[119441-89-3]  · C8H16BBrO2  · (E)-(2-Bromovinyl)diisopropoxyborane  · (MW 234.93)

((E)-vinylation reagent, useful for synthesis of (E)-alkenes, a,b-unsaturated ketones, and a,b-unsaturated esters)

Physical Data: bp 73 °C/12 mmHg.

Solubility: sol ether, THF, CH2Cl2, and CHCl3.

Form Supplied in: colorless liquid; not available commercially.

Analysis of Reagent Purity: purity is analyzed by 1H NMR (CDCl3): d 1.17 (d, J = 6 Hz, 12H), 4.22-4.56 (m, 2H), 6.37 (d, J = 15 Hz, 1H), 7.06 (d, J = 15 Hz, 1H).

Preparative Methods: by the bromoboration of acetylene with tribromoborane,2 followed by the addition of isopropanol to the resulting 2-(bromovinyl)dibromoborane (eq 1).3,4 The reagent is isolated by distillation under nitrogen. Large quantities may be prepared and stored in a refrigerator.

Handling, Storage, and Precautions: moisture sensitive; can be handled in air for a short time but is gradually hydrolyzed to the boronic acid by moisture. It should be stored in a refrigerator under a dry inert gas and transferred by syringe. After prolonged storage it should be distilled under a dry inert gas before use. Sensitive to both base and acid; base treatment causes rapid decomposition by b-elimination.

Synthesis of (E)-Alkenes via Palladium-Catalyzed Stepwise Cross-Coupling Reactions.

Cross-coupling reactions of alkenylboranes with organic halides are known to proceed in the presence of a palladium catalyst and an appropriate base.5 In the absence of base, (E)-(2-bromovinyl)diisopropoxyborane reacts with organozinc compounds as a simple organic halide to give (E)-(1-alkenyl)diisopropoxyboranes. Subsequent addition of base and an organic halide promotes the coupling reaction between the alkenylborane and the organic halide to provide (E)-alkenes stereoselectively (eq 2).4 Though the isolation of the alkenylborane intermediate is possible, the reaction can be carried out without isolation of any intermediates. Alkynyl, alkenyl, alkyl, and aryl groups can be introduced on to both sides of the double bond. This method is an alternative to the cross-coupling reaction of alkenylboranes prepared by the hydroboration of alkynes, and is preferable especially when an alkyne is difficult to synthesize or has functional groups that are sensitive to hydroboration reactions.

a,b-Unsaturated Ketones.

This methodology is effective for the synthesis of a,b-unsaturated ketones using a-alkoxyalkenylzinc chlorides in the first cross-coupling step.6 The reaction proceeds under mild conditions, and is applicable for the synthesis of polyenones such as pseudoionone (eq 3).

This reaction has been applied to the synthesis of prostaglandin B1(PGB1).7 Stepwise cross-coupling of (1-methoxy-1-pentenyl)zinc chloride with (E)-(2-bromovinyl)-diisopropoxyborane followed by 3-bromo-2-[6-(methoxycarbonyl)hexyl]-2-cyclopenten-1-one (1) provides 15-dehydro-PGB1 directly in a one-pot operation (eq 4).

a,b-Unsaturated Esters.

A sequence involving cross-coupling and alkoxycarbonylation reactions of (E)-2-(bromovinyl)diisopropoxyborane gives a,b-unsaturated esters.8 Isolation of alkenylborane intermediates is not necessary, but removal of the solid generated in the first step is required. The second step is carried out under a carbon monoxide atmosphere after the addition of the other reagents (eq 5).

1. (a) Suzuki, A.; Hara, S. Res. Trends Org. Chem. 1990, 77. (b) Suzuki, A. PAC 1986, 58, 629.
2. (a) Lappert, M. F.; Prokai, B. JOM 1964, 1, 384. (b) Hyuga, S.; Chiba, Y.; Yamashina, N.; Hara, S.; Suzuki, A. CL 1987, 1757.
3. Brown, H. C.; Imai, T. OM 1984, 3, 1392.
4. Hyuga, S.; Yamashina, N.; Hara, S.; Suzuki, A. CL 1988, 809.
5. Miyaura, N.; Yamada, K.; Suginome, H.; Suzuki, A. JACS 1985, 107, 972.
6. Ogima, M.; Hyuga, S.; Hara, S.; Suzuki, A. CL 1989, 1959.
7. Hyuga, S.; Hara, S.; Suzuki, A. BCJ 1992, 65, 2303.
8. Yamashina, N.; Hyuga, S.; Hara, S.; Suzuki, A. TL 1989, 30, 6555.

Akira Suzuki & Shoji Hara

Hokkaido University, Sapporo, Japan

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