Comparative study on reducing aromatic aldehydes by using ammonia borane and lithium amidoborane as reducing reagents

Lithium amidoborane (LiNH2BH3) and ammonia borane (NH3BH3) reduce aromatic aldehydes in tetrahydrofuran (THF) through two different pathways. LiNH2BH3 only transfers hydridic hydrogen on boron to aldehydes through a hydroboration process to achieve lithium aminoborate; ammonia borane, on the other hand, transfers both protic and hydridic hydrogens on N and B, respectively, to aldehydes to directly achieve corresponding alcohols. Mechanistic investigations confirm that protic H(N) and hydridic H(B) of ammonia borane participate in the reduction, in which the dissociation of both B-H and N-H bonds is likely to be involved in the rate-determining step.