ON THE MECHANISM OF ACID DECOMPOSITION OF PHENANTHRENE DIMETHOXY PEROXIDE

V. P. Shabrov, A. G. Pechenkin

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

At present, methods have been developed for obtaining 3,8-dimethoxy-4,5,6,7-dibenzo-1,2-dioxacyclooctane (dimethoxy peroxide) by ozonolysis of both pure and technical phenanthrene in methanol [1, 2]. The isolated dimethoxy peroxide must be subjected to decomposition to compounds that can be used as intermediates in organic synthesis. The methods described in the literature for the decomposition of dimethoxy peroxide using organic and inorganic bases provide an almost quantitative conversion of it into diphenic aldehydo acid or its methyl ester [3, 4].

The decomposition of dimethoxy peroxide under the influence of acids has not been described in the literature, although it is known that in industry such a method of decomposing organic peroxide compounds is widely used. A special study carried out by us showed that, in the presence of strong mineral acids, decomposition of dimethoxy peroxide occurs. Elucidation of the mechanism of the cleavage of dimethoxy peroxide that takes place was of particular interest to us, since knowledge of the latter can be successfully used for strictly directed and controlled decomposition of dimethoxy peroxide.

Cleavage of peroxides in such cases most often proceeds by a heterolytic mechanism, since the molecules of the starting substances are quite readily polarized. Considering the structure of the dimethoxy peroxide molecule as a disubstituted dioxiperoxide, we proposed that in polar solvents its decomposition, catalyzed by strong acids, may proceed according to the following scheme:

From the scheme presented for the acid mechanism of cleavage of dimethoxy peroxide it follows that first the proton attacks the peroxide oxygen atom, which in the polarized dimethoxy peroxide molecule causes rupture of the C—O bond with formation of the hydroperoxy carbocation (II). The carbocation formed readily reacts with nucleophilic reagents present in the reaction system. The hydroperoxy ...

group, under the action of acid, is converted through a carbocation into an aldehyde group, with liberation of a molecule of hydrogen peroxide.

If this mechanism is correct, the principal products of cleavage of dimethoxy peroxide in aqueous solutions of strong acids should be diphenic aldehyde and hydrogen peroxide. When aliphatic alcohols are used as the medium for carrying out the acid cleavage of dimethoxy peroxide, one should expect, in accordance with the proposed mechanism, that as a result of nucleophilic addition of alkoxanions to the carbocation, an acetal of diphenic aldehyde will be formed.

Indeed, when the cleavage of dimethoxy peroxide is carried out in aqueous solutions of strong mineral acids, diphenic aldehyde and hydrogen peroxide are obtained in almost quantitative yield, which served as the basis for our development of a new method for preparing diphenic aldehyde [5]. When the acid cleavage of dimethoxy peroxide is carried out in aliphatic alcohols, acetals are formed, which can be isolated from the reaction mixture, since they are fairly stable toward alkalis, whereas dimethoxy peroxide itself and the diphenic aldehyde that is formed are readily converted, upon alkaline hydrolysis, into diphenaldehydic acid and spirolactone.

The reaction of acid decomposition of dimethoxy peroxide requires, in order to proceed, the supply of heat from outside; this was used by us to determine the possibility of stopping the reaction at the intermediate stage of formation of hydroperoxide III, and also of carrying it out in the reverse direction, i.e., the synthesis of dialkoxy peroxide from hydroperoxide. The possibility of formation of dialkoxy peroxide from hydroperoxide is indicated in the works of Bailey [1, 3].

We carried out the acid cleavage of dimethoxy peroxide in ethanol, the reaction being deliberately stopped at the stage of formation of hydroperoxide III (which was monitored by the disappearance of dimethoxy peroxide), and it was observed that, owing to the presence of acid in the system, peroxide I is formed from hydroperoxide III, while the accompanying transesterification of the methoxyl group by ethanol proceeds, ensuring the preparation of diethoxy peroxide.

Since aldehydes with hydrogen peroxide form oxyhydroperoxides, i.e., intermediate products according to the proposed scheme for the decomposition of dimethoxy peroxide, we assumed that the reaction under study is completely reversible. To confirm this, the synthesis of dialkoxy peroxide was carried out starting from diphenic aldehyde, an alcohol, and hydrogen peroxide in the presence of sulfuric acid. The synthetic diethoxy peroxide formed in high yield proved to be identical with the sample obtained by the known procedure by ozonolysis of phenanthrene in ethanol [6].

The results obtained by us show that the investigated reaction of acid cleavage of dimethoxy peroxide can proceed both in the forward and in the reverse direction.

Thus, the data obtained from the study of the reaction of acid cleavage of dimethoxy peroxide and of the reversibility of this process confirm the proposed scheme of the reaction mechanism and make it possible to regulate this process.

Experimental Part

I. Acid cleavage of dimethoxy peroxide.

a) Hydrolysis in methanol.
Into a 50-ml flask are placed 3 g of dimethoxy peroxide; 10 ml of absolute methanol and 0.2 ml of H₂SO₄ \((d = 1.84\ \text{g/cm}^3)\) are added. The contents

the flask is thoroughly mixed, and then the resulting mixture is heated and kept at reflux until the dimethoxy peroxide has completely dissolved (about 80 min). To the resulting solution are added 10 ml of a 10% KOH solution, and after one hour of boiling the methanol is distilled off on a Vigreux column. The residue in the flask is diluted with 5 ml of water and, after cooling to 0°C, filtered through a layer of Al₂O₃ on a glass filter; then the flask and the precipitate are washed with another 5 ml of water. The filtrate obtained is acidified to pH 3, giving 1.72 g of a mixture containing 64% diphenic aldehydic acid and 36% diphenic hydroxy acid.

The acetal is washed from the filter with 10 ml of methanol, and then the mixture is heated for 10 min with 5 ml of a 1% HCl solution and 5 ml of a 10% KOH solution is added. After 20 min of boiling, the solution is filtered and, upon acidification, diphenic hydroxy acid is isolated. Yield 0.41 g, 16.5%; m.p. 134°C. Literature m.p. — 143°C [7]. Diphenic aldehydic acid is absent from the sample.

b) Decomposition of dimethoxy peroxide in ethanol.

Into a 50-ml flask are introduced 3 g of dimethoxy peroxide, 10 ml of ethanol, and 0.2 ml of H₂SO₄ ($d = 1.84$). The contents of the flask are rapidly heated to the boiling point of the alcohol and kept with stirring for 60 sec, during which the dimethoxy peroxide dissolves completely. The resulting solution gives a positive test for hydroperoxide with lead tetraacetate. It is cooled to room temperature, and after 16 h the precipitated crystals of diethoxy peroxide are filtered off. Yield 2.8 g, 84.8%; m.p. 151–153°C. Literature m.p. 163°C [6].

Calculated in % for C₁₈H₂₀O₄: C—72.00; H—6.70; O_act.—5.33.
Found in %: C—72.07; H—6.80; O_act.—5.35.

II. Synthesis of diethoxy peroxide from diphenic aldehyde.

Into a 50-ml flask are placed 3 g of diphenic aldehyde and 10 ml of ethanol. After complete dissolution of the aldehyde, 0.3 ml of H₂SO₄ ($d = 1.84$) is added to the mixture, then 1.5 ml of a 30% H₂O₂ solution, and the mixture is thoroughly stirred. The appearance of small crystals of diethoxy peroxide is observed immediately. After 16 h the precipitate formed is isolated by filtration, traces of sulfuric acid are washed out with alcohol, and the product is dried.

Yield 3.51 g, i.e., 81.7%, calculated on diphenic aldehyde. M.p. 156–158°C. Literature m.p. — 163°C [6].

Calculated in % for C₁₈H₂₀O₄: C—72.00; H—6.70; O_act.—5.33.
Found in %: C—72.43; H—6.86; O_act.—5.30.

Conclusions

1. The acid cleavage of dimethoxy peroxide in ethanol and methanol has been studied.

2. A scheme has been proposed for the heterolytic mechanism of cleavage of dimethoxy peroxide in polar solvents.

3. It has been shown that the acid-cleavage reaction of dimethoxy peroxide is reversible.

References

1. P. S. Bailey, S. B. Mainthia. J. Org. Chem., 23, 8, 1089, 1958.

2. L. P. Kulev, V. A. Salskii, A. K. Lebedev, V. P. Shabrov. Journal of the All-Union Chemical Society named after D. I. Mendeleev, 7, No. 5, 599, 1962.

3. P. S. Bailey, S. S. Bath. J. Am. Chem. Soc., 78, 12, 3811, 1956.

4. A. K. Lebedev, A. N. Novikov. Proceedings of the Tomsk Polytechnic Institute named after S. M. Kirov, 151, 311, 1966.

5. V. P. Shabrov, A. K. Lebedev, A. G. Pechenkin. Proceedings of the Tomsk Polytechnic Institute named after S. M. Kirov (in press).

6. P. S. Bailey. U.S. Patent 2861465 of September 9, 1958; RZhKhim, 9L1165, 1965.

7. M. G. Sturrock et al., J. Org. Chem., 28, 2340, 1963.