[85116-38-7]  · C23H39B  · B-Allyldiisopinocampheylborane  · (MW 324.36) (-)-(Ipc2BAll)


(reagent for the asymmetric allylboration of aldehydes to produce homoallylic alcohols2,3)

Solubility: most often used as the crude preparation in Et2O; however, reactions have also been conducted in CS2, CHCl3, CH2Cl2, toluene, and THF.

Analysis of Reagent Purity: 11B NMR (d +78, diethyl ether).

Preparative Methods: prepared in three steps from either (+)- or (-)-a-pinene (eq 1) (see also B-Methoxydiisopinocampheylborane).

Handling, Storage, and Precautions: removal of magnesium salts from the crude reagent preparation is accomplished by solvent exchange into pentane and filtration. Concentration in vacuo provides the neat allylborane as a colorless liquid that can be stored under argon atmosphere for an extended period of time.

Addition to Aldehydes.

Condensation of B-allyldiisopinocampheylborane (Ipc2BAll) with aldehydes occurs in good yield (50-90%) and with excellent enantioselection (83-96% ee) to provide secondary, homoallylic alcohols.2 Improvements in the original procedure have produced a salt-free reagent preparation that can increase the enantioselectivity of many additions to &egt;99% ee (eqs 2 and 3).3

Several methods have been employed in the reaction workup to decompose the initial borinic ester adduct (eq 4).2,4 An oxidative workup procedure (H2O2, NaOH) delivers the desired alcohol product along with 2 equiv of isopinocampheol. The elimination workup (i-PrCHO, BF3.OEt2) allows for recovery of the starting a-pinene, whereas the amino alcohol workup (ethanolamine or 8-hydroxyquinoline) precipitates the diisopinocampheylborinate-amino alcohol adducts which are removed by filtration, and which can also be recycled.

The (+)- and (-)-Ipc2BAll reagents have been condensed with a variety of aldehyde structures2,5 with no reported reversal of enantiospecificity. Because of this strict fidelity of addition, these reagents have been used in the structure determination of several natural products.6 In the case of a-substituted aldehydes, however, an erosion of diastereofacial selectivity is sometimes observed for mismatched reactant pairs (eqs 5-7).7

These reagents have been used for the conversion of Cs symmetrical chains into chiral, nonracemic products. A double condensation with meso dialdehydes can produce either optically active diol product (eq 8).8

B-Allyldiisopinocampheylborane compares favorably in both yield and enantioselectivity to alternate methods of asymmetric allylboration1 (see also B-Allyldiisocaranylborane; Diisopropyl 2-Allyl-1,3,2-dioxaborolane-4,5-dicarboxylate; (R,R)-2,5-Dimethylborolane).

Addition to Ketones.

Condensation of Ipc2BAll with methyl ketones proceeds with modest to good enantioselectivity. Unlike with aldehydes, the degree of enantioselection is highly dependent upon ketone structure (eq 9).2

B-Methallyldiisopinocampheylborane and Related Reagents.

Derivatives of the reagent containing substituted allyl groups have been used for the synthesis of more highly functionalized homoallylic alcohols (see also B-Crotyldiisopinocampheylborane and B-Allyldiisocaranylborane). In general, these reagents are prepared in a manner analogous to Ipc2BAll or by hydroboration of the corresponding diene with diisopinocampheylborane. For example, reagents containing alkyl substituents, such as B-methallyldiisopinocampheylborane and B-(3,3-dimethylallyl)diisopinocampheylborane, condense with aldehydes to give the corresponding substituted homoallylic alcohols (eqs 10 and 11).9 An alkenyl-substituted derivative, B-2-isoprenyldiisopinocampheylborane, has been used for the asymmetric isoprenylation of aldehydes (eq 12).10

Derivatives containing heteroatoms have allowed for the preparation of allylic 1,2-diols and amino alcohols by this method. Aldehydes react with B-[(Z)-3-(methoxymethyloxy)allyl]diisopinocampheylborane to give monoprotected diol adducts.11 1,2-Diols can be obtained directly by condensation with B-[(E)-3-((diisopropylamino)dimethylsilyl)allyl]diisopinocampheylborane followed by an oxidative reaction workup (eqs 13 and 14).12

Amine-containing reagents, such as B-[(E)-3-(diphenylamino)allyl]diisopinocampheylborane, react in a similar manner to provide 1,2-amino alcohols (eq 15).13 A further variation on this reagent is where the allylic group is contained in a ring. The cyclohexenyl reagent B-(cyclohex-2-enyl)diisopropylcampheylborane and the related cycloheptenyl and cyclooctenyl versions have been described.14 At low temperatures (< -25 °C), these boranes are stable toward allylic isomerization and transfer with complete diastereoselectivity to provide erythro products (eq 16).

1. Brown, H. C.; Ramachandran, P. V. PAC 1991, 63, 307.
2. (a) Brown, H. C.; Jadhav, P. K. JACS 1983, 105, 2092. (b) Jadhav, P. K.; Bhat, K. S.; Perumal, P. T.; Brown, H. C. JOC 1986, 51, 432.
3. Racherla, U. S.; Brown, H. C. JOC 1991, 56, 401.
4. Brown, H. C.; Racherla, U. S.; Liao, Y.; Khanna, V. V. JOC 1992, 57, 6608.
5. For a representation of functionality that is tolerated in this reaction see: (a) Stork, G.; Zhao, K. JACS 1990, 112, 5875. (b) Nicolaou, K. C.; Groneberg, R. D.; Stylianides, N. A.; Miyazaki, T. CC 1990, 1275. (c) Rychnovsky, S. D.; Rodriguez, C. JOC 1992, 57, 4793. (d) Racherla, U. S.; Liao, Y.; Brown, H. C. JOC 1992, 57, 6614.
6. (a) Schreiber, S. L.; Goulet, M. T. JACS 1987, 109, 8120. (b) Nicolaou, K. C.; Ahn, K. H. TL 1989, 30, 1217. (c) Mori, Y.; Kohchi, Y.; Noguchi, H.; Suzuki, M.; Carmeli, S.; Moore, R. E.; Patterson, G. M. L. T 1991, 47, 4889.
7. (a) Brown, H. C.; Bhat, K. S.; Randad, R. S. JOC 1987, 52, 319. (b) Brown, H. C.; Bhat, K. S.; Randad, R. S. JOC 1989, 54, 1570.
8. Wang, Z.; Deschênes, D. JACS 1992, 114, 1090.
9. (a) Brown, H. C.; Jadhav, P. K. TL 1984, 25, 1215. (b) Brown, H. C.; Jadhav, P. K.; Perumal, P. T. TL 1984, 25, 5111. (c) Truesdale, L. K.; Swanson, D.; Sun, R. C. TL 1985, 26, 5009.
10. (a) Brown, H. C.; Randad, R. S. TL 1990, 31, 455. (b) Brown, H. C.; Randad, R. S. T 1990, 46, 4463.
11. (a) Brown, H. C.; Jadhav, P. K.; Bhat, K. S. JACS 1988, 110, 1535. (b) Burgess, K.; Chaplin, D. A.; Henderson, I.; Pan, Y. T.; Elbein, A. D. JOC 1992, 57, 1103. (c) Barrett, A. G. M.; Edmunds, J. J.; Horita, K.; Parkinson, C. J. CC 1992, 1236.
12. (a) Barrett, A. G. M.; Malecha, J. W. JOC 1991, 56, 5243. (b) Barrett, A. G. M.; Edmunds, J. J.; Hendrix, J. A.; Malecha, J. W.; Parkinson, C. J. CC 1992, 1240.
13. Barrett, A. G. M.; Seefeld, M. A. CC 1993, 339.
14. (a) Brown, H. C.; Jadhav, P. K.; Bhat, K. S. JACS 1985, 107, 2564. (b) Brown, H. C.; Bhat, K. S.; Jadhav, P. K. JCS(P1) 1991, 2633.

Mark T. Goulet

Merck Research Laboratories, Rahway, NJ, USA

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