Nature of the Active Species

It is clear that McMurry reactions can be performed with low-valent titanium species in various oxidation states. Pinacol and alkene-forming reductive coupling reactions have been achieved with organometallic complexes of Ti(0), Ti(II), and Ti(III) (see Section 6.2.2.3). While no evidence was found for the presence of 11(0) on reduced Ti02 surfaces active in benzaldehyde coupling, X-ray photoelectron spectroscopy showed that the active site required for gas-solid reductive coupling is an ensemble of Ti cations in the +1, +2, and +3 oxidation states that collectively effect the four-electron reduction [258]. 

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A catalytic system comprising TiCNMe ), LiNCSilVIej) and IMes has been developed for the intermolecular hydroamination of terminal aliphatic alkynes (1-hexyne, 1-octyne, etc.) with anilines [toluene, 100°C, 10 mol% TiCNMe ) ]. Markovnikov products were dominant. Substituted anilines reacted similarly. High conversions (85-95%) were observed with specific anilines. The optimum Ti/IMes/ LiN(SiMe3)2 ratio was 1 2 1. However, the nature of the active species and especially the role of LiN(SiMe3)2 are unclear [74]. 

A series of chiral p-hydroxysulfoximine ligands have been synthesised by Bolm et al. and further investigated for the enantioselective conjugate addition of ZnEt2 to various chalcone derivatives. The most eiScient sulfoximine, depicted in Scheme 2.33, has allowed an enantioselectivity of up to 72% ee to be obtained. These authors assumed a nonmonomeric nature of the active species in solution, as suggested by the asymmetric amplification in the catalysis with a sulfoximine of a low optical purity. 

Nature of the Active Species. It was considered that microanalyses of the starting anchored catalyst and that recovered from a hydrogenation would provide the... 

Chiral ligand 31 has two C-2 symmetrical picolynyl amides. Initial kinetics taught us the reaction was 0.5 order in 31. The kinetics indicated a dimeric nature of the active species. Therefore, systematic modification of the ligand was attempted and the results are summarized in Figure 2.5 [25]. 

The molecular details by which NAMI-A exerts antimetastatic effects in vivo have not been definitely determined, and may occur by multiple mechanisms. The solution chemistry of NAMI-A involves both loss of Cl and DMSO. Interestingly, the antimetastatic activity is retained under a wide variety of experimental conditions producing solvolyzed intermediates.192 The exact nature of the active species may be difficult to resolve. 

This also opens a question about the nature of the active species in, for example, oxidative nucleophilic reactions is it peroxo or superoxo form which nucleophilically attacks an organic substrate These are challenging questions which motivate future investigations. 

The soluble nature of the active species is the basis for this success story as it allows modification and optimization of the catalytic performance on a molecular level. At the same time, it is also the main drawback of the approach, as products and catalysts are contained in the same phase at the end of the reaction. Efficient separation is often crucial, however, because the costly and highly specialized catalysts should be recovered and recycled for economic and environmental reasons. Furthermore, the specifications and applications do not allow even trace amounts of metals or other catalyst components in many products. 

As seen, the anionic and cationic polymerizations are analogous differing mainly on the nature of the active species. The stereochemistry associated with anionic polymerization is also similar to that observed with cationic polymerization. For soluble anionic initiators at low temperatures, syndiotactic formation is favored in polar solvents, whereas isotactic formation is favored in nonpolar solvents. Thus, the stereochemistry of anionic polymerizations appears to be largely dependent on the amount of association the growing chain has with the counterion, analogous with the cationic polymerizations. 

Jordan reported that cationic aluminium alkyl derivatives of amidinates (3) [10] and aminotroponiminates (4) [11] are active in ethene polymerization. The amidinate system was later shown to be more complicated, and the nature of the active species remains unclear [14],... 

The nature of the active species in the anionic polymerization of non-polar monomers, e. g. styrene, has been disclosed to a high degree. The kinetic measurements showed, that the polymerization proceeds in an ideal way, without side-reactions, and that the active species exist in the form of free ions, solvent-sparated and contact ion pairs, which are in a dynamic equilibrium (l -4). For these three species the rate constants and activation parameters (including the activation volumes), as well as the rate constants and equilibrium constants of interconversion have been determined (4-7.) Moreover, it could be shown by many different methods (e. g. conductivity and spectroscopic methods) that the concept of solvent-separated ion pairs can be applied to many ionic compounds in non-aqueous polar solvents (8). 

Recently several studies have focused on the nature of the active species in the polymerization of ethylene and conjugated dienes initiated by the chelate of butyllithium and N,N,N, N -tetramethylethylenediamine (TMEDA). 

T he absorption of molecular hydrogen by aqueous solutions of cyanocobaltate(II) was first reported by Iguchi in 1942 (5). Since then, several groups of workers have sought to determine the nature of the activating species and the product of its hydrogenation. 

Considerable efforts have been made to understand ZSM-5-based catalysts for the selective oxidation of benzene to phenol by nitrous oxide. However, the nature of the active species remains unclear. The most important proposals for the active species are extraframework Fe species [101], Bronsted add sites [102] and Lewis and A1 sites [103, 104]. The activity is usually interpreted in terms of very small, possibly... 

Based on this discussion, it is possible to clarify some aspects of the literature interpretations on the nature of the active species. Lunsford and coworkers [49, 93] published many papers indicating that the active species is colloidal palladium, which implies the easy dissolution of Pd in solution. An analysis of patents clearly reveals that this is not the case for active catalysts and various patents explicitly indicate that there is no leaching of Pd. On the other hand, a colloid would be difficult, if not impossible, to manage in a commercial process and its recovery would be not viable at the very low concentrations of dissolved metal employed. In addition, the presence of even traces of Pd in commercial H202 could be extremely dangerous in terms of the possibility of explosion. Finally, if the solid is a simple reservoir for Pd going into solution, a deactivation is expected with time-on-stream in continuous operations. 

The molecular weight distribution is very broad, and the high-molecular-weight part only contains rara-(isotactic) triads. Further studies with other initiators are described in the same paper [7]. All of the results show a dependence of the microstructure on the initiator being used and allow an insight into the mechanism of the polymerization process. The nature of the active species in these special anionic polymerizations is responsible for the actual course of the polymerization. 

The following mesomeric forms, which can be proposed for the complex of the epoxide with the catalyst active site, seem to present the nature of the active species in a more illustrative way [74] ... 

In discussions about the nature of the active species in the polymerization of dienes by Ziegler/Natta catalyst systems allyl species have already been suggested in the 1960s [273-278]. This discussion has continued through the past decades [139,279-283]. Today, it is widely accepted that Nd-allyl-groups are the key element in the insertion of dienes into the Nd carbon bond. 

To our knowledge there is no study available on the formation and the nature of the active species present in Nd-phosphate-based catalyst systems. Such a study should be designed to elucidate the question why extraordinary small amounts of Al alkyls ( ai/ n < 4) are sufficient for catalyst activation. 

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