Hydroboration. 88. Borane-1,4-Thioxane. A New, Convenient Hydroborating Agent for the Preparation of Representative Borane Reagents

Article abstract of DOI:10.1021/jo00044a036

Borane-1,4-thioxane (BOT) (1), readily synthesized by adding diborane to 1,4-thioxane (2), is a stable liquid at 25 deg C, which crystallizes on cooling to 0 deg C, mp 11-15 deg C.The neat reagent is 8.0 M in borane.It hydroborates alkenes rapidly in 3:1 mole ratio to form the corresponding trialkylborane in excellent yield.By varying the stoichiometric ratio of alkenes to 1 to 1:1 and 2:1 molar ratios, in certain cases monoalkylboranes, such as thexylborane (7) and monoisopinocampheylborane (8), and dialkylboranes, such as disiamylborane (3), dicyclohexylborane (4), diisopinocampheylborane (6), and 9-borabicyclo<3.3.1>nonane (9-BBN) (5), could be synthesized.The hydroboration reaction can be carried out at 25 deg C in a wide variety of solvents, such as tetrahydrofuran (THF), diethyl ether (EE), methylene chloride (CH2Cl2), and pentane, or with neat reagents at 0 and 25 deg C.It has been demonstrated that the presence of 2 does not interfere with the subsequent in situ utilization of these substituted borane reagents for further transformations.Unlike the ligand methyl sulfide, which is completely insoluble in water, 2 is moderately soluble in water (0.3 M).Consequently, it can be washed out with water from solutions in EE, CH2Cl2, and pentane.Therefore, the organoborane containing 2 can be oxidized selectively to alcohol with a controlled quantity of hydrogen peroxide in the presence of excess sodium hydroxide without attack on 2. 2 in turn can then be removed by washing with water.Alternatively, 2 can be oxidized selectively in the presence of the organoborane by aqueous sodium hypochlorite.The resulting sulfoxide is highly soluble in water and is readily washed away from the organoboranes, thus enabling the organoboranes to be utilized for the many transformations it undergoes.In the case of stable and isolable reagents, such as 9-BBN, 2 can be removed either by decantation along with the solvent or by distillation during neat reaction.The two asymmetric hydroborating agents 8 and 6 were utilized for asymmetric syntheses by the hydroboration-oxidation reaction of 1-methyl-1-cyclopentene and cis-butene, respectively., to yield trans-2-methylcyclopentanol and 2-butanol in 73 percent and 97 percent ee.