7i-System

In all cases examined the ( )-isomers of the allylic alcohols reacted satisfactorily in the asymmetric epoxidation step, whereas the epoxidations of the (Z)-isomers were intolerably slow or nonstereoselective. The eryfhro-isomers obtained from the ( )-allylic alcohols may, however, be epimerized in 95% yield to the more stable tlireo-isomers by treatment of the acetonides with potassium carbonate (6a). The competitive -elimination is suppressed by the acetonide protecting group because it maintains orthogonality between the enolate 7i-system and the 8-alkoxy group (cf the Baldwin rules, p. 316). 

Hiickel s rule states that planar cyclic 71 systems involving 4n+2 electrons will be unusually stable ( aromatic ), while cyclic 7i systems with 4n electrons will be unstable ( antiaromatic ). 

It has been proposed that protonation or complex formation at the 2-nitrogen atom of 14 would enhance the polarization of the r,6 -7i system and facilitate the rearrangement leading to new C-C bond formation. The equilibrium between the arylhydrazone and its ene-hydrazine tautomer is continuously promoted to the right by the irreversible rearomatization in stage II of the process. The indolization of arylhydrazones on heating in the presence of (or absence of) solvent under non-catalytic conditions can be rationalized by the formation of the transient intermediate 14 (R = H). Under these thermal conditions, the equilibrium is continuously pushed to the right in favor of indole formation. Some commonly used catalysts in this process are summarized in Table 3.4.1. 

The thienothienopyridines are a relatively little-known class of compound. Interest in these systems arose through the possibility that they occurred in coal-derived products and their extended 7i-systems initiated interest for their interesting optical properties. Additionally, several differently substituted examples have antitumor activity <2002CPB656>, and may serve as DNA intercalating agents <2005MOL279>. 

Thiazyl halide monomers undergo a variety of reactions that can be classified under the general headings (a) reactions involving the 7i-system of the N = S triple bond, (b) nucleophilic substitution, (c) halide abstraction, and (d) halide addition. The cycloaddition of NSF with hexafluoro-1,3-butadiene provides an example of a type (a) reaction. 

Reduction of 40 (E=S/Se) could be achieved with triphenylantimony to form the neutral molecules 42 (E=S/Se). Compounds 42 (E=S and E=Se), which are formally antiaromatic 16 7i-systems, exhibited bond lengths consistent with a quinoid formulation. 

Fig. 1 summarizes cyclizations of the two prototype molecules, (Z) hex-3-ene-l,5-diyne and (Z) hept-3,5,6-triene-l-yne. In all of these processes, bonds are formed from in-plane 7t-orbitals in the presence of an orthogonal 7i-system. However, it is clear that the properties of the newly formed cyclic conjugated systems can be quite... 

For Rh and Ir diphosphine-based catalysts there exist some indications on reactive species and also on hydrogen activation. James and coworkers [43, 85] investigated the Rh-catalyzed DMA-imine hydrogenation and concluded that the imine is -coordinated to the Rh center via the nitrogen lone pair, and not via the 7i-system of the C = N bond. They also suggested that the hydrogen activation occurs after the imine is coordinated. 

Keiluweit M, Kleber M (2009) Molecular-level interactions in soils and sediments the role of aromatic 7i-systems. Environ Sci Technol 43 3421-3429... 

Owing to such orbital interactions, a-silyl substitution causes the decrease in the LUMO level of the 7t-system and enhances the electron accepting ability of the 7i-system. Therefore, the reduction potentials of a-silyl-substituted 7t-systems are less negative than those of the parent jr-systems, although the magnitude of this effect is not large. 

The RC60+ cations are carbenium ions (trivalent carbocations), which are formally conjugated with the entire 7i-system of the fullerene cage. This chapter focuses on recent developments in the author s laboratory regarding the prepara-... 

Several hydrogenated precursors of the desired dianion are possible for more extended 7i-systems (see Table 5.11). 

Friedel-Crafts Alkylations and Mukaiyama-Michael Reactions The metal-catalyzed addition of aromatic substrates to electron-deficient a- and 7i-systems, commonly known as Friedel-Crafts alkylation, has long been established as a powerful strategy for C-C bond formation. Surprisingly, however, relatively... 

In its original form, the Michael addition consisted on the addition of diethyl malonate across the double bond of ethyl cinnamate in the presence of sodium ethoxide to afford a substituted pentanedioic acid ester. Currently, all reactions that involve a 1,4-addition of stabilized carbon nucleophiles to activated 7i-systems are known as Michael additions. Among the various reactants, enolates derived from p-dicarbonyl compounds are substrates of choice due to their easy deprotonation under mild conditions. Recently, Michael addition-based MCRs emerged as highly potential methodologies for the synthesis of polysubstituted heterocycles in the five- to seven-membered series. 

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