Contribution to the Chemistry of Boron, 235. Reactions of Prolinol with Borane–Tetrahydrofuran: Anomalous Dimerization of a 1,3,2-Oxazaborolidine

The reaction of (S)-prolinol (LH) with H3B · THF proceeds via [L2BH2<]BH4, which decomposes with elimination of hydrogen at ca. −40°C to give the BH3 adduct of spirocyclic 7. On heating, 7 loses more H2 to give the “anomalous” dimerization product 11, which features a tetracoordinated B atom in a “tetrahedral” BN2O2 unit and another B atom in an N2BH2 environment. It is also shown that the product resulting from the reaction of pseudoephedrin with H3B·THF undergoes an analogous anomalous dimerization, affording product 8.