[51458-06-1]  · C18H23B  · Dimesitylborane  · (MW 250.19)

(very selective hydroborating reagent for alkenes and alkynes;2 preparation of boron-stabilized alkyl and vinyl carbanions2,3)

Alternate Names: bis(2,4,6-trimethylphenyl)borane; Mes2BH.

Physical Data: mp 162-164 °C.

Solubility: moderate sol THF (37.5 g L-1).

Form Supplied in: white solid.

Preparative Methods: from Mes2BF and LiAlH4;1,2a Mes2BH2Li (Mes2BF + 2 LiH) and benzyl chloride2a or MeI;2b Mes2BF and LiH.2c

Purification: recrystallization from 1,2-dimethoxyethane.1,2a

Handling, Storage, and Precautions: solid stable in air and in THF but reacts with moisture and protic solvents. Best stored under an inert atmosphere. Use in a fume hood.

Hydroboration of Alkenes.

Mes2BH is a highly selective reagent for the hydroboration of alkenes.2a In general, suspensions of the reagent in THF react slowly with mono- (1) and 1,2-disubstituted (2) alkenes (8-25 h, 25-65 °C) and reactions are very sensitive to steric factors. The air-stable (and often crystalline) dimesitylboranes which result may be oxidized to alcohols with alkaline Hydrogen Peroxide (eq 1). Geminally disubstituted alkenes and cycloalkenes react very slowly with Mes2BH.

Alkyl dimesitylboranes of the type produced by these hydroboration reactions also have been found to be useful in the preparation of boron-stabilized carbanions which may be alkylated3 (eq 2) or converted into alkenes,4 glycols,5 or ketones6 upon reaction with carbonyl compounds. The bulky dimesitylboryl group not only imparts stability to its derivatives but sterically promotes the Cg-alkylation of allylic carbanions.7

Hydroboration of Alkynes.

Generally, Mes2BH reacts rapidly (within minutes) with alkynes, giving aldehydes and ketones upon oxidation of resulting vinylboranes (eq 3).2a The slow rate of reaction of Mes2BH with cyclohexanone2c suggests that such relatively rapid hydroborations could be conducted in the presence of many common nonacidic functional groups. Compatability with halides,8 phosphines, and ferrocene9 has been demonstrated. Unprecedented regioselectivity was observed in the hydroboration-oxidation of 2-hexyne (3) where isomeric 2- and 3-hexanone were produced in a ratio of 90:10 (cf. 60:40 with Diborane and 78:22 with 9-Borabicyclo[3.3.1]nonane), making this the most selective hydroborating agent of unsymmetrical alkynes known (see also 4).2a

The directing influence of silyl and stannyl groups has been used regioselectively to prepare a-metallovinylboranes which are precursors of boron-stabilized vinyl carbanions (eq 4).2b

The resistance to boron ate-complex formation imparted by the mesityl groups to certain vinyl dimesitylboranes has been shown to enable metal-halogen exchange-initiated nucleophilic addition reactions to the boron-activated double bond (eq 5).8

Related Reagents.

9-Borabicyclo[3.3.1]nonane; Borane-Dimethyl Sulfide; Borane-Tetrahydrofuran; Dicyclohexylborane; Disiamylborane.

1. Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic: New York, 1988; p 429.
2. (a) Pelter, A.; Simgaram, S.; Brown, H. TL 1983, 24, 1433. (b) Pelter, A.; Smith, K., Parry, D. E.; Jones, K. D. AJC 1992, 45, 57. (c) Hooz, J.; Akiyama, S.; Cedar, F. J.; Bennett, M. J.; Tuggle, R. M. JACS 1974, 96, 274.
3. Pelter, A.; Williams, L.; Wilson, J. W. TL 1983, 24, 627.
4. Pelter, A.; Singaram, B.; Wilson, J. W. TL 1983, 24, 635.
5. Pelter, A.; Buss, D.; Pitchford, A. TL 1985, 26, 5093.
6. Pelter, A.; Smith, K.; Elgendy, S.; Rowlands, M. TL 1989, 30, 5643.
7. Pelter, A.; Singaram, B.; Wilson, J. W. TL 1983, 24, 631.
8. (a) Cooke, M. P., Jr.; Widener, R. K. JACS 1987, 109, 931. (b) Cooke, M. P., Jr. JOC 1992, 57, 1495.
9. Yuan, Z.; Taylor, N. J.; Sun, Y.; Marder, T. B. JOM 1993, 449, 27.

Manning P. Cooke, Jr.

Washington State University, Pullman, WA, USA

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