Catalytic cleavage of methyl oleate or oleic acid

Introduction
The cleavage of oleic acid via ozonolysis is the industrially applied process for the synthesis of azelaic acid from renewables. Alternatives are in demand due to safety concerns of ozone handling. Recently the authors reported on the epoxidation of methyl oleate with molecular oxygen in the presence of aldehydes [1]. Now the cleavage of oleic acid or methyl oleate using this system and additionally OsO4 or potassium osmate as catalysts is presented.

Experimental
In a typical experiment, 2 mmol substrate, 7.5 mmol aldehyde, 0.04 mmol OsO4 or K2OsO4•2H2O and 50 mg azobisisobutyronitrile were placed in a 100 ml Buechi glass autoclave and were dissolved in 20 ml of the appropriate solvent. Then the autoclave was pressurized with 4 bar O2 and stirred at 70-90°C for 1-4 hours. The progress of the reaction was controlled by GC-MS.

Results
Monomethyl azelate 4 (R=CH3) and pelargonic acid 5 were the main products in the Os-catalyzed oxidation of methyl oleate 1 (R=CH3) with O2/aldehyde besides varying amounts of the epoxide 2 and the diol 3, whereas azelaic acid (4, R=H) was solely obtained from oleic acid (1, R=H). However, a comparative application of RuO4 or RuO2 instead of Os-catalysts did not lead to cleavage products.
The simultaneous formation of the cleavage products 4 and 5 as well as of the epoxide 2 and the diol 3 is interpreted mechanistically as parallel reactions (route A-C), this assumption was supported by monitoring the course of reaction. Epoxide 2 seems to be evolved according both to a radicalic and non-radicalic pathway. The formation of 2 in 42% yield by oxidation of methyl oleate in the presence of 2,6-di-tert.-butyl-4-methyl-phenol argues for a non-radicalic mechanism, probably peracid is generated from the aldehyde via Os-catalysis. A further conversion of 2 under these conditions did not lead to 3 but only to a minor degree to 9- or 10-keto derivatives.

Fig. 1. C=C cleavage of oleic acid or methyl oleate
The diol 3 originated supposably by direct Os-catalyzed dihydroxylation of 1 using in situ-formed peracid. This guess was drawn on the increase in yield of 3, if the reaction conditions were adapted to known optimal dihydroxylation conditions (e.g. addition of diazabicyclooctane, solvent DMF or MeCN/H2O/ethyl acetate). A possible acid-catalyzed cleavage of 2 to 3 or of 3 to 4 and 5 was found to a minor degree. The direct cleavage of the double bond of 1 to 4 and 5, most likely via a glycolate complex, was also presumed to be non-radicalic due to significant amounts of products, even under application of a radical scavenger. Also in that case, the reaction of O2 with the aldehyde to peracid is likely. On the basis of the experimental results, the reaction pathways B and C to 4 and 5, discussed in Figure 1, seem not to be the preferred routes at least. In summary, the obtained yields of 5 (R=CH3) amounted to 50-70%. Suitable solvents were acetone or dichloromethane, more polar or aqueous-organic mixtures decreased the yield.
If oleic acid was used as substrate under equal reaction conditions, surprisingly only the formation of 4 and 5 in about 50 % yield (non-optimized) was observed.
The authors gratefully acknowledge the Federal Ministry of Education and Research (BMBF) for financial support (FKZ 22003704).

[1] A. Köckritz, M. Blumenstein, A. Martin, Eur. J. Lipid Sci. Technol. 110, 581-586 (2008).