[22086-45-9]  · C8H14BBr  · 9-Bromo-9-borabicyclo[3.3.1]nonane  · (MW 200.91)

(selective reagent for the cleavage of ethers2 and deoxygenation of sulfoxides;3 adds regioselectively and stereoselectively to terminal alkynes;4 undergoes conjugate addition to a,b-unsaturated ketones;5 converts vinyltins to vinylboranes10)

Alternate Name: Br-9-BBN.

Physical Data: colorless liquid; bp 54-59 °C/0.3-0.4 mmHg; mp -17 °C; d 1.315 g cm-3.

Solubility: sol pentane, CH2Cl2; dec ethers, alcohols.

Form Supplied in: 1.0 M solution in CH2Cl2.

Analysis of Reagent Purity: the reagent exhibits a single resonance in its 11B NMR (neat) spectrum at d 83.9 (s) ppm.2

Handling, Storage, and Precautions: the reagent is pyrophoric and reacts violently with moisture and protic solvents. Individuals should thoroughly familiarize themselves with the special handling techniques required for such reagents prior to use.1 Use in a well-ventilated fume hood.

Selective Cleavage of Ethers.

In methylene chloride solution, Br-9-BBN readily cleaves a variety of common ethers at 25 °C.2 For dibutyl ether the reagent reacts smoothly to produce 1-bromobutane quantitatively in 24 h. This behavior contrasts to that of polyfunctional reagents such as Boron Tribromide whose rate is markedly slowed after two of its three bromides have been used. The bromine is selectively transferred to tertiary and secondary vs. primary groups, the process occurring with 73% inversion in the case of (R)-(-)-2-methoxyoctane (eq 1). Alkyl aryl ethers are also selectively cleaved, with no Claisen products from allyl phenyl ether as is observed with Boron Trichloride. One of the methoxy groups in hydroquinone dimethyl ether can be selectively cleaved with the reagent, whereas the methylenedioxy moiety must be completely removed with two equivalents of the reagent under more forcing conditions.

Deoxygenation of Sulfoxides.

Br-9-BBN rapidly and cleanly deoxygenates sulfoxides with no accompanying rearrangement in CH2Cl2 at or below 0 °C.3 The reagent gives isolated yields which equal or exceed those from either Boron Tribromide or Bromodimethylborane, all of which provide the corresponding sulfides in excellent yields even with polyfunctionalized substrates (eq 2).

Addition to Terminal Alkynes.

The syn bromoboration of terminal acetylenes with Br-9-BBN provides a simple route to (Z)-2-bromovinylboranes (eq 3).4 The addition is highly regioselective (98-99%) even for functionalized substrates such as propargyl bromide. Reaction occurs over ca. 3 h at 0 °C in CH2Cl2 and both ester and alkenic functionalities can be tolerated in the alkyne. The process has been effectively used in the asymmetric synthesis of (-)-ipsenol.4b

The (Z)-2-bromovinyl-9-BBN adducts undergo additional conversions which lead to stereoselective carbon-carbon bond formation. For example, the iodination of the alkynyl borate complexes derived from these vinylboranes produces the corresponding (Z)-brominated enynes in 53-72% yield for representative combinations (eq 4).6 However, it must be mentioned that for certain applications (e.g. oxidation,7 halogenation,8 Pd-catalyzed cross coupling9) the use of the 1:1 adducts derived from the addition of BBr3 to 1-alkynes is preferable to the corresponding 9-BBN derivatives.

Conjugate Addition to a,b-Unsaturated Ketones and Aldol Reactions.

Br-9-BBN undergoes 1,4-addition to conjugated enones to produce g-brominated (Z)-enolboranes which undergo stereoselective crossed aldol reactions with a variety of aldehydes.5 Elimination occurs under the reaction conditions leading to a-bromomethylated enones in good overall yields (50-60%) in a one-pot sequence (eq 5). The stereochemistry and successful substrates suggest that the addition takes place through the cisoid form of the starting enone.

Vinylboranes from Vinylstannanes.

Like Cl-9-BBN, Br-9-BBN undergoes transmetalation with vinylstannanes to produce vinylboranes (eq 6), the trans-1,2-bis-9-BBN derivative being less stable than its BMe2 or catecholboryl analog (see B-Bromocatecholborane).10

1. (a) Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic: London, 1988. (b) Brown, H. C.; Midland, M. M.; Levy, A. B.; Kramer, G. W. Organic Synthesis via Boranes; Wiley: New York, 1975.
2. Bhatt, M. V. JOM 1978, 156, 221.
3. Guindon, Y.; Atkinson, J. G.; Morton, H. E. JOC 1984, 49, 4538.
4. (a) Hara, S.; Dojo, H.; Takinami, S.; Suzuki, A. TL 1983, 24, 731. (b) Ikeda, N.; Arai, I.; Yamamoto, H. JACS 1986, 108, 483.
5. Shimizu, H.; Hara, S.; Suzuki, A. SC 1990, 20, 549.
6. Hara, S.; Satoh, Y.; Ishiguro, H.; Suzuki, A. TL 1983, 24, 735.
7. Satoh, Y.; Tayano, T.; Koshino, H.; Hara, S.; Suzuki, A. S 1985, 406.
8. Hara, S.; Kato, T.; Shimizu, H.; Suzuki, A. TL 1985, 26, 1065.
9. Satoh, Y.; Serizawa, H.; Miyaura, N.; Hara, S.; Suzuki, A. TL 1988, 29, 1811.
10. Singleton, D. A.; Redman, A. M. TL 1994, 35, 509.

John A. Soderquist

University of Puerto Rico, Rio Piedras, Puerto Rico

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.