Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2002-10-04
2004-06-15
Kumar, Shailendra (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S322000, C568S361000, C568S402000
Reexamination Certificate
active
06750371
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention provides a continuous process for the oxidation of alcohols with the aid of nitroxyl compounds as catalysts in multiphase systems.
2. Background Art
The oxidation of alcohols to aldehydes or ketones is an important transformation in organic chemistry, since compounds having a high reaction potential are formed from readily available alcohols. Such transformations are therefore of great importance in industrial processes. Catalytic processes are particularly advantageous. A process frequently employed in industry is the gas-phase dehydrogenation of alcohols. However, only volatile compounds can be used in this process. The catalyst systems employed are useful with only a few substrates, and the reaction conditions have to be matched specifically to these substrates. Oxidations employing nitroxyl compounds, in particular TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) and its derivatives (Review: A. E. J. de Nooy, A. C. Besemer and H. V. Bekkum, SYNTHESIS 1996, 1153), as oxidation catalysts are more generally applicable. Numerous TEMPO derivatives, including TEMPO derivatives on polymeric supports, e.g. polyamine immobilized piperidinyl oxyl “PIPO”, Dijksman et al., Synlett 2001, 102, EP 1103537, have been described as oxidation catalysts. These TEMPO-catalyzed oxidations are frequently carried out in two-phase systems, e.g. methylene chloride/water, as disclosed by P. L. Anelli, C. Biffi, F. Montanari and S. Quici, J. Org. Chem. 1987, 52, 2559. The oxidation of alcohols using sodium hypochlorite or sodium hypobromite as oxidant has been studied in detail. Aldehydes can be obtained from primary alcohols, and ketones from secondary alcohols by this process.
When the syntheses are, as is customary, carried out batchwise, the oxidant dissolved in the aqueous phase is added to the organic phase containing the alcohol to be oxidized and the nitroxyl compound. A disadvantage of this existing procedure is that the heat of reaction of the strongly exothermic nitroxyl-catalyzed oxidation process can be removed only with great difficulty, particularly in the case of large batches. This necessitates an increase in the contact time of the two phases and thus the duration of the process, as a result of which secondary reactions, e.g. reaction of alkali-labile groups such as ester group saponification, become significantly more prominent. For the purposes of the present invention, the contact time is the time over which the phases participating in the reaction are mixed.
A further secondary reaction which occurs in batch reactions involves formation of hemiacetals with the starting alcohol and further oxidation to the corresponding ester in accordance with equation 1.
This reaction becomes increasingly prominent with increasing reaction time and phase contact time, so that the achievable yields decrease with increasing batch size, rendering industrial production impossible. A further secondary reaction which decreases the yield of the aldehyde is the further oxidation of the aldehyde to the corresponding carboxylic acid.
For example, the reaction of 20 g of 2-n-butyryloxyethanol formed only 37% of the desired aldehyde together with 12% of 2-n-butyryloxyethyl 2-n-butyryloxyacetate, 4% of butyric acid and 41% of 2-n-butyryloxyacetic acid, as shown in Comparative Example C9 herein. On doubling the batch size, the yield of aldehyde decreased to only 12% and the amount of 2-n-butyryloxyethyl 2-n-butyryloxyacetate was as high as 45% as shown in Comparative Example C10.
EP 0340703 and DE 4007923 describe TEMPO-catalyzed oxidation reactions using sodium hypochlorite solution in a batch reaction. The contact times are more than 15 minutes and particularly more than 30 minutes, since the post-reaction alone required these amounts of time. Although the possibility of carrying out this process continuously was considered, no continuous process was exemplified.
DE 10029597 describes an oxidation process employing a polymer-enlarged TEMPO derivative, likewise as a batch process. The possibility of carrying out the process continuously is envisaged only in conjunction with the use of a membrane reactor, but is not exemplified. Here too, contact times of at least 30 minutes are always employed. DE 69415345 (EP 0734392) employs contact times of 5 hours. In DE 19605039 (EP 0801073), a contact time of at least 45 minutes is employed. In EP 1103537, the after-reaction time alone is 20 minutes. In WO 01/90111, contact times as high as 4 hours were used, and 2 hours were involved with the after-reaction alone.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide a process for the oxidation of an alcohol to an aldehyde or a ketone in the presence of a nitroxyl compound as catalyst, which process gives higher yields of aldehydes or ketones.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
This and other objects are achieved by a process in which the alcohol to be oxidized in an organic liquid phase is reacted in the presence of a nitroxyl compound with an aqueous phase comprising the oxidant, wherein the reaction is carried out continuously at a contact time of the phases of from 0.1 s to a maximum of 15 minutes with intensive mixing of the phases.
It has been surprisingly discovered that simply converting the batch process into a continuous process does not improve the yields. Although no ester formation is then observed, the further oxidation of the aldehyde to the carboxylic acid occurs to an increased extent. The simple conversion of the batch process into a continuous process therefore does not represent a solution to the yield problems. This can also be seen from the Comparative Examples C11 and C12 of the present patent application.
In the process of the invention, the combination of the use of a continuous process together with contact times below 15 minutes is essential. The above-described secondary reactions are suppressed efficiently only by adherence to both process requirements.
The continuous reactors required for the process are known to those skilled in the art. An overview of the most important embodiments are given in, for example, “Ullmann's Encyclopedia of Industrial Chemistry”, Vol. B4. The process can be carried out, for example, in continuously operated tube reactors, in continuously operated loop reactors, in continuously operated stirred vessels or cascades of stirred vessels, or by means of centrifugal pumps. In a particularly preferred embodiment, the two phases are combined in a static mixing element and then directed through a tube reactor.
The contact time of the phases is preferably from 1 s to 5 minutes, more preferably from 1 s to 2 minutes, and most preferably from 1 s to 30 s.
Intensive mixing of the phases is preferably achieved by establishing turbulent flow in the reaction mixture. Particular preference is given to turbulent flow at a Reynolds number Re of from 800 to 20,000. Intensive mixing or turbulent flow can be achieved by means of all known mixing systems, e.g. static mixing elements or stirrers. Various mixing systems can also be combined to reach or exceed the Reynolds number.
The organic liquid phase comprises the alcohol and, if desired, one or more organic solvents. Suitable organic solvents include, for example, linear or branched saturated or unsaturated aliphatic hydrocarbons having 1-20 carbon atoms, cyclic aliphatic saturated or unsaturated hydrocarbons having 5-20 carbon atoms or aromatic hydrocarbons having 5-20 carbon atoms, in each of which one or more hydrogen atoms or one or more carbon atoms may be replaced by heteroatoms.
Preference is given to linear or branched saturated or unsaturated aliphatic hydrocarbons having 1-16 carbon atoms, cyclic aliphatic saturated or unsaturated hydrocarbons having 5-16 carbon atoms or aromatic hydrocarbons having 6-16 carbon atoms, in each of which one or more hydrogens may be replaced, independently of one another, by F, Cl, Br, NO
2
or CN, or one or more CH
2
groups may be replaced, independently of one another
Fritz-Langhals Elke
Petersen Hermann
Stohrer Juergen
Brooks & Kushman P.C.
Consortium fuer Elektrochemische Industrie GmbH
Kumar Shailendra
Witherspoon Sikarl A.
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