Hydroboration. 76. Hydroboration of Cyclic Dienes with Representative Hydroborating Agents

Article abstract of DOI:10.1021/jo00354a005

A detailed study was made of the hydroboration of cyclic dienes with representative hydroborating agents: borane-methyl sulfide (BMS), 9-borabicyclo<3.3.1>nonane (9-BBN), disiamylborane (Sia2BH), dibromo-borane-methyl sulfide (Br2BH*SMe2), and dibromoborane (Br2BH). 1,4-Cyclohexadiene essentially undergoes monohydroboration with a stoichiometric amount of the representative hydroborating agents, whereas 1,5-cyclooctadiene gives the dihydroboration product predominantly.The rapid dimerization of 1,3-cyclopentadiene introduces a complication into its hydroboration.However, monomeric cyclopentadiene undergoes hydroboration primarily to the homoallylic derivative with little of the desired allylic product.Hydroboration of 1,3-cyclohexadiene with a stoichiometric amount of the hydroborating agent furnishes mainly the allylboranes, which upon treatment with acetaldehyde followed by oxidation, furnishes 1-(2-cyclohexenyl)-1-ethanol in good yield.Similary, hydroboration of 1,3-cycloheptadiene and 1,3-cyclooctadiene furnished dihydroborated products preferentially, with lower yields of the corresponding allylboranes, characterized as 1-(2-cycloheptenyl)-1-ethanol and 1-(2-cyclooctenyl)-1-ethanol, respectively, following reaction with acetaldehyde.