Diethylzinc1

Et2Zn

[557-20-0]  · C4H10Zn  · Diethylzinc  · (MW 123.50)

(useful organometallic reagent for organic synthesis;1 polymerization catalyst)

Physical Data: mp -28 °C; bp 118 °C; bp 27 °C/30 mmHg; d 1.187 g cm-3.

Solubility: sol most organic solvents; reacts with acidic hydrogens; pyrophoric; reacts violently with water.

Form Supplied in: colorless liquid; available as 1.0 M solution in hexanes and 1.1 M solution in toluene, or neat in a metal cylinder.

Analysis of Reagent Purity: can be titrated by iodometric methods4 or by complexometry.5

Preparative Methods: prepared by the reaction of Zinc/Copper Couple with Ethyl Iodide followed by distillation (81-84%).2 It can also be prepared via a B/Zn exchange reaction.3

Handling, Storage, and Precautions: highly flammable liquid; spontaneous ignition in air; reacts violently with water. Should be kept under an inert atmosphere.6 Neat diethylzinc is best destroyed by first diluting with THF, followed by slow addition of ethanol at 0 °C.

Addition to Carbonyl Compounds.

Diethylzinc, like other dialkylzincs, reacts only slowly with carbonyl compounds. Carbon Dioxide can be used as protecting gas for reactions performed with diethylzinc and the formation of a zinc propionate is observed only under a high pressure of CO2 and at higher temperatures.7 Whereas the reaction of Acetaldehyde with neat diethylzinc is complete within a few hours, the reaction with higher homologs requires several days.8 Higher diorganozinc derivatives do not add to aldehydes in solution. The addition of magnesium salts to diethylzinc considerably accelerates the addition to the carbonyl group (eq 1).9 Similarly, the addition of a tetraalkylammonium chloride or of a catalytic amount of N,N,N,N-Tetramethylethylenediamine accelerates the addition reaction and limits the formation of reduction product.10

Neat diethylzinc does not add to ketones and its reaction with benzophenone produces only diphenylmethanol.11 As in the case of aldehydes, the addition of diethylzinc is accelerated by the presence of magnesium salts.9 With 1,2-diketones, diethylzinc undergoes an addition in the reverse way to the carbonyl group, producing an a-alkoxy ketone (eq 2).12 A similar reaction with a-diimines leads to zincated enamines which can be converted to indolizidines after the addition of an aldehyde and heating (eq 3).13

The presence of a heteroatom at the a-position to the carbonyl group accelerates the addition and diethylzinc adds to the aldehyde (1) via a chelate controlled transition state producing the alcohol (2), which was further converted to exo-brevicomin (eq 4).14 The addition of diethylzinc to (-)-menthyl phenylglyoxylate gives, after saponification, an a-hydroxy acid with a good diastereoselectivity (eq 5).15 The low reactivity of diethylzinc with aldehydes has been advantageously exploited by performing the addition reaction in the presence of a chiral catalyst.16

Mukaiyama showed that the chiral proline derivative (3) catalyzes the addition of diethylzinc to benzaldehyde, unfortunately without any asymmetric induction.17 Oguni found a few years later that (S)-leucinol (4) also catalyzes the reactions and the same addition proceeds with 48% ee.18 Since this discovery, a variety of amino alcohols were found to catalyze the addition of diethylzinc to aldehydes with high enantioselectivity (up to 99% ee).16,19-34 (-)-3-Exo-(dimethylamino)isoborneol, (-)-DAIB (5), has a particularly high catalytic activity. In the presence of 2 mol % of this catalyst, the addition of diethylzinc to benzaldehyde in toluene is complete within 6 h at 0 °C (98% ee).20 This catalyst is effective for aromatic aldehydes but gives lower enantioselectivities with aliphatic aldehydes. A series of readily available pyrrolidinylmethanols (6), derived from proline, give excellent results both with aliphatic and aromatic aldehydes (eq 6).21 Remarkably, chiral quaternary ammonium salts such as (7) catalyze in the solid state the addition of benzaldehyde to diethylzinc with a good enantioselectivity (eq 7).22 Chiral metal complexes can also be used as catalysts, and the reaction of diethylzinc with aldehydes in the presence of a chiral oxazoborolidine produces ethylated secondary alcohols in 52-95% ee.23

Very active titanium catalysts (8) and (9) have been developed respectively by Yoshioka and Ohno,24 and Seebach.25 Their high catalytic activity allows the extension to the enantioselective addition of higher dialkylzincs (eq 8)25 and to both functionalized aldehydes and dialkylzincs (eqs 9-11).26,27 The excellent functional group tolerance allows the use of many functionalized aldehydes.

A new method for the enantioselective synthesis of lactones is based on the enantioselective addition of 3- or 4-formyl esters in the presence of catalytic amounts of the amino alcohol (10) (eq 12).28 g-Hydroxy ketones can be prepared enantioselectively by the addition of diethylzinc to the ketoaldehyde (11) (eq 13).29 Optically active hydroxy aldehydes are obtained with high enantioselectivity by the addition of diethylzinc to monoprotected dialdehydes (eq 14).30 A variety of other functionalized aldehydes have been ethylated under similar conditions (eqs 15 and 16).31

The addition of diethylzinc to activated imines such as N-diphenylphosphinoylimines of type (12) proceeds in good yields and excellent enantioselectivity, allowing the synthesis of enantiomerically enriched secondary amines (eq 17).32a An enantioselective ethylation on N-(amidobenzyl)benzotriazoles catalyzed by chiral amino alcohols is also possible.32b

Polymer-bound chiral catalysts33 and chiral catalysts supported an silica gel and alumina34 have been successfully employed for the enantioselective addition of diethylzinc to aldehydes. Little success was obtained in the addition of diethylzinc to esters or lactones. However, the addition to Diethyl Oxalate produces ethyl 2-ethyl-2-hydroxybutanoate in 75% yield.9 The reaction with acid chlorides1,35 or anhydrides1,36 proceeds in some cases satisfactorily;35,36 however, Pd0 37 or CuI 26,38 catalysis gives more predictable results and allows the extension of the reaction to higher functionalized homologs (eq 18).26,38

Addition of Diethylzinc to Double or Triple Bonds.

Diethylzinc, like higher homologs, is able to add to activated alkynes such as propiolic esters in the presence of a THF soluble copper salt such as CuCN.2LiCl (eq 19).26 Less reactive alkynes such as 1-(methylthio)-1-hexyne or phenylacetylene also react with the reagent EtCu(CN)ZnEt.2LiCl, leading stereospecifically to the syn adducts (eq 20).39 Diethylzinc can be added to unfunctionalized alkynes in the presence of ZrCp2I2.40 In the case of terminal alkynes, the addition shows a low regioselectivity, producing a mixture of alkenylzinc species (eq 21).40 Remarkably, the addition to internal alkynes proceeds well, leading after iodolysis to tetrasubstituted alkenyl iodides (eq 22).40 The addition to alkenes is more difficult. However, the intramolecular carbozincation of alkenes mediated by a palladium or nickel catalysis is possible (eq 23).41 The role of diethylzinc in this reaction is to transmetalate the palladium(II) intermediate back to an organozinc reagent and to generate the Pd0 catalyst. This Pd catalysis can also be used for an efficient generation of primary alkylzinc iodides (eq 24).41 In the absence of palladium salts, the iodine-exchange reaction requires a temperature of 50 °C.26,27b This I/Zn exchange reaction constitutes a unique method for preparing functionalized dialkylzincs (eq 25).26 The Michael addition of diethylzinc to a,b-unsaturated ketones proceeds well in the presence of a nickel catalyst. In the presence of a chiral nickel catalyst, good to excellent enantioselectivities are observed (eq 26).42

Cyclopropanation.

The reaction of diethylzinc with Diiodomethane rapidly produces ICH2ZnEt (see Ethyliodomethylzinc and Iodomethylzinc Iodide).1,43 A further exchange can occur, leading to (ICH2)2Zn.43 Both of these iodomethylzinc derivatives are excellent cyclopropanation reagents, and their generation with diethylzinc (Furukawa method) allows convenient cyclopropanation reactions (eq 27).44

The reaction gives especially good results with electron-rich alkenes (eq 28).45 The cyclopropanation was originally performed under the strict absence of oxygen. It was later found that the presence of air accelerates the reaction and improves the yields (eq 29).1,46 An extension of the cyclopropanation using higher 1,1-diiodoalkanes is possible, and leads preferentially to the syn products (eqs 30 and 31).47 An anti stereoselectivity is observed with allylic alcohols (eq 32).47 Benzene and polynuclear aromatic compounds are cyclopropanated by 1,1-Diiodoethane and diethylzinc in moderate yields (eq 33).48 Highly diastereoselective cyclopropanations can be performed with chiral allylic and homoallylic alcohols and acetal derivatives (eq 34).49 The cyclopropanation of silylated ketene acetals using Bromoform and diethylzinc allows the generation of polyfunctional zinc carbenoids and the performance of intramolecular cyclopropanations (eq 35).50

Miscellaneous Reactions.

Diethylzinc undergoes a variety of substitution reactions with tertiary halides.51 The elimination of HX is an important side reaction. By treating dithioacetals with Diiodomethane and diethylzinc, vinyl sulfides are obtained in satisfactory yields.52 A direct treatment of diethylzinc with a dithioacetal produces the substitution product.53 Ketocarbenoids have also been generated from a,a-dibromo ketones (eq 36).54 Finally, diethylzinc has been used to prepare zinc enolates (eq 37)55 or mixed lithium-zinc enolates (eq 38).56

Related Reagents.

Diethylzinc-Bromoform-Oxygen; Diethylzinc-Iodoform.


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Paul Knochel

Philipps-Universität, Marburg, Germany



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