Synthesis of Saturated Alcohols

Hydroboration-oxidation is now a standard method for anb -Markovnikov s cis-hydration [1] of alkenes from the less hindered side of the double bond. The hydroboration of olefins, mainly in THF solvent, affords conveniently the corresponding organoboranes. Oxidation of the intermediate organoboranes is most conveniently carried out by successive addition of 3 M NaOH and 30% hydrogen peroxide (depicted as [O], Eq. 6.1). Other oxidation procedures, e.g., HjOj in buffer solutions, HjOj in NaOAc, per acids, and trimethyl-N-oxides are used in certain cases when the organoboranes are unstable to water or contain functionalities sensitive to alkaline medium. 

Among all boranes, 9-BBN shows the highest sensitivity to subtle differences in steric environment Though, disiamylborane is sterically more hindered than 9-BBN, but surprisingly, the hydroboration of 1-hexene with 9-BBN in THF at 25 °C is more regioselective than is disiamylborane. The comparative data are given in Table 6.1 [2]. 

In general, the intermediate organoboranes on usual oxidation afford the corresponding alcohols, essentially in quantitative yields. Only in the case of 2,3-dimethyl-2-butene is the primary alcohol ( 0.5%) derived from an isomer-ized organoborane. The results are summarized in Table 6.2 [2]. 

3-Dimethyl-l-butene 2 25 3.3- Dimethyl-l-butenol, 99.7 3.3- Dunethyl-2-butanol, 0.3 

3-Dimethyl-2-butene 8 65 2.3- Dimethyl-2-butanol, 99.5 2.3- Dimethyl-1-butanol, 0.5 

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