[56050-29-4] · C16H32BLi · Lithium 9,9-Dibutyl-9-borabicyclo[3.3.1]nonanate · (MW 242.23)
Physical Data: white gelatinous solid in hexane (reagent generated in situ).
Solubility: insol to sparingly sol hexane; sol MeOH, THF, benzene.2-5
Preparative Methods: a slurry of the reagent (1) is prepared by the dropwise addition of n-Butyllithium (14.4 mL of 1.39 M, 20.0 mmol) in hexane to B-Bu-9-BBN (3.76 g, 20.0 mmol) in hexane (25 mL) at 0 °C under a nitrogen atmosphere.2
Analysis of Reagent Purity: exhibits a single resonance in its 11B NMR (THF) spectrum at d -18.5 (s) ppm.5
Handling, Storage, and Precautions: the generation of (1) involves handling air-sensitive reagents which can be pyrophoric (i.e. BuLi, B-Bu-9-BBN) and consequently should be performed in a well ventilated hood. Individuals should thoroughly familiarize themselves with the special handling techniques required for such reagents prior to using (1).1
Exhibiting unique behavior among organoborane-based reducing agents, (1) serves as an a-hydride source reducing allylic, benzylic, and tertiary halides to hydrocarbons with the accompanying rearrangement of the bicyclo[3.3.1]nonanate skeleton to cis-bicyclo[3.3.0]oct-1-yl(dibutyl)borane (2) (eqs 1, 2).2-5 Although significantly more reactive than other common tetraalkylborates, the reagent fails to reduce primary, secondary, or aryl halides, a feature which can be utilized in selective reductions. These reductions, which are thought to involve carbocationic rather than radical intermediates,2b are carried out in hexane, a medium in which (1) is only sparingly soluble.
In THF solution, the reduction of acetyl chloride occurs at low temperature to produce ethyl acetate and (2).5a The reagent reduces neither esters nor nitriles. While useful for the reduction of both aldehydes and ketones, the former are reduced selectively in admixtures. Moreover, methyl ketones are reduced faster than their straight or branched chain isomers, and also faster than aryl ketones (eqs 3-5).
Cycloalkanones are reduced from their least hindered face to give cis-cyclohexanols, a process enhanced with added MeOH to suppress equilibration. However, with added lithium salts (i.e. LiOMe, LiBr) or excess ketone in the absence of MeOH, a Meerwein-Ponndorf-Verley type equilibration occurs to give the more thermodynamically favored isomer (eq 6). Methanol is thought to inhibit the formation of alkoxyborate complexes while lithium ions are believed to function as Lewis acids, making the carbonyl a better hydride acceptor.
Unsymmetrical epoxides are reduced by (1) in a regioselective manner, the course of which is determined by the nature of the substituents. Aromatic derivatives give primary alcohols whereas aliphatic derivatives exhibit the opposite selectivity, suggesting that a steric basis for the regiochemistry is more important in these substrates (eq 7).
John A. Soderquist & Pedro I. Ortiz
University of Puerto Rico, Rio Pedras, Puerto Rico