Effect of Temperature on Borane Reduction of Representative Malonic Acids

Article abstract of DOI:10.1021/jo00266a037

The controlled reaction of phenylmalonic acid (1a) with borane-THF at appropriate temperature makes available (phenylmalonyldioxy)borane <2a, PhCH(CO2)2BH>, which has been characterized.Reduction of 2a or of the corresponding acid 1a proceeds at 0 deg C at the same rate.The reaction, however, is incomplete, showing only 33percent product formed in 4 h.Diethylmalonic acid (1b), which possesses no α-hydrogen, and the (diethylmalonyldioxy)borane (2b) are reduced at the same rate to the corresponding cyclic dialkoxyborane 5b.These results suggest that the reduction proceeds through intermediate formation of 2.The rates of the borane reduction for a series of malonic acids (1a-h) have been systematically studied and compared at 0 deg C and at -20 deg C.The reductions of aromatic substituted malonic acids (1f-h) are quite sluggish with substantial (46-72percent) α-metalation occurring at 0 deg C.Aliphatic alkylmalonic acids (1d-e) are reduced in 24 h with 34-40percent of α-metalation.At -20 deg C, most malonic acids are completely reduced in times ranging from 24 h for aliphatic (1c-e) to 3 days for aromatic (1f-h) compounds, with only 2-24percent α-metalation.The reduction of 1b at either 0 deg C or -20 deg C is completed in 8 h.At an appropriate lower temperature the reduction successfully competes with the α-metalation.