Sodium Borohydride Reduction of Esters in Toluene Using the All-at-Once Addition Method with an Alcohol Activator

The sodium borohydride (NaBH4) reduction of esters in toluene (also in 2-MeTHF and CPME) has been accomplished using 2-methoxyethanol (2-ME) as an activator. In previous mixed-solvent methods, it was necessary to add methanol dropwise to NaBH4 to suppress the formation of inactive species. In contrast, the present method enables the all-at-once addition of 2-ME. The most plausible active species is likely to be NaBH(OCH2CH2OCH3)3, and its reducing ability was assessed by DFT calculations.