Selective catalytic oxybromination of hydroxyaromatic compounds
A method for selectively brominating hydroxyaromatic compounds is disclosed. Hydroxyaromatic compounds are contacted with oxygen and a bromine source, in an acidic medium, in the presence of a catalyst selected from the group of compounds and mixtures of compounds of Group IV–VIII transition metals of the Periodic Table of Elements. The selectivity of mono-brominated products produced, predominantly in the para-position, from the method using transition metal compounds as catalysts is significantly higher than that of known methods using other catalysts. Thus, there is a significant reduction in the production of undesirable dibrominated and more highly brominated by-products.
1. A method for preparing a brominated hydroxyaromatic compound which comprises contacting a hydroxyaromatic compound with oxygen and a bromine compound selected from the group consisting of hydrogen bromide, elemental bromine, ionic bromide salts, and mixtures thereof, in a polar solvent and an acidic medium, in the presence of a catalyst selected from the group of compounds and mixtures of compounds of Group IV–VIII transition metals of the Periodic Table of Elements.
2. The method of claim 1 , wherein said Group IV–VIII transition metals are selected from the group consisting of vanadium, titanium, molybdenum, tungsten, and iron.
3. The method of claim 1 , wherein said catalyst is selected from the group consisting of sodium metavanadate, bis(acetylacetonate)oxovanadium, bis(acetylacetonate)oxotitanium, sodium molybdenum oxide dihydrate, iron bromide (FeBr 2 ), tungstic acid (H 2 WO 4 .xH 2 O), and mixtures thereof.
4. The method of claim 1 , wherein said catalyst comprises a compound of vanadium in the form of a neutral complex, cationic salt, or anionic salt.
5. The method of claim 1 , wherein said catalyst comprises a mixture of a compound of vanadium and a compound of molybdenum or tungsten.
6. The method of claim 5 , wherein a molar ratio of said compound of vanadium to said compound of molybdenum or tungsten ranging from about 1:0.5 to about 1:6 is employed.
7. The method of claim 1 , wherein a nitrate salt is added to said catalyst.
8. The method of claim 7 , wherein said nitrate salt is sodium nitrate.
9. The method of claim 7 , wherein said catalyst is a compound of vanadium.
10. The method of claim 7 , wherein a molar ratio of said nitrate salt to said catalyst ranging from about 1:1 to about 1:4 is employed.
11. The method of claim 1 , wherein said acidic medium is anhydrous.
12. The method of claim 11 , wherein said bromine compound is anhydrous hydrogen bromide or an anhydrous ionic bromide salt, and wherein water is added to said anhydrous acidic medium, and wherein a molar ratio of water to anhydrous hydrogen bromide or anhydrous ionic bromide salt ranging from about 0.1:1 to about 2:1 is employed.
13. The method of claim 1 wherein the hydroxyaromatic compound has the formula
wherein each R is independently hydrogen or C 1-4 alkyl.
14. The method of claim 1 , wherein said hydroxyaromatic compound is phenol, o-cresol, or m-cresol.
15. The method of claim 1 , wherein the bromine compound is hydrogen bromide.
16. The method of claim 1 , wherein the bromine compound is elemental bromine having formula Br 2 .
17. The method of claim 1 , wherein said oxygen is provided by air.
18. The method of claim 1 , wherein oxygen under pressure is employed.
19. The method of claim 1 , wherein flowing oxygen is employed.
20. The method of claim 1 , wherein the polar solvent is acetonitrile, dimethyl sulfoxide, chloroform, o-dichlorobenzene, ethyl acetate, water, phenol, o-cresol, m-cresol, propionic acid or acetic acid.
21. The method of claim 1 , wherein the polar solvent is acetic acid.
22. The method of claim 1 , wherein a temperature in the range of about 20–150° C. is employed.
23. The method of claim 1 , wherein said bromine compound is an ionic bromide salt and wherein a molar ratio of said ionic bromide salt to said hydroxyaromatic compound less than 1:1 is employed.
24. The method of claim 1 , wherein said bromine compound is elemental bromine and wherein a molar ratio of said elemental bromine to said hydroxyaromatic compound less than 1:2 is employed.
25. The method of claim 1 , wherein a molar ratio of said hydroxyaromatic compound to said catalyst ranging from about 1:1 to about 500:1 is employed.
26. A method for preparing 4-bromophenol, 4-bromo-o-cresol or 4-bromo-m-cresol, which comprises contacting phenol, o-cresol or m-cresol, respectively, with air and hydrogen bromide in a polar solvent and an acidic medium, in the presence of a catalyst selected from the group of compounds and mixtures of compounds of Group IV–VIII transition metals of the Periodic Table of Elements.