Mild and Efficient Strontium Chloride Hexahydrate-Catalyzed Conversion of Ketones and Aldehydes into Corresponding gem- Dihydroperoxides by Aqueous H2O2

SrCl2·6H2O has been shown to act as an efficient catalyst for the conversion of aldehydes or ketones into the corresponding gem-dihydroperoxides (DHPs) by treatment with aqueous H2O2 (30%) in acetonitrile. The reactions proceed under mild and neutral conditions at room temperature to afford good to excellent yields of product.


Results and Discussion
As part of our ongoing efforts to develop new methods for the synthesis of DHPs, we report here another new and highly efficient and inexpensive catalyst SrCl 2 ·6H 2 O to promote the synthesis of gem-DHPs from ketones and aldehydes employing aqueous H 2 O 2 (30%) at room temperature. To achieve suitable reaction conditions, i.e., lower reaction times and higher yields, for the conversion of the ketones and aldehydes into their corresponding DHPs, various Lewis acid catalysts and solvents were investigated using 3-pentanone, cyclohexanone, acetophenone, and benzaldehyde as test compounds at room temperature (Table 1). As can be seen in Table 1, the reaction worked best in terms of yield and reaction time with aqueous H 2 O 2 (30%) when SrCl 2 ·6H 2 O (10 mol %) was used as a catalyst. The other catalysts such as SbCl 3 , CeO 2 and CrCl 3 ·6H 2 O gave moderate to low yields while KF-Al 2 O 3 was found to be completely unsuitable for the synthesis of these DHPs. Effects of the solvents such as CH 2 Cl 2 , Et 2 O, MeCN and AcOEt on the yields of the products were tested and the results are summarized in Table 1. Acetonitrile appeared as a much better solvent compared with other ones. This suggests that solvent polarity plays an important role in the synthesis of DHPs.
This success encouraged us to extend these reaction conditions to a variety of cyclic and acyclic aliphatic ketones 1a-g using aqueous H 2 O 2 (30%) in the presence of 10 mol% amount of SrCl 2 ·6H 2 O as a chosen catalyst in acetonitrile at room temperature. The corresponding gem-dihydroperoxides 2a-g were produced in high to excellent yields (90-98%) within 3-12 h ( Table 2, Scheme 1). Similarly, aromatic ketones 1h-j and aromatic aldehydes 1l-p were converted to their corresponding gem-DHPs 2h-j and 2l-p in (45-68%) and (52-75%) yields respectively (Table 1). However, under the same reaction condition no conversion to gem-DHP was observed for benzophenone 1k and it was recovered almost intact after 12 hours. This can possibly be accounted for by the strong resonance stabilization and steric effects exerted by two phenyl groups.

Scheme 1. Synthesis of gem-dihydroperoxides 2a-g.
As previously reported by Rieche [40] and Žmitek et al. [41], we observed in the present protocol that simple, nonaromatic aldehydes such as octanal 3q and dihydrocinnamaldehyde 3r, which easily undergo hydration [42], reacted differently from the ketones and aromatic aldehydes. Under the same reaction conditions which converted ketones and aromatic aldehydes into their corresponding DHPs, both alkyl aldehydes-octanal 3q and dihydrocinnamaldehyde 3r-were not converted into their corresponding DHPs but instead into hydroxyl-hydroperoxides 4q and 4r in high yields (Table 1, Scheme 2), that is the addition of only one molecule of hydrogen peroxide to the carbonyl group has occurred. This implies that our protocol can furnish another hitherto unreported approach to 1,1-hydroxyhydroperoxides from aliphatic aldehydes.

General
Chemicals were obtained from Merck. FT-IR spectra were recorded on a Shimadzu 435-U-04 spectrophotometer (KBr pellets). 1

General procedure for synthesis of gem-dihydroperoxides
Caution: Peroxidic compounds are potentially explosive and require precautions in handling (shields, fume hoods, absence of transition metal salts and heating).
A mixture of carbonyl substrates 1 or 3 (1 mmol), 30% aqueous H 2 O 2 (3 mL) and SrCl 2 ·6H 2 O (0.1 mmol) in MeCN (4 mL) was stirred at room temperature for 3-10 h (Table 1). After the completion of the reaction, the mixture was diluted with water (5 mL), extracted with EtOAc (3 × 5 mL). The combined organic layer was washed with saturated aqueous sodium bicarbonate solution (3 mL), dried over anhydrous Na 2 SO 4 and concentrated in vacuo. The residue was purified by column chromatography (silica gel, hexane-EtOAc) to afford pure gem-dihydroperoxides 2 or hydroxyl-hydroperoxides 4 ( Table 1). The products were characterized on the basis of their physical properties and spectral ( 1 H-, 13 C-NMR and MS) analyses and compared with literature data [32,33,37,40,41]. The spectral ( 1 H-, 13 C-NMR and MS) data of some representative products are given below.

Conclusions
In summary, a new efficient homogeneous catalyst SrCl 2 ·6H 2 O has been shown to promote the synthesis of gem-dihydroperoxides from aliphatic and aromatic ketones and aldehydes using aqueous H 2 O 2 (30%) in acetonitrile at room temperature. The attractive features of this new approach are the readily available and non-expensive catalyst, high yields of the products, mild reaction conditions and the operational simplicity of the procedure.