Reactions of Amine-Boranes with Oxalic Acid: Substitution on the N or B Atom Leads to Different Spiroborate Compounds

A mild and convenient reaction of amine-boranes and oxalic acid to bi- or tricyclic spiroborate compounds is presented. The substitution position (on the N or B atom) of amine-boranes plays an important role on the divergence of reaction pathways: ammonia-borane (NH3BH3, AB) or N-substituted amine-boranes NHnMe3-nBH3 (n = 0, 1, 2) give bicyclic spiroborate compounds 1a-d, whereas B-substituted amine-borane NH3BH2Me leads to tricyclic spiroborate compound 3. Fortunately, acyloxyboranes were detected as important intermediates by NMR spectroscopy in the reaction of ammonia-borane and oxalic acid, uncovering a stepwise dehydrogenation pathway.