Enantioselective Reduction of Ketones by in situ Generated Borane and Cbs Catalyst: Economical Alternative to Bh₃-Thf

References

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• Typical procedure: Freshly ground LiH (0.2 g, 25 mmol) was suspended in dry THF (20 cm3. ) and BF3Oet2 (4.75 cm3., 5.30 g, 37.5 mmol) was added. After the vigorous reaction had subsided the mixture was stirred for 30 min at ambient temperature, after which MeCBS 1 (1 M in toluene, 0.5 cm3., 0.5 mmol) was added, followed by 4‐bromoacetophenone (1.99 g, 10 mmol) in THF (5 cm3. ) over 60 min using a syringe pump. The mixture was stirred for a further 5 min, then quenched with water and diluted with ether. The aqueous layer was extracted twice with Et2O, the combined organic layers washed with water, saturated NaHC03 (x2), water then brine, dried over MgSO4, concentrated and distilled to afford 1.60 g (80 %) of 4‐bromophenylethanol (97% ee by GC)
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• Problems associated with the use of isolated borane complexes have rather limited application of CBS catalysis at fine chemical scales. We are aware of only a handful of commercial processes (Sipsy, Merck and Pfizer)
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