INTRODUCTION:
Arenetellurinic acid anhydrides have been known since many years ago, their chemical properties have been little investigated.1 They are expected to have a potential : oxidizing ability like organotelluroxides2 and tellurones3 due to their similar labile Te-O bonds. Barton et al. reported4 that aryl tellurinic acid anhydrides were oxidizing agents towards thiol, hydroquinones etc. p-Methoxybenzenetellurinic acid anhydride is readily accessible by treatment of anisole with tellurium tetrachlorides,5 followed by alkaline hydrolysis of the resulting p-methoxy phenyltellurium trichloride.1 It is insoluble in usual solvents except acetic acid, but as an oxidation with it proceeds, it is reduced to readily soluble bis(p-methoxyphenyl) ditelluride. Arenetellurinic acid anhydrides have recently been used as mild and selective oxidizing agents.6,7
The readily accessible arenetellurinic anhydrides exhibit oxidizing properties similar, in many aspects, to those of the diaryl telluroxides.4,8,9
From competitive experiments performed to establish, the relative oxidizing capacity of some anhydrides and telluroxides, the following order was determined: (2naphthylTeO)2O>(p-MeOC6H4)2TeO>(p-MeOC6H4TeO)2O >(p-PhOC6H4TeO)2O.
Although arenetellurinic acids exhibit no catalytic activity on the epoxidation of olefins, the anchored tellurinic acids, prepared by condensation of tellurium tetrachloride with divinylbenzene-styrene copolymers, followed by hydrolysis of the TeCl3 group, catalyze such epoxidation.10
Kinetic data show that the epoxidation is accelerated with increasing alkyl substitution, as usually occurs with other electrophilic reagents. The oxidation is stereospecific with retention of the cis/trans geometry in the formed epoxide, the attack occurring mainly on the less hindered face (eg. 3-methylcyclohexene). The catalytic activity increases with the degree of cross-linking in the supporting copolymers.
Phenyltellurinyl trifluoroacetate adds to alkenes in the presence of ethyl carbamate giving p-(phenyltellurinyl) alkylcarbamates.14 However, if the reaction is effected in refluxing 1,2-dichloroethane, 2-oxazolidines, an important class of heterocyclic compounds with wide applications15 are formed in high yield.16-18 The reaction is regio and stereoselective, the initial amidotellurinylation proceeds by a Markovnikov addition via an anti opening of an epitelluride intermediate. The cyclization occurs via a back side attack of the carbonyl oxygen on to the carbon bearing the tellurium moiety, resulting therefore a net syn-addition to the olefin.19
The phenyltellurinyl trifluoromethanesulfonate PhTe(O)O3SCF3 is an outstanding reagent. Due to the high leaving ability of its counter groups, it is reactive in refluxing chloroform even in the absence of a Lewis acid.
The utility of the described amidotellurinylation can be further illustrated by the elimination of benzenetellurinic acid from the intermediate telluroxides to form amido olefins.
Non-terminal acetylenes, e.g. 2-nonyne, I-phenyl-l-popyne, and diphenylacetylene, are usually found to be inert towards tellurinic anhydrides. However, if the reaction is performed in the presence of catalytic amounts of sulfuric acid, diphenylacetylene is converted into benzil. Furthermore, it was found other more that tellurinic anhydrides in refluxing acetic acid20 catalyse the hydration of terminal acetylenes to ketones.
Allylbenzene reacts with arenetellurinic anhydrides in refluxing acetic acid furnishing 2-acetoxyalkyl (aryl) tellurium diacetates.21,22 It seems that the aryltellurinyl acetate is the actual active species and that the product is formed by the acetolysis of an intermediate epi-oxytelluronium intermediate. This reaction is therefore different from that effected in the presence of sulfuric acid which furnishes diacetoxylated products via a tellurenylation of the olefin.
When an alkene bearing an hydroxyl group at a suitable position is submitted to this procedure, an intramolecular reaction occurs leading to tellurinylated cyclic ethers. Due to its untractability the products are isolated as the corresponding tellurides after reduction with hydrazine hydrate.
The reaction requires a high temperature (110°C) and often competes with acetoxytellurinylation. However, the addition of Lewis acids such as diethyl ether-boron trifluoride complex or titanium (IV) chloride greatly enhances the reaction which is completed in 30 minutes in chloroform at room temperature. In accordance with the Baldwin rules, formation of five-membered rings is favored over four- and sixmembered rings. The obtained telluroethers can be converted to the tellurium free ethers by treatment with tributyltin hydride.
Recently, we have reported23 the use of p-hydroxyphenyl and 3-methyl-4-hydroxyphenyl tellurinic acid anhydrides as oxidizing agents towards some organic substrates.
Only eleven tellurinic acid, RTe(O)OH, are known. They have been prepared by hydrolysis of organyl tellurium trichloride or oxidation of diorganyl tellurides. Tellurinic acid chloride, bromides, iodides, nitrates and anhydrides RTe(O) X [X= CI, Br, I, NO3,RTe(O)O] comprise known derivatives.
(1) Oxidation of dioranyl tellurides to tellurinic acid
(2) Hydrolysis of organyl tellurium trihalides by water
(3) Hydrolysis of organyl tellurium trihalides in alkaline medium
(4) Disproportionation of tellurenyl compounds in sodium hydroxide solution.
(5) Oxication of diorganyl ditellurides by nitric acid chlorides and nitrates
(6) Nitration of aromatic tellurinic acids
(7) Hydrolysis of tellurinic acid chlorides and nitrates
p-Hydroxyphenyltellurium(IV) Trichloride36-37
TeCl4 (7.5 g, 27.8 mmol) and phenol (10 g, 106.4 mmol) in carbon tetrachloride (150 ml) were refluxed in an atmosphere of dry N2 for about 12 h until evolution of HCl ceased. The amount of HCl evolved corresponded to the loss of one equivalent of chlorine atoms per mole of TeCl4. The product was filtered, washed thoroughly with carbon tetrachloride to remove excess of phenol and then with benzene to remove any unreacted TeCL4. It was recrystallized from acetonitrile to give a yellowish green solid.
Analysis, % found (calculated): Te = 38.88 (38.95), CI = 32.07 (32.56), C = 21.85 (22.03), H = 1.64 (1.53), Melting point = 221-224°c (lit.6 m.pt. 222°C)
3-Methyl-4-hydroxyphenyltellurium (IV) Trichloride36,37
TeCl4 (5.0 g, 18 mmol) and a-cresol (4.0 g, 37 mmol) in 100 ml of carbon tetrachloride were refluxed on a water bath under an atmosphere of dry N2 for about 12 h until the evolution of HCl ceased. The amount of HCl liberated corresponded to the loss of one equivalent of chlorine per mole of TeCI4. The product was filtered and washed thoroughly with carbon tetrachloride and benzene to remove excess of cresol and TeCl4, respectively. It was recrystallized from acetonitrile to give a yellow product.
Analysis, % found (calculated): Te = 37.10 (37.40), CI = 31.00 (31.20), C = 24.60 (26.60), H = 2.10 (2.10), Melting point = 199-202 °c (lit.6 m. pt. 201°C)
p-Hydroxyphenyltellurinic Acid Anhydride36,37
A 10% aqueous NaOH solution (100 ml) was slowly added to a vigorously stirred solution of p-hydroxyphenyl tellurium (IV) trichloride (5 g) in THF (50 ml). The mixture was stirred at room temperature for 8 hours and then the THF was distilled off. The cooled colourless mixture was acidified with acetic acid and the precipitate filtered and dried to afford p-hydroxyphenyl tellurinic acid anhydride as a white crystalline powder.
Analysis, % found (calculated): Te = 37.24 (37.02),
C = 42.54 (43.10), H = 3.45 (3.79)
Melting point = 215-220 °c (lit.37 m. pt. 217°C)
3-Methyl-4-hydroxypbenyltellurinic Acid Anbydride36
A 10% aqueous NaOH (100 ml) solution was slowly added to a vigorously stirred solution of 3-methyl-4-hydroxyphenyl tellurium (IV) trichloride (12 g) in THF (50 ml). The mixture was stirred under reflux for 5 hours and the THF was distilled off. The precipitate was filtered from the cooled mixture, washed with H2O and dried in vacuo to afford 3-methyl-4-hydroxyphenyl tellurinic acid anhydride.
Analysis, % found (calculated): Te = 36.10 (36.92), C = 43.45 (43.94), H = 3.97 (3.21)
Melting point = 280-285 °c (lit.6 m. pt. 282°C) .
p-Hydroxyphenyl Tellurinic Acid Anhydride, (RTeO)2O, As Oxidizing Reagent
Oxidation of Benzoin: Formation of Benzil
Yield = 80%
m. pt. 95-95°C (lit.41 m.pt. 95°C)
Oxidation of 4-Methoxybenzylalcohol: Formation of 4-Methoxybenzaldehyde (Anisaldehyde):
4-Methoxybenzylacohol (0.276 g, 2 mmo!) and p-hydroxyphenyl tellurinic acid anhydride (0.504 g, 2 mmol) were refluxed in xylene (5 mL) for 22 h under nitrogen atmosphere. The reaction was continued till the completion of the reaction, as monitored by TLC. After the completion of the reaction, the contents were filtered and evaporation of filtrate followed by chromatographic separation (Si02, hexane / toluene (1:1) from a small amount of p-hydroxyphenyl ditelluride, gave purple crystal of p-anisaldehyde in 75% yield whose identity was confirmed by comparison with authentic sample.38-40
Yield : 75%
m.pt. 245-247 °C (lit.42 m.p. 246 C)
Oxidation of 4-Nitrobenzylalcohol: Formation of 4-Nitrobenzaldehyde
4-Nitrobenzylalcohol (0.306 g, 2 mmol) and p-hydroxyphenyl tellurinic acid anhydride (0,504 g, 2 mmol) were refluxed in toluene (5.mL) for 24 h under nitrogen atmosphere. The reaction was continued till the completion of the reaction, as monitored by TLC. After the completion of the reaction, the contents were filtered and evaporation of filterate followed by chromatographic separation (Si02, hexane/toluene (1:1) from a small amount of p-hydroxyphenyl ditelluride, gave 4-nitrobenzaldehyde in 82% yield. Its structure was confirmed by direct comparison with authentic sample.38-40
Yield : 82%
m.pt. 105-107 °C (lit.43 m.p. 106 °C)
IR (KBr, cm-1). 1680 (C=O), 1535 (NO2)
1H NMR (CDCl3, ppm), 8. (d, 2H), 8.39 (d, 2H), 10.1 (s, 1H)
Oxidation of Triphenylphosphine: Formation of Triphenylphosphine oxide
Triphenylphosphine (0.261 g, I mmol) in dichloromethane (4 mL) and p-hydroxyphenyl tellurinic acid anhydride (0.504 g, 2 mmol) were stirred at room temperature for 24 h under nitrogen atmosphere. The reaction was continued till the completion of the reaction, as monitored by TLC. After the completion of the reaction.
Oxidation of Thiophenol: Formation of Diphenyl disulfide
Yield : 72%
m.pt. 72 -74°C
Oxidation of Triphenylphosphine: Formation of Triphenylphosphine oxide
Yield : 82%
m.pt. 154 to 156°C
Oxidation of Hydroquinone: Formation of p-Benzoquinone
Yield : 85%
m.pt. 113 to 115°C
3-Methyl-4-Hydroxyphenyl Tellurinic Acid Anhydride, (RTeO)2O, As Oxidizing Reagent
Oxidation of Benzoin: Formation of Benzil
Yield : 90%
m.pt. 94-95°C
Oxidation of 4-Methoxybenzylalcohol : Formation of 4-Methoxy-benzaildehyde (p-Anisaldehyde)
Yield : 85%
m.pt. : 245-247°C
Oxidation of 4-Nitrobenzylalcohol: Formation of 4-Nitrobenzaldehyde
Yield : 95%
m.pt. : 105-107°C
Oxidation of Thiophenol: Formation of Diphenyl disulphide
Yield: 85%
m.pt. : 72-74°C
Oxidation of Triphenylphosphine: Formation of Triphenylphosphine oxide
Yield : 90%
m.pt. : 154-156°C
Oxidation of Hydroquinone: Formation of p-Benzoquinone
Yield: 90%
m.pt. : 113-115°C
RESULTS AND DISCUSSION:
Phenol and a-cresol appear to undergo Friedal-Crafts type condensation reaction with tellurium tetrachloride whereby the TeCl3 + unit attacks a position para to a hydroxyl group in the aromatic ring. The formation of hydroxyaryltellurium (IV) trichlorides can . be represented as
TeC4 + R-H RTeCl3 + HCI
(R - H = phenol, a-cresol)
These hydroxyaryltellurium (IV) trichlorides upon alkaline hydrolysis yield the corresponding tellurinic acids:
RTeCl3 + 3 NaOH RTe(O)OH + 3 NaCI + H2O
These tellurinic acids, upon dehydration give the tellurinic acid anhydrides.
2 RTe(O)OH (RTeO)2O + H2O
These readily accessible arenetellurinic anhydrides i.e. p-hydroxyphenyl-tellurinic and 3-methyl-4-hydroxyphenyltellwimic anhydrides have been investigated for their oxidizing properties towards six organic substrates viz. benzoin, 4-methoxybenzyl alcohol, 4-nitrobenzylalcohol, thiophenol, triphenylphosphine and hydroquinone.
These hydroxyaryltellurinic anhydrides should exhibit a certain degree of basic character by virtue of polar nature of tellurium-oxygen bonds.
It has been observed that oxidizing property is more in case of 3-methyl-4-hydroxyphenyltellurinic acid anhydride as compared to p-hydroxyphenyl tellurinic acid anhydride. Also a comparison of oxidizing property of these anhydrides with those of corresponding telluroxides, show these acid anhydrides to be weaker oxidizing agents than the respective telluroxides.
CONCLUSION
p-Hydroxyphenyl and 3-methyl-4-hydroxyphenyl tellurinic anhydrides have been obtained by alkaline hydrolysis of corresponding hydroxyaryltellurium (IV) trichlorides, which in turn were obtained by direct reactions of tellurium tetrachloride with phenol and a-cresol, respectively.
These two tellurinic acid anhydrides have been investigated for their oxidizing property towards six organic substrates. They oxidize benzoin to benzil, 4-methoxy and 4-nitrobenzylalcohol to corresponding benzaldehydes, thiophenol to diphenyl disulphide, triphenylphosphine to triphenylphosphine oxide and hydro quinone to p-benzoquinone. It has been observed that 3-methyl-4-hydroxyphenyl tellurinic anhydride is a better oxidizing agent than p-hydroxyphenyl tellurinic anhydride in these reactions. Also, the tellurinic anhydrides in general, are poor oxidizing reagents compared to the corresponding telluroxides.
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Received on 10.12.2012 Modified on 15.12.2012
Accepted on 18.12.2012 © AJRC All right reserved
Asian J. Research Chem. 5(12): Dec., 2012; Page 1452-1456