[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.

Preparative Methods: NB-Enantrane2 may be prepared by hydroboration of nopol benzyl ether (commercially available, or from nopol and benzyl bromide1a) with 9-Borabicyclo[3.3.1]nonane (9-BBN) (eq 1).

Handling, Storage, and Precautions: NB-Enantrane is an air-sensitive reagent and must be handled under an inert atmosphere. Use in a fume hood.

Asymmetric Reductions.

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

Borohydride Reagent.

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.

Related Reagents.

(+)-B-Chlorodiisopinocampheylborane; B-3-Pinanyl-9-borabicyclo[3.3.1]nonane.

1. (a) Midland, M. M.; Kazubski, A. JOC 1982, 47, 2814. (b) Midland, M. M. CRV 1989, 89, 1553.
2. NB-Enantrane is a trademark of Aldrich Chemical Company.
3. Brown, H. C.; Ramachandran, P. V.; Weissman, S. A.; Swaminathan, S. J. JOC 1990, 55, 6328.
4. Midland, M. M.; Kazubski, A.; Woodling, R. E. JOC 1991, 56, 1068.

M. Mark Midland

University of California, Riverside, USA

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