[7789-33-5] · BrI · Iodine Monobromide · (MW 206.80)
Physical Data: mp 40 °C; bp 116 °C (dec); d 4.416 g cm-3.
Solubility: sol alcohol, ether, CS2, acetic acid, acetone, CH2Cl2, MeCN, DME, toluene (but with limited solubility below -80 °C); sol H2O, but hydrolyzes to HBr and IOH.
Form Supplied in: brownish-black crystals or very hard, black solid; widely available (usually 97-98% purity).
Handling, Storage, and Precautions: corrosive solid and vapor; readily absorbed through skin and mucous membranes; use in a fume hood and wear gloves and appropriate protective clothing; air-, moisture-, and light-sensitive; should be stored refrigerated (under N2) in a tightly sealed amber bottle. Forms explosive mixtures with potassium and sodium, and reacts extremely exothermically with phosphorus and tin.
Iodine monobromide provides very good diastereoselectivity in electrophilic cyclizations of homoallylic carbonates (eq 1). Isolated yields of a-iodo cyclic carbonates are excellent, and these products can be easily and efficiently converted into the corresponding epoxides (eq 2). Optimal diastereoselectivity is obtained from reactions performed in toluene at -80 to -85 °C. Use of Iodine/MeCN6 or Iodine Monochloride/CH2Cl2 yields much lower diastereoselectivity than iodine monobromide under similar conditions. Results of an equilibration experiment strongly indicate that the diastereoselectivity in iodine monobromide-induced cyclization stems from kinetic control.
Iodine monobromide addition across alkenes is very facile, yielding vicinal bromoiodoalkanes. In general, addition occurs with high anti stereospecificity, and mechanistic studies have implicated an iodonium ion intermediate in the overall reaction sequence. The regiochemistry of iodine monobromide addition is very sensitive to the initial alkene structure, such that anti-Markovnikov products are possible (Table 1).
Iodine monobromide is an effective monobrominating reagent for steroidal aldehydes (eq 3).3 The ratio of diastereomeric products is in part a function of the reaction site's proximity to an asymmetric center, with b-orientation favored for the entering bromine. In an analogous application, methyl 3-oxo-5b-cholanate has been monobrominated (IBr/MeCO2H; 50% yield) to produce the 2b-bromo ketone.4
Cleavage of carbon-metal bonds by iodine monobromide typically produces the organic iodide and the metal bromide. Cleavage of organotin,5a-c organoarsenic,5d organoantimony,5d organogermanium,5e and organolead5e bonds has been investigated (see also Iodine Monochloride for use in cleaving carbon-metal bonds).
Iodine monobromide has been used for brominating phenol and aryl ether derivatives.7 Ethers and esters have been cleaved by catalytic iodine monobromide in the presence of Bromotrimethylsilane.8 Aryl acetals can be directly oxidized to the corresponding esters in moderate yield.9
Ronald G. Brisbois & Randall A. Wanke
Hamline University, St. Paul, MN, USA