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Borane (BH3) is a highly versatile reagent with reproducible applications in organic synthesis. Herbert C Brown (1912-2004) was awarded the Nobel Prize in Chemistry in 1979 as a recognition of his work in borane chemistry. Borane is a strong Lewis acid with an empty p-orbital on the boron atom and exists as either diborane dimer (B2H6) or as a complex with Lewis bases.1-3 Borane complexes are commonly used as regio-, chemo- and stereoselective reducing agents for carboxylic acids, aldehydes, ketones, amides and olefins.
Because it is a pyrophoric gas, borane is usually prepared as a Lewis acid-base complex. Borane-tetrahydrofuran (BH3.THF)4 and borane-methyl sulfide BMS5 are widely used as hydroboration agents. Both reagents have disadvantages beside the advantages.6,7
Borane-THF is a valuable hydroboration reagent and can be used as a reducing reagent. However, it is not very stable over a period of time. Stabilized BH3.THF solutions permit storage for a longer time, but it still requires a relatively dilute solution in THF, which is a serious drawback to the commercial use of this reagent.6
Borane-Me2S is more stable than BH3.THF and is widely used as hydroboration and reducing reagent. However, it yields a product which contains free methyl sulfide, is highly volatile, flammable and has a very noxious odour. Also, it is insoluble in water and as a result cannot be removed by washing with water.6,8
Herbert C. Brown and co-workers introduced 1,4-oxathiane as a less volatile borane adduct.9,10 They studied the utility of several liquid organic sulfides as borane carriers,11 bis(hydroxymethyl)sulfide having an advantage of being water soluble.12 In 2001 a fluorinated dimethyl sulfide was described as a convenient, odourless and recyclable borane carrier.13 There are several studies referring to different borane reagents with bis-sulfides3 and polymeric adducts.14 We believe that there is a serious interest to find safe and readily handled borane carriers.
The aim is to synthesise easily handled solid borane adducts having high molarity of borane, mild odour and low volatility and to use such adducts as selective hydroborating and reducing reagents.
The last report showed that we have successfully prepared polymer 1 in quantitative yield by the reaction of 1,3-dibromoprpane with sodium sulfide (Equation 1). The structure of polymeric 1 was confirmed by 1H NMR spectroscopy.
Then we have prepared the adduct 1a by passing BH3 gas, generated from BF3.OEt2 and NaBH4, into a solution of 1 in dichloromethane as a solvent at room temperature for 2 h. The solvent was removed under inert atmosphere and the solid left was kept under nitrogen. The structure of adduct 1a is represented in (Figure 2). The structure of the adduct 1a was confirmed by 11B NMR.
Stability of the adduct 1a has been studied, by measuring the volume of the hydrogen librated by the hydrolysis of the adduct. The hydrolysis and hydrogen volume analysis were repeated several times over a period up to 46 days. We found that 1 g of the fresh adduct 1a contains 7.5 mmol borane, while, after 46 days the borane content was reduced to 5.6 mmol per gram. This result clearly indicates that 1a is highly stable over a long period of time (up to 46 days) and 25% of borane was oxidised or hydrolysed.
By this we have prepared new solid borane adduct which has a significant stability, mild odour and reasonable solubility in THF. Also we can weigh it by using simple balance opened to air.
To study the synthetic applicability of the prepared adduct our attention was next turned to use it as a hydroboration reagent in hydrboration-oxidation of alkenes. Four different alkenes (1-octene, 1-hexene, styrene and cyclopentene) were used and 1a was used as hydroboration reagent. The procedure involved the addition of the alkene to the solution of 1a in THF at 0 oC, left to stir overnight a room temperature and worked up with alkaline hydrogen peroxide. The produced alcohols were analysed by GC. Results (see Table 1), show that the adduct is working selectively.
Table 1: Hydroboration-oxidation of representative alkenes with 1a (0.5 g)
Amount of the alkene (mmol)
Product yield (%)
Furthermore we have studied the ability of using the prepared adduct as a reducing reagent. Right now three different carbonyl compounds (caproic acid, benzaldehyde and acetophenone) were used and 1a was used as a reducing reagent. The procedure involved the addition of the carbonyl compound to the solution of 1a in THF at 0 oC, left to stir 2 hours at room temperature and worked up by the addition of 1.5 mL methanol to quench the unreacted borane. The produced alcohols were analysed by GC. Results (see Table 2), show that the adduct is working as a good reducing reagent that can reduce such carbonyl compounds to the correspondence alcohols quantitatively.
Table 2: Reduction of representative carbonyl compounds with 1a (0.5 g)
Amount of the compound (mmol)
Product yield (%)
Hydroboration of 1-Hexene Using 1a: Representative Procedure for Hydroboration of Alkenes
In an oven dried 50 ml flask protected by a rubber septum, 1a (0.5 g) was placed. The flask was cooled to 0 oC (ice-bath) under a stream of nitrogen and freshly distilled THF (14 ml) was added slowly. 1-Hexene (0.606 g, 7.2 mmol) was slowly added. The mixture was stirred at 0 oC for 30 min and then at room temperature overnight. The reaction mixture was cooled to 0 oC; NaOH (3 ml, 3 M) was added, followed by slow addition of hydrogen peroxide (3 ml, 30%). The mixture was further stirred at room temperature for 1 h to ensure complete oxidation. Hexadecane (1 ml, 0.77 g) was added, followed by addition of diethyl ether (10 ml) to precipitate the polymeric sulfide. A sample from the organic layer was analyzed by GC.
Reduction of Benzaldehyde Using 1a: Representative Procedure for Reduction of Carbonyl Compounds
In an oven dried 50 ml flask protected by a rubber septum, 1a (0.5 g) was placed. The flask was cooled to 0 oC (ice-bath) under a stream of nitrogen and freshly distilled THF (14 ml) was added slowly. Benzaldehyde (0.276 g, 2.6 mmol) was slowly added. The mixture was stirred at 0 oC for 30 min and then at room temperature for 2 hours. The flask was hen cooled to 0 oC and methanol (1.5 mL) was added to quench the excess borane. Subsequently, diethyl ether (10mL) was added to precipitate the released polymeric sulfide. The mixture was then filtrated and washed with brine. A sample from the organic layer was analyzed by GC.