Electron-donating center

The alternative Polonovski reaction mode, in which C —C bond cleavage is observed, can occur if two conditions are met (equation 6) (i) the C —C bond to be broken is activated towards cleavage by an adjacent electron-donating center (double bond, aromatic ring or heteroatom) and (ii) the C —C and N—O bonds are antiperiplanar. 

The rate of oxidation of N3- by HO is nearly diffusion controlled (Table VI), so that with only 10 mmol dm of N3-, the half-life of reaction 48 is just 6 ns. The overall result is that the yield ( uimol J ) of N3 is 0.55, with a residual yield of hydrogen atoms of 0.06. The H atoms react only slowly with the basic solution components, and adjustments for their contribution can be made. The rate constant for oxidation of phenol is about two orders of magnitude smaller than that for oxidation of phenoxide, again illustrating the powerful effect of protonation of the electron donating center. 

A second variant, the enzyme from Ralstonia eutropha H16 (previously Alcaligenes eutrophus HI6), is monomeric (encoded by norB) and lacks the small subunit that binds the electron donating centers of the other enzymes. Residues proposed to ligate hemes b and Fes are conserved in norB of R. eutropha and the enzyme contains heme b and nonheme Fe in a 2 1 ratio. The enzyme has an extended N-terminus of 280 amino acids that is essential for activity, with a proposed role in allowing quinols rather than cytochrome c to function as electron donor. These NORs, which are also membrane bound, have biochemical and spectral properties indicating that the catalytic center is the same as in prototype NOR. 

In Other words one may state that the hydrogen bond is the Lewis acid-Lewis base interaction which leads to the formation of the A-H... B link where A-H and B play the role of the Lewis acid (electron accepting) and Lewis base (electron donating) centers, respectively. Further the numerous properties of this link may be specified, as for example that B may be mono-center (atom of any species or ion) or multicenter (ir-electron or o-electron system). 

Pyridine is more nucleophilic than an alcohol toward the carbonyl center of an acyl chloride. The product that results, an acylpyridinium ion, is, in turn, more reactive toward an alcohol than the original acyl chloride. The conditions required for nucleophilic catalysis therefore exist, and acylation of the alcohol by acyl chloride is faster in the presence of pyridine than in its absence. Among the evidence that supports this mechanism is spectroscopic observation of the acetylpyridinium ion. An even more effective catalyst is 4-dimeftiyIaminopyridine (DMAP), which functions in the same wsy but is more reactive because of the electron-donating dimethylamino substituent. ... 

For the Birch reduction of mono-substituted aromatic substrates the substituents generally influence the course of the reduction process. Electron-donating substituents (e.g. alkyl or alkoxyl groups) lead to products with the substituent located at a double bond carbon center. The reduction of methoxybenzene (anisole) 7 yields 1-methoxycyclohexa-1,4-diene 8 ... 

Treatment of a-alkoxy-substituted iron acyl complexes 20 with bromine in the presence of an alcohol produces free acetals 22 with loss of stereochemistry at the center derived from the a-carbon of the starting complexl2,49. Electron donation from the alkoxy group allows formation of the oxonium intermediate 21, which is captured by the alcohol to generate the product acetal. 

It has been postulated that the stability of free radicals is enhanced by the presence at the radical center of both an electron-donating and an electron-with-drawing group.This is called the push-pull or captodative effect (see also pp. 159). The effect arises from increased resonance, for example ... 

Electron-donating orbitals are those occupied by electrons, i.e., bonding orbitals of bonds, non-bonding orbitals of lone pairs, HOMOs of molecnles, gronps and others. Electron-accepting orbitals are vacant orbitals, i.e., antibonding orbitals of bonds, vacant atomic orbitals on cationic centers, LUMOs of molecnles, gronps, etc. 

The orbital phase theory is applicable to the singlet diradicals [20]. The electron configuration of the singlet states of the cross- (TMM) and linear (BD) conjugate diradicals is shown in Scheme 9, where the mechanism of the delocalization of a and P spins between the radical centers through the double bond are separately illustrated by the arrows. The cyclic [-a-Tr-b-T -] interaction is readily seen to occur for the spin delocalizations. The p orbital a) in one radical center and the n orbital are occupied by a spins, and therefore, electron-donating orbitals. The p orbital (b) in the other radical center and the ii orbital are not occupied by a spins. 

In the singlet state of Jt-type 1,3-diradical (e.g., TM, 2), there may also exist the through-space interaction between radical centers, i.e., p...q interaction (Fig. 9), in addition to the previously addressed cyclic -p-o -q-o- orbital interactions (Fig. 6). The through-space interaction is indispensable for the bond formation between the radical centers. The corresponding delocalization of the a-spin electron is shown in Fig. 9a. Clearly, the involvement of the through-space p... q interaction gives rise to two cyclic orbital interactions, -p-o -q- and -p-o-q-. From Fig. 9, one can find that the cyclic -p-o -q- orbital interaction can satisfy the phase continuity requirements for the a-spin electron the electron-donating radical orbital, p (D) can... 

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