Hydroboration of Acyclic Alkenes

Simple alkenes, such as 1-hexene, undergo hydroboration with BH, to place 6% of the boron at the 2 position and 94% on the terminal position and this distribution is not significantly influenced by branching [1]. 

Moreover, the polyfunctional nature of BHj, its relative low selectivity and its low steric requirements cause various difficulties. To overcome these problems, various types of borane derivatives have been developed [2]. The regioselectivity [2] of addition of these boranes to an alkene is dependent on both steric and electronic effects exerted by the substituents present on C=C and also on the bulkiness of the hydroborating agent [3], 

However, the instability of most of the hydroborating agents and their derivatives has limited their use in hydroboration and their subsequent synthetic applications. On the other hand, both 9-BBN and B-R-9-BBN compounds exhibit remarkable thermal stabilities. The hydroboration with 9-BBN is carried out at 25 °C or at a higher temperature to achieve essentially complete hydroboration of alkenes of widely different structural types [4] as compared with disiamylbo-rane, where the reaction is normally carried out at 0 °C. In general, the hydroboration with 9-BBN is carried out in THF as the solvent, but carbon tetrachloride, benzene, and hexane are also employed for the reaction. 

The reactions of 9-BBN with terminal alkenes in THF are complete at room temperature in 2 h, whereas the internal alkenes undergo hydroboration at re- 

9-BBN exhibits a remarkable regiospecificity in the hydroboration of alkenes [6] (Chart 5.1). 

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