Tin(IV) Chloride-Zinc Chloride1

SnCl4-ZnCl2
(SnCl4)

[7646-78-8]  · Cl4Sn  · Tin(IV) Chloride-Zinc Chloride  · (MW 260.51) (ZnCl2)

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

(catalyst for bond formation between silyl nucleophiles and carbonyl groups or enones1)

Physical Data: SnCl4: mp -33 °C; bp 114.1 °C; d 2.226 g cm-3; ZnCl2: mp ca. 290 °C; bp 732 °C.

Solubility: both reagents react violently with water. SnCl4: sol cold H2O; dec hot H2O; sol alcohol, ether, CCl4, benzene, toluene, acetone. ZnCl2: sol H2O (432 g/100 g at 25 °C, 614 g/100 g at 100 °C); 2% HCl (4 g/1 mL); alcohol (1 g/1.3 mL); glycerol (1 g/2 mL); freely sol acetone.

Form Supplied in: blend not commercially available. Anhydrous SnCl4: colorless liquid; 1 M soln CH2Cl2 or heptane; widely available. Anhydrous ZnCl2: white, odorless granules, lumps, or rods; 1 M soln in Et2O, 0.5 M soln in THF; widely available.

Purification: SnCl4: heat to reflux with mercury or P2O5 for several hours, then distil under reduced nitrogen pressure into receiver with P2O5. Redistil. Typical impurities: hydrates. ZnCl2: heat to reflux in dioxane (100 g/800 mL) with zinc dust (10 g), filter hot, and cool to precipitate ZnCl2. Crystallize from dioxane. Impurities: H2O, zinc oxychloride.

Handling, Storage, and Precautions: both reagents are hygroscopic and should be stored in a glove box or over P2O5 to minimize exposure to moisture. Containers should be flushed with N2 or Ar and tightly sealed. Perform all manipulations under N2 or Ar. Solvating SnCl4 with H2O is highly exothermic. Use in a fume hood.

Introduction.

The Tin(IV) Chloride-Zinc Chloride blend1 is one of many Lewis acid blends, such as Sn(OTf)2-Bu3SnF,2 SnCl4-Sn(OTf)2,3 SbCl5-Sn(OTf)2,4 TMSCl-SnCl2,5 TrCl-SnCl2 (Tr = trityl),6 SnO-TMSOTf,7 and GaCl3-AgClO4,8 which are effective catalysts in carbon-carbon bond forming reactions. The active catalyst is believed to be +ZnCl-SnCl5- which is formed prior to the addition of organic reactants. Single Lewis acids (SnCl4, TiCl4, etc.) promote these reactions, but do not catalyze them.9

Ethynylation of Acetals and Aldehydes.1

The SnCl4-ZnCl2 blend is the most useful catalyst (10 mol %) for the preparation of secondary propargylic ethers from 1-trimethylsilyl-1-alkynes and acetals (eq 1).1 Conventional promoters such as TrCl-SnCl2 and TMSCl-SnCl2 are not effective, and SnII-SnIV, SnII-TiIV, and ZnII-TiIV blends provide lower yields. Moderate yields (29-53%) are obtained for acetals with large alkoxy groups (R2); however, cyclic acetals, such as 1,3-dioxolane, do not react. Aromatic and conjugated dimethyl acetals provide dipropargyl derivatives as side products. Ethynylation of aldehydes is accomplished by forming intermediate hemiacetal-like compounds from aldehydes and alkoxytrimethylsilanes in the presence of the Lewis acid blend. These intermediates then undergo reaction with 1-trimethylsilyl-1-alkynes to form the desired secondary propargylic ethers (eq 2).

Allylation of Secondary Propargylic Ethers and Aldehydes.1

Propargylic ethers are allylated by Allyltrimethylsilane in the presence of the SnCl4-ZnCl2 blend (eq 3). Thus acetals can be transformed to 1,5-enynes in one pot with sequential nucleophilic additions. The blend also catalyzes the allylation of aldehydes by allyltrimethylsilane, yielding homoallylic alcohols in good yields (61-74%).

Aldol and Michael Reactions.1

The SnCl4-ZnCl2 blend is an effective catalyst in the aldol reaction of silyl enol ethers with aldehydes (eq 4), acetals (eq 5), or ketones. The product anti/syn ratios are variable (32:69 to 89:11). The blend also catalyzes the Michael addition of silyl enol ethers with a,b-unsaturated ketones (eq 6), yielding alkylation products (84-100%) with anti selectivity (anti/syn = 55:45 to 87:23)

Related Reagents.

Tin(IV) Chloride; Zinc Chloride.


1. (a) Hayashi, M.; Inubushi, A.; Mukaiyama, T. BCJ 1988, 61, 4037. (b) Hayashi, M.; Inubushi, A.; Mukaiyama, T. CL 1987, 1975.
2. (a) Mukaiyama, T.; Uchiro, H.; Kobayashi, S. CL 1989, 1001. (b) Kobayashi, S.; Mukaiyama, T. CL 1989, 297. (c) Kobayashi, S.; Uchiro, H.; Fujishita, Y.; Shiina, I.; Mukaiyama, T. JACS 1991, 113, 4247.
3. Mukaiyama, T.; Shimpuku, T.; Takashima, T.; Kobayashi, S. CL 1989, 145.
4. Kobayashi, S.; Tamura, M.; Mukaiyama, T. CL 1988, 91.
5. (a) Iwasawa, N.; Mukaiyama, T. CL 1987, 463. (b) Mukaiyama, T.; Wariishi, K.; Saito, Y.; Hayashi, M.; Kobayashi, S. CL 1988, 1101.
6. (a) Mukaiyama, T.; Kobayashi, S.; Tamura, M.; Sagawa, Y. CL 1987, 491. (b) Mukaiyama, T.; Sugumi, H.; Uchiro, H.; Kobayashi, S. CL 1988, 1291.
7. Mukaiyama, T.; Uchiro, H.; Kobayashi, S. CL 1990, 1147.
8. Mukaiyama, T.; Ohno, T.; Nishimura, T.; Suda, S.; Kobayashi, S. CL 1991, 1059.
9. (a) Mukaiyama, T.; Narasaka, K.; Banno, K. CL 1973, 1011. (b) Mukaiyama, T. AG(E) 1977, 16, 817. (c) Narasaka, K.; Soai, K.; Aikawa, Y.; Mukaiyama, T. BCJ 1976, 49, 779.

Stephen Castellino

Rhône-Poulenc Ag. Co., Research Triangle Park, NC, USA

David E. Volk

North Dakota State University, Fargo, ND, USA



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