Push-pull olefins

The possibility of simultaneously finding an electron-hole and an electron pair in a -system substituted by an electron-withdrawing group (NO2) and/or electron-donating group (NH2) has been examined with cis- and trans-isomers of 10 (push-pull olefins) and for non-vicinal positions40. 

It is rather difficult to generate donor-substituted carbenes (path b), and methylena-tion of push-pull-olefins (path c) is not very efficient due to the low reactivity of these alkenes. Therefore these two alternative [2 + l]-cycloadditions have been of relatively low importance so far. This is also true for the addition of suitable nucleophiles to cyclopropenes activated by electronwithdrawing substituents (path e). 

Uracil derivatives 92 are usually aminomethylated at position 3, although the reaction with formaldehyde alone shows an equilibrium constant for hydroxymethylation in position 1, which is about twice the value obtained for the reaction at position 3. The C-aminomethylation reaction takes place at position 5, when unsubstituted, as these substrates resemble the structure of push-pull olefines (67, Table 8). ... 

Push-pull-Olefine vom Typ I cyclisieren mit Hydrazin-Hydrat fiber die Ester-Gruppe, wahrend mit Phenyl-hydrazin die Cyan-Gruppe (fur R = SCH3) reagiert469 497 ... 

Unlike pyridine (85MI1), tri-rm-butylazete (101) has, as judged from its reactivity, olefinic character [88AG(E)(27)272]. As in the case of cyclobutadiene, the push-pull substitution [69CC240 88AG(E)(24)1437] promotes the stabilization of azete, as has been demonstrated by the isolation of the first thermodynamically stable substituted azete, tris(dimethylamino)azete... 

In the NMR spectra, a push-pull effect of octahydropyrido[2,3- pyrimidine 28 was observed in the olefinic C-4a and G-8a due to the electronic behavior of their substituents. Thus, C a appears at low 6 values (88-93 ppm) and C-8a gives peaks at lower field (146.0-148.5 ppm) <1996JHC45>. 

The nature of the bonding in Zeise s anion and other r -olefin complexes is illustrated in Fig. 18.2. Without the push-pull mechanism, the 7r electrons of the olefin would have little or no tendency to allow themselves... 

The catalyst diagnostician can justifiably inquire if the carbonium ion participation and migration by some push-pull mechanism is the only mechanism on which catalyst construction can be based, or do we have some alternatives. Up to the present no alternatives have been published, however, the relatively simple mechanism described above, can be modified in that the initial reaction of carbonium ion formation can be also looked upon as a two step process, whereby a paraffin is first dehydrogenated and the resulting olefin promptly forms a carbonium ion. The overall result is the same. 

Among the difference types of olefins known with barriers to rotation amenable to study by dynamic H NMR technique, the reported rotational barriers of push-pull ethylenes containing potentially heteroaromatic systems are rather low, ca. 50 kJ-mol [85MI1 88AHC(43)173], Moreover, Elguero and co-workers have studied the rotational barriers around the C—C interannular bond of several 2-(4-pyridyl)benzazoles and their pyri-dinium salts (areno-analogues of amides), since they are too low to measure by H NMR (60 MHz) at 173 K (77H911). 

Phosphinocarbene or 2 -phosphaacetylene 4, which is in resonance with an ylide form and with a form containing phosphoms carbon triple bond, is a distillable red oil. Electronic and more importantly steric effects make these two compounds so stable. Carbene 4 adds to various electron-deficient olefins such as styrene and substituted styrenes. Bertrand et al. have made excellent use of the push-pull motif to produce the isolable carbenes 5 and 6, which are stable at low temperature in solutions of electron-donor solvents (THF (tetrahydrofuran), diethyl ether, toluene) but dimerizes in pentane solution. Some persistent carbenes are used as ancillary ligands in organometallic chemistry and in catalysis, for example, the ruthenium-based Grubbs catalyst and palladium-based catalysts for cross-coupling reactions. 

Wang et al have used the AMl/FF approach for static properties and ZlNDO/SCl to study the -hyperpolarizabdity of some azulene derivatives. High values are obtained and it is concluded that the azulene ring is a more efficient conjugation bridge between donor and acceptor than either benzene or thiophene. They use the same methods in a study of push-pull polyenes containing nonaromatic cycfic olefines and for tetra-substituted benzenes and pyrazines. 

In the meantime, the Wittig olefination of carbohydrate lactones with stabilized phosphoranes at high temperature was discovered. This was an efficient reaction which worked with y- and 5- lactones in excellent yields. This way another new class of exo-glycals 3 was available, as Z/E mixtures but with an interesting push-pull substitution of the double bond (Scheme 1). ... 

A new method for the determination of the configuration and conformation of push-pull perfluoroalkyl-containing olefinic derivatives by the use of the long-range coupling, Vcf, has been discussed by El Kharrat et al. ... 

Diederich and coworkers have developed a new class of push-pull chro-mophores resulting from the formal [2-1-2] cycloaddition-retroelectrocyclization (CA-REC) reaction between electron-rich alkynes and electron-poor olefins such as tetracyanoethylene (TCNE) [134]. A highly functionalized 6,6-dicyanopentafulvene 226 with an intense, low-energy charge-transfer band is obtained by a novel ring-closure reaction of a push-pull chromophore 227 (Scheme 6.61a) [135]. Later, they also reported the synthesis of new fulvene-based push-pull chromophores 228 and 229 via the formal [2-1-2] CA-REC reaction of dicyanopentafulvenes 222 and 230 with electron-rich alkynes (Scheme 6.61b,c) [136]. 

Scheme 11.10. Cation-anion coupling technique for the generation of the OPVs 27a-c having olefinic push-pull segments (a) CH2 group deprotonated by DMF, (b)...

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