Push-pull substitution

Fig. 4. Push-pull substituted diphosphanes, bonding parameters in Angstroms and degrees...

The investigations indicate that, in agreement with the calculations on the cations, the push-pull substituted diphosphenes tend to form a bridged structure of one ligand the other substituent can easily depart under formation of an ion pair structure. 

Theoretical calculations by Mixon and Cioslowski (1991) suggested that a push-pull substituted ethane (H2N)3C—C(N02)3 exists as an ion pair (H2N)3C C(N02)3. Recently, a similar hexamethylguanidinium-... 

Generally, push-pull substituted cydopropanes as 2-364 are flexible building blocks and represent an equivalent for 1,4-dicarbonyl compounds. They show a pronounced tendency to undergo ring opening [202]. 

The concept of captodative substitution implies the simultaneous action of a captor (acceptor) and a donor substituent on a molecule. Furthermore, in the definition of Viehe et al. (1979), which was given for free radicals, both substituents are bonded to the same or to two vinylogous carbon atoms, i.e. 1,1- and 1,3-substitution, and so forth is considered. One might, however, also include 1,2-, 1,4-,. .. disubstitution, a situation which is more often referred to as push-pull substitution. Before discussing captodative substituent effects it might be helpful to analyse the terms capto and dative in more detail. 

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... 

Some years later, the first stable thiocarbonyl ylides 9 and 10 were prepared by the reaction of thiourea with cyano-substituted oxiranes (19,20) or by addition of Rh-di(tosyl)carbenoid to benzo-l,2-dithiole-3-thione (21), respectively. Enhanced stability and the low reactivity of 9 and 10, which enables their isolation in crystalline form, results from the push-pull substitution at the two termini [cf. also (22)]. Another class of stable thiocarbonyl ylides that are also able to afford [3 + 2]-cycloaddition products are the mesoionic 1,3-dithiole-4-ones of type 11 (23,24). 

U. Schoberl, J. Salbeck, and J. Daub, Precursor compounds for organic metals and nonlinear optical devices from carbohydrate-derived hydroxymethylfurfuraldehyde Furanoid electron donors and push/pull substituted compounds, Adv. Mater., 4 (1992) 41-44. 

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,. 

Any substitution reaction (i.e. by tertiary or secondary amines or by metallated secondary amines) is greatly facilitated when the jS-carbon carries an electron-with-drawing or simply a conjugative group. Accordingly, chlorocyanoacetylene reacts smoothly at —50° with free secondary amines to cyanoynamines, an example of push-pull substituted acetylenes (12)27... 

In push-pull substituted / -enaminones 84 to 87 due to through conjugation the C=C distance is elongated to about 1.380 A. In other push-pull systems 99 to 103 the C=C bond lengths are in the range of 1.348 to 1.405 A. In both classes the C—N distances vary between 1.303 and 1.390 A. 

In the case of doubly push-pull substituted enamines 88 to 92 and 106 to 107, instead of through conjugation a tendency to adopt a zwitterionic form is observed. This form is characterized by a very long C=C distance between 1.407 to 1.482 A, very short C—N lengths of 1.312 to 1.352 A and shows various angles of twist around the C=C bond. 

Donor/acceptor-substituted phosphole 22 exhibits classical properties, namely the phosphorus atom has a pyramidal geometry and the aromatic character of the heterole is similar to that of cyclopentadiene <2000JOC2631>. Due to the push-pull substitution pattern, significant delocalization of the endocyclic 7t-electron density over the entire system... 

Push-pull substituted azobenzenes are nice examples of solvent-dependent E)/ (Z)-isomerization reactions [527-530, 561, 729]. For instance, the thermal cis-to-trans isomerization of 4-(dimethylamino)-4 -nitroazobenzene exhibits a rate enhancement of ca. 10 on changing the solvent from -hexane to formamide [528]. 

Since the former theoretical predictions of push-pull substituted phthalocyanines as candidates for second-order NLO properties, some work has been devoted to prepare and study different substituted derivatives with the aim to establish the key structural parameters affecting the NLO response. Some revisions have already been done on the second-order NLO behavior of phthalocyanines [16, 17, 31]. For most of the unsymmetrically substituted push-pull compounds (planar conjugated 7T-electron systems in the XZ plane), 2 symmetry may be assumed due to the... 

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,... 

Reactions suitable for the kinetics aiming at the purpose mentioned above needs to satisfy one condition. The TST must be valid for a fairly wide range of pressure and temperature. Furthermore it is desirable that the reactant can be generated in situ and the reaction can be followed spectroscopically to obtain reliable rate constants. As the first set of reactions, thermal ZfE isomerization of three N-benzylideneanilines (benzaldehyde anUs), i.e. N-[4-(dimethylamino) benzylidene]-4-nitroaniline (DBNA), N-[4-(dimethylamino)benzylidene]-4-ethoxy-carbonylaniline (DBEA), and N-[4-(dimethylamino)benzylidene]-4-bromoaniline (DBBA), and two push-pull substituted azobenzenes, i.e. 4-(dimethylamino)-4 -nitroazobenzene (DNAB) and 4 -(dimethylamino)-2-methoxy-4-nitroazobenzene (DMNAB) as shown in Scheme 3.2 were selected. 

A similar but mechanistically different isomerization of two push-pull substituted azobenzenes shown in Scheme 3.2 was also studied. Although it is possible for azobenzenes to isomerize by the same nitrogen-inversion mechanism as the N-benzylideneanilines [40], rotational isomerization via a highly dipolar activated complex 3.2 becomes predominant [41, 42] in polar solvents provided that the compound contains a strongly electron-donating dialkylamino group as in DNAB and DMNAB. 

Pressure-induced retardation clearly occurred at higher pressure when the reaction temperature was raised. The results leave httle doubt about the validity of TST at low pressures and its failure at higher pressures in the rotational isomerization of push-pull substituted azobenzenes in GTA and MPD [43]. The observed rate constants are plotted against in Figs 3.14 and 3.15. 

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