Organoborane, from alkenes reaction with

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). 

When secondary alkyl alanes are heated above 100°C, equilibria are established which result in extensive isomerization to the 1-alkyl alanes (40, 94). The isomerization can be catalyzed by Ti(IV) or Zr(IV) salts (9). Since A1H addition to the C=C bond is more selective than for the corresponding organoboranes, it is possible to prepare 1-alkenes from alkenes with C=C bonds in an internal position. Simultaneously, however, the A1—C bond is able to react with the C=C double bond under these conditions (see Section V,C,l,a) i.e., synthetic reactions occur, which lead to considerable loss of the alkene used. With organoboranes this hardly occurs at all. 

The required organoborane is obtained from the reaction of an alkene with catecholbo-rane. Because alkenes are readily available, this method can be used to prepare a wide variety of alkyl benzenes. 

Stereoselectivity in addition reactions of borane with bicyclic alkenes is similar to that observed for reduction of bicyclic ketones with hydride reducing agents. Boranes add to norbornene derivatives from the exo face (27 gave 85% exo attack upon reaction with diborane). In 27-30, all values are percentage yields of the organoborane products obtained by reaction from that face. Interestingly, only 80% exo attack was observed when 27 reacted with disiamylborane, and the size of the borane appears to have little effect on selectivity. 

Organoboranes are formed from the reaction of alkenes and BHg or, in some cases, with other organoboranes. Are organoboranes useful in other reactions The experimental example in Section 10.4.3 reports that monoalkylboranes 56 and 57 are converted to alcohols 52 and 53 by treatment with hydrogen peroxide and sodium hydroxide. This transformation is a formal oxidation (see Chapter 17). Several other reactions can transform an alkylborane into different functional groups, but this chapter will focus only on oxidation to an alcohol. 

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