Organoboranes ionic reactions

Compared with other organometallics, organoborane compounds are relatively inert to chemical transformations, particularly to ionic reactions, because of the less polar nature of the carbon-boron bond. 

Organoboranes react with bromine to form brominated boranes,l44 but they also react with brominated substrates. a-Bromo esters, for example, react with organoboranes to give coupling products if a base is added to the bromination reaction.Adding a base suppresses radical bromination, allowing an ionic reaction to... 

The normal synthetic pathway for hydroboration is reaction with an ambiphilic nucleophile of which the simplest example is hydroperoxide ion. This elicits a 1,2-migration of an alkyl group from boron to oxygen with concurrent loss of hydroxide ion. The step occurs with essentially complete retention of configuration. In similar vein, ambiphilic species with the structure NH2X may be used in amination, so that the overall reaction is an addition of ammonia to the alkene with the regio- and chemoselectivity driven by the hydroboration step. A majority of reactions of organoboranes can be rationalized in terms of these ionic mechanistic pathways, or closely related protocols (Scheme 2). 

Oxidations of organoboranes involve numerous reagents and several different general mechanisms, most of which parallel those wluch occur for other types of oiganoborane reactions. The mechanisms fall under three broad headings (i) ionic, with a 1,2-shift fiom boron to a heteroatom (equations 1-3) (ii) radical and (iii) electrocyclic. 

As described in 1.7.4.1.4, ionic hydrides react with organoboranes to afford trialkylhydroborates. In some cases this reaction does not occur however, treatment with t-C H Li results in the formation of the desired compounds ... 

Organoboranes participate in either ionic or radical reactions [1]. However, there are a few reports on the apphcations of organoboranes as initiators for radical reactions [2, 3]. It is reported [4] that in the presence of a catalytic amount of 9-BBN or B-hexyl-9-BBN, or to some extent B-methoxy-9-BBN, initiates the radical addition of alkanethiols to alkenes under very mild conditions to provide the corresponding dialkylsulfides almost in quantitative yields (Scheme 14.1). On the other hand, the radical reactions of alkenes with thiols are generally initiated by thermal decomposition of peroxides or azo compounds, by UV radiations, or by radiolysis [5]. The reaction initiated by 9-BBN is completely inhibited in the presence of galvinoxyl, a radical trapping agent. This confirms that 9-BBN participates in the initiation of radical addition (Table 14.1) [4]. 

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