[10139-47-6] · I2Zn · Zinc Iodide · (MW 319.19)
Physical Data: mp 446 °C; bp 625 °C (dec); d 4.740 g cm-3.
Solubility: sol H2O (1 g/0.3 mL), glycerol (1 g/2 mL); freely sol EtOH, Et2O.
Form Supplied in: white, odorless, granular solid; principal impurities are H2O and iodine.
Analysis of Reagent Purity: mp.
Purification: heat to 300 °C under vacuum for 1 h, then sublime.
Handling, Storage, and Precautions: very hygroscopic and light sensitive; store under anhydrous conditions in the absence of light.
Organozinc reagents may be prepared through transmetalation of organolithium, organomagnesium, and organocopper species with ZnI2, although Zinc Chloride and Zinc Bromide are used far more frequently.1a It is more usually the case that organozinc iodides are prepared by zinc insertion into alkyl iodides using Zinc/Copper Couple.1a These zinc reagents possess reactivity analogous to the other organozinc halides, finding particular use in palladium-catalyzed coupling reactions (eq 1).4 An alternative method for the preparation of the Simmons-Smith reagent (Iodomethylzinc Iodide) involving the treatment of Diazomethane with ZnI2 has been reported (eq 2).5
The catalytic effect of ZnI2 on the Diels-Alder reaction has been noted (eq 3),6 but its use in such cycloaddition reactions is rare compared with ZnCl2 and ZnBr2.
Catalysis of aldol condensation reactions using silyl ketene acetals and ZnI2 has been the subject of several studies.7 It is observed that ZnI2 favors the activation of functionalized carbonyl compounds via b-chelates to impart useful stereoselectivity (eq 4).7a In analogous fashion, diastereoselective additions to imines and nitrones have been reported which offer useful access to b-lactams (eq 5).8 a,b-Unsaturated esters are subject to conjugate addition reactions by silyl ketene acetals, also through the agency of ZnI2 activation (eq 6).9
An important role for ZnI2 has been found in the catalysis of R3SiCN addition to ketones and aldehydes to afford silyl protected cyanohydrins.10 This is a very general reaction that is effective even with very hindered carbonyl compounds (eq 7).11 Diastereoselective cyanohydrin formation has been reported when these reaction conditions are applied to asymmetric carbonyl substrates (eq 8).2a
The Lewis acidity of ZnI2 may be exploited to activate various carbon-heteroatom bonds to nucleophilic substitution. Treatment of epoxides and oxetanes with Cyanotrimethylsilane/ZnI2 results in selective C-N bond formation with ring opening (eq 9).12 Similarly, C-S and C-Se bonds may be formed by treating cyclic ethers with ZnI2 and RSSiMe3 and RSeSiMe3, respectively (eq 10).13
Treatment of 4-acetoxy- and 4-sulfoxyazetidin-2-ones with ZnI2 results in the formation of the corresponding imine or iminium species which subsequently suffers silyl-mediated addition. These reactions result in the formation of trans substitution (eqs 11 and 12).14,15
Substitution reactions of orthoesters3b and acetals,3c including anomeric bond formation in carbohydrates (eq 13),16 have been catalyzed by ZnI2.
Allyl and aryl ketones, aldehydes, and alcohols are reduced to the corresponding hydrocarbon by Sodium Cyanoborohydride/ZnI2 (eq 14).17 This is a reasonably reactive reducing mixture which will also attack nitro and ester groups.
Methyl and benzyl ethers may be cleaved by the combination of (Phenylthio)trimethylsilane/ZnI2 (eq 15).18 Similar cleavage of alkyl and benzyl ethers takes place using Acetic Anhydride/ZnI2 to afford the corresponding acetate (eq 16).19
Glenn J. McGarvey
University of Virginia, Charlottesville, VA, USA