Selective oxidation of alcohols with hexadecyl silica supported cupric nitrate

ELSEVIER

Reactive & Functional Polymers 29 (1996)193- 195

REACTIVE & FUNCTIONAL POLYMERS

Short Communication

Selective oxidation of alcohols with hexadecyl silica supported cupric nitrate Farajollah

Mohanazadeh

Department of Chemisq,

*, Shohreh

Ghamsari

Mazandaran Universit4: EO. Box 311, Babolsar; Iran

Received 23 June 1995; accepted in revised form 21 December 1995

Abstract Preparation of hexadecyl silica supported cupric nitrate is described. This oxidant, in carbon tetrachloride, alcohols to their corresponding carbonyl compounds. It oxidizes primary alcohols in the presence of secondary with absolute chemoselectivity in high yields. Keywords:

Hexadecyl

silica; Cupric nitrate; Alcohol; Aldehyde;

1. Introduction Oxidation of alcohols is an important process in organic chemistry. Endless interest in the development of new methods for the mild and selective oxidation of alcohols has prompted reports of many new monomeric and polymeric reagents which are effective in accomplishing this transformation [ 11. Advantages frequently claimed in favour of supported reagents compared with their homogeneous counterparts are increased yields, mild conditions, and clean and rapid reactions. Also, when polymers are used in combination with other reagents the intrinsic reactivity of the reagent can be well controlled to realize highly chemoselective reactions [2,3] and, in some cases, even the nature of reagent can be changed completely to present valuable new reactions [4,5]. Although the application of inorganic polymers with polar surface in organic synthesis [6] * Corresponding author.

oxidizes alcohols

Ketone; Oxidation

has been a subject of extensive investigation, no attention has been paid to these polymers in the form of hydrophobic surface. In this paper, we wish to report the preparation and use of hexadecyl silica supported cupric nitrate as the new reagent for selective oxidation of primary alcohols in the presence of secondary alcohols. 2. Result and discussion Hexadecyl silica was obtained from refluxing silica gel with hexadecanol in toluene according to the method reported in the literature [7]. After filtering, the white powder was Soxhlet extracted 18 hours with toluene and dried under vacuum. Carbon analysis of the product indicated the degree of functionalization to be 0.3 mmole of hexadecyl per gram of support. Hexadecyl silica-Cu(NOs)z was acquired from mixing of hexadecyl silica with Cu(NO3)2 in acetone and then, solvent was evaporated under reduced pressure at room temperature. The resulting powder was very stable at room temperature and

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& Functional Polymers 29 (1996) 193-195

Table 1 Oxidation of alcohols with hexadecyl silica-Cu(NOs)2 Entry

1 2 3 4 5 6 I 8 9 10 II 12 13 14 15 16 17 18 I9 20 21

Substrate

benzyl alcohol benzyl alcohol benzyl alcohol cinnamyl alcohol cinnamyl alcohol cinnamyl alcohol I-hexadecanol 1-hexadecanol I-hexadecanol

I-octanol 1-0ctanol

1-heptanol 2-phenylethanol cyclohexanol cyclohexanol cyclohexanol benzoin benzoin 1,3-dibenzoyloxy-2-propanol I-phenyl-I ,2-ethanadiol 4-( 1-hydroxyethyl)-benzyl alcohol

Solvent

cc14 CHC13 cc4

cc14 CHCI, cc14

cc4 CHCl3 cc14 cc4 CHC13 cc14 cc4 cc14 CHC13 cc14 cc14 cc4 cc14 cc4 cc14

Time a (min)

Yield b and recovery b procuct

substrate

5 5 30 5 5 30 15 15 60 I5 I5 15 15 180 180 60 600 360 540 I5 I5

95 80 90 95 85 92 80 60 50 80 53 15 95 90 76 96 55 86 45 90 95

5 20 IO 5 15 8 20 40 50 20 47 25 5 IO 24 4 45 14 55 10 5

c

c

c

c c c

a The course of reaction was checked by TLC. b Yield of isolated pure compound. c Nonfunctionalized silica gel.

could be stored as a bench top reagent for months without appreciable change in its reactivity. This reagent was used for oxidation of alcohols (Table 1). Primary alcohols were converted to aldehydes in high yields. Secondary alcohols reacted slowly with this oxidizing agent and their corresponding car-bony1compounds were produced in low yields. A comparison of the results presented in this article with those reported with silicaCu(N03)~ [S], clearly indicated that hexadecyl silica-Cu(NO& is a more effective reagent for primary alcohols than silica-Cu(NOs)z. For example, the reaction time for oxidation of benzyl alcohol with this reagent is 5 min (95%). This time for silica-Cu(NO& is 30 min (90%). It should be mentioned that the rate of oxidation of secondary alcohols with hexadecyl silicaCu(NO3)2 is slower than with silica-Cu(NOs)z. No overoxidation of the aldehydes to the corresponding carboxylic acids was observed. Nonpolar solvents such as CC14 or CHC13 are suitable for the reaction, but CC14 was better for

this purpose. However, the reactivity of unsupported cupric nitrate was found to be very low in CC14or CHC13.Oxidation of benzyl alcohol with Cu(N03)2 at boiling point of CC14 and CHCls during 2 h was, respectively, 8% and 9%. Selective oxidation of primary alcohol in the presence of secondary alcohol was demonstrated by the conversion of 1-phenyl- 1,2-ethanediol to 2-hydroxy-2-phenyl-1 -ethanal. Also, this selectivity was observed in conversion of 4-( lhydroxyethyl)-benzyl alcohol to 4-( 1-hydroxyethyl)-benzaldehyde. 3. Experimental ’H-NMR spectra were recorded on a Varian EM 360A NMR spectrometer using tetramethylsilane as the internal standard. IR spectra were taken on a Perkin-Elmer 267 spectrophotometer. Thin layer chromatography was performed on silica gel (Macerey-Nagel Co., Plygram Sil G/uv). All organic solutions were dried over MgSO4 or Na2S04 and solvents were removed under re-

E Mohanazadeh, S. Ghamsari/Reactive

duced pressure on a Buchi rotary evaporator. All oxidation products are known compounds, they were identified by comparison of their spectral (IR, NMR) and physical data with those of authentic samples. 3.1. Preparation of the reagent A solution of Cu(NO& (5 g, 26 mmole) in acetone (50 ml) was added to hexadecyl silica (10 g) and stirred at room temperature for 30 min. Solvent was then evaporated under reduced pressure at room temperature to give the supported reagent (1.7 mmole of nitrate/g of adsorbent), which was used for the oxidation of alcohols. 3.2. General procedure for oxidation qf alcohols A solution of alcohol (10 mmole) in CC14 or CHCls (50 ml) was treated with the reagent (11.7 g, 20 mmole). The heterogeneous reaction mixture refluxed for the indicated time. At the end

& Functional Polymers 29 (1996) 193-195

19.5

of reaction, the green-blue reagent was converted to a dark-yellow solid. During the course of the reaction cupric nitrate was converted to cuprous oxide. The reaction mixture was filtered. The filtrate was evaporated to leave the product. The crude product was chromatographed through a short column of silica gel to leave a pure carbonyl compound. References [II R.C. Larock, Comprehensive Organic Transformations: A Guide to Functional Group Preparations. VCH Publishers, Inc., New York, NY, 1989, p. 604. I21 Y. Kamitori, M. Hojo, R. Masuda, T. Inoue and T. Izumi, Tetrahedron Lett., 23 (1982) 4585. r31 Y. Kamitori, M. Hojo, R. Masuda, T. Izumi and T. moue, Synthesis, 5 (1983) 387. [41 M. Hojo and R. Masuda, Synthesis, 10 (1976) 678. [51 M. Hojo and R. Masuda, Tetrahedron Lett., 17 (1967) 613. I61 DC. Sherrington and P Hodge, Synthesis and Separations Using Functional Polymers. John Wiley and Sons Ltd, Chichester, 1988. [71 M. Baverez and J. Bastick, J. Bull. Chim. Fr., 12 (1965) 3662. @I T. Nishiguchi and F. Asano, J. Org. Chem., 54 (1989) 1531.