Tetracyanoethylene, diene

Endo adducts are usually favored by iateractions between the double bonds of the diene and the carbonyl groups of the dienophile. As was mentioned ia the section on alkylation, the reaction of pyrrole compounds and maleic anhydride results ia a substitution at the 2-position of the pyrrole ring (34,44). Thiophene [110-02-1] forms a cycloaddition adduct with maleic anhydride but only under severe pressures and around 100°C (45). Addition of electron-withdrawiag substituents about the double bond of maleic anhydride increases rates of cycloaddition. Both a-(carbomethoxy)maleic anhydride [69327-00-0] and a-(phenylsulfonyl) maleic anhydride [120789-76-6] react with 1,3-dienes, styrenes, and vinyl ethers much faster than tetracyanoethylene [670-54-2] (46). 

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

Compare electrostatic potential maps for the following Diels-Alder transition states cyclopentadiene+ethene, cyclopentadiene+acrylonitrile and cyclopentadiene+ tetracyanoethylene, with those of reactants cyclopentadiene, ethene, acrylonitrile and tetracyanoethylene. Are electrons transferred from diene to dienophile in the transition states (relative to reactants) or vice versa For which reaction is the transfer the greatest The least Quantify your conclusion by measuring the total charge on the diene and dienophile components in the three transition states. 

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. 

The comparison of rates of cycloaddition of maleic anhydride, tetracyanoethylene, and styrene to PPA shows that the latter, irrespective of the presence of electronegative groups, behaves in these reactions not as an electron-poor diene system. This fact, together with the composition of side products (giving evidence of PPA decarboxylation), allows the assumption to be made that the cycloaddition of dienophiles involves mainly decarboxylated polyene sections of cis-transoid structure213, 266. This is in agreement with the fact that PPA with predominant trans-transoid configuration interacts with these dienophiles at a substantially lower rate. The ultimate amounts of the dienophile combined with PPA of this structure is also considerably smaller. 

In another aspect of the mechanism, the effects of electron-donating and electron-withdrawing substituents (p. 1065) indicate that the diene is behaving as a nucleophile and the dienophile as an electrophile. However, this can be reversed. Perchlorocyclopentadiene reacts better with cyclopentene than with maleic anhydride and not at all with tetracyanoethylene, though the latter is normally the most reactive dienophile known. It is apparent, then, that this diene is the electrophile in its Diels-Alder reactions. Reactions of this type are said to proceed with inverse electron demand ... 

Alkenes of all types can be converted to cyclopropane derivatives by this reaction (though difficulty may be encountered with sterically hindered ones). Even tetracyanoethylene, which responds very poorly to electrophilic attack, gives cyclopropane derivatives with carbenes.Conjugated dienes give 1,2 addition ... 

Another type of steric effect results from interactions between diene substituents. Adoption of the s-cis conformation of the diene in the TS brings the d.v-oricnlcd 1- and 4-substituents on a diene close together. /(-1,3-Pcnladicnc is 103 times more reactive than 4-methyl-l,3-pentadiene toward the very reactive dienophile tetracyanoethylene. This is because the unfavorable interaction between the additional methyl substituent and the C(l) hydrogen in the s-cis conformation raises the energy of the TS.20... 

Chemical separation of conjugated dienes and other polyunsaturated hydrocarbons is based on the availability of tt delocalized electrons. The use of a strong dienophile (e.g. tetracyanoethylene, TCNE) will derivatize only conjugated dienes, thus separating the polyunsaturated compounds into two groups. However, such derivatization is not always reversible since a retro-Diels-Alder reaction may require a high temperature. Hence, the retrieved compounds may be the thermostable ones and not those present in the initially analysed mixture. 

In the presence of electron-deficient dienophiles such as tetracyanoethylene (TCNE), however, vinylallene 198 acts as a diene to afford cross [4+2]-cycloadducts. In the case of 1,4-naphthoquinone, the adduct 199 was converted to 1-methylanthra-quinone 200 in the presence of charcoal [163],... 

Dienes such as 3 exhibit an even greater sensitivity to add than do 6 and 7. [16] Since formation of the [4.4.4]propelladiene 10 competes very favorably with conversion to 9, the transition state for aromatization is not particularly advantaged (Scheme 3-2). To our amazement, tetracyanoethylene has been found to be capable of promoting the identical chemical transformations. [17]... 

Tetracyanoethylene complexes, disilanes, 816 Tetracyclone, bleaching, 734-5 Tetraethylammonium ozonide, 736 Tetragermabuta-1,3-diene, 825, 826 Tetrahedral distortion acychc organic peroxides, 103 alkyl hydroperoxides, 110 anion ligands, 119... 

Isopropenylbenzofuran (124, Scheme 30) affords good yields of the adducts 123 and 125 on separate reaction with maleic anhydride and tetracyanoethylene. With but-3-en-2-one, 2-isopropenylbenzofuran (124, Scheme 31) affords the adducts 126 and 127 in a combined yield of 29%. When the crude product was dehydrogenated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in boiling benzene, the aromatized product 128 (6%) was obtained. It was accompanied by the dicyanodibenzofuran 129, which was found to arise from the excess diene present in the reaction mixture. A speculative mechanism is shown. 

Cycloaddition of 9-vinylcarbazoIe to tetracyanoethylene took place in benzene solution giving 103, but in methanol the diene 104 was obtained cyclobutane (103) itself was transformed into 104 in methanol. Other... 

Furthermore, the synthetic utility of 2,6-divinyl-l,4-dithiin 68 as a reactive diene in Diels-Alder reactions was reported with tetracyanoethylene, maleic anhydride, A -phenylmaleimide, and dimethyl acetylenedicarboxylate (DMAD) and allowed the preparation of various dihydrothianthrene derivatives (Equation 9) <2003S849>. 

The first reference to 2-trimethylsilyloxy-l,3-butadiene (1) was a report2 of its reaction with tetracyanoethylene by Cazeau and Frainnet without mention of any experimental details. Later, Conia3 reported its synthesis in 50% yield with only a reference made to the usual House procedure4 for silyl enol ethers. The diene 1 has also been prepared using lithium diisopropylamide as base and chlorotrimethylsilane in tetrahydrofuran-ether (1 1) in yields up to 65%, but on a smaller scale.s... 

A final and interesting example of the use of vinylpyrroles as diene systems in Diels-Alder cycloadditions is the reaction of divinylporphyrins with DMAD and other activated dienophiles [/V-phenylmaleimide (NPMI), Tetracyanoethylene (TCNE)], which provide a general route to bacterichlorin-like chromophores for the treatment of malignant tumors (86JOC1094 91TL2875). 

It was shown that the diene reacted only slowly with tetracyanoethylene, even slower than with maleic anhydride. The reaction of the ortho adduct with maleic anhydride led to the same 1 2 adduct as the photochemical reaction between benzene and maleic anhydride. The previous failure to trap the diene with tetracyanoethylene resulted from an unexpectedly low dienophilic reactivity of this compound toward the ortho adduct in the presence of benzene. Tetracyanoethylene is apparently completely complexed in benzene solution. 

TCNE = Tetracyanoethylene Dipdba = 4,4 -diisopropyl(di-benzylideneacetone) TMM = Trimethylenemethane dppm = (Diphenylphosphino)methane MMLCT = Metal-metal bond to ligand charge-transfer Tp = Hydrido(trispy-razolyl)borate Tp = Hydridotris(3,5-dimethyl-pyrazolyl)bo-rate BAr = (3,5-triflnoromethylphenyl)borate ttab = l,2,4,5-tetrakis(l-iV-7-azaindolyl)benzene tmeda = Tetra-methylethylenediamine bpma = Bis(pyridyhnethyl)amine TFE = Triflnoroethanol dtbpm = Bis(di-terr-butylphosphi-no)methane dmpe = Bis(dimethylphosphino)ethane dcpe = Bis(dicyclohexylphosphino)ethane triphos = Bis(2-diphe-nylphosphinoethyl)phenylphosphine COD = 1,5-cycloocta-diene dppbts = (Diphenylphosphinobutane)tosylate sodium PPE = Poly(jo-phenylene ethylene). 

Reaction with cycloociatetraene. With most dienophiles, cyclooctatetraene (I) reacts through the quasiplanar diene system of the valence tautomer bicyclo[4.2.0]-octatrienc (2). For example, tetracyanoethylene reacts in this way to give (3). However,... 

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