Process for preparing 2-cycloalkenones

The present invention relates to a process for preparing 2-cyclo-alkenones of the formula I ##STR1## which comprises subjecting a 2-alkoxycycloalkanone and/or a 2-hydroxyalkanone ketal, in which n is an integer from 0 to 20, m is 0, 1, 2 or 3 and R is alkyl, alkylaryl, aryl, arylalkyl, alkenyl or alkynyl, to an elimination reaction in the gas phase on an acidic heterogeneous catalyst.

The present invention relates to a process for preparing 2-cycloalkenones 
starting from 2-alkoxycycloalkanones or 2-hydroxycycloalkanone ketals by 
an elimination reaction in the gas phase on an acidic heterogeneous 
catalyst. 
2-Cycloalkenones are starting materials or intermediates with many possible 
uses, inter alia for preparing pharmacologically active substances. 
Examples which may be mentioned are the preparation of carbazoles and 
carbazole derivatives as antiinflammatory agents with relevance in 
veterinary medicine as disclosed in EP-A-008 446, and the preparation of 
prostaglandins starting from 2-cyclopentenones. 
Nevertheless, the known processes for the industrial preparation of 
2-cycloalkenones have considerable disadvantages. Known processes based on 
precursors which can be obtained easily and cheaply in industrial 
quantities are industrially complicated or result in byproducts which are 
difficult to utilize. Examples thereof are the Birch reduction of anisole, 
a complicated multistage process which uses sodium and liquid ammonia, 
provides only moderate yields and, moreover, is confined to the 
preparation of 2-cyclohexenone, and the oxidation of cycloalkenes with 
peroxides (J. Mlochowski, S. B. Said, Polish J. Chem. 1997, 71, p. 149) 
which, because of low selectivity, results in a large number of byproducts 
which are difficult to remove. 
An industrial process is disclosed in DE-A-21 50 294. This entails thermal 
elimination of hydrogen halide from 2-halocyclohexanone or the relevant 
carboxylic acid from 2-acyloxycyclohexanone. The serious disadvantages of 
this process will be indicated taking the example of the preparation of 
2-cyclohexenone from 2-chlorocyclohexanone. In this case, the 
cyclohexanone must be first ketalized, then chlorinated, 
dehydrochlorinated and subsequently subjected to an acidic ketal cleavage. 
In addition, the chlorination results in regioisomers and polychlorination 
products, which must be removed or may result in the formation of mixtures 
of 2- and 3-cyclohexenone products which in turn require complicated 
separation or isomerization. 
It is an object of the present invention to provide a widely applicable 
process for preparing 2-cycloalkenones which is based on precursors which 
can easily be obtained in industrial quantities and affords the required 
cycloalkenones in the minimum number of stages. 
We have found that this object is achieved by preparing the required 
2-cycloalkenones in a simple manner, in one stage and in good yields by 
elimination, with acidic heterogeneous catalysis, from the easily 
obtainable 2-alkoxycycloalkanones or 2-hydroxycycloalkanone ketals. 
The present invention therefore relates to a process for preparing 
2-cycloalkenones of the formula I 
##STR2## 
which comprises subjecting a 2-alkoxycycloalkanone of the formula II 
##STR3## 
or a 2-hydroxycycloalkanone ketal of the formula III 
##STR4## 
where n is an integer from 0 to 20, preferably 1 to 8, particularly 
preferably 2 and 3, 
m is 0, 1, 2 or 3, 
the R radicals are, independently of one another, alkyl, alkylaryl, aryl, 
arylalkyl, alkenyl or alkynyl, it being possible for the aryl radicals to 
have one or two substituents which are selected, independently of one 
another, from alkyl, hydroxyl or alkoxy, 
the R' radicals are, independently of one another, alkyl or alkenyl or, in 
formula III, may together be C.sub.1 -C.sub.5 -alkylene or C.sub.2 
-C.sub.5 -alkenylene, 
or a mixture of the compounds of the formulae II and III to an elimination 
reaction in the gas phase on an acidic heterogeneous catalyst. 
Alkyl (also in alkylaryl, alkoxy etc.) comprises straight-chain or branched 
alkyl groups which preferably have 1 to 12, in particular 1 to 6, carbon 
atoms. Examples are methyl, ethyl, n-propyl, isopropyl, n-butyl, 
sec-butyl, isobutyl, t-butyl, n-hexyl, 2-ethylhexyl and n-dodecyl. 
Aryl is, in particular, phenyl or naphthyl. 
Alkenyl comprises straight-chain or branched alkenyl groups preferably 
having 2 to 6 carbon atoms. Examples are allyl, methallyl, 2-butenyl and 
3-butenyl. 
Alkynyl comprises straight-chain or branched alkynyl groups preferably 
having 2 to 6 carbon atoms. Examples are 2-propynyl, 2-butynyl and 
3-butynyl. 
The preferred meanings in the above formulae are: 
R is H (m=0), alkyl or aryl, which is unsubstituted or substituted by one 
or two alkyl, hydroxyl or alkoxy radicals. R is particularly preferably H; 
m is 0 or 1; 
n is 2 to 5, in particular 2 to 4; and 
R' is alkyl or C.sub.1 -C.sub.5 -alkylene. 
It is particularly preferred to prepare 2-cyclopentenone and 
2-cyclohexenone respectively from 2-hydroxycyclopentanone C.sub.1 -C.sub.6 
-dialkyl ketal and 2-hydroxycyclohexanone C.sub.1 -C.sub.6 -dialkyl ketal 
or C.sub.1 -C.sub.6 -alkoxycyclopentanone and C.sub.1 -C.sub.6 
-alkoxycyclohexanone. 
It is preferred according to the invention to use an acidic heterogeneous 
catalyst which comprises at least one oxide or phosphate, or a mixture of 
oxides and phosphates, or mixed oxides of elements of groups 3 to 6 and 13 
to 15 of the Periodic Table, numbered according to their current IU 
recommendation. The acidic heterogeneous catalyst comprises in particular 
oxides of titanium, zirconium, aluminum, silicon, and/or phosphates of 
zirconium, lanthanum and cerium, particularly preferably TiO.sub.2, 
ZrO.sub.2, Al.sub.2 O.sub.3, SiO2 and LaPO.sub.4. 
The acidic heterogeneous catalyst is preferably additionally impregnated 
with one or more acidic compounds. Suitable acidic compounds comprise 
mineral acids such as phosphoric acid, nitric acid, sulfuric acid, 
hydrochloric acid, their condensed forms such as pyrophosphoric acid, 
pyrosulfuric acid, and their metal or ammonium salts. By metals are meant 
the elements of groups 1 to 14 of the Periodic Table, numbered according 
to the current IU recommendation, but especially the alkali metals and 
alkaline earth metals. The term ammonium stands for the protonated or 
alkylated form of ammonia or of a mono-, di- or trialkylamine, it being 
possible for the alkyl radicals independently of one another to have 1 to 
20, in particular 1 to 6, carbon atoms. The acidic heterogeneous catalyst 
may, where appropriate, be applied to a carrier. Suitable carriers are 
silicon dioxide (silica gel), aluminum oxide, silicon carbide, silicon 
nitride, graphite, zeolites and other solids which are inert under the 
reaction conditions. 
The amount of acidic compound is generally in the range from 1 to 70% by 
weight, preferably 5 to 50% by weight, based on the oxide or phosphate. 
The novel process is generally carried out continuously, in particular in a 
tubular reactor. The catalyst is moreover generally used in particulate 
form, e.g. in the form of extrudates, tablets or beads. Use as a fixed bed 
is preferred. 
The acidic heterogeneous catalysts used according to the invention are also 
subject to the usual gradual deactivation on use in gas-phase reactions, 
but they can advantageously be completely regenerated thermally in the 
reactor in the presence of atmospheric oxygen, preferably at temperatures 
above 300.degree. C., particularly preferably at temperatures above 
400.degree. C., so that regular operation of the reactor is ensured 
without time-consuming and costly replacement of the catalyst. 
It is possible in the novel process for the starting materials to be passed 
without solvent or together with the vapor of a solvent over the acidic 
heterogeneous catalyst. Examples of suitable solvents are water, alcohols, 
such as methanol, ethanol, isopropanol etc., ethers such as 
tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol 
dimethyl ether etc., and dimethylformamide and N-methylpyrrolidinone. The 
novel process is preferably carried out without the addition of solvents 
or in the presence of water. 
The novel process can be carried out in a wide range of temperatures and 
pressures. The skilled worker will choose them so that all the reactants 
and products and the solvent which is employed where appropriate are 
present in vapor form, and an adequate space-time yield is achieved. The 
novel elimination reaction is preferably carried out at a temperature in 
the range from 200 to 500.degree. C., particularly preferably in the range 
from 300 to 450.degree. C. The preferred pressures are in the range from 
10 mbar to 5 bar, and it is particularly preferably carried out under 
atmospheric pressure. 
It is possible to prepare the 2-alkoxycycloalkanones (II) and 
2-hydroxycycloalkanone ketals (III) required for this process in a variety 
of ways, for example by ketalization of the appropriate 
hydroxycycloalkanone derived, where appropriate, from an acyloin 
condensation (X. Creary, A. J. Rollin, J. Org. Chem. 1977, 42, 4231). 
An important advantage of the novel process is, however, that starting 
materials of the formulae II and III can easily be prepared in industrial 
quantities by an electrochemical reaction between the appropriate 
cycloalkanone and the appropriate alcohols. One process for this is 
disclosed in EP-A-460 451, which is incorporated herein by reference. 
The product obtained according to the invention may, if desired, be 
purified in a usual manner, e.g. by distillation. It is advisable to use 
fractional distillation. Depending on the boiling point of the product 
vacuum distillation is used. Generally, distillation is carried out at a 
pressure in the range of about 10 mbar to 1 bar. 
It has been found to be advantageous to separate off the water contained in 
the feed material during distillation by adding a suitable azeotrope 
forming agent, e.g. toluene. 
Prior to distillation possibly present acid traces may be neutralized with 
bases such as alkali hydroxides, alkali carbonates or alkali bicarbonates, 
e.g. sodium or potassium hydroxide, carbonate or bicarbonate. 
The following examples are intended to illustrate the novel process 
without, however, restricting the scope of the invention.

EXAMPLES 
Example 1 
Silicon dioxide impregnated with 20% by weight phosphoric acid was used as 
catalyst. The catalyst was in the form of 4 mm pellets. 0.1 mol/h nitrogen 
and 15 ml/h liquid 2-hydroxycyclohexanone dimethyl ketal were metered from 
above into a electrically heated silica glass tubular reactor which was 
arranged vertically and which contained at the bottom 10 ml of silica 
rings, then the catalyst (27.5 g) as pellets and subsequently a further 20 
ml of silica rings, at an internal temperature of 350.degree. C. The top 
layer of silica rings acted as vaporizer and caused negligible chemical 
reaction of the starting material. The gases emerging at the lower end 
were fed to a condenser operated with cooling water. The conversion of the 
2-hydroxycyclohexanone dimethyl ketal was&gt;99%. The condensate contained 
2-cyclohexenone in a yield of about 63% of theory. The 2-cyclohexenone was 
obtained with a purity&gt;99% after distillation. 
Example 2 
The process was carried out as in Example 1 but using in place of pure 
2-hydroxycyclohexanone dimethyl ketal a 1:1 mixture thereof with 
2-methoxycyclohexanone as starting material. The condensate contained 64% 
2-cyclohexenone.