Tetracyanoethene, reaction with

It was demonstrated some time ago that vinylic and cross-conjugated double bonds of protoporphyrin (IX) dimethyl ester (80a) constitute a diene system capable of undergoing Diels-Alder reactions with activated dieno-philes (68CC697). Isobacteriochlorins 81 and 82a were obtained in a reaction with dimethyl acetylenedicarboxylate (ACDE) and tetracyanoethene (TCNE), respectively (73JCS(PI)1424). 

Diphenylcyclopropene does not undergo an ene reaction with simple alkenes or with enophiles such as diphenylacetylene however, with the electron-poor alkenes tetracyanoethene or 1,2-dibenzoylethene, and alkynes such as dimethyl acetylenedicarboxylate, moderate yields of ene products were observed. ... 

Tetracyanoethene reacted with benzylidenecyclopropane and (diphenylmethylidene)cyclo-propane in a similar manner. Rather than [2+2] cycloadducts only products deriving from a Diels-Alder reaction in which a phenyl ring is part of the diene system 20 were isolated. 

In allylidenecyclopropanes, the semicyclic double bond can be part of a diene system capable of undergoing Diels-Alder reactions with appropriate dienophiles such as tetracyanoethene, maleic anhydride, benzoquinone, dimethyl (E)- and (Z)-but-2-enedioate and acetylenedicarb-oxylate. ... 

The dithienylethene (83) showed photochromic properties. Due to the enhanced electron density of the alkynyl moiety the closed-ring isomer (84) underwent the addition reaction with tetracyanoethene to give (85). The resulting closed-ring form (85) isomerized to (86). ... 

Fe, Ru and Os - The synthesis and X-ray crystal structure of the n-bonded tetracyanoethene complex [Fe(NO)2(PPh3)(ti2-TCNE)] has been reported36. Whereas the complex [Fe(depe)2(N2>] [depe = 1,2 bis(diethylphosphinoethane)] was shown32 to react with ethene to form [Fe(depe)2(Tl2-C2H4)], reaction with 2-methylstyrene led to activation of the alkene C-H bond to afford a mixture of cis and trans [FeH(depe)2(CH=C6H4CH3)]. The selective formation of alkene rt-complexes or C-H activated products was explained in terms of steric considerations. 

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). 

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

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). 

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

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. 

A special kind of cyclization is the reaction of enaminonitriles and tetracyanoethene, which give 2-amino-3,4,5-tricyanopyridines, with evolution of HCN102 (equation 71). 

A prototype cycloaddition involving a zwitterionic intermediate is the reaction of -butyl vinyl ether with tetracyanoethene, as shown in Eq. (5-33) [94-98]. 

Nevertheless, there are some examples known with larger, although still moderate, solvent effects [124, 125, 129, 538-540]. Over a range of solvents from o-xylene to trichloromethane, the reaction rates for the addition of tetracyanoethene to anthracene have been found to increase by a factor of 70 [125]. In the case of the reaction between cyclopentadiene and acrolein, changing the solvent from ethyl acetate to acetic acid causes a 35-fold acceleration in rate [129]. A strongly dipolar activated complex is unlikely, as reflected by this small sensitivity to solvent. These data are more consistent with the following mechanism first the diene and dienophile form an EPD/EPA com-... 

Examples of the solvent-influenced competition between concerted [4 -I- 2]Diels-Alder type cycloaddition reactions and 1,4-dipolar reaction pathways with zwitterionic intermediates can be found in references [677-679], For example, in solvents of low polarity (CHCI3, CH2CI2), homofuran reacts with tetracyanoethene to form the seven-membered [4 -I- 2]cycloadduct A in quantitative yield. In solvents of high polarity (CH3CN), however, the [2- -2]cycloadduct B predominates, formed via a 1,4-dipolar activated complex and a zwitterionic intermediate [679] cf. Eq. (5-142). 

In the reactions of the pyrroles 102 with tetracyanoethene, one of the nitrile groups in the latter compound was replaced by a 2- or 3-pyrrole moiety to form tricyanoethenylpyrroles 103 (R = H) or 104 (R = Me) in quantitative yields. The reaction proceeded as an addition-elimination sequence (Equation (27)) (00RJO1504, 01ARK(ix)37). 

---