[81971-15-5] · C26H39B · 2-[2-[(Benzyloxy)ethyl]-6,6-dimethylbicyclo[3.3.1]-3-nonyl]-9-borabicyclo[3.3.1]nonane · (MW 378.41)
(asymmetric reducing agent1)
Alternate Name: NB-Enantrane®.
Solubility: sol most organic solvents.
Form Supplied in: 0.5M solution in THF; commercially available.
Handling, Storage, and Precautions: NB-Enantrane is an air-sensitive reagent and must be handled under an inert atmosphere. Use in a fume hood.
NB-Enantrane is an analog of B-3-Pinanyl-9-borabicyclo[3.3.1]nonane (Alpine-Borane1b). Since the absolute configuration of nopol is the opposite of (+)-a-pinene, the reagent provides the opposite mode of asymmetric induction. The reagent provides higher asymmetric induction than Alpine-Borane for cases where the two groups flanking the ketone are relatively small, such as methyl or ethyl alkynyl ketones (eq 2).
Reduction occurs via transfer of the tertiary hydride, which is b to the boron. Since this is the hydride added during the hydroboration process, labeled material may be obtained by using labeled 9-BBN in the initial preparation. The absolute configuration of the product may be predicted by using a simple six-membered ring model (1a) for the transition state. The larger group is placed away from the reagent in this arrangement.
The reagent presumably leads to higher asymmetric induction because of the increased size of the nopol side chain. Brown has shown that replacing the vinylic methyl group of a-pinene with larger groups also leads to higher asymmetric induction (eq 3).3
Treatment of NB-enantrane with t-Butyllithium provides the lithium trialkylborohydride NB-Enantride® (eq 4).4 This reagent is an effective asymmetric reducing agent for acetophenone and alkyl methyl ketones such as 2-octanone (eq 5). Few reagents show selectivity for such alkyl ketones.
M. Mark Midland
University of California, Riverside, USA