Boranes, tris bromination

Biphenylcarboxylic acid [[1,1 -Biphenyl] -2-carboxylic acid], 83 Boron tnfluonde, etherate, [Borane, tri-fluoro-, compd. with l,l -oxybis-[ethane] (11)], 10 Bromine, 108... 

The reactions of trialkylboranes with bromine and iodine are gready accelerated by bases. The use of sodium methoxide in methanol gives good yields of the corresponding alkyl bromides or iodides. AH three primary alkyl groups are utilized in the bromination reaction and only two in the iodination reaction. Secondary groups are less reactive and the yields are lower. Both Br and I reactions proceed with predominant inversion of configuration thus, for example, tri( X(9-2-norbomyl)borane yields >75% endo product (237,238). In contrast, the dark reaction of bromine with tri( X(9-2-norbomyl)borane yields cleanly X(9-2-norbomyl bromide (239). Consequentiy, the dark bromination complements the base-induced bromination. 

Tris(glycosyl)boranes and dicyclohexylboranes are iodinated with Nal/Chloramine T or brominated with NaBr/Chloramine T to yield iodinated and brominated sugars, respectively946. BC13 is used for the selective chlorination of carbohydrates947. 

The brominolysis of trialkylboranes is unusual among die reactions of m ganobatanes in that it proceeds mainly widi inversion. Thus, tris-exo-2-norboniylborane gives 75% of e/ierythro isomers of tris(3,3-dimethyl-I-butyl-l,2-d2)borane in the presence of NaOMe (equation 57). ... 

An alternative indirect but efficient method for the bromination of all three groups of tri(primaiy alkyl)boranes involves initial reaction with mercury(II) acetate followed by in situ bromination. Alkenyldialkylboranes react with bromine to give bromoalkenes via an addition-elimination mechanism. The meftod of elimination controls the stereochemistry of the product bromoalkenes (Scheme 1). For reasons which are not clear, exactly opposite stereochemical results are obtained from (aryl-ethenyl)dialkylboranes as compared with (alkylethenyl)dialkylbotanes (Scheme 1). ... 

Tris(pentafluorophenyl)borane, a compound first made by Stone et al. in 1963, has in recent years become one of the most widely applied boron reagents, as a highly effective activator for metallocene-based olefin polymerisation catalysts. There are several reasons for this it is a strong Lewis acid, unlike boron trihalides it is resistant to hydrolysis, and it possesses strong B-C and C-F bonds which make it essentially inert to chemical attack for example, it can be recovered unchanged from neat elemental bromine. 

An alternative indirect but efficient method for the bromination of all three groups of tri(primary alkyl)boranes involves initial reaction widt mercury(II) acetate followed by in situ bromination. 

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