New Direct ¹¹B NMR-Based Analysis of Organoboranes through Their Potassium Borohydrides

Abstract

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.

Synthesis and reactivity of sulfonamides containing boronate esters

“…This result is interesting as it suggests that a weak interaction of the amide hydrogen with the neighbouring boron atom may be occurring. However, the 11 B NMR spectrum for 1a shows a broad peak at δ 29 ppm, signifying that the boron atom lies in a trigonal environment [31]. An X-ray diffraction study on 1a was conducted to confirm the formation of these sulfonamides (Fig.…”

On the Practical Limits of Determining Isolated Product Yields and Ratios of Stereoisomers: Reflections, Analysis, and Redemption

“…The data in Tables 5 and 6 demonstrate that HPLC and NMR are superior to GC analysis (uncalibrated) for ratios of diastereomers up to 95:5. Various methods of determination of diastereomer ratios (dr) by NMR as well as enantiomer ratios (er) via Mosher esters have been published, including methods based on 11 B NMR. 11 The publications generally claim accuracy of 1-3% as determined by careful integration.…”

“…Various methods of determination of diastereomer ratios (dr) by NMR as well as enantiomer ratios (er) via Mosher esters have been published, including methods based on 11 B NMR. 11 The publications generally claim accuracy of 1-3% as determined by careful integration. 12 In this regard it is interesting to note that the old method of cutting and weighing of NMR peak areas is just as accurate as or even more accurate than integration.…”

One-Step Conversion of Potassium Organotrifluoroborates to Metal Organoborohydrides

“…Modification of the steric, stereochemical, hydrophobic, and electronic influences of organic substituents permits creation of MOBs with reactivity ranging from milder than NaBH 4 , with the creation of sodium cyanoboro­hydride and sodium triacetoxy­borohydride, to much stronger than LiAlH 4 , with the invention of “Super Hydride” LiBEt 3 H . Others, including Singaram, Paetzold, Soderquist, and Noth, furthered this work. The recent explosion of frustrated Lewis pair chemistry takes advantage of the hydride donation ability of organoborohydrides with the first examples of metal-free catalytic hydrogenation.…”