Coordinating process

In the process of radical polymerization a monomolecular short stop of the kinetic chain arises from the delocalization of the unpaired electron along the conjugated chain and from the competition of the developing polyconjugated system with the monomer for the delivery of rr-electrons to the nf-orbitals of a transition metal catalyst in the ionic coordination process. Such a deactivation of the active center may also be due to an interaction with the conjugated bonds of systems which have already been formed. 

The need for multiple desolvation of the metal ion in some systems may provide a barrier to complex formation which is reflected by lower formation rates - especially for inflexible macrocycles such as the porphyrins. Because of the high energies involved, multiple desolvation will be unlikely to occur before metal-ion insertion occurs rather, for flexible ligands, solvent loss will follow a stepwise pattern reflecting the successive binding of the donor atoms. However, because of the additional constraints in cyclic systems (relative to open-chain ones), there may be no alternative to simultaneous (multiple) desolvation during the coordination process. 

Coordination chemistry is predominantly thought of in terms of compounds prepared and characterized in the condensed phases. The gas phase is just another phase in which coordination chemistry can be observed and studied. There is a need for a better understanding of coordination processes in the gas phase the development of instrumentation is enabling us to increase our knowledge in this area. 

Mn(II) oxidation is enhanced in the presence of lepidocrocite (y-FeOOH). The oxidation of Mn(II) on y-FeOOH can be understood in terms of the coupling of surface coordination processes and redox reactions on the surface. Ca2+, Mg2+, Cl, S042-, phosphate, silicate, salicylate, and phthalate affect Mn(II) oxidation in the presence of y-FeOOH. These effects can be explained in terms of the influence these ions have on the binding of Mn(II) species to the surface. Extrapolation of the laboratory results to the conditions prevailing in natural waters predicts that the factors which most influence Mn(II) oxidation rates are pH, temperature, the amount of surface, ionic strength, and Mg2+ and Cl" concentrations. 

Following olefin coordination, the Chalk-Harrod mechanism proceeds by olefin insertion into the M-H bond, whereas with the modified Chalk-Harrod mechanism, olefin coordination is followed by insertion into the M-Si bond. This step distinguishes the two mechanisms. Thus, the coordination of styrene to the hydridosilyl complex to form an olefin 7t-complex may be the first step of the catalytic cycle that discriminates between the two mechanisms. We have examined this coordination process as well as the relative energies of the many isomers of the 7i-complex that are possible. 

The first of these reactions is the photochemical cleavage of one Cr-CO bond, which is followed by the coordination of a solvent molecule (S) to Cr(CO)5. This coordination process is extremely fast (it takes less than 25 ps). The second (slower) step is the replacement of the solvent molecule by ligand L (reaction 13.34). ArV was assumed to be negligible for both reactions. 

Species such as XXV, XXVI, or XXVII readily form coordination complexes when treated with AuCl, H20So(C0)j q, Idn(CO)3(r -C5Hj), Fe(C0)3(PhCH=CHC(0)CH3>, or [RhCl(CO)2]2 ( ) Tw results are of special interest. First, the skeletal nitrogen atoms in XXV-XXVII do not participate in the coordination process. Presumably, they are effectively shielded by the aryloxy units and are of low basicity. Second, coordinatlve crosslinking can occur when two phosphine residues bind to one metal atom. Ligand-exchange reactions were detected for the rhodium-bound species. The tri-osmium cluster adducts of XXV, XXVI, and XXVII are catalysts for the isomerization of 1-hexane to 2-hexene. 

Propagation with the anionic coordination initiators, especially the aluminum and zinc initiators such as aluminum isopropoxide or the metalloprophyrins such as VI, involves covalent propagation in which the epoxide monomer is inserted into a metal-oxygen bond [Penczek and Duda, 1993 Penczek et al., 1995 Szwarc and Van Beylen, 1993] (Eq. 7-9). The propagation is categorized as an anionic coordination process since one can visualize... 

With a tungsten pentacarbonyl catalyst, the calculated mechanisms are summarized in Scheme 4.15 [26]. Coordination of the 4-pentyn-l-ol substrate to the pentacarbonyl tungsten leads to the formation of the 7i-alkyne-W(CO)5 adduct Wl. This coordination process was calculated to be exothermic by 24.3 kcal mol. The cydoisomerization leading to a five-membered-ring exo product starts with the 7i-complex Wl via a one-step process with a barrier of 46.5 kcal mol (path a of Scheme 4.15). The barrier calculated here is comparable with that calculated for the catalyst-free process. From Wl to W3, the tungsten metal center does not play a significant role in the isomerization process. 

Hydroxytryptamine (5-HT), dopamine, and norepinephrine play important roles as central neurotrans-mitters in the process of erection. Still other substances or hormones, such as endorphins, oxytocin, vasopressin, adrenocorticotropic hormone (ACTH) and related peptides, and prolactin, appear to participate in the complex and coordinated process of penile erection. Central nonadrenergic neurons also may influence male sexual behavior. 

The data in Figure 1 show no dramatic concentration dependence for the interaction of either tetrahydrofuran or 2,5-dimethyltetrahydrofuran with poly(styryl)lithium. The absence of distinct breaks within the range of R values from 0.2 to 2 can be regarded as evidence that the initial base coordination process (eq 1) is followed by successive coordination with other tetrahydrofuran molecules as shown in eq 2. 

The relatively large difference in heats between tetra-hydrofuran and 2,5-dimethyltetrahydrofuran (2.2 kcal/mole) indicates that the base coordination process for poly(styryl)lithium is quite sensitive to the steric requirements of the base... 

This supposition is supported by results for linking reactions of polymeric organolithium compounds which indicate that the steric requirements of a poly(styryl) chain end are larger than those for a poly(dienyl) chain end ( 4,25). Since a larger sensitivity to base steric requirements is exhibited by poly-(isoprenyl)lithium and it is known that the coordination process for poly(styryl)lithium involves coordination to give the unassociated species (eq 1), it is concluded that tetrahydrofuran coordination with poly(isoprenyl)lithium must involve interaction with an associated species (presumably the dimer) to explain the large sensitivity to the steric requirements of the base. 

At the present time the paucity of data renders it impossible to give a complete picture of the various ways in which coordination may affect the reactivity of aromatic ligands. It is possible to indicate the effects which the coordination process may have on the aromatic reactivity, however, to show how masking and polarization may affect reactivities, and to show at least a few instances where coordination may provide useful synthetic routes to some aromatic compounds. 

The coordination process may either stabilize or destabilize aromatic Schiff bases. If nickel (II) salts are added to ammoniacal solutions of salicylaldehyde, the precipitate obtained is the inner complex salt of nickel (II) and salicylaldimine (61). If beryllium chloride is added to the Schiff base derived from 2-hydroxy-l-naphthaldehyde and ethylamine, however, the Schiff base is decomposed and the inner complex of beryllium (II) and 2-hydroxy-1-naphthaldehyde is obtained (59). Here the strength of the coordinate bonds formed with the metal seems to determine which complex will be formed. 

When the donor atom is a part of the aromatic system, one would expect more obvious differences in reactivity. At present relatively little comparative information is available on such heterocyclic systems. Only on pyridine and its derivatives are there any reasonably extensive data. For pyridine a wide variety of coordination processes are available and pyridine-N-oxide as well as metallic complexes and complexes with nonmetallic Lewis acids must be considered. For comparative purposes the great reluctance with which pyridine undergoes electrophilic... 

An early indication that replication is a highly coordinated process in which the parent strands are simultaneously unwound and replicated was provided by... 

One form of asymmetric division caused by unfavourable conditions is spoliation, which is a highly coordinated process ultimately producing spores which are resistant to adverse environmental conditions. When conditions are favourable, out-growth to vegetative cells then occurs. Streptomycetes and the blue-green bacteria are exceptions to this relatively simple cellular morphology and each of these types of bacteria shows various forms of cellular differentiation, both biochemical and morphological 7,8). 

Although metal-olefin complexation can be a source of enandoselection, reactions exploiting this mechanistic motif have not been developed much. Due to the facile enantioface interconversion process, the origin of the enantioselection often reverts back to Type C alkylation (Figure 8E, 1). To transfer chiral recognition of the coordination process to the ee of the product, kinetic trapping of the incipient 7t-allyl complex is required prior to any isomerization process. For this reason, few successful examples have come from the use of more reactive heteroatom nucleophiles (N, O and S) and/or intramolecular reactions. 

This conclusion provides an explanation for the calorimetric observation that base coordination of poly(isoprenyl)lithium is more sensitive to the steric requirements of the base (AAH = 3.2 kcal/mole) than is the coordination process for poly(styryl) lithium (AAH — 2.2 kcal/mole), since monomeric poly(isopropenyl) lithium would be expected to be less hindered than unassociated poly(styryl)lithium. However, dimeric poIy(isoprenyl)lithium could very well be more hindered toward base coordination (Eq. (13)) than monomeric poly(styryl)lithium (Eq. (11)). 

The decreased steric requirements for this base coordination process (AAH = 2.1 kcal/mole) compared to the analogous interaction for poIy(isoprenyl)iithium (AAH = 3.2 kcal/mole) are consistent with a po y(butadienyl)lithium chain end being less sterically demanding than a poly(isoprenyl)lithium chain end. Several factors can be considered in favor of the same degree of association for the base adduct for... 

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