Conjugate systems, and

The pyridine-like nitrogen of the 2H-pyrrol-2-yiidene unit tends to withdraw electrons from the conjugated system and deactivates it in reactions with electrophiles. The add-catalyzed condensations described above for pyrroles and dipyrromethanes therefore do not occur with dipyrromethenes. Vilsmeier formylation, for example, is only successful with pyrroles and dipyrromethanes but not with dipyrromethenes. 

The examples that have been presented in this section illustrate the approach that is used to describe structure and reactivity effects within the framework of MO description of structure. In the chapters that follow, both valence bond theory and MO theory will be used in the discussion of structure and reactivity. Qualitative valence bond terminology is normally most straightforward for saturated systems. MO theory provides useful insights into conjugated systems and into effects that depend upon the symmetry of the molecules under discussion. 

One criterion of aromaticity is the ring current, which is indicated by a chemical shift difference between protons, in the plane of the conjugated system and those above or below the plane. The chemical shifts of two isomeric hydrocarbons are given below. In qualitative terms, which appears to be more aromatic (Because the chemical shift depends on the geometric relationship to the ring current, a quantitative calculation would be necessary to confirm the correctness of this qualitative impression.) Does Hiickel MO theory predict a difference in the aromaticity of these two compounds ... 

Conjugated Systems and Their Physicochemical Properties, in Advances in Quantum Chemistry. Vol. 17, ed. P. -O. Lbwdin, Academic Press, New York, 1985, pp. 251 and references therein. 

The analogous structures 309, 310, and 311 have been suggested for the pyrophthalones. Early work favored structure 309 (see reference 385 and references therein). Later, ultraviolet spectral data showed the presence of an extended conjugated system, and comparison with the spectra of the A -methyl compounds suggested structure 310. ... 

We have also used poly(propynoic acid) in our studies of the photochemical interaction of PCSs with dienophiles, such as maleic anhydride, tetracyanoethylene, and styrene. This photochemical reaction of Diels-Alder type is accompanied by the breakdown of the conjugation system and the formation of slightly colored adducts266. Together with the cycloaddition reaction, photodegradation of PPA and its adducts takes place. A cycloaddition reaction is always preceded by the formation of a donor-acceptor complex of a PCS with a dienophile. 

Delocalisation takes place (cf. 1,3-dienes, p. 13), so that an electron-deficient atom results at C3, as well as at C, as in a simple carbonyl compound. The difference between this transmission via a conjugated system, and the inductive effect in saturated system, is that here the effect suffers much less diminution by its transmission, and the polarity at adjacent carbon atoms alternates. 

The ladder polysilanes are highly a-conjugated systems, and they are easily oxidized and reduced to give unique oxidation products and persistent radical anions. 

This comparison demonstrates that the exocyclic double bond length is little affected by the cyclic strain, which was also found for the radialenes see Section n.E. However, significant deviations were found for conjugated systems, and the same holds for the linear and branched dienes and polyenes. 

The development of -conjugation system and high degree of orientation in the PAV LB films arise from the orientational orderliness of the precursor chains in the LB films. The precursor polymers should form two-dimensional orientation... 

Abstract Described are the synthetic routes to precisely defined molecular wires which are of discrete length and constitution. They are fully conjugated systems and are expected to have nearly linear current-voltage response curves. Their ends are functionalized with molecular alligator clips, based on chalconides and isonitriles, for adhesion between proximal probes. Both solution and solid-phase approaches have been used to prepare these molecular wires that are based on oligofthiophene ethynylene)s and oligofphe-nylene ethynylene)s. Molecular device syntheses are also described that would be expected to have nonlinear current-voltage responses. 

Isoxazolo[2,3-4]pyridines 44, isothiazolo[2,3-4]pyridines 46, and their fully saturated derivatives 45 and 47 (Scheme 16) were discussed in CHEC(1984) <1984CHEC(6)613> and CHEC-II(1996) <1996CHEC-II(8)249>. Very little information was available on the isothiazolo[2,3- ]pyridine ring system while most of the informations given on the oxygenated parent, isoxazolo[2,3- ]pyridines, concerned the fully saturated system. Careful examination of the literature clearly shows that the situation did not change much almost no references have been reported on isothiazolo[2,3- ]pyridines and most of the work done in the last decade concerns the synthesis and reactivity of hexahydro-isoxazolo[2,3- ] pyridines 45. Therefore, this chapter will briefly describe the new reactions of fully conjugated systems and will focus on the partially/completely saturated derivatives. 

Aromatic molecules are highly conjugated but have stabilities greater than those shown by (i) noncyclic conjugated systems and (ii) cyclic but non-aromatic... 

Authors [143] suppose that sulfur atoms are not affected by oxidation and only act as activating double bond substituents. However, the analysis of the electronic structure of tetrathiofulvalene shows that, resulting from the interaction of tt CC) and 7t(S) orbitals, HOMO of this molecule is an antibonding combination of jr and riji orbitals with the predominant contribution being from the latter. Indeed, the substitution of one or more S atoms with Se results in a less conjugated system and, as a consequence, in higher oxidation potentials [144]. 

After a plate has been exposed to the mobile-phase solvent for the required time, the compounds present can be viewed by several methods. Polynuclear aromatic hydrocarbons, other compounds with conjugated systems, and compounds containing heteroatoms (nitrogen, oxygen, or sulfur) can be viewed with long-and short-wave ultraviolet light. The unaided eye can see other material, or the plates can be developed in iodine. Iodine has an affinity for most petroleum compounds, including the saturated hydrocarbons, and stains the compounds a reddish-brown color. 

Phenylene-linked bis(diradicals), where the diradical is a carbene or a ni-trene, are relatively simple and experimentally accessible and it is possible to vary the carbene site by chemical substitution. Thus, such compounds are reasonable models for the spin-spin interactions across conjugated systems and, in particular, across aromatic rings. This chapter is devoted in the recent efforts to generate and study such systems in order to progress our understanding of more complex cases. 

---