[7440-66-6]  · Zn  · Zinc-1,2-Dibromoethane  · (MW 65.39) (BrCH2CH2Br)

[106-93-4]  · C2H4Br2  · Zinc-1,2-Dibromoethane  · (MW 187.86)

(combination reagent used for the regio- and cis-stereospecific reduction of alkynes)

Physical Data: zinc: mp 419.5 °C; d 7.140 g cm-3; 1,2-dibromoethane: mp 9-10 °C; bp 131-132 °C; d 2.18 g cm-3.

Solubility: 1,2-dibromoethane: sol alcohol, ether, acetone, benzene.

Form Supplied in: zinc: dust, foil, granular, mossy, pieces, powder, shot, splatter, sticks, wire; 1,2-dibromoethane: clear oil.

Preparative Method: the reducing agent is prepared by refluxing Zn and 1,2-dibromoethane in ethanol until evolution of ethylene has subsided.

Purification: the reducing agent is not purified prior to use.

Handling, Storage, and Precautions: for specific information, consult Zinc and 1,2-Dibromoethane.

General Discussion.

1,2-Dibromoethane-activated zinc serves as a versatile, mild reducing agent for alkynes. The products are exclusively (Z)-alkenes with yields ranging from 70-90%.1 In substrates containing two differentiated alkyne linkages the reagent exhibits selectivity. Conjugated triple bonds are reduced preferentially over nonconjugated bonds. Terminal alkynes are reduced over internal alkynes (eq 1). However, if the terminal alkyne has an appending silyl group, the internal bond is reduced (eq 2).

Hetero-substituted diynes yield products with the triple bond nearest the heteroatom being reduced. Sulfur does not poison the reaction: 2-thienylacetylene was converted to 2-vinylthiophene (80%). Other heteroatoms (OR, NR2) are also stable under these conditions.

The activity of this reagent can be improved, at the expense of selectivity, by adding Copper(I) Bromide/Lithium Bromide to the zinc/1,2-dibromoethane mixture prior to the addition of the substrate.2 Under these conditions, monoalkynes containing a heteroatom (OR, NR2) are reduced to (Z)-alkenes. Sulfides are not reduced under these conditions.

1. Aerssens, M. H. P. J.; Brandsma, L. CC 1984, 735.
2. Aerssens, M. H. P. J.; van der Heiden, R.; Heus, M.; Brandsma, L. SC 1990, 20, 3421.

Brian A. Roden

Abbott Laboratories, North Chicago, IL, USA

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