with a cocatalyst, crown ether 18-6. This is catalysis by solid/liquid phase transfer. The reaction is carried out at atmospheric pressure and at 100° C. but the high toxicity and the high cost of the phase transfer cocatalyst, the crown ether, are major disadvantages. Furthermore, the mean rate of formation of ether per mole of catalyst is of the order of 0.03 mol per hour, which is low. Another catalytic system was then proposed, in order to operate with nontoxic reactants. This is still phase transfer catalysis. Such a process is described in Industrial & Engineering Chemistry Research, volume 27, pages 1565-1571 (1988). Tundo et al. use, as catalytic system, polyethylene glycol adsorbed on a solid stationary bed composed either of potassium carbonate or of &agr;-alumina beads. In the latter case, potassium carbonate is also adsorbed on the &agr;-alumina beads. This process exhibits the disadvantage of using a complex catalytic system with at least two constituents, polyethylene glycol and potassium carbonate. The simultaneous presence of these two constituents is essential in obtaining a good yield. This is because, by way of comparison, when the catalytic system is composed of &agr;-alumina beads covered solely with 5% by weight of potassium carbonate, the conversion of phenol to anisole, as indicated in FIG. 4, is 25%, which is very low. Furthermore, the heterogeneity of the reaction mixture reduces the efficiency of the reaction. The mean rate of formation of the ether per mole of catalyst is consequently only 0.13 mol per hour.
A similar continuous process was envisaged by Bomben et al. in an article which appeared in Industrial & Engineering Chemistry Research, Volume 38, pages 2075-2079 (1999). The authors of this article use, as catalytic system, a stirred catalytic bed composed of polyethylene glycol and of potassium carbonate. The disadvantages related to such a process remain still the complexity of the two-component catalytic system and the low value of the mean rate of formation of ether per mole of catalyst, which is only 0.7 mol/mol.h.
Another process, disclosed in Japanese Application JP 06145091, consists in reacting a phenol compound, such as phenol or hydroquinone, and an alkyl carbonate. The catalyst used is an alkali metal salt, in particular potassium carbonate. The reaction necessarily takes place in the presence of a nitrogenous organic solvent, such as pyridine, a formamide or an alkylacetamide. The authors have shown, in Comparative Example 1, that the yield was zero when the reaction was carried out without solvent. Furthermore, the reaction time remains long and the yields are low.
A person skilled in the art is therefore still looking for a process for the synthesis of aryl alkyl ethers which is inexpensive and selective, with a good yield, and for which the reaction conditions are mild, in particular as regards the pressure and the temperature.
Such a process is a subject-matter of the present invention.
The invention relates to a process for the synthesis of aryl alkyl monoethers by reaction of a phenol compound, comprising one or more hydroxyl groups attached to the aromatic cyclic system, and of a dialkyl carbonate, characterized in that the said process is carried out without solvent, at a pressure of between 0.93×10
5
Pa and 1.07×10
5
Pa, at a temperature of between 100° C. and 200° C., in the presence of a catalyst chosen from the group consisting of alkaline carbonates and alkaline hydroxides and in that the dialkyl carbonate is added gradually to the reaction mixture.
This process exhibits the advantage of being simple and inexpensive and makes it possible to obtain ethers with a very good yield.
This is because it is a solvent-free process using only a single catalyst. The use of a limited number of constituents thus reduces the cost. Furthermore, the operating conditions, in particular as regards the pressure and the temperature, are easy to implement industrially.
The reaction is selective. This is because, when the phenol compound comprises only one hydroxyl group on the aromatic cyclic system, only the corresponding aryl alkyl ether is obtained, without formation of by-products.
When the phenol compound comprises two or more hydroxyl groups attached to the aromatic cyclic system, an aryl alkyl monoether is predominantly obtained. Only small amounts of polyethers are obtained.
Another advantage of this process is the high value of the mean rate of formation of the ether with respect to the amount of catalyst used, which is of the order of 2 to 6 mol of ether formed per hour and per mole of catalyst used. With the already existing processes, the mean rate was less than or equal to 1 mol of ether formed per hour and per mole of catalyst.
This process makes it possible to selectively obtain a wide range of aryl alkyl monoethers starting from a dialkyl carbonate and a phenol compound.
The phenol compound is preferably chosen from the compounds of formula (I)
in which R
2
, R
3
, R
4
, R
5
and R
6
, which are identical or different, each represent
a hydrogen atom,
a substituted or unsubstituted, saturated or unsaturated, C
1
to C
20
alkyl radical,
a substituted or unsubstituted aryl or aralkyl group,
a halogen atom,
a nitrile or nitro group or a group of formula:
 in which R
7
is a C
1
to C
20
aliphatic radical, a C
7
to C
12
aralkyl radical or a C
6
to C
14
aromatic radical, R
8
is a C
1
to C
20
aliphatic radical, a C
7
to C
12
aralkyl radical or a C
6
to C
14
aromatic radical, and R
9
is a hydrogen atom,
it being possible for two adjacent radicals, for example R
2
R
3
or R
3
R
4
or R
4
R
5
or R
5
R
6
, to be connected to one another to form a saturated or unsaturated aliphatic ring, an aromatic ring or a saturated or unsaturated heterocycle which are unsubstituted or substituted by the groups as described for R
2
to R
6
. The substituents of the R
2
to R
6
radicals are chosen in particular from halogen atoms, nitrile or nitro groups, or groups of fo

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