Rc-System

The use of the term leuco dye is a common paradox. Leuco color formers are materials that undergo controlled chemical or physical changes resulting in a shift from a colorless state to an intense color. The preparation of leuco color formers takes advantage of the very nature of colored materials themselves. The existence of extended conjugated -system in dyes is responsible for the absorption in the visible region. The chemistry of such rc-system is noted for facile reactivity, particularly to reactions such as reduction, oxidation, and hydrolysis (not hydrolytic cleavage). When n-... 

The reactivity of the various azolides as well as the order of reactivities within this group can be explained on the basis of the quasi-aromatic character of the azole rc-system the lone electron pairs on the acyl-substituted nitrogens N(l) are part of the cyclic tc-system of the azole units, leading to a partial positive charge on N(l) that interferes with the normal carboxamide resonance and exerts an electron-withdrawing effect on the... 

We have only just started to explore empirical access to FMO parameters based on these similarities 52). Recently, others have reported empirical equations for calculating IP s and EA s for a variety of Ji-bonded systems 64). This approach used a large number of parameters for the underlying rc-system, heteroatom substitution, and the substituents on the 7t-system. However, we aim at calculating FMO parameters from fundamental atomic data while taking due account of the bond structure of a molecule. 

The solid complex of TV-methylbenzothiazol -2(3H)-selcnonc (mbts) with Te(Ph)Br contains two symmetry-independent molecules, both of which pack in a way that weak dimers through T-T contacts are formed (Figure 29). In molecule A the distance between the Te atom and the neighbouring conjugated ji-system is 365.6 pm, and the Te atom is positioned over the central carbon atom of selenoamide function (Te- Se 413.8 Te- -S 396.1 Te- -N 393.4 pm). In molecule B, the distance from Te to the rc-system is 370.5 pm, and the Te atom is shifted away from the central carbon atom towards the endocyclic sulfur atom (Te- -Se 422.7 Te- -S 390.6 Te- -N 450.2 pm). Related T-T contacts are a general kind of packing for Se and Te-centred T-structures. 

Several reports have appeared concerning the addition of elemental phosphorus to rc-systems, either involving addition-elimination or direct addition across a ji-linkage. These proceed in reasonable yield for the preparation of pure organophosphorus materials. 

Grigg s group synthesized a unique bicyclic P-lactam 108 via an intramolecular Heck reaction from 107 [87, 88]. The 7-membered ring was formed via an unusual insertion at C(3) of furan, an aromatic rc-system. 

For the central violene rc-system, we chose benzene with acetylene spacers for decreasing steric hindrance between the large cationic centers. Therefore, the tetracation 254 salt was designed as a first example for a hybrid with a cyanine unit at the both termini (27). As illustrated in Figure 22, tetracation 254 exemplifies a new hybrid system, which produces another cyanine substructure 252 by two-electron transfer. In this case, overall, four electron transfer should afford a fully neutralized species such as a neutral diradical. 

Our initial studies focused on the transition metal-catalyzed [4+4] cycloaddition reactions of bis-dienes. These reactions are thermally forbidden, but occur photochemically in some specific, constrained systems. While the transition metal-catalyzed intermole-cular [4+4] cycloaddition of simple dienes is industrially important [7], this process generally does not work well with more complex substituted dienes and had not been explored intramolecularly. In the first studies on the intramolecular metal-catalyzed [4+4] cycloaddition, the reaction was found to proceed with high regio-, stereo-, and facial selectivity. The synthesis of (+)-asteriscanoHde (12) (Scheme 13.4a) [8] is illustrative of the utihty and step economy of this reaction. Recognition of the broader utiHty of adding dienes across rc-systems (not just across other dienes) led to further studies on the use of transition metal catalysts to facilitate otherwise difficult Diels-Alder reactions [9]. For example, the attempted thermal cycloaddition of diene-yne 15 leads only... 

In this chapter we provide an overview of studies originating from the above work and directed at the design and development of three new metal-catalyzed cycloaddition reactions, namely the [5+2] cycloaddition of vinylcyclopropanes (VCPs) and rc-systems, the [6+2] cycloaddition of vinylcyclobutanones and re-systems, and the three-component, [5+2+1] cycloaddition of VCPs, rc-systems, and CO. These new reactions provide fundamentally new approaches to a range of problems in seven- and eight-membered ring synthesis. 

Mechanistic analysis of the reaction of substituted cyclopropanes reveals that each re-gioisomeric cycloadduct correlates uniquely with a metal-n -system complex (Scheme 13.11, A and A ). Metal coordination to either of the two vinyl rc-system faces of 122 followed by oxidative coupling leads to diastereomeric metaUacyclopentenes B or B ... 

Figure 7.2 (A) Bond lengths for the five different sets of C-C bonds and favored electronic resonance structure of [(Et3P)2Pt]6C5o determined by X-ray crystal structure analysis. (B) 48-Electron rc-system produced by the saturation of six double bonds in the octahedral sites of leaving eight linked benzenoid hexagons.

Adsorbate Electronic Structure and Bonding on Metal Surfaces rc-System... 

If we analyze the case of hexatriene to cyclohexadiene conversion, the situation is just the reverse. The thermal reaction should be disrotatory and the photochemical reaction conrotatory. The butadiene belongs to (4ri)n system and hexadiene to (4n +2)rc system, and a generalization of the systems may be attempted. 

With these definitions, useful descriptions of non-planar rc-systems and potentially homoconjugated systems have been developed. The descriptions are particularly attractive to experimentalists because orbital overlap is accepted as a major contributing factor to bonding and it is easy to visualize. Although Haddon did not formally define homoaromaticity in his work63 one can use his threshold value of S to define homoaromaticity in the following way ... 

So far we have only defined what suprafacial and antarafacial mean on rc-systems (Fig. 2.7), but we need to see how o-bonds are treated by the Woodward-Hoffmann rules. Just as a suprafacial event on a rc-bond has overlap developing to the two upper lobes that contribute to the bond, so with o-bonds (Fig. 3.5a), overlap that develops to the two large lobes of the sp3... 

Probably, also in the anion 31 and in the alkylation transition state 12b one methyl group is turned inside and shields the bottom side of the molecule effectively. Since these alkyl halides give the highest inductions that contain an extended rc-system (benzyl, cinnamyl, phenylpropargyl, etc.) it is plausible to assume also for the incoming alkyl group the folded conformation (cf. 12b). 

H-NMR spectra of 4-azaazulenones 5-8 are shown in Table II. The average chemical shift (5) of 5 differs from those of 6-8 by 0.74-0.94 ppm, thus indicating a destabilization of the rc-system of 5, which unlike 6-8 does not... 

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