[1611-82-1] · C4H9BrO · t-Butyl Hypobromite · (MW 153.02)
Physical Data: bp 44-45 °C/85 mmHg; mp -27 to -28 °C; d 1.33 g cm-3.
Solubility: sol ether, CCl4, CH2Cl2.
Analysis of Reagent Purity: the reagent can be assayed by thiosulfate titration of iodine liberated by the addition of the reagent to a solution of KI in aq HCl.5
Preparative Methods: prepared in 43% yield on a 0.18 mol scale by addition of t-butyl alcohol to a solution of Ag2SO4 and bromine in water.3 The product is isolated by distillation. Since much material is lost during the distillation, many workers have found it more convenient to prepare and use the reagent in solution. In one procedure the reagent is generated in CH2Cl2 solution from Hg2O, bromine, and t-butyl alcohol.6 It has also been prepared from NaBr, NaOCl, t-butyl alcohol, and acetic acid.2
Handling, Storage, and Precautions: reported3 to be stable for weeks at 0 °C in the dark. It decomposes at 85 °C, upon irradiation or by washing with 5% aq NaHCO3 solution. It also decomposes upon repeated washing with water. Should be handled in a fume hood.
t-BuOBr has been used for the alkoxybromination of alkenes,2,7 though there appears to be little advantage over the use of the more readily available Bromine or N-Bromosuccinimide. For unsymmetrical alkenes the additions follow Markovnikov's rule. For terminal alkenes, selectivities are on the order of 2:1 (eq 1).
Substituted and unsubstituted 3,4-dihydro-2H-pyrans have been alkoxybrominated by this reagent (eq 2).8 Little diastereoselectivity was seen in substituted examples.
There are a few examples in the literature9 of the preparation of b-bromo t-butyl ethers with this reagent. It has been proposed10 that it would be possible to catalyze these reactions with Boron Trifluoride Etherate, but no examples have been reported. The analogous chloro10 and iodo11 compounds have been prepared from the chloro and iodo reagents.
t-BuOBr has occasionally been used for the radical chain bromination of alkanes.3 The radical chain carrier in these reactions is the t-butoxy radical. This radical is less selective than the bromine radical, and so these reactions often give a more random distribution of products than reagents such as NBS.12
t-BuOBr has occasionally been used for allylic bromination reactions. Alden and Davies reported4 that the allylic bromination of 2-methylenebicyclo[2.2.1]heptane with t-BuOBr gives only the exo diastereomer (eq 3), whereas NBS gives exo/endo mixtures.
t-BuOBr has been used to brominate nitroalkanes,13 for aromatic bromination,14 and for bromolactonizations.15
Bruce L. Finkelstein
E. I. DuPont de Nemours, Newark, DE, USA