9-BORABICYCLO[3.3.1]NONANE

Analysis

Analysis of Reagent Purity: the melting point of 9-BBN-H dimer is very sensitive to trace amounts of impurities. Recrystallization from dimethoxyethane is recommended for samples melting below 146 °C. The dimer exhibits a single 11B NMR (C6D6) resonance at δ 28 ppm and 13C NMR signals at 20.2 (br), 24.3 (t), and 33.6 (t) ppm.

Uses

9-Borabicyclo[3.3.1]nonane (9-BBN-H) is a highly selective, stable hydroborating agent; anti-Markovnikov hydration of alkenes and alkynes; effective ligation for alkyl-, aryl-, allyl-, allenyl-, alkenyl- and alkynylboranes; forms stable dialkylboryl derivatives, borinate esters, and haloboranes; organoboranes from hydroboration and organometallic reagents; precursor to boracycles. It can selectively reduce conjugated enones to allylic alcohols; organoborane derivatives for α-alkylation and arylation of α-halo ketones, nitriles, and esters; vinylation and alkynylation of carbonyl compounds; conjugate addition to enones; homologation; asymmetric reduction; Diels-Alder reactions; enolboranes for crossed aldol condensations; Suzuki-Miyaura coupling.

Solubility in organics

9-Borabicyclo[3.3.1]nonane is sparingly soluble in cyclohexane, dimethoxyethane, diglyme, dioxane (<0.1 M at 25 °C); soluble in THF, ether, hexane, benzene, toluene, CCl4, CHCl3, CH2Cl2, SMe2 (ca. 0.2-0.6 M at 25 °C). It reacts with alcohols, acetals, aldehydes, and ketones.

storage

the crystalline 9-BBN-H dimer can be handled in the atmosphere for brief periods without significant decomposition. However, the reagent should be stored under an inert atmosphere, preferably below 0 °C. Under these conditions the reagent is indefinitely stable. In solution, 9- BBN is more susceptible both to hydrolysis and oxidation, and contact with the open atmosphere should be rigorously avoided. Many 9-BBN derivatives are pyrophoric and/or susceptible to hydrolysis so that individuals planning to use 9-BBN-H dimer should thoroughly familiarize themselves with the special techniques required for the safe handling of such reagents prior to their use.1b The reagent should be used in a well-ventilated hood.

Purification Methods

It is available as the solid dimer or in tetrahydrofuran solution. The solid is relatively stable and can be purified by distillation in a vacuum (as dimer) and by recrystallisation from tetrahydrofuran (solubility at room temperature is 9.5%, 0.78M), filter off the solid under N2, wash it with dry pentane and dry it in vacuo at ca 100o. The solid is a dimer (IR 1567cm-1), stable in air (for ca 2 months), and can be heated for 24hours at 200o in an inert atmosphere without loss of hydride activity. It is a dimer in tetrahydrofuran solution also (IR 1567cm-1). It is sensitive to H2O and air (O2) in solution. Its concentration in solution can be determined by reaction with MeOH and measuring the volume of H2 liberated, or it can be oxidised to cis-cyclooctane-1,5-diol (m 73.5-74.5o). [IR: Knights & Brown J Am Chem Soc 90 5280 1968, Brown et al. J Am Chem Soc 96 7765 1974, Brown et al. J Org Chem 41 1778 1976, Brown & Chen J Org Chem 46 3978 1981, Fieser & Fieser Reagents for Org Synth 2 31, 3 24, 10 48, 15 43, 17, 49.] Borane pyridine complex [110 -51 -0] M 92.9, m 8-10o, 10-11o, b 86o/7mm, 100-1 0 1o/12mm, d 4 0.785. Dissolve it in Et2O and wash it with H2O in which it is insoluble. Evaporate the Et2O and distil the residual oil to gives better than 99.8% purity. Its vapour pressure is less than 0.1mm at room temperature. [Taylor et al. J Am Chem Soc 77 1506 1955, Beilstein 20 IV 2235.]