Zinc1

Zn

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

(reducing agent;2 used for preparation of organozinc reagents,1,3 Reformatsky reagents,4 and the Simmons-Smith reagent (cyclopropanation)5)

Physical Data: mp 419 °C; bp 907 °C; d 7.14 g cm-3.

Solubility: insol organic solvents; reacts with aqueous acidic solutions.

Form Supplied in: dust, foil, granular, wire, mossy, rod; widely available at low cost.

Handling, Storage, and Precautions: slowly oxidizes in air; no toxic properties are associated with zinc and zinc organometallics; in several cases the metal requires an activation procedure before use.6

Reduction of Carbon-Carbon Multiple Bonds.

Whereas isolated double bonds are rarely reduced by zinc, triple bonds are cleanly converted to alkenes using either Zinc/Copper Couple or Zinc Amalgam.7 A regio- as well as stereospecific reduction of a wide range of alkynic derivatives can be performed using zinc powder (eq 1).8 The reduction of propargylic alcohols proves to be especially efficient (eq 2).9 Also, the selective cis reduction of conjugated dienynes and trienynes proceeds well with Zn(Cu/Ag).10 The presence of a leaving group at the propargylic position leads to the formation of allenes.11 The conjugation of a double bond with an electron-withdrawing substituent considerably facilitates the reduction of the double bond.12 The reduction of a,b-unsaturated ketones produces the corresponding saturated ketones.13 Nickel catalysis allows the reduction of unsaturated aldehydes, ketones, and esters in an aqueous medium under ultrasonic irradiation (eq 3).14

Reduction of Carbonyl Groups.

Zinc reduces ketones to either alcohols or to a methylene unit, depending on the reaction conditions and the nature of the substrate. For example, conjugation is required if reduction to a hydroxy group is desired. The reduction of aryl ketones provides benzylic alcohols (eq 4)15 and a-diketones can be converted selectively to a-hydroxy ketones (eq 5).16 The reduction of the carbonyl group of nonconjugated ketones to a methylene unit with zinc and hydrochloric acid in organic solvents such as ether, acetic anhydride, or benzene-ethanol proceeds in satisfactory yields with a wide range of ketones (Clemmensen reaction) (eqs 6-8).17 The Clemmensen reduction of aromatic a-hydroxy ketones gives conjugated alkenes.18 Finally, the Clemmensen reduction can also be performed by using zinc and Chlorotrimethylsilane in an aprotic medium, leading to alkenes (eq 9).19 This variation has been exploited for the preparation of alkenes (eq 10)19b and has been used in new cyclization reactions (eqs 11 and 12).20 Trimethylsilyl ethers can be regioselectively prepared by the zinc reduction of a-chloro ketones in the presence of TMSCl.21 Mixed pinacol products have been prepared by using Zn(Cu) as the reducing agent (eq 13).22

Reduction of Carbon-Oxygen Bonds.

Carbon-oxygen bonds situated a to an unsaturation are easily reduced with zinc in an acidic medium. In the case of a-hydroxy ketones, ketones are obtained in good yields (eq 14).23 A wide range of allylic or benzylic ethers, acetates, and alcohols are reduced with zinc (eq 15).24,25 The reduction of epoxides can lead to either alcohols (eq 16)26 or alkenes.26b-e In the presence of catalytic amounts of Pd0 and zinc dust, allylic acetates are coupled to give 1,5-dienes (eq 17).27 Under similar reaction conditions and in the presence of an aldehyde, homoallylic alcohols are obtained in satisfactory yields (eq 18).27b-d

Reduction of Carbon-Halide Bonds.

Alkyl and alkenyl halides are readily reduced with zinc under various reaction conditions. The reduction produces, as an intermediate, an organic radical which can undergo carbon-carbon bond formation (Barbier reaction)28 or can be further reduced, usually under acidic conditions. Aliphatic iodides or bromides and benzylic chlorides react readily with Zinc-Acetic Acid, providing the corresponding hydrocarbon.29 Although aromatic halides are reduced less easily, the tribromothiophene (1) is reduced selectively to the bromide (2) (eq 19).29e,g Various b-chloro enones are cleanly reduced to enones with Zinc/Silver Couple in methanol at rt (eq 20).29f a-Dihalo ketones are reduced smoothly, allowing the preparation of a variety of ketones (eqs 21 and 22).30 The reductive couplings of a-bromo ketones, tropylium, and 1,3-dithiolylium cations have been observed.31 In the case of 1,3-dihalides, cyclopropanes are obtained in good yields.32 If the reduction of the carbon-halide bond is performed in the presence of an electrophile, a radical addition often occurs. Thus phenacyl halides can be coupled with methylenecyclohexanes (eq 23).33a Performing the reaction in the presence of an unsaturated ketone provides the 1,4-adducts. Interestingly, the reduction proceeds well in an aqueous medium supporting a radical mechanism, since zinc organometallics react instantaneously with water but only very sluggishly with enones (eq 24).33b-h The addition of Chloromethyl Methyl Ether to 1,2-bis-silyl enol ethers in the presence of zinc leads to ring-enlarged 1,3-cycloalkanediones after acidic treatment.34 An interesting three-component reaction has been described (eq 25).34b A wide range of allylic halides undergo Barbier-type addition to carbonyl groups (eqs 26 and 27).35,36 The reduction of a,a-dihalo ketones with a zinc-copper couple in the presence of a diene such as Isoprene provides cycloaddition products via a zinc oxyallyl cation.37

Reduction of Carbon-Nitrogen and Carbon-Sulfur Bonds.

Aldimines and oximes are converted to amines, and various heterocycles bearing carbon-nitrogen double bonds are reduced with zinc under acidic conditions.38 Cyanamides can be cleanly cleaved leading to amines,39a and the zinc reduction of acylnitriles provides a-amino ketone derivatives (eq 28).39b Aromatic amides can be reduced with zinc dust to aromatic aldehydes.39c Activated carbon-sulfur bonds a to a carbonyl group40a,b and sulfur ylides40c,d can be reduced with zinc.

Reduction at Heteroatoms.2

Nitrogen-oxygen bonds of oximes,41 nitro,42 and nitroso43 groups are readily reduced by zinc in acidic medium. Zinc in acetic acid has often been used for the workup procedure of alkene ozonolysis to afford aldehydes or ketones.2 Sulfinates and thiols can be obtained selectively by the reduction of aromatic sulfonyl chlorides or disulfides.44

Dehalogenation and Related Reactions.6c,45

Zinc dust is a very efficient reducing agent for the dehalogenation of 1,2-dihalides or 1-halo-2-alkoxy derivatives, leading to alkenes. The reaction allows an access to highly reactive ketenes,46 alkenes,47 or alkynes48 not readily available by standard methods (eqs 29-31). The reduction of b-alkoxy halides using Zinc-Graphite proved to be especially interesting when applied to sugar derivatives (eq 32).6c,45e,49 The dehalogenation using zinc is such a straightforward and chemoselective reaction that several protecting groups have been devised which use this reaction as a deblocking step.50

The Reformatsky Reaction.4

The insertion of zinc into a-halo esters produces zinc ester enolates which react readily with aldehydes or ketones, leading to aldol products. Historically, this reaction has been important since it allowed the first quantitative generation of an ester enolate. However, several modern synthetic methods for the stereoselective preparation of aldol products using metal enolates compete favorably with the Reformatsky reaction.51 The nature of the zinc activation6 has proved to be important for fast and quantitative zinc insertion. Remarkably, the Zn(Ag) couple on graphite reacts with ethyl bromoacetate at -78 °C within 20 min,52a whereas Rieke zinc requires 1 h at 25 °C,52b as does zinc generated from the reaction of Zinc Chloride with Lithium under ultrasound irradiation52c (eq 33).52a Interesting synthetic applications have been reported (eqs 34 and 35).4,52 4-Bromocrotonate reacts with ketones and Zn(Cu) with solvent-dependent regioselectivity.52f See also Ethyl Bromozincacetate.

The Simmons-Smith Reaction.5,53

Cut Zn foil readily inserts into Diiodomethane providing iodomethylzinc iodide,53 which cyclopropanates a wide range of alkenes in good yields (see Ethylzinc Iodide, Iodomethylzinc Iodide, Diethylzinc, Ethyliodomethylzinc). The in situ generation of iodomethylzinc iodide is often used. The Zn(Ag) couple has proved to be especially active for cyclopropanations (eq 36).53d

Preparation of Organozinc Reagents.1,3a

The insertion of zinc into organic halides provides the most general synthesis of organozinc halides. Primary and secondary organic iodides react with zinc dust (2-3 equiv) in THF between 20 °C and 50 °C, leading to organozinc iodides in high yields.3a,54a-c Benzylic chlorides and bromides react under even milder conditions, providing the corresponding benzylic zinc halides without the formation of significant amounts of Wurtz coupling products.54d -f Two remarkable properties characterize organozinc reagents: (i) their high functional group compatibility, which allows the preparation of polyfunctional organometallic zinc species bearing almost all common functional groups with the exception of nitro, azido, or hydroxy functions (see the reagents 3,54b 4,54g,h 5,54i 6,54j 7,54k-o 8,54p 9,54n,o 10, 11,54q and 12-14,54f and eq 37); and (ii) their ability to undergo transmetallation with other metallic salts, such as copper salts, thus giving polyfunctional copper reagents which react readily with a wide range of electrophiles (enones,54b,r aldehydes,54s alkynes,54t-v nitro alkenes,54w-y allylic halides,54b,z alkynyl halides,54g acid chlorides,54b,aa and alkylidenemalonates54ab). Similarly, efficient transmetalations with PdII salts allow the coupling reactions to be performed (eq 38).55,56 Zinc insertion also proceeds well with various polyfluorinated alkyl iodides57 and with primary alkyl and benzyl phosphates and mesylates.58 Alkenyl and aromatic halides undergo the zinc insertion far less readily and require the use of polar solvents59 or highly activated zinc.60 The use of a sacrificial zinc electrode offers an interesting alternative.61 Allylic zinc halides are formed under very mild conditions and, contrary to other classes of organozinc reagents, display a high reactivity toward organic electrophiles (comparable to organomagnesium species).36e-h,54a,62 A wide range of synthetic applications of zinc reagents for the formation of carbon-carbon bonds has been reported (eqs 39-44).56,60a,63-66 Diorganomercurials also react with zinc dust, providing diorganozincs.67,68

Related Reagents.

Dibromomethane-Zinc-Titanium(IV) Chloride; Dichlorobis(cyclopentadienyl)zirconium-Zinc-Dibromomethane; Diiodomethane-Zinc-Titanium(IV) Chloride; Molybdenum(V) Chloride-Zinc; Niobium(V) Chloride-Zinc; Phosphorus(III) Bromide-Copper(I) Bromide-Zinc; Potassium Hexachloroosmate(IV)-Zinc; Titanium(IV) Chloride-Zinc; Zinc-Acetic Acid; Zinc Amalgam; Zinc-Copper(II) Acetate-Silver Nitrate; Zinc-Copper(I) Chloride; Zinc/Copper Couple; Zinc-1,2-Dibromoethane; Zinc-Dimethylformamide; Zinc-Graphite; Zinc/Nickel Couple; Zinc/Silver Couple; Zinc-Zinc Chloride.


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Paul Knochel

Philipps-Universität Marburg, Germany



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