Terminal olefins are oxidised to ketone in high yields under mild conditions using palladium chloride and cyclodextrins as catalysts in two-phase systems; cyclodextrins show substrate selectivity.

Cyclodextrins (CDs) form inclusion compounds with various organic molecules¹ and they have been extensively studied as a model of enzymes². However, in most of the reactions reported CDs are not catalysts and so their applications to organic syntheses have been limited. Previously we reported that CDs form inclusion compounds with some organometallic complexes³. Now we found a system in which CDs function as catalysts and show marked substrate selectivity. Here we report the first example of the use of CDs as inclusion catalysts for effecting reactions catalysed by transition metal complexes in aqueous-organic two-phase systems.

We have examined the behaviour of CDs in the oxidation of terminal olefins to the corresponding methyl ketones. Although ethylene can be oxidized to acetaldehyde by PdCl₂ in an aqueous system (Wacker oxidation), higher alpha-olefins are oxidized to methyl ketones only very slowly or not oxidized in entirely aqueous solutions of PdCl₂.

Now we have found that on addition of CD into this system, terminal olefins are smoothly oxidized under mild conditions to give the corresponding methyl ketones in high yields and that cyclodextrins show substrate selectivity.

In a typical experiment, dec-1-ene (25 mmol) was added to an aqueous solution of alpha-CD (1.0 mmol), PdCl₂ (1.0 mmol), and CuCl₂(10 mmol) at 75 °C. When oxygen was bubbled though an aqueous solution and stirred vigoursly, decan-2-one was obtained in 76% yield. No oxidation products were obtained without alpha-CD.

The results on the oxidation of some long-chain olefins by the present catalytic system are summarized in Table 1 and illustrated in Fig. 1, which shows that the yield of ketone depends strongly on the substrate employed and high yields are obtained with the substrate having C₈-C₁₀ structures. The yield suddenly drops when the carbon number of the substrate exceed ten, indicating that the PdCl₂ system shows high substrate selectivity. Essentially no reations of internal olefins were observed under the present conditions.

Fig. 1 [I'll just give a data set - I'll try to post a pic soon :)] Effects of carbon number on the yields of ketones. Open system, 70 °C, 8 h. Closed system, 60 °C, 8 h, 1 atm.

It should be noted that alpha-CD is effective as catalyst and it can be recovered simply by cooling the solution and can be reused. Beta-CD is also effective as the catalyst and shows similar selectivity. The metal catalyst can be reused without any reduction in yields. Oxygen can be replaced by air.

Recently, Alper et al. reported that the reaction was carried in two-phase systems with quaternary ammonium salts as phase transfer catalysts and CCl₄ as solvent⁴. In our system isolation of the products from catalyst or solvent is quite easy because no organic solvents are required, and the CD and PdCl₂ catalysts are soluble only in an aqueous phase.

One possible pathway of the reaction may be the initial formation of CD complexes with substrates. CD extracts substrates from the organic phase allowing them to form complexes with PdCl₂. Then the substrates undergo oxidation to yield ketones. Unlike with the phase transfer catalysts generally used, such as quarternary ammonium salts, the reaction proceeds in the aqueous phase in our system.

The results show that this new system consisting of cyclodextrin and PdCl₂ is effective and selective catalysts for oxidation of long-chain alpha-olefins to ketones in a two-phase system and that separation of the products from catalysts or solvents is quite easy.^5,6