TCNE

Hayashi M, Yang T-S, Yu J, Mebel A, Chang R, Lin S H, Rubtsov I V and Yoshihara K 1998 Vibronic and vibrational coherence and relaxation dynamics in the TCNE-HMB complex J. Phys. Chem. A 102 4256-65... 

Arylthiazoles derivatives are good subjects for the study of these transfers. Thus the absorption wavelengths and the enthalpies of formation of a series of charge-transfer complexes of the type arylthiazole-TCNE, have been determined (147). The results are given in Table IIM3. 

Tetracyanoethylene is colorless but forms intensely colored complexes with olefins or aromatic hydrocarbons, eg, benzene solutions are yellow, xylene solutions are orange, and mesitylene solutions are red. The colors arise from complexes of a Lewis acid—base type, with partial transfer of a TT-electron from the aromatic hydrocarbon to TCNE (8). TCNE is conveniendy prepared in the laboratory from malononitrile [109-77-3] (1) by debromination of dibromoma1 ononitrile [1855-23-0] (2) with copper powder (9). The debromination can also be done by pyrolysis at ca 500°C (10). 

With substances that give up an electron more readily than aromatic hydrocarbons, such as potassium, nickel carbonyl, cyanide ion, or iodide ion, complete transfer of an electron occurs and the TCNE anion radical is formed (11). Potassium iodide is a particulady usefiil reagent for this purpose, and merely dissolving potassium iodide in an acetonitrile solution of TCNE causes the potassium salt of the anion radical to precipitate as bronze-colored crystals. 

The reaction of bis(benzene)vanadium [12129-72-5] with TCNE affords an insoluble amorphous black soHd that exhibits field-dependent magnetization and hysteresis at room temperature, an organic-based magnet (12). The anion radical is quite stable in the soHd state. It is paramagnetic, and its intense electron paramagnetic resonance (epr) spectmm has nine principal lines with the intensity ratios expected for four equivalent N nuclei (13) and may be used as an internal reference in epr work (see Magnetic spin resonance). 

In a study of the relative rates of addition of 20 dienophiles to cyclopentadiene, TCNE was at the head of the Hst, eg, it added 7700 times as rapidly as maleic anhydride (15). Reaction with most 1,3-dienes takes place rapidly and in high yield at room temperature. TCNE has often been used to characterize 1,3-dienes, including those that are unstable and difficult to isolate (16). 

Although a C—CN bond is normally strong, one or two cyano groups in TCNE can be replaced easily, about as easily as the one in an acyl cyanide. The replacing group can be hydroxyl, alkoxyl, amino, or a nucleophilic aryl group. Thus hydrolysis of TCNE under neutral or mildly acidic conditions leads to tricyanoethenol [27062-39-17, a strong acid isolated only in the form of salts (18). 

Heating TCNE with an alcohol in the presence of a mild base such as urea causes replacement of either one (19) or two (20) cyano groups by alkoxyl. The products with ethanol are 1-ethoxy-1,2,2-tricyanoethylene [69155-32-4] and l,l-bisethoxy-2,2-dicyanoethylene [17618-65-4]. 

Aromatic compounds that are sufftciendy nucleophilic to condense with benzenediazonium chloride and form azo compounds generally condense with TCNE, eg, the reaction of /V, /V- dim ethyl a n i1 in e proceeds stepwise (21,22). 

A few percent TCNE added during formation of urethane foams imparts enough conductivity to dissipate electrostatic charges. Airplane fuel tanks filled with this foam stiU have about the same volume for fuel but do not build up static charges (29). 

Hexacyanobutadiene [5104-27-4] (4), 1,3-butadiene-1,1,2,3,4,4-hexacarbonitrile, is prepared in good yield by a two-step process from the disodium salt of tetracyanoethane (30). It is like TCNE in forming colored TT-complexes and an anion radical. 

Hexacyanoethane [4383-67-9] ethanehexacaibonitiile, is quite unstable and readily decomposes to TCNE and cyanogen [460-19-5J (NC—CN) (43). It is prepared as follows ... 

Tetracyanobenzoquinone [4032-03-5] 3,6-dioxo-l,4-cyclohexadiene-l,2,4,5-tetracarbonitrile, is a remarkably strong oxidizing agent for a quinone it abstracts hydrogen from tetralin or ethanol even at room temperature (50). It is a stronger TT-acid than TCNE because it forms more deeply colored TT-complexes with aromatic hydrocarbons. 

Tetracyanoethylene oxide [3189-43-3] (8), oxiranetetracarbonitnle, is the most notable member of the class of oxacyanocarbons (57). It is made by treating TCNE with hydrogen peroxide in acetonitrile. In reactions unprecedented for olefin oxides, it adds to olefins to form 2,2,5,5-tetracyanotetrahydrofuran [3041-31-4] in the case of ethylene, acetylenes, and aromatic hydrocarbons via cleavage of the ring C—C bond. The benzene adduct (9) is 3t ,7t -dihydro-l,l,3,3-phthalantetracarbonitrile [3041-36-9], C22HgN O. 

The double bond in A7-methoxycarbonyl-2-azetine (237 Z = COaMe) undergoes acid or photocatalyzed hydration and subsequent ring opening to give the aldehyde (238). In cycloadditions it is inert to TCNE and diphenylisobenzofuran but it does react with dipyridyltetrazine (77CC806). 

Dipping solution Dissolve 0.5 g tetracyanoethylene (TCNE) in 100 ml dichloro-methane. 

Experimentally, the rates of Diels-Alder reactions between electron-rich dienes and electron-poor dienophiles generally increase with increased alkyl substitution on the diene. This is because alkyl groups act as electron donors and lead to buildup of electron density on the diene. An exception to this is the reaction of Z,Z-hexa-2,4-diene with tetracyanoethylene (TCNE), which is actually slower than the corresponding addition involving E-penta-1,3-diene. 

Repeat your analysis for Z,Z-hexa-2,4-diene, and again calculate the energy to twist the diene into the same conformation as seen in the Diels-Alder transition state (Z,Z-hexa-2,4-diene+TCNE). Compare the two twisting energies , and rationalize the observed relative rates for the two cycloaddition reactions. 

Examine conformational energy profiles for Z-penta-1,3-diene and E,E-hexa-2,4-diene together with transition-state geometries for cycloadditions with TCNE (Z-penta-1,3-diene+TCNE and E,E-hexa-2,4-diene+TCNE, respectively). Predict the rates of Diels-Alder reactions involving these two dienes, relative to that for cycloaddition of E-penta-1,3-diene with TCNE. 

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