[670-54-2]  · C6N4  · Tetracyanoethylene  · (MW 128.10)

(highly electron-deficient and strongly electrophilic reagent; principal reaction types are addition to its double bond and replacement of the cyano group;1 forms complexes with organometallic reagents2)

Alternate Name: TCNE.

Physical Data: mp 200-202 °C (sealed capillary); bp 223 °C/760 mmHg; sublimes at 130 °C/5 mmHg or 60-65 °C/0.2 mmHg.

Form Supplied in: colorless crystals.

Preparative Methods: the preferred synthetic preparation of TCNE involves the debromination of the KBr complex of dibromomalononitrile with Cu powder in boiling benzene (62% yield).3

Purification: recrystallize from 1,2-dichloroethane or chlorobenzene, followed by sublimation (130 °C/5 mmHg).

Handling, Storage, and Precautions: extremely toxic and irritant.4 It must be handled in a well ventilated hood, with rubber gloves, due to appreciable vapor pressure of the reagent at room temperature; sensitive to moist air.

Addition Reactions.

TCNE can be reduced to tetracyanoethane by a variety of reagents (H2/Pd, HI, H2S, thiols, HSCH2CO2H). It was found that, as hydrogen donors, several benzyl alcohols5 and hydroheterocycles6 could be dehydrogenated by TCNE (Scheme 1).

The chlorination of TCNE leads to the formation of 1,2-dichlorocyanoethane (Scheme 1).7

TCNE reacts readily with sulfurous acid to give 1,1,2,2-tetracyanosulfonic acid, of which a number of salts have been reported (Scheme 1).8

Diazomethane reacts with TCNE to give a pyrazoline which splits off a molecule of nitrogen to give 1,1,2,2-tetracyanocyclopropane (Scheme 1).9

The a-cyanoisopropyl radical reacts with TCNE to give 2,3,3,4,4,5-hexacyano-2,5-dimethylhexane (Scheme 1).8

The allene tetramer 1,3,5,7-tetramethylenecyclooctane undergoes an unusual 1,7-addition of TCNE to give 3,3,4,4-tetracyano-8,11-dimethylenetricyclo[]dodecane (Scheme 1).10

TCNE adds to ketones containing an a-hydrogen to give tetracyanopropyl ketones (Scheme 1).8 It is an acid-catalyzed Michael-type addition. However, the reaction of other active methylene compounds with TCNE depends on the basicity of the reaction medium.11 Thus the reaction of TCNE with 2,4-pentanedione can give different products (Scheme 2).12 TCNE can also react with cyclic ketones.1b

The oxidation of TCNE with aqueous Hydrogen Peroxide is the method of choice for the synthesis of tetracyanoethylene oxide (TCNEO) (Scheme 1).13 The reaction of TCNEO with alkenes proceeds via a 1,3-dipolar cycloaddition pathway.14 TCNEO can also react with nucleophilic reagents15 (dialkyl sulfide,16 nitrogenated tertiary aromatic bases,14e,15,17 diphenylcyclopropenone, diphenylcyclopropenenethione,18 or thiophene19 (Scheme 3).

Synthesis of Heterocycles.

TCNE has proven to be a useful starting material for the synthesis of five- or six-membered heterocycles, including thiophenes, pyrroles,20 isoxazoles, pyrazoles, pyridines, and pyrimidines.21 The required reaction partner can be an enolic ketone, amine, imine, hydrazone, or an a,b-unsaturated azo compound. The reaction generally proceeds via a Diels-Alder or 1,3-dipolar cycloaddition. The reaction of TCNE with acyclic ketones yields either 4H-pyrans12 or dihydropyrroles;22 enolic 1,3-cyclohexanediones with TCNE also produce pyran derivatives.23 Substituted pyrimidines are obtained from the reaction of barbituric acid with TCNE, and pyrazolopyridines are obtained from aminopyrazoles.24 Cyclization of 1,1,2,2-tetracyanoethane with substituted azomethines leads to dihydropyrrole or pyrrole derivatives.

Replacement Reactions.

Tricyanovinylation occurs at either the N atom (aliphatic amines) (eq 1) or the para position (aromatic amines) (eq 2).25 (Tricyanovinyl)amines have found widespread interest because of their optical26 and dyeing25 properties, and their ability to yield charge-transfer complexes with donors and acceptors.27 Hassan recently reported that TCNE also forms another tricyanovinylation product, (PhCH2)2NC6H4C(CN)=C(CN)2, with N,N-dibenzylaniline.28

An example of retrograde Michael addition reaction is illustrated by 2H,3H-thieno[3,2-b]pyrrol-3-one. Instead of the expected tricyanovinyl derivative, 2-dicyanomethylene-2H,3H-thieno[3,2-b]pyrrol-3-one was obtained (eq 3).29

Cycloaddition Reactions.

Diels-Alder and Related [4 + 2] Cycloaddition Reactions.

TCNE is a reactive dienophile and undergoes Diels-Alder type cycloadditions with conjugated dienes. The Diels-Alder reaction of TCNE occurs even with sluggish dienes such as 2-vinylnaphthalene, spontaneously and rapidly, without added catalyst or external heat.8

The regiochemistry of concerted, thermal cycloadditions is not fully understood, and the stereoselectivity involves either a torsional preference or p-facial attack on the diene entering the [4 + 2] cycloaddition.30

Recently, Fessner et al. have investigated the p-facial diastereoselectivity in the Diels-Alder reaction by studying the directing influence of proximate unsaturation.31 They used the model dienes (1) and (2) to differentiate the regioselectivity observed with various dienophiles. Based on steric and electronic considerations, (1) is expected to yield the endo-syn (syn with respect to the four-membered ring) adduct and (2) the endo-anti product with various dienophiles. Dienophiles such as maleic anhydride and p-benzoquinone gave the expected syn and anti cycloadducts with (1) and (2), respectively. However, TCNE predominantly gave the syn product with both (1) and (2). The authors attribute this to an anti preorienting charge-transfer complex involving the electron-depleted orthogonal p-bonds of the dienophile and the suitably oriented p-moieties of (2).

Porter et al. have investigated the cycloaddition reactions of TCNE with 3-vinylindene, 3-vinylbenzo[b]thiophene, and 3-vinylindole.32 The normal [4 + 2] Diels-Alder cycloaddition reaction is observed with the unsubstituted compounds. However, alkylation of the diene can alter the course of the reaction. Thus (3) and (4) yield the [2 + 2] cycloadduct (5) on reaction with TCNE and a rearrangement accompanies the cycloaddition of TCNE to 2-methyl-3-vinylbenzo[b]thiophene (6) to give adduct (7) (eqs 4 and 5).

Pindur et al. have demonstrated the versatility of [4 + 2] cycloaddition and elimination processes of TCNE in their studies of the addition reactions such as 1,4-cycloaddition, cycloreversion, sigmatropic rearrangements, and elimination processes of TCNE with dimethylpyrano[3,4-b]indol-3-one and 1-methylpyrido[3,4-b]indol-3-one.33 Pindur et al. have also studied the stereoselectivity and regioselectivity of the Diels-Alder reaction of TCNE with (E/Z)-2-methoxy-substituted 3-vinylindoles. TCNE reacted with (Z)-(8) to produce (9a):(10) in a 1:1 ratio and with (E)-(8) to give (9b):(10) in a 1:1 ratio.34

Homo-Diels-Alder Reaction.

The cycloaddition of dienophiles to bridged 1,4-dienes with interrupted conjugation (homo DA reaction) is a useful synthetic reaction for the preparation of tetracyclic compounds (eq 6).35

Alder Ene Reaction.

The ene reaction is the indirect, substituting addition of a compound having a double bond (enophile) to an alkene possessing an allylic hydrogen atom (ene).36 Trimethyl-substituted isodicyclopentadiene reacts readily with TCNE in an ene reaction involving a zwitterionic intermediate when the reaction is performed in ethyl acetate. However, TCNE adds to isodicyclopentadiene in refluxing xylene to produce the [4 + 2] cycloadduct in 51% yield.37

Thermal [2 + 2] Cycloaddition.

It is well established that TCNE undergoes nonconcerted, ionic, thermal, [2 + 2] cycloaddition reactions with electron-rich alkenes (eq 7).38 The reaction is believed to involve a zwitterion and not a biradical intermediate. This is supported by strong dependency on solvent polarity,38c incomplete stereochemistry,39 and trapping of the zwitterion with alcohol to form an acetal.40

Other Cycloadditions.

Cyclopropylethylene derivatives have been found to undergo facile cycloadditions with TCNE to form either cyclobutane41 or cyclopentane derivatives. This thermal, ring-opening cycloaddition was reviewed in 1984.42

The reaction of TCNE with substituted 1,3-butadienes leads to competing [4 + 2] and [2 + 2] cycloadditions. The regioselectivity of the reaction depends on substitution of both diene termini, and on solvent polarity. 2,7-Dimethyl-2,4,6-octatriene undergoes cycloaddition with TCNE to give not only the expected Diels-Alder adduct but also the [2 + 2] cycloadduct. Formation of the latter adduct is favored by more polar solvents. The less-substituted 2-methyl-2,4,6-octatriene gives only the [4 + 2] cycloadduct (eq 8).43

TCNE can react with [4]annulenes, fulvenes, bullvalenes, and cyclooctatetraenes.1d

Photochemical Reactions.

Brief reviews on the photochemical reactions of organic compounds in the presence of TCNE are available.1e,44 The photocycloaddition of alkenes to benzenoids gives a variety of products. Such electron-poor alkenes as a,b-unsaturated acid derivatives generally give [2 + 2] cycloadducts.45

Additional cycloaddition type reactions have been described (cycloaddition with aromatic hydrocarbons, cyclophanes, mesoionic compounds, heterocyclic compounds, and stereoselective addition with exocyclic dienes).1e

Organometallic Reactions.

With organometallics, TCNE shows interesting properties such as insertion into metal-hydrogen, metal-carbon, and even metal-metal bonds.

Reactions with Main Group Organometallics.

The reaction of TCNE with the main group organometallic compounds generally proceeds by pathways involving the formation of charge-transfer complexes or fragmentation products. Formation of cycloadducts can also be observed.46 A review on TCNE charge-transfer complexes of phenyl derivatives of silicon, germanium, tin, and lead has been published.46b

It has been found that s-electrons of low ionization energy, such as the Si-Si and Ge-Ge bonds, can be donated to certain p-acceptors, such as TCNE. Charge-transfer complexes of silicon and germanium with TCNE include polymethylpolysilanes,47 bimetallic compounds [(Et3Ge)2Hg],48 methylphenylsilanes,49 aryltrisilanes,50 phenylsilacyclobutanes,51 1,2-disilacycloalkanes,52 silacyclopentenes,53 cyclopolysilanes,54 silatranes,55 trimethylsilyl-substituted benzene,56 naphthalene, anthracene, and phenanthrene derivatives,56b phenylsilanes57 and -germanes,57,58 benzylsilanes and -germanes,59 silicon-containing ethers,60 and peralkylcyclopropylsilanes.61 Furans and thiophene containing silyl,62 germyl, stannyl, and plumbyl have also been examined.62b

Reactions of tetraalkyl organometallics derived from silicon, germanium, tin, and lead with TCNE have been described and involve fragmentation of cation radicals following electron transfer.63

The Diels-Alder cycloaddition of TCNE to organometallics derived from silicon, germanium, tin, and mercury is not common but has been described.64 Moreover, the reaction products are not easy to predict.

Reactions with Metallocenes.

Iron metallocenes are regarded as p-bases of high reactivity. They are able to form charge-transfer complexes with TCNE (p-acid) in which the p-cyclopentadienyl rings or the metal can act as an electron donor (eq 9). These complexes have been shown to have a benzenoid structure.65 In some cases the TCNE radical-anion salt is obtained (complete electron transfer occurs).66 Neither ruthenocene nor osmocene form stable complexes with TCNE. However, it can proceed smoothly under photochemical activation.67 Substituted vinylferrocenes react readily with TCNE in a [2 + 2] cycloaddition type reaction.68

Reaction with Metal-Coordinated Alkenes and Alkynes.

The electrophile TCNE may act either as a one-electron oxidant towards organotransition metal complexes (eq 10),69 or undergo a variety of reactions such as metal-carbon insertion (eq 11)70 and [2 + 2] cycloaddition to coordinated polyalkenes (eq 12).71

Additional applications of TCNE in organometallic chemistry include platinum-family complexes with TCNE,2,72 reactions of tricarbonyliron complexes with TCNE,2 and diverse behavior of TCNE towards transition metal-carbon s-bonded complexes.2

Related Reagents.

Acrylonitrile; 1,4-Benzoquinone; Dimethyl Acetylenedicarboxylate; Maleic Anhydride.

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Laurent Deloux, Poonam Aggarwal & Morris Srebnik

University of Toledo, OH, USA

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.