Such a method is described by Semmelhack, M. F. et al. (J. Am. Chem. Soc., 106, pp. 3374 - 3376 (1984)). In this method copper (I) chloride and oxygen are used in, for example, dimethylformamide (DMF) as solvent. The method described is suitable for oxidizing benzylic and allylic alcohols. This publication also describes the use of TEMPO. It is reported that apart from the very easily oxi- dizable p-methoxybenzyl alcohol, most alcohols require the catalysts to be present in a concentration of minimally 5 - 10 mol. %.

The method described has the disadvantage that only easily oxidizable alcohols are specifically oxidized to form an aldehyde (or ketone). This method is not suitable for alcohols that are more difficult to oxidize, such as aliphatic alcohols (which, if they comprise a double bond, do not have the hydroxyl group on an allylic carbon atom) and, for example, cyclic alcohols having diverse func- tional groups, e. g. steroid alcohols and saccharide deri- vatives.

It is the object of the present invention to pro- vide a method of the kind mentioned in the preamble, by which a variety of primary and secondary alcohols can be oxidized to form aldehyde or ketone respectively, includ- ing alcohols that are difficult to oxidize, e. g. cyclic alcohols having diverse functional groups, e. g. steroid alcohols and saccharide derivatives.

To this end the method according to the invention is characterized in that the alcohol is contacted with catalytic amounts of i) a ruthenium ion; and ii) a sub- stantially stable N-0 free radical compound as the nitro- xylic compound wherein two atoms bound to the nitrogen atom are not themselves hydrogen carriers.

Surprisingly, it has been shown that this object can be achieved by combining a ruthenium ion on the one hand and a substantially stable N-0 free radical compound on the other hand. It is also surprising that with this combination double bonds are not oxidized. Ruthenium can occur in various states of oxidation. It can be conveni- ently added in the form of a salt, such as RuCl3. A ruthenium dichloride tris (triphenylphosphine) complex (RuCl2 (PPh3) 3) appears to be even more effective. The required amount of ruthenium catalyst in relation to alco- hol is small, and is, for example, less than 4%, e. g. less than 2%. In most case, the radical will be present in sub- stantially equimolar amounts in relation to the ruthenium catalyst. Conversion is conveniently effectuated at elev- ated temperatures, e. g. at minimally 40°C, preferably at minimally 60°C, more preferably at 80°C, and most prefer- ably at minimally 100°C. As will become apparent from the accompanying examples, no (detectable amount of) carbo- xylic acid is formed.

According to a preferred embodiment, the free rad- ical compound has the general formula I: wherein A represents a 5-8-membered ring having in addi- tion to the nitrogen atom 0-3 additional hetero-atoms selected from the group consisting of N, 0, and S; R1 and R2, independently of each other, represent an alkyl group having 1-3 carbon atoms, which alkyl group may optionally be substituted with 1-3 halogen atoms selected from fluorine and chlorine; or a halogen atom selected from fluorine and chlorine;

R1'and R2', independently of each other, have the same meaning as R1 and R2, represent a bond, or are part of a 5-8-membered ring fused with the A ring, which 5-8- membered ring may comprise 1-2 hetero-atoms selected from oxygen, nitrogen and sulphur, or which together with R1 and R2 respectively, represent a =0 or =S group, such as suitably TEMPO.

Such free radical compounds have been shown to be very effective.

According to an alternative embodiment the free radical compound comprises an anion of Fremy's salt or a derivative thereof.

Such a free radical compound is particularly suit- able for the oxidation of alcohols in an aqueous environ- ment. Suitable derivatives are again those compounds of which one SO3 group is replaced by a residue with at least an atom selected from carbon, nitrogen, and sulphur, and bound to the nitrogen atom, and wherein the atom of the residue bound to the nitrogen atom does not carry a hydro- gen atom. The residue could, for example, be a CF3 group.

The present invention will now be elucidated with reference to the following exemplary embodiments.

In all the experiments described the yield and sel- ectivity were measured. For the Examples 1 through 9 this was performed with the aid of a Varian 3400CX GC apparatus equipped with a Chrompack CP-WAX 52 CB column (50 m x 0.53 mm). For the Examples 10 through 15 a Varian 3600 GC appa- ratus was used equipped with a Chrompack CP-Sil 5 CB col- umn (50 m x 0.53 mm). Detection was achieved by means of flame ionisation measurement.

Example 1: Preparation of octanal 1.95 g (15.0 mmoles) of 1-octanol, 37.9 mg (0.15 mmoles Ru) of Ruz'3 hydrate (Johnson Matthey P. L. C., Royston, U. K.) and 70.3 mg (0.45 mmoles) of TEMPO were dissolved in 30 ml chlorobenzene. The solution was heated under stirring to 100°C and kept for 7 hours under a 100% oxygen atmosphere (1 bar). After cooling to ambient tem-

perature, the reaction mixture was analyzed by means of GC.

Example 2: Preparation of octanal The method of Example 1 was repeated but RuCl3 hydrate was replaced by 114 mg (0.15 mmoles) of ruthe- niumdichloride tris (triphenylphosphine) complex (RuCl2 (PPh3) 3).

Control experiment 3: Preparation of octanal The method described in Example 2 was repeated but without TEMPO.

Control experiment 4: Preparation of octanal The method described in Example 2 was repeated but in the absence of RuCl2 (PPh3) 3.

Control experiment 5: Preparation of octanal The method described in Example 2 was repeated but in the absence of both RuCl2 (PPh3) 3 and TEMPO.

Example 6: Preparation of 2-octanon 1.95 g (15.0 mmoles) of 2-octanol, 144 mg (0.15 <BR> <BR> <BR> mmoles) of RuC12 (PPh3) 3 and 70.3 mg (0.45 mmoles) of TEMPO were dissolved in 30 ml of chlorobenzene. The solution was transferred to a high-pressure reactor and heated under stirring to 100°C and the pressure raised to 10 bars. For 7 hours an 8% by volume of oxygen/nitrogen mixture was injected (10 ml/min). The reaction solution was subsequently cooled to room temperature and analyzed by means of GC.

Example 7: Preparation of 2-octanon 144 mg (0.15 mmoles) of RuCl2 (PPh3) 3 and 70.3 mg (0.45 mmoles) of TEMPO were dissolved in 30 ml (188 mmoles) of 2-octanol. Oxidation was carried out in the same manner described in Example 6.

Example 8: Preparation of cyclooctanon 2.57 g (20.0 mmoles) of cyclooctanol, 192 mg (0.20 <BR> <BR> <BR> mmoles) of RuCl2 (PPh3) 3 and 93.8 mg (0.60 mmoles) of TEMPO were dissolved in 40 ml of chlorobenzene. The composition was prepared in the same manner described in Example 6.

Example 9: Preparation of 2-adamantanon The method described in Example 8 was repeated but cyclooctanol was replaced by 3.05 g (20.0 mmoles) of 2- adamantanol.

Example 10: Preparation of benzaldehvde The method described in Example 6 was repeated but 2-octanol was replaced by 1.62 g (15.0 mmoles) of benzyl alcohol. The reaction time was 5 hours instead of 7 hours.

Control experiment 11: Preparation of benzaldehyde The method of Example 10 was repeated but in the absence of TEMPO.

Control experiment 12: Preparation of benzaldehvde The method of Example 10 was repeated but in the absence of the ruthenium catalyst.

Control experiment 13: Preparation of benzaldehyde The method of Example 10 was repeated but in the absence of both ruthenium catalyst and TEMPO.

Example 14: Preparation of p-methoxybenzaldehyde 2.76 g (20.0 mmoles) of p-methoxybenzyl alcohol, 192 mg (0.20 mmoles) of RuCl2 (PPh3) 3 and 93.8 mg (0.60 mmoles) of TEMPO were dissolved in 40 ml chlorobenzene.

The solution was transferred to a high-pressure reactor and heated under stirring to 100°C and the pressure raised to 10 bars. For 3 hours an 8% by volume oxygen/nitrogen mixture was injected (10 ml/min). The reaction solution was subsequently cooled to room temperature and analyzed by means of GC.

Example 15: Preparation of p-nitrobenzaldehvde 3.06 g (20.0 mmoles) of p-nitrobenzyl alcohol, 192 <BR> <BR> <BR> mg (0.20 mmoles) of RuCl2 (PPh3) 3 and 93.8 mg (0.60 mmoles) of TEMPO were dissolved in 40 ml chlorobenzene. The compo- sition was further prepared as described in Example 10, with the difference that the oxygen/nitrogen mixture was injected for 7 hours.

Example 16 : Preparation of acetophenone The method of Example 15 was repeated, with the difference that 2.45 g (20.0 mmoles) of 1-phenyl ethanol was used instead of p-nitrobenzyl alcohol.

Example 17: Preparation of 3-methyl-2-butenal The method of Example 10 was repeated but benzyl alcohol was replaced by 1.29 g (15.0 mmoles) of 3-methyl- 2-butenol.

All GC data from the above-mentioned experiemnts are summarized in the table below.

T A B L E

Expriment No. Yield % Selectivity % 1 52 100 2 70 100 02 ~ 41 0 5¹ 0 - 6 75 100 7 6, 5 100 65 100 9 92 100 10 100 100 11¹ 12 100 12¹ 0 - 31 O0 - 14 100 100 15 68 100 16 91 100 0 100 17 50 100 1 control experiment ² In this experiment 0,45 mmol of dioctylether was formed (yield 6%)