Tetracyanoethene

These reactions are found to be promoted by electron-donating substituents in the diene, and by electron-withdrawing substituents in the alkene, the dienophile. Reactions are normally poor with simple, unsubstituted alkenes thus butadiene (63) reacts with ethene only at 200° under pressure, and even then to the extent of but 18 %, compared with 100% yield with maleic anhydride (79) in benzene at 15°. Other common dienophiles include cyclohexadiene-l,4-dione (p-benzoquinone, 83), propenal (acrolein, 84), tetracyanoethene (85), benzyne (86, cf. p. 175), and also suitably substituted alkynes, e.g. diethyl butyne-l,4-dioate ( acetylenedicarboxylic ester , 87) ... 

Reaction of 4,4-diphenyl- and 4,4-di(p-tolyl)dithienosilole (DTS) <1998JOM487, 1998CL1233, 1999JOM1453, 2002JOM137> 25a,b with excess tetracyanoethene (TCNE) 168 in DMF afforded coupling products 4,4-diphenyl-and 4,4-di(p-tolyl)-2-(tricyanoethenyl)dithienosilole 169 in 85% and 87% yields, respectively <20020L1891> (Scheme 20). 

Tetracyano ligands have been used to bridge between four Ru(NH3)5 moieties. The complexes [ Ru(NH3)5 4(/i-L)] + (L = tetracyanoethene, tetracyano-p-quinodimethane, 1,2,4,5-tetracyano-benzene, 2,3,5,6-tetracyanopyrazine) exhibit intense, long-wavelength electronic absorptions. Oxidation to [ Ru(NH3)5 4(yU-L)] °" " and reduction to [ Ru(NH3)5 4(//-L)] + and [ Ru(NH3)5 4-(/i-L)] + can readily be achieved. The species are fully delocalized with partially reduced ligands or partially oxidized Ru centers. Treatment of [5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato] cobalt(II) or [5,10,15,20-tetrakis(4-cyano-2,6,-dimethylphenyl)porphyrinato]cobalt(II) with [Ru-(NH3)5(0S02CF3)] introduces cyano-bound pendant Ru (NH3)5 groups to the porphyrinato complexes. ... 

Ethylcarbazole reacted at C-3 with tetracyanoethene in hot dimethyl-formamide generating 142. ° A series of aryl chlorovinyl ketones reacted with 9-methylcarbazole at the 3-position to give monosubstitution products 143 ulitizing aluminium chloride catalysis. ... 

In the absence of a dipolarophile, thiocarbonyl ylide 84 undergoes a 1,4-hydrogen shift to give the naphthoannelated thiophene derivative 85. Many examples of related syntheses have been reported (139-143). The photolysis of tetraarylthiiranes in the presence of tetracyanoethene represents an approach to tetrahydrothiophenes via a SET mechanism (75,76). 

To 1,2-dimethyl-3-vinylindole (0.5 g, 2.9 mmol) in dry benzene (30 ml) was added tetracyanoethene (0.37 g, 2.9 minol). The initial dark green color faded after 5 min. The solution was concentrated to 10 mL and crystals of the product separated upon standing yield 0.68 g (79%). 

Interestingly, in these cases, the use of aluminum trichloride as a Lewis acid catalyst promoted regiospecificity and only the l,l-dicyclopropyl-2-mcthylenecyclobutanes were obtained (see Section 1.3.4.2.). Furthermore, the catalyzed cycloadditions proceeded under milder conditions (room temperature vs. 200 °C for noncatalyzed cycloadditions). In the same study, the cycloaddition of 1,1-dimethylallene (14) with tetracyanoethene proceeded regiospecifically giving 3-(l-methylethylidene)cyclobutane-l,l,2,2-tetracarbonitrile (15) in 71% yield.11,12... 

The reaction of tetrakis(ethylsulfanyi)aiiene (32) with tetracyanoethene reportedly gives the corresponding cyclobutane 33, although no experimental or physical data were reported.24... 

RB = Rose Bengal E = eosine TPP = tetraphenylporphyrin MB = methylene blue TONE = tetracyanoethene. b 7-Oxadispiro[2.0.2.1]heptane (62%) was also formed. 

Markl has shown (68TL3257) that l-phenyl-2,5-dimethylarsole acts as a diene in the Diels-Alder reaction. When acetylenedicarboxylates are used as the dienophile the intermediate arsolene is unstable. The adduct with tetracyanoethene was, however, isolated as the stable crystalline solid (50 m.p. 157 °C) (reaction 10). At higher temperatures 1,2,5-triphenylarsole reacts similarly as a diene with diphenylacetylene (81TL4713). 

Exercise 13-4 What products would you expect from the Diels-Alder addition of tetracyanoethene to c/s,frans-2,4-hexadiene and c/s,c/ s-2,4-hexadiene Explain. 

Tetracyanoethene undergoes [2 + 2] cycloaddition with cis- and trans-1-methoxypropene. The following facts are known about these reactions. 

The cIs- and frans-1-methoxypropenes are interconverted by tetracyanoethene at a rate comparable to the [2 + 2] addition rate with tetracyanoethene. 

Exercise 22-40 The rate of the Diels-Alder addition between cyclooctatetraene and tetracyanoethene is proportional to the tetracyanoethene concentration, [C2 (CN)4], at low concentrations of the addends but becomes independent of [C2(CN)4] at high concentrations. Write a mechanism that accounts for this behavior. 

Exercise 24-15 Tetracyanoethene in benzene forms an orange solution, but when this solution is mixed with a solution of anthracene in benzene, a brilliant blue-green color is produced, which fades rapidly colorless crystals of a compound of composition C14H10-C2(CN)4 then are deposited. Explain the color changes that occur and write a structure for the crystalline product. 

Exercise 31-2 The cyclobutadiene iron complex, 10, has been prepared optically active, and when oxidized with Ce(IV) in the presence of tetracyanoethene gives a mixture of cyclobutadiene cycloadducts, all of which are optically inactive. 

In a first successful approach, Kochi, Renzepis, and co-workers [41] chose EDA complexes of 9-cyanoanthracene (14) and tetracyanoethene (TCNE, 15) since their charge transfer (CT) absorption bands are well separated from the absorption bands of the monomers. Excitation with a 25 ps laser pulse produced two transient absorption bands near 460 and 750 nm, which decayed simultaneously within ca. 60 ps. As was shown in the chloranil-enolether system 9—10, cf. Fig. 6), the transients can be identified with the arene radical cation (14a+ ) and the olefin radical anion (/5- ), respectively (Scheme 5). 

This is the case in the system anthracene donor — tetracyanoethene (TCNE) acceptor [86], Replacing TCNE for tetranitromethane, donor-acceptor addition (in this case aromatic nitration) occurs [44b, 87]. 

First, we note that the charge ordering of the solvent can impose itself on the distribution of products in reaction. Chiappe and Pieraccini [40] report that in their study of electron transfer between Micheler s ketone and tetracyanoethene, they observed that the formation of a radical ion pair to be preferred over formation of a single, neutral complex. Such a preference is only observed for the most highly polar molecular liquids, and is analogous to the spontaneous ionization of metal atoms in fused salts noted above. This represents a novel phenomenon for moderately polar solvents, though its generality is unclear at this time. 

The last factor often is the one that determines the reaction rates of [4+2]-cycloadditions. This factor allows one to understand, for example, why the cycloadditions of ethene or acetylene with butadiene (cf. Figure 15.1) occur only under rather drastic conditions, while the analogous cycloadditions of tetracyanoethene or acetylenedicarboxylic acid esters are relatively rapid. As will be seen, a simple orbital interaction between the reagents at the sites where the new a bonds are formed is responsible for this advantageous reduction of the activation energies of the latter two reactions. 

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