(Hive Bee)
01-08-04 19:07
No 481193
(Rated as: excellent)
For an excellent, general overview: Post 439780 (Aurelius: "Compilation on GHB/GBL", Methods Discourse)
J. Chem. Research (S), 2002, 538–539
No DOI found
M. Salavati-Niasari* and H. Najafian
Department of Chemistry, University of Kashan, Kashan, Iran
Abstract: Copper (II) Complexes of the 14-membered hexaaza-macrocycles; 1, 8-dimethyl-; 1, 8-diethyl-; 1, 8-dipropyl-; 1,8-dibutyliso-; 1, 8-dibutylsec- and 1, 8-dibenzyl- 1, 3, 6, 8, 10, 13-hexaazacyclotetra decane ([Cu{R2[14]aneN6}](ClO4)2) catalysed efficiently the selective oxidation of tetrahydroforan into the corresponding gamma-butyrolactone and a small amount of tetrahydrofuran-2-ol and 4-hydroxybutyraldehyde, using dil. H2O2 as the oxidant. The conversion of this catalytic oxidation process can reach 98.6% when [Cu{(Benzyl)2 [14] ane N6}](ClO4)2 is used.
Keywords: hexaazamacrocycle, oxidation, tetrahydrofuran, copper
One of the major current challenges in synthetic organic chemistry is the selective oxidation of organic compounds using O2, TBHP, H2O2 and PhIO as oxygen donors in the presence of transition metal catalysts.1 The selective oxidative functionalisation at the alpha C–H bond of ethers is one of the most useful reactions in organic synthesis, because it provides for the efficient preparation of esters (or lactones in the case of cyclic ethers).2 Such conversions are usually accomplished by the use of either stochiometric amounts of CrO3, Pb (CH3COO)4, and RuO3 as oxidants,3,4 or catalytic amounts of RuO4 in the presence of OCl– or IO4–.5 Most recently, several new oxidation systems have been described using transition metal complexes for the transformation of ethers to esters,6 tetrahydrofuran was also oxidised to the corresponding hydroxy-aldehyde with Co(II) porphyrin by employing a combination of dioxygen and 2-methylpropanal.7 Furthermore, Sudalai et al. have been reported that titanium silicates (TS-1, TS-2) catalysed efficiently the selective oxidation of tetrahydrofuran with H2O2 as the oxidant to gamma-Butyrolactone.8 In our recent publications, we reported the role of some transition metals and their complexes included within zeolite Y as catalysts.9–11 In both cases, it was observed that these catalysts were able to transfer oxygen from TBHP and H2O2 to substrate and hydroxylate the hydrocarbons. Recently, we found that air-stable iron (III) and manganese (II) bipyridine complexes included in zeolite Y and bentonite can easily catalyse the oxidation of tetrahydrofuran (THF) under H2O2 and TBHP as oxidant to give tetrahydrofuran-2-ol, tetrahydrofuran-2-one with minor amounts of 2,3- dihydrotetrafuran.10 In this study we synthesised several Cu (II) complexes with ligands of 14-membered hexaaza macrocycles and find that they catalyse the oxidation of tetrahydroforan with hydrogen peroxide.
In a typical procedure, a mixture of catalyst (0.2 g) and tetrahydrofuran (10 ml) was stirred in a 50 ml round bottom twonecked flask equipped with a condenser and dropping funnel for 30 min, under nitrogen atmosphere. Then 8 ml of the hydrogenperoxide (30% in H2O) was added through the dropping funnel. The mixture was then refluxed for 8 h. The reaction mixture was analysed by GC and GC–MS. The product yields were determined by GC analysis using naphthalene as internal standard. The solvent was removed under reduced pressure and the residue purified by chromatography to give gamma-butyrolactone (1), which was further confirmed by 1H NMR analysis, a small amount of the corresponding tetrahydrofuran-2-ol (2) and 4-hydroxybutyraldehyde (3) (Scheme 1).
All catalysts were also examined under the same reaction conditions and their conversions and selectivity are summarised in Table 1. It is clear that the conversion highly depends on the solubility of the catalyst used in THF, that is the homogeneous reaction phase gave a higher conversion. Soluble 1,9-dibenzyl-1, 3, 7, 9, 11, 15-hexaazacyclo hexadecane copper (II) in THF usually give a high conversion, but [Cu {(CH3)2 [14] ane N6}] 2+ and [Cu{[14] ane N6}] 2+ catalysts only give a low conversion.
Table 1 Substrate conversions and product selectivities in the oxidation of tetrahydrofuran with H2O2 in the presence of 14-membered hexaaza macrocyclic copper (II) complexes
| Catalyst | Conversion | Yield/% (1) | Yield/% (2) | Yield/% (3) |
| [Cu{[14] ane N6}](ClO4)2 | 54.2 | 68.7 | 18.4 | 12.9 |
| [Cu{(CH3)2[14] ane N6}](ClO4)2 | 58.6 | 76.5 | 16.7 | 6.8 |
| [Cu{(C2H5)2[14] ane N6}](ClO4)2 | 61.4 | 81.6 | 13.6 | 4.8 |
| [Cu{(butyliso)2[14] ane N6}](ClO4)2 | 71.5 | 84.3 | 10.5 | 5.2 |
| [Cu{(butylsec)2[14] ane N6}](ClO4)2 | 72.3 | 88.4 | 6.7 | 4.9 |
| [Cu{(benzyl)2[14] ane N6}](ClO4)2 | 98.6 | 100 | - | - |
Experimental
All the materials were of commercial reagent grade. Tetrahydrofuran was purified by standard procedures.12 Melting points were obtained with a Yanagimoto micro melting point apparatus and are uncorrected. Gas chromatographic analyses of products were performed on a Philips, Pu-4400 chromatograph, 1.5 m, 3% OV-17 column and GC-MS (Varian 3400 chromatograph, 25m CBP-5 column coupled with a QP 1100EX MAT INCOF 50, 70ev). 1H NMR spectra were determined for solution in CDCl3 with tetramethylsilane as internal standard on a Bruker AC 80. All the solid compounds reported in this paper gave satisfactory C, H, N microanalyses with a Perkin-Elmer Model 240 analyzer. The 1, 8-dimethyl-, and 1, 8-diethyl-1, 3, 6, 8, 10, 13-hexaaza cyclotetradecane copper (II) complex were prepared according to the literature.13
Synthesis
Caution: Some of compounds containing perchlorate anions must be regarded as potenially explosive and should be handled with caution.
1, 8-dipropyl, 1, 8-dibutyliso-; 1, 8-dibutylsec- and 1, 8-dibenzyl- 1, 3, 6, 8, 10, 13- hexaazacyclotetradecane copper (II) perchlorate were synthesised according to the following procedures.13 To a stirred methanol solution (75 ml ) of CuCl2*2H2O (4 g, 23 mmol) were slowly added 99% ethylenediamine (2.82 g, 47 mmol), 36% formaldehyde (9.4 ml) and RNH2 (R= pr, butyl iso, butyl sec, benzyl ) (46 mmol). The mixture was heated at reflux for 24 h until a deep blue-violet solution resulted. The solution was cooled to room temperature and filtered to remove copper hydroxide. Excess perchloric acid or lithium perchlorate dissolved in methanol was added to the filtrate, and the mixture was kept in the refrigerator until purple-red crystals formed. The crystals were filtered, washed with methanol, and air dried. The crystals were recrystallised from hot water.
[Cu{(pr)2 [14] ane N6}](ClO4): Yield: ~ 35 %. Anal. Calcd for CuC14H34N6Cl2O8: Cu, 11.58; C, 30.63; H, 6.24; N, 15.31. Found: Cu, 11.49; C, 30.51; H, 6.13; N, 15.42 %. IR (KBr): 3214 cm-1[sup] (íN-H). UV-Vis (CH3NO2): 496 nm (å = 80 M[sup]-1 cm-1[sup]) [Cu{(butyliso)2 [14]ane N6}](ClO4)2,Yield: ~ 46%. Anal. Calcd for CuC16H38N6Cl2O8: Cu, 11.01; C, 33.31; H, 6.64; N, 14.57. Found: Cu, 10.92; C, 33.21; H, 6.53; N, 14.63 %. IR (KBr) : 3228 cm[sup]-1 (íN-H) . UV-Vis (CH3NO2): 487 nm (å = 79 M-1 cm-1). [Cu{(Benzyl)2 [14] ane N6}](ClO4)2: Yield: ~ 59 %. Anal. Calcd for CuC22H34N6Cl2O8: Cu, 9.85; C, 40.97; H, 5.31; N, 13.03. Found: Cu, 9.79; C, 40.86; H, 5.27; N, 13.17 %. IR (KBr) : 3234 cm-1 (íN-H). UV-Vis (CH3NO2): 491 nm ( å = 83M-1 cm-1). gamma-Butyrolactane: íH (CDCl3) 2.26 (2H, q, J 6.6, CH2), 2.46 (2H, t, J 7.0, CH2) and 4.32 (2H, t, J 6.9 Hz, CH2).
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