Bromides, alkyl from organoboranes

Alkyl bromides can also be obtained from organoboranes by treatment with bromine or bromine chloride 170). 

Several papers have appeared describing the synthesis of alkyl halides from organoboranes. Alkyl chlorides are obtained by the reaction of trialkylboranes with dichloramine-T, and in somewhat lower yield with iV,N-dichlorourethane/ whereas alkyl bromides are produced in excellent yield when the boranes derived from terminal alkenes and dicyclohexylborane are treated with either Bt2 or BrCl. Alkyl iodides are obtained from the same boranes by the use of ICl-NaQAc " or iodide ion-chloramine-T/ and this latter method has been applied to the radioiodination of olefins (with Vinyl iodides result from... 

The Overall process of reduction of an alkene to an alkane, or of reduction of an alkyne to either an al-kene or an alkane, via hydroboration requires that the intermediate organoborane be protonolyzed (Section 3.10.1.1). However, simple trialkylboranes are remarkably resistant to hydrolysis. For example, trimethylborane gives only 69% of hydroxydimethylborane after 7 h at 180 °C with 1 equiv. of water. Similar resistance is shown to alcohols, phenols and amines, and even mineral acids do not completely protonolyze trialkylboranes with any ease. Thus, aqueous or anhydrous hydrogen bromide removes only one alkyl group from tributylborane after reflux for 1 h. ... 

The asymmetric hydrosilylation of alkenes mentioned earlier (Scheme 1) can be used to prepare optically active bromides by halogenation of the alkyl-pentafluorosilicate intermediates (1) with inversion of configuration using N-bromosuccinimide. Treatment of organoboranes with ICl provides a rapid mild route from alkenes to anti-Markovnikov iodides (Scheme 37), although all three alkyl groups of the organoborane do not react with the ICl. " ... 

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