Zinc-Copper(II) Acetate-Silver Nitrate


[7440-66-6]  · Zn  · Zinc-Copper(II) Acetate-Silver Nitrate  · (MW 65.39) (AgNO3)

[7761-88-8]  · AgNO3  · Zinc-Copper(II) Acetate-Silver Nitrate  · (MW 169.88) (Cu(OAc)2.H2O)

[6046-93-1]  · C4H8CuO5  · Zinc-Copper(II) Acetate-Silver Nitrate  · (MW 199.67)

(reagent for selective reduction of activated or conjugated triple bonds to (Z)-alkenes;1,2 compatible with many functional groups)

Physical Data: see Zinc, Copper(II) Acetate, and Silver(I) Nitrate.

Solubility: insol organic solvents and water; reacts with dilute acids and bases.

Preparative Method: argon is passed for ca. 15 min through a well stirred suspension of commercial zinc dust (5.0 g, ca. <60 mm; >230 mesh ASTM) in water (30 mL). Cu(OAc)2.H2O (0.5 g) is added and after 15 min AgNO3 (0.5 g) is introduced (exothermic reaction). The mixture is stirred for 30 min, and the activated metal is collected by suction filtration under argon. The dark solid is washed with water (2 × 30 mL), MeOH (2 × 30 mL), acetone (2 × 30 mL), and Et2O (2 × 30 mL). The Et2O-moist Zn dust is transferred into 20 mL MeOH/H2O (1:1, v/v) and is ready for use.

Handling, Storage, and Precautions: the reagent has to be freshly prepared prior to use. The activated metal may be briefly stored covered with solvent (e.g. MeOH/H2O; 1/1, v/v) in the absence of oxygen. The dry reagent is pyrophoric!

Reduction of Triple Bonds.

Zinc metal, previously activated by successive treatment with Cu(OAc)2 (10%) and AgNO3 (10%), reduces conjugated or activated triple bonds stereospecifically to (Z)-alkenes.1,2 Best results are obtained in a polar protic solvent like MeOH/H2O (1:1, v/v) under neutral conditions. The conditions are exceptionally mild and most reductions can be conducted at 25 °C using an excess of activated metal (ca. 1:10 -> 100). As a consequence, even a thermolabile, acid- and base-sensitive tetraene like the naturally occurring 2Z,4Z,6E,8Z-undecatetraene (eq 1) can be prepared without competing isomerization and polymerization. Unlike Lindlar's catalyst (see Palladium on Calcium Carbonate (Lead Poisoned)), Zn(Cu/Ag) does not reduce simple, nonactivated alkynes. However, nonconjugated propargylic alcohols are readily reduced at 25 °C.

The reduction of (Z)-enynes is effected at 25 °C, but temperatures of 40-50 °C are necessary for simple (E)-enynes. The reduction of highly unsaturated dienediynes (eqs 1 and 2) proceeds at 25 °C independent of the enyne stereochemistry.1 Overreduced products are usually not observed. Both a- and b-branched enynes react sluggishly at elevated temperatures (40-50 °C; eq 3).3

The stereospecific reduction of conjugated enynes to (Z)-alkenes is ideally suited for the synthesis of leukotriene B4 (eq 4)4,5 and other fatty acid-derived polyenoic metabolites like (5S,12S)-DiHETE,6 (12R)- or (12S)-HETE,7 and (13S)-HODE.8 Hydrogenolytic fission of allylic alcohols, ethers, and esters does not occur.9,10 Reactive halides may be incompatible with the reagent. Ketones and esters are tolerated; however, aldehydes are reduced to alcohols. In general, Zn(Cu/Ag) exhibits a somewhat higher reactivity, but similar selectivity, to Rieke zinc metal which can be used for analogous reductions of propargylic alcohols at 65 °C in polar protic solvents.11 Introduction of deuterium atoms is possible using deuterated solvents (MeOD/D2O) and Zn(Cu/Ag), but complete deuteration is difficult to achieve. A recently disclosed stereoselective synthesis of conjugated (E)-trienes from 1.6-dibenzoate-2,4-dienes utilizes Zn(Cu/Ag) for the generation of the dienediol intermediates from readily available diynoic precursors (eq 5).12 The sequence has been extended to LTB4.10

1. Boland, W.; Schroer, N.; Sieler, C.; Feigel, M. HCA 1987, 70, 1025.
2. Keitel, J.; Fischer-Lui, I.; Boland, W.; Müller, D. G. HCA 1990, 73, 2101.
3. Frenzel, M.; Dettner, K.; Boland, W.; Erbes, P. E 1990, 46, 542.
4. Treilhou, M.; Fauve, A.; Pougny, J.-R.; Promé, J.-C.; Veschambre, H. JOC 1992, 57, 3203, and references cited therein.
5. Chemin, D.; Linstrumelle, G. T 1992, 48, 1943.
6. Chemin, D.; Alami, M.; Linstrumelle, G. TL 1992, 33, 2681.
7. Chemin, D.; Gueugnot, S.; Linstrumelle, G. T 1992, 48, 4369.
8. Chemin, D.; Linstrumelle, G. S 1993, 377.
9. Wender, P. A.; Mascareñas, J. L. JOC 1991, 56, 6267.
10. Solladié, G.; Stone, G. B.; Hamdouchi, C. TL 1993, 34, 1807.
11. Chou, W.-N.; Clark, D. L.; White, J. B. TL 1991, 32, 299.
12. Solladié, G.; Stone, G. B.; Rubio, A. TL 1993, 34, 1803.

Wilhelm Boland & Sabine Pantke

University of Karlsruhe, Germany

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