Zinc-Zinc Chloride

Zn-ZnCl2
(Zn)

[7440-66-6]  · Zn  · Zinc-Zinc Chloride  · (MW 65.39) (ZnCl2)

[7646-85-7]  · Cl2Zn  · Zinc-Zinc Chloride  · (MW 136.29)

(mild reducing agent; causes reductive elimination of vicinal heteroatoms;1-3 reduces activated carbonyls4 and alkenes;5 reductively opens some strained rings6)

Physical Data: see Zinc and Zinc Chloride.

Form Supplied in: ZnCl2 is available both as solid and in ether solution. The latter has not been used in most of the following examples, but is very likely to be suitable in many cases. Various grades of Zn or ZnCl2 can find use. Utilization under moist conditions (as well as anhydrous) has been noted.

Reductive Vicinal Eliminations.1-3

Zinc powder in the presence of catalytic amounts of ZnCl2 causes reductive elimination of vicinal halogens (except fluorine, which is inert to these conditions), thereby yielding alkenes1 or alkynes.2 These reactions, performed in alcoholic solvents, are essentially similar to those effected by Zinc-Acetic Acid. Similar treatment of bromo ether (1) also results in alkene formation (eq 1).3

Reduction.4,5

The keto group of a-ketoamides,4 and a single carbonyl of a-diketones,4 are reduced efficiently to the alcohol by Zn/ZnCl2 in EtOH. This reaction, of course, can be carried out by a host of other reagents. Quinones4 are also reduced to hydroquinones, in a reaction claimed to be superior to the procedure using Chromium(II) Chloride.7 The presence of ZnCl2, however, is not crucial to this reaction; it can be omitted, or replaced by a variety of other metal salts.4 Activated alkenes may also be reduced, as shown in eqs 25a and 35b.

Reductive Ring Opening.6

Strained (three- or four-membered) carbocyclic ring systems (2) may be reductively opened by Zn/ZnCl2 in protic solvents, if substituted by carbonyl groups (eq 4). For four-membered rings, two carbonyl substituents (i.e. R2 = COR3) are required;6a a similar reaction can also be effected by Zn/AcOH.8 Three-membered rings react much more readily;6a the greater strain in this case permits cleavage of (2) (R2 = aryl) by a very much extended reaction time,6c and even with Zn/AcOH.6c

Reductive Coupling.9

Aryl aldehydes, on exposure to Zn/ZnCl2, usually react to give a mixture of the respective benzyl alcohol and the dimerized a-glycol (3) (R = H), the latter being favored by absence of solvent during the reaction. In aq THF, solvent composition affects the isomeric composition of the glycols. Acetophenones and diaryl ketones yield the glycols in excellent yields. This reagent is insensitive to oxygen, and reacts efficiently at room temperature, giving appreciable advantage over other favored reagents such as Aluminum, Titanium(IV) Chloride, or RZ021-.9


1. (a) Sauer, J. OSC 1963, 4, 268. (b) Tarrant, P.; Lilyquist, M. R. JACS 1955, 77, 3640.
2. Finnegan, W. G.; Norris, W. P. JOC 1963, 28, 1139.
3. Grummit, O.; Budewitz, E. P.; Chudd, C. C. OSC 1963, 4, 748.
4. Toda, F.; Tanaka, K.; Tange, H. JCS(P1) 1989, 1555.
5. (a) Toda, F.; Iida, K. CL 1976, 695. (b) Kishigami, S.; Tanaka, K.; Toda, F. CL 1990, 1877.
6. (a) Dekker, J.; Martins, F. J. C.; Kruger, J. A. TL 1975, 2489. (b) Dekker, J.; Martins, F. J. C.; Kruger, J. A.; Goosen, A. J. TL 1974, 3721. (c) Murphy, W. S.; Wattanasin, S. JCS(P1) 1986, 1445.
7. Hanson, J. R.; Mehta, S. JCS(C) 1969, 2349.
8. Wenkert, E.; Yoder, J. E. JOC 1970, 35, 2985.
9. Tanaka, K.; Kishigami, S.; Toda, F. JOC 1990, 55, 2981.

Peter Ham

SmithKline Beecham Pharmaceuticals, Harlow, UK



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