[111865-47-5] · C10H16Br3N · Benzyltrimethylammonium Tribromide · (MW 389.96)
Physical Data: mp 100-101 °C.
Solubility: sol CH2Cl2, DMSO, DMF; slightly sol MeOH, AcOH, AcOEt, CHCl3; insol hexane, benzene, CCl4, H2O.
Form Supplied in: orange-red needles.
Analysis of Reagent Purity: 1H NMR (CD3CN) d 3.14 (s, 9H, 3 CH3), 4.50 (s, 2H, CH2), 7.52 (s, 5H, C6H5).
Preparative Methods: to a solution of benzyltrimethylam-monium chloride (11 g, 0.06 mol) and NaBrO3 (4.5 g, 0.03 mol) in water (100 mL) is added aq. HBr (47%, 0.18 mol) under stirring at room temperature; yield 18.2 g (78%).2 For an alternative method, see Kajigaeshi et al.3
Purification: recrystallize from CH2Cl2-Et2O (10:1).
Handling, Storage, and Precautions: stable solid, but decomposes slightly upon standing for a long time in contact with air or water. This reagent should be handled in a fume hood.
This reagent (1) can be regarded as a solid form of Bromine. The end points of its reactions can be detected by the decolorization of the reagent. Accordingly, the electrophilic bromo substitution of aromatic compounds with (1) has been extensively investigated (eq 1).
The reaction of phenols with a calculated amount of (1) in CH2Cl2-MeOH at room temperature gives mono-, di-, or tribromo-substituted phenols, selectively.3 Because the presence of MeOH markedly facilitates the brominations, it can be assumed that the active species is methyl hypobromite (MeOBr) produced from the reaction of (1) with MeOH.4 Polystyrene-bound (1) can also be used as the brominating reagent.5
The reaction of aromatic amines with (1) in CH2Cl2-MeOH containing Calcium Carbonate powder at room temperature gives bromo-substituted aromatic amines.6 Polystyrene-bound (1) is also used for the preparation of polybromo-substituted aromatic amines.7
The reaction of aromatic ethers with a stoichiometric amount of (1) in CH2Cl2-MeOH or AcOH-ZnCl2 under mild conditions gives, selectively, mono-, di-, or tribromo-substituted aromatic ethers.8 The reagent is only slightly soluble in acetic acid at room temperature, but the addition of Zinc Chloride increases its solubility, allowing the bromination of aromatic ethers to proceed smoothly under mild conditions. An equimolar amount of ZnCl2 with respect to (1) is required.9
Bromo-substituted acetanilides have been obtained from the reaction of acetanilides with (1) in CH2Cl2-MeOH10 or AcOH-ZnCl2.11
Arenes are brominated smoothly by (1) in AcOH with the aid of the Lewis acid ZnCl2.9 The reaction of thiophenes with (1) in AcOH-ZnCl2 under mild conditions gives bromo-substituted thiophenes.12
The reaction of arenes with a calculated amount of (1) in refluxing benzene in the presence of AIBN gives a-bromo-substituted arenes.13 The reaction of acetophenones with 1 equiv of (1) gives a-bromo-substituted acetophenones and with 2 equiv of (1) gives a,a-dibromo-substituted acetophenones.2
The reaction of alkenes with (1) in aprotic solvents gives 1,2-dibromo adducts via a stereospecific anti addition; in protic solvents such as MeOH and AcOH it gives the corresponding dibromo adducts along with considerable amounts of solvent-incorporated products in a regioselective manner.14
This reagent has been conveniently used as a mild oxidizing agent in place of liquid bromine. Specifically, the oxidation of primary alcohols or ethers to dimeric esters,15 a,o-diols or cyclic ethers to lactones,15 secondary alcohols to ketones,15 benzyl alcohols to benzaldehydes or benzoic acids,16 1,4-benzenediols to 2,5-cyclohexadiene-1,4-diones,17 sulfides to sulfoxides18 and thiols to disulfides19 have been reported. By using (1), the haloform reaction of methyl ketones,20 N-bromination of amides,21 and Hofmann degradation of amides22 have also been achieved.
Yamaguchi University, Ube, Japan
Ube Technical College, Ube, Japan