Method for producing 2-substituted 4-hydroxy-4-methyl-tetrahydropyrans, said method using recycling

Abstract

The present invention relates to a process for the preparation of 2-substituted 4-hydroxy-4-methyltetrahydropyrans.

Claims

1. A process for the preparation of 2-substituted 4-hydroxy-4-methyltetrahydropyrans of the formula (I) ##STR00009## in which R.sup.1 is a straight-chain or branched C.sub.1-C.sub.12-alkyl, a straight-chain, or branched C.sub.2-C.sub.12-alkenyl, a cycloalkyl having in total 3 to 20 carbon atoms, optionally substituted with C.sub.1-C.sub.12-alkyl and/or C.sub.1-C.sub.12-alkoxy, or an aryl having in total 6 to 20 carbon atoms, optionally substituted with C.sub.1-C.sub.12-alkyl and/or C.sub.1-C.sub.12-alkoxy, the process comprising a reaction of 3-methylbut-3-en-1-ol of the formula (III) ##STR00010## with an aldehyde of the formula (IV)
R.sup.1CHO(IV) where R.sup.1 in the formula (IV) has the meaning above, in a reactor with at least one downstream separating column in the presence of an acidic catalyst, wherein a portion of a discharge stream from the reactor is returned to the reactor, and another portion of the discharge stream is directed to the separating column; wherein heat is removed from the portion of the discharge stream prior to returning to the reactor.

2. The process according to claim 1, wherein the reaction takes place continuously.

3. The process according to claim 1, wherein the reaction is conducted in the presence of a solvent.

4. The process according to claim 1, wherein heat is supplied to the portion of the discharge stream that is directed to the separating column.

5. The process according to claim 1, wherein at least one stream from the top of the at least one separating column is returned to the reactor.

6. The process according to claim 1, wherein the reaction is carried out adiabatically.

7. The process according to claim 1, wherein the reactor used is isothermal.

8. The process according to claim 7, wherein the reactor comprises an internally arranged heat exchanger.

9. The process according to claim 1, wherein the reactor is a fixed-bed reactor.

10. The process according to claim 1, wherein the radical R is isobutyl or phenyl.

11. The process according to claim 1, wherein the acidic catalyst is selected from the group consisting of hydrochloric acid, sulfuric acid, methanesulfonic acid, p-toluenesulfonic acid and strongly acidic cation exchangers.

12. The process according to claim 1, wherein the acidic catalyst is a strongly acidic cation exchanger.

13. The process according to claim 1, wherein the alcohol of the formula (III) and the aldehyde of the formula (IV) are used in a molar ratio in the range from 0.7:1 to 2:1.

14. The process according to claim 1, wherein the reaction is conducted in the presence of 3% by weight to 15% by weight of water, based on the amount of the reaction mixture of components formulae (III) and (IV) and the water.

15. The process according to claim 1, wherein the reaction is carried out at a temperature in the range from 0 C. to 70 C.

16. The process according to claim 1, wherein the reaction is carried out at a pressure in the range from 1 bar to 15 bar.

17. A process for the preparation of 2-substituted 4-hydroxy-4-methyltetrahydropyrans of the formula (1) ##STR00011## in which R.sup.1 is a straight-chain or branched C.sub.1-C.sub.12-alkyl, a straight-chain, or branched C.sub.2-C.sub.12-alkenyl, a cycloalkyl having in total 3 to 20 carbon atoms, optionally substituted with C.sub.1-C.sub.12-alkyl and/or C.sub.1-C.sub.12-alkoxy, or an aryl having in total 6 to 20 carbon atoms, optionally substituted with C.sub.1-C.sub.12-alkyl and/or C.sub.1-C.sub.12-alkoxy, the process comprising reacting 3-methylbut-3-en-1-ol of the formula (III) ##STR00012## with an aldehyde of the formula (IV)
R.sup.1CHO(IV) in a reactor, in the presence of an acidic catalyst, and the alcohol of the formula (III) and the aldehyde of the formula (IV) are present in a molar ratio in a range from 0.7:1 to 2:1, and the reaction is conducted in the presence of 3% by weight to 15% by weight of water, based on the amount of the reaction mixture of components formulae (III) and (IV) and the water, wherein a portion of a discharge stream from the reactor is recycled to the reactor, and another portion of the discharge stream is directed to at least one separating column; wherein heat is removed from the portion of the discharge stream prior to returning to the reactor.

18. The process according to claim 17, wherein the reactor is a fixed-bed reactor, and the acidic catalyst is selected from the group consisting of hydrochloric acid, sulfuric acid, methanesulfonic acid, p-toluenesulfonic acid and strongly acidic cation exchangers.

19. The process according to claim 18, wherein the reaction is carried out at a temperature in the range from 20 C. to 70 C., and a pressure in the range from 1 bar to 15 bar, and R1 is selected from the group consisting of a straight-chain or branched C.sub.1-C.sub.12-alkyl, a straight-chain or branched C.sub.2-C.sub.12-alkyl, and phenyl.

Description

DESCRIPTION OF THE FIGURE

(1) The process according to the invention is explained in more detail by reference to the FIGURE below without limiting it to this embodiment.

(2) In FIG. 1 the following reference numerals are used: 1 Reactor 2 Cooling unit 5 Pump 8 Separating column A Isoprenol B Aldehyde C Water D Recirculation stream E Feed F Top product G Starting material

(3) The process according to the invention can be carried out with a reactor. The reactor can optionally be operated with interim cooling. Here, the reaction takes place in the back-mixed reactor system. In the back-mixed reactor, some or all of the circulation stream can be cooled. Division into two or more beds, optionally also with interim cooling, can likewise be realized in the one reactor.

(4) After emerging from the reactor, at least one distillation stage follows. These can be operated in parallel or in series. Preferably, they are connected in series.

(5) The FIGURE shows an embodiment of the process according to the invention with a reactor (1) and a separating column (8). The three starting materials isoprenol (A), aldehyde (B) and water (C) are introduced into the reactor (1) via three feeds. A discharge from the reactor (1) is removed via a line and the pump (5) and is divided into two part streams. A recirculation stream (D) is fed to the main reactor (1) via the cooling unit (2) together with the starting materials (A), (B) and (C). A feed (E) is passed to the separating column (8). The starting material (G) is removed as discharge at the bottom of the separating column (8) and optionally fed to a work-up stage. At the top of the separating column (8), the top product (F) is removed and returned at least in part to the reactor (1).

(6) The reactor (1) is preferably configured as a fixed-bed reactor. In this connection, it is operated in loop mode, whereas the separating column (8) is operated in a straight pass. In the arrangement shown in the FIGURE, the reactor (1) and the separating column (8) are connected in series such that the temperature profile above the catalyst bed can be adjusted via the back-mixing in the reactor (1). As a result, a large temperature increase at the start of the reaction can be prevented.

(7) The examples below serve to illustrate the invention without limiting it in any way.

EXAMPLES

Example 1 (Continuous Process with Partial Conversion and Recirculation)

(8) An apparatus consisting of a reactor and a laboratory column was used. The reactor used was a jacketed reactor made of RA4 without heating medium for an adiabatic procedure with a length of 150 cm and an internal diameter of 2.6 cm.

(9) The reactor was filled with 230 g (305 ml) of the cation exchanger. The cation exchanger was washed prior to use firstly several times with water, then once with methanol and finally with water so as to be methanol-free. The reactor was then conditioned by introducing a mixture of pyranol:water in a mass ratio of 95:5. The main reactor was then operated back-mixed with a recirculation stream of 2000 g/h, the recirculated stream being cooled to a temperature of 25 C. before reentering the main reactor. After conditioning the cation exchanger to the stated pyranol/water mixture, a mixture of isovaleraldehyde:isoprenol:water in a mass ratio of 45:50:5 was introduced at 25 C. and in a total quantitative stream of 100 g/h. This gave a crude product with a selectivity of 78.5% with regard to isovaleraldehyde with the following composition:

(10) Isovaleraldehyde: 10.25 GC % by weight,

(11) Isoprenol: 13.95 GC % by weight,

(12) Dihydropyran isomers: 6.98 GC % by weight,

(13) 1,3-Dioxane: 3.18 GC % by weight,

(14) Acetal: 0.63 GC % by weight,

(15) trans-Pyranol: 14.60 GC % by weight,

(16) cis-Pyranol: 40.07 GC % by weight,

(17) Water: 6.8% by weight (according to Karl Fischer).

(18) The reaction mixture was then subjected to the simultaneously running, continuous distillative separation, where the top product, consisting of isovaleraldehyde, isoprenol and water, was admixed with the feed of the main reactor and thereby returned. Simultaneously, the feed of the starting materials to the main stream was reduced by the corresponding amount.