Push-pull dienes

Interesdng push-pull dienes, Tchallryldinino-4-nitro-1,3-butddienes, < dpphcddon of this ddchdon-eliminddon redcdon fEq 4 22 ... 

Interesting push-pull dienes, 1-dialkylamino-4-nitro-l,3-butadienes, are prepared by the application of this addition-elimination reaction. (Eq. 4.22).28... 

Fluorinated alkyl cyanides, such as trifluoroacetonitrile, pentafluoropropionitrile, per-fluorobutyronitrile and chlorodifluoroacetonitrile, react with butadiene in the gas phase at 350-400 °C to afford pyridines in high yields (equation 82)72. The push-pull diene 150 and electron-rich cyanides (acetonitrile or acrylonitrile) furnish pyridines (equation 83)73. 

The stereochemistry of the interesting group of compounds known as push-pull dienes has not been much studied, although a fair number are available. The earliest studies stem from the interest in the influence of the number of intervening double bonds in vinylogous amides (76) on the barrier to C—N rotation. It was... 

Electron withdrawing substituents such as aldehyde or imonium functions at the other end of the alkene or the 1,3-diene transform enamines and 1,3-dienamines into push-pull systems characteristic of dyes. Electron release (push) of the donor group shields the / , S, e,... carbons and electron withdrawal (pull) of the acceptor group (carbonyl oxygen or imonium nitrogen) deshields in the a, y,... position [343],... 

Indoles, which are especially electron-rich and thus unsuitable for ordinary Diels-Alder reactions, have performed successfully in the cation-radical reaction as dienophiles (Scheme 44)107 and as dienes (Scheme 45)126. Interestingly, the site of annulation (across the C—C or the C—N bond) in vinylindole cation radicals (functioning as dienes for eneamine dienophiles) may be manipulated by varying the substituent on the enamine and thereby altering its push-pull nature (Scheme 45). 

Alkenes substituted with an electron-withdrawing (Z) and an electron-donating group (X) will be less reactive than the unsubstituted alkene if the substituents are vicinal (Z-C=C-X push-pull alkenes), but will generally become highly reactive when these two substituents are bound to the same carbon atom. The latter type of alkene (ZXC=CR2 1,1-captodative alkenes) readily dimerize to yield cyclobutanes and can react with 1,3-dienes to yield products of [2 + 2] and/or [2 + 4] cycloaddi-... 

This opens up the possibility of a systematic investigation of pericyclic reactions not only for model cases of parent unsubstituted systems, but for inclusion if zwitterionic contributions also enable the analysis of the eventual mechanistic changes induced by the polar substitution. As an example, the push-pull substituted Diels-Alder system will be analysed, in which the diene component is substituted in position 1 by a donor, and dienophilic component in position 6 by an acceptor substitution. In order to avoid the problems with the relative wieght of individual limiting structures of the intermediate (Eq. 30), the coulombic integrals modelling the substitution in the HMO wave function were arbitrarily set to a = 3/ and a = — 3) so that there is sufficient polarity in the system to secure the approximation of the intermediate by pure zwitterionic structure Z,. 

Diels-Alder reactions. The amino group at Ci renders this position electrophilic, whereas C4 is nucleophilic (push-pull diene). The diene (1) can react as a vinyl keteniminium chloride (la) in reactions with simple nitriles such as acetonitrile or acrylonitrile. After treatment with base a pyridine derivative (2) is obtained. The... 

Formation of a cation radical of the diene may catalyze Diels-Alder reactions with certain dienophiles thus oxidation of substituted vinylindoles to the cation radical in the presence of a push-pull dienophile may form different heterocyclic compounds by the Diels-Alder reaction, for instance pyrido[l,2i7]indoles from vinylindoles and yS-enamino-esters [33]. Similarly, anodic oxidation in MeCN of certain oxazolidines in presence of vinyl ether leads to derivatives of oxazepines in a catalytic reaction in which the ring-opened radical cation of the oxazolidine adds to the vinyl ether and the radical cation of the resulting oxazepine oxidizes the oxazolidine [34]. 

Push-pull stabilized benzocyclobutadienes are not known, but several benzo-cyclobutadienes are isolable as stable transition metal complexes9, notably 29, synthesized by Pettit and his co-workers41. The diene character of the uncomplexed... 

Kiesel, M., Haug. E., and Kantlehner, W., Qrthoamidcs. Part 50. Contribution to the chemistry of propionaldehyde aminals. Synthesis and transformalions to push-pull-substituted buta-1,3-dienes, cyclobutanes, vinylogous amidinium salts and 1,2,3-triazoles, J. Prakt. Chem./Chem.-Ztg., 339, 159,... 

Protoporphyrin II dimethyl ester, 77 Pseudomonic acid, 115 Pseudotropane, 81 Push-pull dienes, 149 Pyrazine, 320 Pyrazole, 311 Pyrazolone, 170 Pyridazine, 313-317 Pyridine, 2, 302, 312-313 product, 310 substituted, 146, 245 synthesis, 60 Pyridinium salt, 311... 

Ghosez et al. have recently prepared so-called push-pull dienes (Scheme 6-II).7 These dienes are sufficiently electrophilic to react at relatively low temperatures with unactivated, electron-rich nitriles, affording initial [4 + 2] adducts which tautomerize to 2-aminopyridines. This method would seem to offer a potentially efficient approach to synthesis of certain substituted pyridines. An s-tetrazine dicarboxylate is also a sufficiently electrophilic diene to combine with an N-substituted cyana-mide to afford a triazene in good yield [Eq. (5)].8... 

The Au(I)- or Ag(I)-catalysed intermolecular hetero-Diels—Alder cycloaddition of push-pull l,3-dien-5-ynes (76) with aldimines or silylaldimines (77) produced 5,6-dihydropyridin-2-ones (78) with high diastereo- and regio-selectivities (Scheme 24). A copper-catalysed intramolecular aza-Diels—Alder reaction has been used to produce dihydrochromeno[4,3-Mpyrrolo[3,2-/ quinolmes in good yields. The cinchonidine-derived quaternary ammonium catalyst, A-2, 3, 4 -trifluorobenzyl-0-benzylcinchonidinum bromide (79), catalysed the aza-Diels—Alder reactions between... 

This opens the possibility of the systematic investigation of pericycUc reactions not only for the model cases of parent unsubstituted systems but inclusion of zwitterionic contributions also enables the analysis of eventual mechanistic changes induced by the polar substitution. As an example, the push-pull substituted Diels-Alder system will be analysed, in which the diene component is substituted in position 1 by donor and dienophilic component in position 6 by acceptor substitution. 

More functionalized substrates have also been considered recently [94], giving, for instance, access to 5-alkoxy-2 ,4 -dienones that are regarded as useful 1,4-disubstituted push-pull dienes. 

It was with the independent development of the "push-pull cannula" and the chemltrode that a technique was made available to collect and examine the local perfusate from brain nuclear masses. This technique, however, has been utilized surprisingly little. It was found that ACh was released when the tip of the cannula was in the cortex, but not dien it was in the underlying vdilte matter O. Furthermore, the rate of ACh... 

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