[38932-80-8] · C12H36Br3N · Tetra-n-butylammonium Tribromide · (MW 482.23)
(mild and efficient brominating agent; useful for synthesis of vicinal dibromides from alkenes and alkynes; synthesis of a-bromo acetals; determination of multiple bonds; oxidation in nonaqueous medium)
Alternate Name: TBABr3.
Physical Data: mp 84 °C (76 °C when crude); l
Solubility: insol H2O; sol anhydrous acetic acid (50 g/100 mL), MeCN (300 g/100 mL), CHCl3, trichloroethylene (10 g/100 mL), THF, dioxane; very sol DMF, DMSO; slightly sol alcohols; sol acetone with slow reaction, giving lachrymatory solution.
Form Supplied in: anhydrous, orange nonhygroscopic stable solid; purity more than 98%; stable bromine degree (1 g = 2.07 mequiv Br2). Red crystals when recrystallized from DMF solution.
Handling, Storage, and Precautions: avoid washing reaction vessels with acetone prior to using with this reagent. Should be handled in a fume hood.
The application of pyridinium, quinolinium, or trimethylphenylammonium tribromide as brominating reagents is known but little used.1 Tetra-n-butylammonium tribromide allows easy double bond bromination under mild conditions and with very good yields (Table 1).2 For example, an equimolar amount of TBABr3 is slowly added to a solution of alkene in CHCl3 to give, after solvent evaporation, a residue which is washed with 5% aqueous Na2S2O3 solution and ethanol, and then filtered and recrystallized. Yields are generally in the range 85-95%.
The bromination of alkynes is usually difficult and gives mixtures of products.3 The use of TBABr3 in CCl4, at rt leads to the (E) isomer of 1,2-dibromo alkenes in high yields (85-95%) (Table 2).4
a-Bromo acetals are useful reagents for the synthesis of a,b-unsaturated ketones.5 These derivatives are usually prepared in two steps including bromination.5,6 and yields are often poor. TBABr3 is a convenient reagent which gives a-bromo acetals from ketones when used with ethylene glycol.2 Thus, to a solution of 120 mL of dry ethylene glycol and 0.1 mol of ketone is added an equimolar amount of TBABr3 until the red solution turns yellow (generally 6-10 h). The mixture is then poured into 70 mL of pentane, 70 mL of 10% Na2CO3 and 20 mL of 5% aqueous Na2S2O3. The organic phase is separated, water washed, dried (Na2SO4), and concentrated. The desired a-bromo acetals are obtained in yields of 75-80% (Table 3).
4-Bromo-1,1-diphenylbuten-3-one can be prepared in a one-pot synthesis from 1,1-diphenyl-3-ethylenedioxy-1-butanol by the action of TBABr3 in dry CHCl3.7
The reaction of TBABr3 with aminopyridine and aromatic amines PhNR1R2 (R1 = H, Me, Ph, Cy; R2 = H; R1 = R2 = Et) in an aprotic and nonbasic solvent at 20 °C gives exclusively the para-brominated compounds in high yields (95%).8
Phenols were brominated in the para position (95%) using TBABr3 in chloroform (or in the ortho position if the para position was substituted).8a,9
A method for the determination of the unsaturation number in organic compounds is based on bromination with TBABr3 in nonpolar media followed by electrochemical monitoring of Br3-.10
Michel J. L. Fournier
Université Pierre & Marie Curie, Paris, France