New direct 11B NMR-based analysis of organoboranes through their potassium borohydrides

Representative organoborane mixtures were quantitatively converted to their borohydrides through their reaction with activated KH (KH), permitting their detailed analysis by (11)B NMR. Through the treatment of commercial KH with a THF solution of lithium aluminum hydride (LAH), a dramatic change in the surface morphology results as revealed by scanning electron microscopy (SEM). Energy dispersed spectroscopy (EDS) was employed to reveal that the LAH treatment deposits a significant amount of an unknown aluminum-containing species on the surface of the KH, which imparts a unique reactivity to the KH. Even highly hindered organoboranes are quantitatively converted to their borohydrides by replacing electronegative groups (e.g., OR, halogen) with hydrogen, retaining only the carbon ligation. Through this simple KH treatment, complex organoborane reaction mixtures are converted to the corresponding borohydrides whose (11)B NMR spectra normally exhibit resolved signals for the individual species present. The integration of these signals provides quantitative information on the relative amounts of each component of the mixture. New generalities for the effect of alpha-, beta-, and gamma-substituents have also been determined that provide a new, simple technique for the determination of the isomeric distribution in organoborane mixtures resulting from common organoborane processes (e.g., hydroboration). Moreover, the (1)H-coupled (11)B NMR spectra of these mixtures reveal the extent of alkylation for each species present. Representative organoboranes were examined by this new technique permitting a simple and convenient quantitative analysis of the regio- and diastereomeric composition of a variety of asymmetric organoborane processes. Previously unknown details of pinene-based hydroborations and reductions are revealed for the first time employing the KH (11)B NMR technique.