Electron rich systems

Most studies have concerned the kinetics of arenethiyl radicals with monomers including S and its derivatives468 47" and MMA.469,473 The radicals have electrophilic character and add more rapidly to electron-rich systems (Tabic 3.10). Relative reactivities of the monomers towards the ben/.oy Ithiyl radical have also been examined.453... 

Iminocarbene complexes of chromium and tungsten are useful isolable synthetic equivalents to nitrile ylides having the advantage that the range of 1,3-dipo-larophiles is not limited to electron-acceptor substrates and can be extended to electronically neutral as well as to electron-rich systems [56] (Scheme 18). 

This special propensity of F for pi donor action in electron-rich systems has been previously discussed (20). For the substituents Me, Cl, and Br,... 

The best cofactors are typically flavonoid derivatives that contain many hydroxyl groups, the most favorable at position 3 of the flavones. The strongest cofactors have electron-rich systems that associate with electron-poor compounds such as the flavylium cation. 

In some electron-rich systems, the usual trend that the isomer shift increases with the number of valence electrons appears to be inverted. This is the case for the comparison of planar iron porphyrins in the formal oxidation states (I) and (0), which are obtained by one- and two-electron reduction of iron(II) porphyrins in... 

The description of the carbon-oxygen double bond is analogous, but in addition to the cr-bonds there are unshared pairs of electrons on oxygen so that two excited states are possible, n-n and n-n. For n-n excitation the resultant half-vacant orbital on oxygen should possess electrophilic reactivity, and the electron rich -system should have nucleophilic characteristics. 62>... 

Indolizine is an electron-rich system and its reactions involve mainly electrophilic substitutions, which occur about as readily as for indole and go preferentially at the C-3 position, but may also take place at the C-l. Consistent with their similarity with pyrroles rather than pyridines, indolizines are not attacked by nucleophiles nor are there examples of nucleophilic displacement of halide-substituted systems. 

Nitrogen substituted porphyrazines were the second type of heteroatom-deriva-tized pz macrocycle reported and were prepared from the readily derivatized diami-nomalconitrilc (DAMN) (7). Octakis(dimethylamino)porphyrazines are extremely electron-rich systems and have been used to prepare charge-transfer complexes with Cgo, as well as to peripherally chelate metals or convert to crown appended systems (38, 39). The unsymmetrical dimethylaminoporphyrazine analogues have also been reported (29), as well as the first example of the desymmetrized seco-pz from the dimethylaminoporphyrazine (8, 40). The nitrogen substituted porphyrazines are discussed in Section V. 

The fragment attached to pyridoxal will be the same in both cases, and after hydrolysis it is released as the amino acid glycine. In case this seems a bit complicated, consider the reverse reaction, which would be attack of an electron-rich system on to the carbonyl group of an aldehyde, i.e. an aldol reaction. Therefore, what we are seeing here is merely a reverse aldol-type reaction (see Section 10.3). 

These observations indicate that when the metal complex is electron-rich, the allenylidene-metal complexes are stable (VI and VII), even on heating or protonation [42]. However, with less electron-rich systems (e.g., PPh3 ligands instead of PCy3 or NHC) the corresponding allenylidene complex was never observed, to the profit of the indenylidene complex VIII. These results suggested that the allenylidene-ruthenium complex is a transient species that rearranges into the indenylidene complex VIII, as was observed for a C5 cumulenylidene [48]. 

The stability of azole carbenes can be attributed to electronic factors which operate in both the Tran d CT-frameworks (92JA5530). In the TT-framework, electron donation into the carbene out-of-plane p-orbital by the electron-rich system moderates the typical electrophilic reactivity of carbenes. In the o-framework, additional stability for the carbene electron pair may be gained from the o-electron-withdrawal effects on the carbene center by the more electronegative nitrogens, which moderates the carbene nucleophilic reactivity. The combination of these a- and TT-effects serves to increase the singlet-triplet gap and stabilize the singlet carbene over the more reactive triplet state. For carbenes with bulky substituents (tert-butyl, 1-adamantyl, etc.) steric effects provide additional stabilization. 

These electron-rich systems usually show little tendency to react with nucleophiles, but 1,2-dithiins suffer nucleophilic attack at sulfur followed by ring cleavage. 

The fully unsaturated compounds are electron-rich systems and as such are highly reactive towards electrophiles. Principal reactions occur via attack at a ring sulfur atom, if available,... 

Stoddart and coworkers reported that a macrocyclic bisviologen acceptor (28) acts as a versatile host for -electron rich systems, such as diphenol methyl ethers, dinaphthol methyl ethers, and aromatic amino acids [99-102]. They also discovered that TTF (2) and 28 form a 1 1 complex in which... 

The spin-free valence bond method applications to metallic and electron rich systems... 

In addition to those complexes listed in Table IV, a number of electron-rich systems involving lithium (5J) have been studied, and in these cases unusually large coordination shifts are observed. For example the value... 

An interesting observation was made by Morris et al. [158,159] who reported that the electron-rich system [Ru(IBu )(PPh3)3H] showed no inclination for intramolecular C-H activation. It seems that the same electronic preferences are working for reductive elimination and C-X activation. 

---