[99397-92-9]  · C24H41B  · B-Crotyldiisopinocampheylborane  · (MW 340.40) (+)-(E)

[99438-29-6] (-)-(Z)

[99438-30-9] (-)-(E)


(reagent for the asymmetric crotylboration of aldehydes to produce either syn- or anti-b-methylhomoallylic alcohols2)

Solubility: reagent is prepared and used in situ at -78 °C in THF.

Preparative Methods: prepared from B-Methoxydiisopinocampheylborane and either (Z)- or (E)-crotylpotassium (eq 1).

Handling, Storage, and Precautions: because of the facile isomerization of allylic boranes, crotylborane reagents should be generated and reacted at -78 °C.

Addition to Aldehydes.

B-Crotyldiisopinocampheylboranes (Ipc2BCrt) condense with aldehydes at low temperatures (-78 °C) to provide b-methylhomoallylic alcohols in 60-80% yield and with excellent enantio- (90-94% ee) and diastereoselection (&egt;99% de).2 Several workup procedures have been employed to decompose the initial borinate adduct and facilitate recovery of the product from the chiral auxiliary (see B-Allyldiisopinocampheylborane).3 This reaction is stereospecific, with (Z)-crotylboranes giving syn products, and (E)-crotylboranes providing the anti isomers. By varying the configuration of Ipc2BOMe (+ or -) and the crotylpotassium alkene isomer (Z or E) used in the reagent preparation, all four isomers of the product b-methylhomoallylic alcohols can be obtained (eqs 2 and 3).

Ipc2BCrt has been used with a variety of aldehyde structures in the synthesis of natural products containing polypropionate-derived units (eq 4).4,5

Addition to a-Substituted Aldehydes.

Addition of Ipc2BCrt to a-substituted aldehydes allows the preparation of extended polypropionate arrays. In this reaction the diastereofacial selectivity is good, even in the case of mismatched double asymmetric reactions (eqs 5 and 6).6

As with simple allylboration (see B-Allyldiisopinocampheylborane), the Ipc2BCrt reagents provide comparable or, in many cases, superior levels of enantioselection compared to alternative methods of crotylboration6b (see also B-Allyldiisocaranylborane, Diisopropyl 2-Allyl-1,3,2-dioxaborolane-4,5-dicarboxylate and (R,R)-2,5-Dimethylborolane).

1. Brown, H. C.; Ramachandran, P. V. PAC 1991, 63, 307.
2. (a) Brown, H. C.; Bhat, K. S. JACS 1986, 108, 293. (b) Brown, H. C.; Bhat, K. S. JACS 1986, 108, 5919.
3. Brown, H. C.; Racherla, U. S.; Liao, Y.; Khanna, V. V. JOC 1992, 57, 6608.
4. Boeckman, R. K.; Charette, A. B.; Asberom, T.; Johnston, B. H. JACS 1987, 109, 7553.
5. For other synthetic applications of this reagent, see: (a) Merifield, E.; Steel, P. G.; Thomas, E. J. CC 1987, 1826. (b) Diez-Martin, D.; Grice, P.; Kolb, H. C.; Ley, S. V.; Madin, A. SL 1990, 327. (c) Merifield, E.; Thomas, E. J. CC 1990, 464. (d) Ireland, R. E.; Wipf, P.; Roper, T. D. JOC 1990, 55, 2284. (e) Barrett, A. G. M.; Lebold, S. A. JOC 1990, 55, 5818.
6. (a) Brown, H. C.; Bhat, K. S.; Randad, R. S. JOC 1987, 52, 3701. (b) Brown, H. C.; Bhat, K.; Randad, R. S. JOC 1989, 54, 1570.

Mark T. Goulet

Merck Research Laboratories, Rahway, NJ, USA

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