External donor

The temperature at which a cycloaddition reaction of a neopentylsilene takes place (detected by the elimination of LiCl) has turned out to be dependent on the reaction partners added as substrate. This implies that an interaction between the substrate and A or B or the substrate and C occurs somewhere along the reaction pathway depicted above. For the system Cl3SiCH=CH2/LiBut/R2C=NR it was observed that the imine initiates and supports the salt elimination from the species A/B. Based on the knowledge that silenes are stabilized by external donors [1] we conclude that with carbon unsaturated compounds x-donor interactions instead of cr-donor complexes may be possible as well for the lithiated species (D) as for the silene itself (E). 

Aromatic Systems with an External Donor Atom... 

A third type of copper center, first recognized in cytochrome c oxidase (see Fig. 18-10) is called CuA or purple CuA. Each copper ion is bonded to an imidazole and two cysteines serve as bridging ligands. The two copper ions are about 0.24 nm apart, and the two Cu2+ ions together can accept a single electron from an external donor such as cytochrome c or azurin to give a half-reduced form.521a/b... 

The P — Sn coordination in 271a-d is relatively weak, as could be concluded from the observation that intermolecular coordination of external donors like pyridine, HMPA and DMF could compete with intramolecular P —> Sn interaction204. In solution of 268b and 271d a fast equilibrium exists between tetracoordinated open-chain and pentacoordinated cyclic structures201,643. 

External donors are introduced together with the organometallic cocatalyst. In the absence of external donor, the organoaluminium component (e. g. Et3Al) rapidly reacts with the internal donor. The external donor reduces the rate of internal donor consumption and/or substitutes its loss [286], Chemically, external donors are very similar to internal donors, and they may even be identical. 

Stopped-flow UV-vis absorption and rapid freeze-qnench (RFQ) EPR and Mossbauer stndies have shown that the reaction pathway diagrammed in Fignre 4 for formation of the tyrosyl radical is essentially accurate except that the diiron(IV) species labeled Q in Figured has never been detected in R2. Instead, an intermediate labeled U (not shown in Figured), occurring prior to X, has properties consistent with a protonated tryptophan cation radical. This radical may shuttle an electron from an external donor to the diiron site in order to reach intermediate In... 

It is well established that green sulfur bacteria carry out linear electron flow from external donors such as S , S , 8203 and 803 to ferredoxin and NAD (see Ref. 36 and Chapter 9). The use of 02-uptake as a valid assay of linear flow in membrane preparations has also been demonstrated [61]. 8ulfide oxidation is mediated by a soluble flavocytochrome c-553 ( , -1-0.09 V) [36,62], and thiosulfate oxidation seems to require the soluble Cyt c-551 == 0.14 V) [36]. 8oluble Cyt... 

On the other hand, Spitz 45,97) found strong variations in the deactivation index upon variation of concentration and nature of the external donor. Therefore,... 

Actually, studies on the propylene polymerization at atmospheric pressure carried out in our laboratories 101 > have demonstrated that R0 and the deactivation rate depend, in a complex manner, on both the organoaluminum and external donor concentrations (see Sect. 6.1.2 and 6.1.3). The kinetic curves obtained cannot be reduced to a single model for the deactivation of active centers according to a simple 1 st and 2nd order law, but rather they seem to follow a more complicated behavior. This is not surprising if one considers that the decay of polymerization rate is probably the effect of an evolution, in time, of a plurality of different catalytic species having different stability, reactivity and stereospecificity (see Sect. 6.3). 

With similar binary and ternary catalysts (but using EB as internal and MPT as external donor), however, rather different results were obtained in our laboratories. With the binary catalyst, a two-step increase of the isotacticity was noticed, the first step (up to MPT/TEA 0.2) being associated mainly with a strong decrease in the atactic productivity, the second (at MPT/TEA > 0.2) with a slightly selective decrease in both the atactic and the isotactic productivity (Fig. 38). 

A comparison between the productivity trends of binary and ternary catalysts in the presence of an external donor proves to be quite interesting. While the atactic fraction productivity has a similar behavior for both catalysts, the binary catalyst shows a decrease of isotactic productivity with increased D/Al ratio the ternary catalyst, on the other hand, shows a maximum (see Fig. 40). This is still in agreement with the Burfield model. In fact, these results can be accounted for by assuming that the internal donor gives rise to the formation of a new type of isospecific center, characterized by kA and kD values different from those present in binary catalysts. The presence of a maximum in productivity is consistent with the formation of complexes between the aluminum alkyl and the donor and the resulting modification of the adsorption equilibrium, as suggested by Burfield1S6>. 

This mechanism with coupled one-electron intermolecular electron transfer from the external donor and intramolecular multi-electron transfer from the catalyst to coordinated N2 is, presumably, more efficient than the simpler mechanism considered above with one-electron and multi-electron transfers separated in time. In this mechanism the strongest reductant, which is of necessity the external reducing agent, is used for direct reduction of the substrate, whereas for consecutive one-electron and multi-electron processes its reducing power is used only to prepare the reduced form of the catalyst. 

The first electron acceptor from the external donor is the Fe4S4 cluster situated between two subunits of Fe protein. From Fe4S4 an electron travels first to a P-cluster, then to FeMo cofactor, and finally to an activated substrate molecule. According to the X-ray data the shortest distance from the Fe4S4 cluster of the Fe protein to the P cluster of the MoFe protein is ca 18 A, that from P cluster to FeMo cofactor is ca 14 A, and the distance from Fe4S4 to FeMoco is ca 32 A. The P-cluster lies between Fe4S4 and FeMoco. Thus, electron transfer in nitrogenase can be presented as ... 

For the process in vivo flavodoxin is the external electron donor. For isolated nitrogenase, dithionite 8204 is traditionally used as electron donor. If dithionite is the external donor, the oxidized iron protein with 2MgADP (after the electron is transferred) then dissociates from the MoFe protein. This dissociation, which initially seemed necessary for enzyme function, is, however, apparently a result of the salt effect of dithionite, the concentration of which for effective electron transfer must be sufficiently large. 

Cytochrome cd nitrite reductase from Paracoceus pantotrophus has a different mechanism, with two identical subunits, each with domains containing a c-type cytochrome heme and a dj-type cytochrome heme. Electrons from external donors enter through the c heme the d heme is the site of nitrite reduction to NO and oxygen reduction to water. One of the puzzles of the mechanism is how the NO can escape from... 

Internal/external donor effects and the nature of the active species 1035... 

Most recently, a further family of MgCl2-supported catalysts has been developed,344 345 in which the internal donor is a succinate rather than a phthalate ester. As is the case with the phthalate-based catalysts, an alkoxysilane is used as external donor. The essential difference between these catalysts is that the succinate-based systems produce PP having much broader molecular mass distribution, discussed in Section 4.09.3.4. 

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