Selective Reductions. 30. Effect of Cation and Solvent on the Reactivity of Saline Borohydrides for Reduction of Carboxylic Esters. Improved Procedures for the Conversion of Esters to Alcohols by Metal Borohydrides

Article abstract of DOI:10.1021/jo00145a018

A comparative study of the relative reactivity of saline borohydrides (Li, Na, Ca) for the reduction of carboxylic esters has been made in selected solvents (ether, tetrahydrofuran, diglyme, 2-propanol, and ethanol) at 25 deg C.In ether solvents the reactivity follows the trend LiBH4 > Ca(BH4)2 > NaBH4.On the other hand, in alcohol solvents the order of reactivity is Ca(BH4)2 > LiBH4 > NaBH4.The reactivities of LiBH4 in ethyl ether and THF, of Ca(BH4)2 in THF and 2-propanol, and of NaBH4 in ethanol proved to be promising for the reduction of esters.However, alcoholsolvents are not useful for reductions at elevated temperatures because the decomposition of the reagents becomes competitive with the reduction.A convenient synthetic procedure has been developed for the rapid conversion of esters to alcohols by using LiBH4 in ethyl ether, LiBH4 in THF, and Ca(BH4)2 in THF and utilizing essentially stoichiometric amounts of the reagents.The procedure involves adding toluene to the reaction mixture and bringing the temperature to 100 deg C while allowing the solvent do distill off.Following completion of the reaction, toluene is readily removed under vacuum and the reaction product hydrolyzed.These reductions were generally complete in 0.5 - 2.0 h, and high yields of alcohols (73-96percent) were isolated.A number of ester derivatives, including compounds containing nitro, halo, cyano, and alkoxy groups, diesters, and lactones were reduced by this procedure.The study demonstrated the high selectivity of these reagents, permitting the rapid reduction of the ester group in the presence of many substituents.However, unsaturated esters undergo simultaneous hydroboration when reduced by this procedure.