Potassium 9-Siamyl-9-boratabicyclo[3.3.1]nonane1

(R = siamyl, 3-methyl-2-butyl)

[125392-39-4]  · C13H26BK  · Potassium 9-Siamyl-9-boratabicyclo[3.3.1]nonane  · (MW 232.30) (R = n-Bu)

[119875-90-0]  · C12H24BK  · Potassium 9-Butyl-9-boratabicyclo[3.3.1]nonane  · (MW 218.27) (R = t-Bu)

[119875-89-7]  · C12H24BK  · Potassium 9-t-Butyl-9-boratabicyclo[3.3.1]nonane  · (MW 218.27)

(selective reducing agent for ketones2 and aromatic nitriles to aldehydes3)

Physical Data: colorless in THF solution.

Solubility: sol THF; evolves hydrogen in protic solvents.1

Analysis of Reagent Purity: the reagents exhibit typical upfield 11B NMR resonances in THF (e.g. KHR-9-BBN: R = n-Hx, d -13.2 (d, JB-H = 70 Hz); R = Ipc, -6.1 (d, JB-H = 73 Hz); R = t-Bu, -10.4 (t, JB-H = 75 Hz) ppm).2,4

Preparative Methods: prepared as stable THF solutions from the reaction of a purified suspension of Potassium Hydride in THF with the appropriate B-alkyl-9-BBN.4 KH is purified by removal of the mineral oil by successive pentane washing and decantation of the supernatant under an inert atmosphere, treatment with a solution of Lithium Aluminum Hydride in THF followed by additional washings with fresh dry THF. This provides KH which reacts with B-alkyl-9-BBN in THF to form the borohydrides in <30 min at 25 °C. It is recommended that the initial addition be carried out at 0 °C to prevent an exothermic reaction with unhindered derivatives. Stable solutions of KHB(O-i-Pr)3 can be used stoichiometrically to prepare KHR-9-BBN from B-R-9-BBN or, alternatively, as a catalyst for the KH method.5

Handling, Storage, and Precautions: THF solutions of the reagent are indefinitely stable at 25 °C when stored under a nitrogen atmosphere. Exposure to atmospheric oxygen, moisture, or protic solvents results in its potentially dangerous rapid oxidation and/or the exothermic generation of hydrogen gas. The generation and use of KHR-9-BBN involves the handling of dangerously pyrophoric or flammable reagents (i.e. KH, LiAlH4, or B-R-9-BBN). Individuals should thoroughly familiarize themselves with the special handling techniques required for such reagents prior to their use.1 The reagent should be used in a well ventilated hood.

Selective Reduction of Ketones.

Cyclic and bicyclic ketones are quantitatively reduced to the corresponding alcohols with KHR-9-BBN reagents, the diastereofacial selectivities being determined both by steric and conformational effects in the substrate and the steric bulk of the B-R substituent in the reagent (eq 1).2 Thus KH(t-Bu)-9-BBN exhibits selectivities which are comparable to the high selectivity Lithium Trisiamylborohydride reagent, exceeding those of less hindered borohydrides such as Lithium Tri-s-butylborohydride or KH(OR)-9-BBN.

Aldehydes from Nitriles.

Aromatic nitriles are efficiently reduced to aldehydes (60-98%) with 1 equiv of KH(Sia)-9-BBN (24 h, 25 °C; eq 2).3 In competitive experiments, the selective reduction of aromatic nitriles can be accomplished in the presence of their aliphatic counterparts which are recovered intact. The reagent also readily reduces acid chlorides, esters, lactones, aldehydes, ketones, and epoxides to the corresponding alcohols. Amines, carboxylic acids, oximes, and aliphatic nitro compounds are not reduced.2b

Related Reagents.

Lithium 9-boratabicyclo[3.3.1]nonane; Lithium Tri-s-butylborohydride; Lithium Trisiamylborohydride.

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. (c) Brown, H. C.; Krishnamurthy, S. T 1979, 35, 567.
2. (a) Cha, J. S.; Yoon, M. S.; Kim, Y. S.; Lee, K. W. TL 1988, 29, 1069. (b) Cha, J. S.; Yoon, M. S.; Lee, K. W.; Lee, J. C. Bull. Korean Chem. Soc. 1989, 10, 75. (c) See also: Cho, B. T.; Chun, Y. S. TA 1992, 3, 73.
3. (a) Cha, J. S.; Yoon, M. S. TL 1989, 30, 3677. (b) Babler, J. H. SC 1989, 19, 355.
4. (a) Soderquist, J. A.; Rivera, I. TL 1988, 29, 3195. See also: (b) Brown, C. A.; Krishnamurthy, S. JOM 1978, 156, 111. (c) Brown, H. C.; Singaram, B.; Mathew, C. P. JOC 1981, 46, 2712.
5. (a) Brown, C. A.; Hubbard, J. L. JACS 1979, 101, 3964. (b) Brown, C. A.; Krishnamurthy, S. JOC 1986, 51, 238.

John A. Soderquist & Eduvigis González

University of Puerto Rico, Rio Piedras, Puerto Rico

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