Norbomyl bromide

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. 

Representative reductions of alkyl halides with LiBEtyH are shown in Table 4. Most alkyl halides, including chlorides, are reduced under mild conditions in essentially quantitative yields. Even hindered alkyl halides, such as neopentyl bromide and cAro-norbomyl bromide, undergo facile reduction to the corresponding alkanes in more than 96% yield with this reagent. 

Another important factor to deduce the involvement of radicals in a transition metal-catalyzed process is the integrity of stereocenters. In oxidative addition or Sr 2-type processes the stereochemical information - retention or inversion, respectively - is preserved for optically active substrates like sec-butyl bromide (Sect. 2.2), while racemic products are observed when radical intermediates are generated. On the other hand, stereochemical convergence is observed for strongly biased diastereomeric substrates, such as exo- and endo-norbomyl substrates 25 (Fig 9) The reactions occur almost exclusively at the exo-face of the norbomyl... 

Norbomyl Cations.— The kinetics of solvolysis of 1-substituted 2-exo-norbornyl bromides (121) in 20% ethanol are reported. The rate constants, relative to that when R = H, are R = COjMe, 6.2 x 10 R = CN, 2.1 x lO"" and R = CH2OH, 2.0 X 10 These can be correlated with Taft substituent constants with correlation coefficient 0.999 and p = — 3.62. It is suggested that there is no 1—6 bond participation and that an unbridged cation is formed. Extrapolation from these three data predicts (if the mechanism were unaltered) a rate for the case of R = H 100 times lower than that found. It is concluded that this factor of 100 is due to participation and that all existing evidence supports a bridged structure for the intermediate in the solvolysis of unsubstituted 2-exo-norbornyl derivatives as well as for the long-lived ion obtainable in superacid media. ... 

The initial studies centered on the 1-norbomyl halides 8. On irradiation in CH3OH, they afford a mixture of the reduction product norbornane (9) and the ether 10, with the former predominating from bromide 8b and the latter from iodide 8a. The reduction product 9 arises via abstraction of a hydrogen atom by 1-norbornyl radical from the medium and ether 10 via nucleophilic trapping of the 1 -norbornyl cation. Irradiation of either hahde in CH3OD afforded ether 10 with no detectable incorporation of deuterium, indicating that it does not arise via acid-catalyzed addition of the alcohol to an initially formed unsaturated intermediate such as the bridgehead alkene 12 or the propeUane 13. 

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