[92055-65-7]  · C23H39B  · B-Allyldiisocaranylborane  · (MW 326.38) (2-dIcr2BAll)

[124821-92-7]  · C23H39B  · B-Allyldiisocaranylborane  · (MW 326.38) (4-dIcr2BCrtZ)

[103818-03-7]  · C24H41B  · B-Crotyldiisocaranylborane  · (MW 340.41) (4-dIcr2BCrtE)

[103882-37-7]  · C24H41B  · B-Allyldiisocaranylborane  · (MW 340.41) (2-dIcr2BCrtZ)

[130540-34-0]  · C24H41B  · B-Allyldiisocaranylborane  · (MW 340.41) (2-dIcr2BCrtE)

[130609-25-5]  · C24H41B  · B-Allyldiisocaranylborane  · (MW 340.41)

(reagents for the asymmetric allyl- and crotylboration of aldehydes to produce secondary homoallylic alcohols2 and b-methylhomoallylic alcohols3)

Solubility: these reagents are prepared and used in situ at -78 °C in Et2O (Icr2BAll) or THF (Icr2BCrt).

Preparative Methods: both reagents are derived from the corresponding B-methoxydiisocaranylborane which is prepared in two steps from (+)-3- or (+)-2-carene (eqs 1 and 2). Note: only the (+) isomer of 3-carene occurs naturally; (+)-2-carene is obtained by base-catalyzed isomerization, and both are commercially available.

Allylboration of Aldehydes.

The B-allyldiisocaranylboranes (Icr2BAll) condense with aldehydes with exceptional levels of enantioselectivity (98- &egt; 99% ee) to form, upon workup, secondary homoallylic alcohols.2 While both regioisomers of this reagent (2-dIcr2BAll and 4-dIpc2BAll) are derived from the (+)-terpene, they exhibit complementary reactivity to form enantiomeric products (eqs 3 and 4). The degree of enantioselection achieved in the condensation of Icr2BAll with aldehydes is superior to other means of allylboration1 (see also B-Allyldiisopinocampheylborane; Diisopropyl 2-Allyl-1,3,2-dioxaborolane-4,5-dicarboxylate; (R,R)-2,5-Dimethylborolane).

Crotylboration of Aldehydes.

The crotyl analogs of this reagent (Icr2BCrt) likewise provide very high levels of enantio- and diastereoselectivity (&egt;99% de) when condensed with aldehydes.3 By varying the geometry of the crotyl group (Z or E) and the (+)-carene isomer used in the reagent preparation (2- or 3-carene), all four of the possible b-methyl homoallylic alcohol products can be obtained (eqs 5 and 6). As with allylborations, the condensation of B-crotyldiisocaranylboranes with aldehydes provide higher levels of enantioselection than the other crotylboration reagents3 (see also B-Crotyldiisopinocampheylborane; (Z) and (E)-Diisopropyl 2-Allyl-1,3,2-dioxaborolane-4,5-dicarboxylate; (R,R)-2,5-Dimethylborolane).

1. Brown, H. C.; Ramachandran, P. V. PAC 1991, 63, 307.
2. (a) Brown, H. C.; Jadhav, P. K. JOC 1984, 49, 4089. (b) Brown, H. C.; Randad, R. S.; Bhat, K. S.; Zaidlewicz, M.; Racherla, U. S. JACS 1990, 112, 2389. (c) Racherla, U. S.; Brown, H. C. JOC 1991, 56, 401.
3. (a) Brown, H. C.; Bhat, K. S. JACS 1986, 108, 5919. (b) Brown, H. C.; Randad, R. S. T 1990, 46, 4457.

Mark T. Goulet

Merck Research Laboratories, Rahway, NJ, USA

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