[21512-92-5] · C4H10Cl2OZn · Zinc Chloride Etherate · (MW 210.43)
(Lewis acid catalyst solution that is considerably more active than ZnCl2 in either diethyl ether or dichloromethane)
Physical Data: density depends on solvent composition; see Table 1.
Solubility: sol a variety of inert solvents.
Form Supplied in: 2.2 M solution in dichloromethane.
Preparative Methods: Zinc Chloride (20.0 g, 0.147 mol) is dissolved in dry diethyl ether with initial cooling (exothermic!) and shaking, which takes up to 12 h for the more concentrated solutions. These solutions (which are very viscous when less than 30 mL of ether are used) are diluted with CH2Cl2 to obtain the 1.47 M catalyst solutions shown in Table 1.
Handling, Storage, and Precautions: the solutions described in Table 1 can be stored at ambient temperature for several months. Precipitation of ZnCl2 takes place when solutions with a [Et2O]/[ZnCl2] ratio <2 are diluted with CH2Cl2 at room temperature. Separation can be avoided by cooling the catalyst solution at -70 °C before addition of the extra solvent and the reactants. The reagent is relatively insensitive toward traces of moisture;1 tentative addition of water causes a similar reduction in the catalytic activity as the same molar amount of ether.1
Because of its low solubility in inert solvents (e.g. chlorinated hydrocarbons), zinc chloride acts as a weak Lewis acid catalyst for a variety of organic reactions.2 It is highly soluble in ether and related solvents,3 but the catalytic activity of such solutions is low because of coordinative saturation of the catalytic sites. Highly active forms of ZnCl2 can be obtained, however, when small amounts of ether are used to solubilize ZnCl2 in inert solvents, and it has been shown that solutions of ZnCl2.Et2O in dichloromethane possess a catalytic activity that is 103-104 times greater than that of the ether-free reagent. Dichloromethane solutions, in which the molar ratio [Et2O]/[ZnCl2] exceeds 5-8, possess a lower catalytic activity than heterogeneous ZnCl2/CH2Cl2 systems.
Zinc chloride etherate in dichloromethane has most frequently been used to catalyze additions of alkyl,4 benzyl,4,5 allyl6 and propargyl chlorides7 to alkenes and dienes (eqs 1-3). The major advantage of this catalyst system is that it works well for polymerization-sensitive compounds such as allylic chlorides or conjugated p-systems. The scope of these reactions has been discussed.4
Carbocationic cycloadditions of allyl6,8 and propargyl cations9 with alkenes and dienes have been induced with this reagent (eqs 4 and 5).
Acetylcyclopentadienes have been obtained by ZnCl2.OEt2-catalyzed reactions of allyl chlorides with 2,4-pentanedione (eq 6).10
Alkoxyalkylations of allylsilanes with a-halogeno ethers and acetals have been reported.11 In addition, ZnCl2.OEt2/CH2Cl2 is useful for C-glycosidations, as demonstrated by reaction of the O-benzyl protected glucopyranosyl trichloroacetimidate (1) with Furan to give the 2-glucosylfuran (2) (eq 7).12 Similar results have been reported for related heteroarenes.12
Zinc chloride etherate in 1,2-dichloroethane has been found to be superior to Tin(IV) Chloride in O-glycosidation reactions. While benzoate (3) and the sensitive enynol (4) gave the acetal (5) in 61% yield in presence of ZnCl2.OEt2 (eq 8), the corresponding reaction with SnCl4 gave only 23% of the desired acetal.13
Dialkoxyalkylations of silyl enol ethers have been achieved with orthoesters in the presence of this catalyst (eqs 9 and 10).14
While the reactions of orthoformates with allylsilanes in presence of Titanium(IV) Chloride (>1 equiv) gave only bisallylic ethers, the corresponding reactions with catalytic amounts of ZnCl2.OEt2/CH2Cl2 gave homoallyl acetals selectively (eq 11).15
Acylations of silylated enol ethers and alkenes have also been reported (eq 12).16
The reaction of (6) with (7) with formation of the dihydropyran (8) took place exclusively with ZnCl2.OEt2 as the catalyst while the thermal reaction of (6) with (7) predominantly gave an ene product (eq 13).17
Catalytic amounts of ZnCl2.OEt2 in CH2Cl2 provide a selective and simple preparative procedure to convert citronellal (9) in high yields into the isomers of isopulegol (10) (eq 14).18
Attempted deprotection of 2-(trimethylsilyl)ethyl ether (11) with anhydrous Tetra-n-butylammonium Fluoride under various conditions resulted in complete decomposition. Treatment of (11) with zinc chloride etherate, however, afforded racemic terrein (rac-12) in almost quantitative yield (eq 15).19
Herbert Mayr & Gisela Hagen-Bartl
Technische Hochschule Darmstadt, Germany