Dehydroboration-rehydroboration

In this displacement an olefin replaces another olefin from an organoborane. If the olefin added is less volatile than the olefin displaced from the organoborane, the equilibrium may be shifted to produce the more volatile olefin in high yield. The rate of displacement is independent of the concentration of the displacing olefin, in agreement with the dehydroboration-rehydroboration mechanism, in which the dehydroboration is the ratedetermining step . 

Isomerization of allylboranes proceeds not by a dehydroboration-rehydroboration mechanism that would involve the intermediacy of allenes, but by an allylic rearrangement or 1,3-metallotropy that is a symmetry-allowed concerted process. The probable HOMO-LUMO interactions suggested for the 1,3-metallotropy are shown in formulas XXI and XXH. 

The activation of non-activated C-H bonds is an important research field 14), In most cases, transition metal complexes have been used for this purpose 14-19). In this chapter, we wish to describe a stereoselective allylic C-H activation involving the thermal rearrangement of organoboranes (Scheme 7) 20-22). The observed stereochemistry may be best explained by a dehydroboration-rehydroboration mechanism, but mechanistic studies indicate that a more complex pathway involving a second molecule of BH3 may be involved. 

On the other hand, the bulkier 2-(triisopropylsilyl)ethanol is prepared only from triisopropyl(a-methoxyvinyl)silane. This is based on Larsons findings that silyl enol ethers [2] undergo hydroboration with 9-BBN, followed by P-elimina-tion of the B-methoxy-9-BBN and subsequent rehydroboration. Consequently, this one-pot procedure [1] of hydroboration-dehydroboration-hydroboration gives the desired alcohol, 2-(triisopropylsilyl)ethanol (Chart 6.10) in 89% isolated yield. As evident in Chart 6.10, (a-methoxyvinyl)silane acts as the key intermediate for the preparation of either 1- or 2-(trialkylsilyl)ethanols. 

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