Intermolecular complex catalysis

As shown above, quantitative structure-reactivity analysis is very useful in elucidating the mechanisms of cyclodextrin complexation and cyclodextrin catalysis. This method enables us to separate several intermolecular interactions, acting simultaneously,... 

Lewis-acid catalysis is effective in intermolecular as well as intramolecular /zomo-Diels-Alder reactions. Thus, complex polycyclic compounds 93 have been obtained in good yield by the cycloaddition of norbornadiene-derived dienynes 92 by using cobalt catalyst, whereas no reaction occurred under thermal conditions [91] (Scheme 3.18). 

A few additional Pd-catalyzed schemes have been employed for Ilac type cyclization chemistry. Palladium-phenanthroline complexes were used by the Ragaini group to prepare indoles via the intermolecular cyclization of nitroarenes and alkynes in the presence of carbon monoxide <06JOC3748>. Jia and Zhu employed Pd-catalysis for the annulation of o-haloanilines with aldehydes <06JOC7826>. A one-pot Ugi/Heck reaction was employed in the preparation of polysubstituted indoles from a four-component reaction system of acrylic aldehydes, bromoanilines, acids, and isocyanides <06TL4683>. 

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

The second important solvent effect on Lewis acid-Lewis base equilibria concerns the interactions with the Lewis base. Since water is also a good electron-pair acceptor129, Lewis-type interactions are competitive. This often seriously hampers the efficiency of Lewis acid catalysis in water. Thirdly, the intermolecular association of a solvent affects the Lewis acid-base equilibrium242. Upon complexation, one or more solvent molecules that were initially coordinated to the Lewis acid or the Lewis base are liberated into the bulk liquid phase, which is an entropically favourable process. This effect is more pronounced in aprotic than in protic solvents which usually have higher cohesive energy densities. The unfavourable entropy changes in protic solvents are somewhat counterbalanced by the formation of new hydrogen bonds in the bulk liquid. 

MD simulations can aid in the understanding of enzymic reactions by providing new insights into the structures and intermolecular interactions fundamental to the chemical catalysis. By studying the structures from the simulation of the lysozyme-(GlcNAc)g complex, we have proposed an alternative to the accepted mechanism which accounts for the available experimental observations. The proposal of this lysozyme mechanism illustrates one way in which simulations can serve to generate new ideas which can be explored by experiment and computation. 

Intermolecular oxygen atom transfer from a metal complex to an organic substrate is an archetypical reaction step in oxidation catalysis. As the transformation of O2 into metal 0x0 groups by oxidative addition is a well-precedented process (Sect. 2.2), its combination with transfer of the oxygen atom to an oxidizable substrate ( S ) constitutes a catalytic cycle for aerobic oxidations (Eq. 21). Examples of such cycles exist in organometallic chemistry, by virtue of 0x0 complexes with carbon-based ancillary hgands. 

The consecutive formation of o-hydroxybenzophenone (Figure 3) occurred by Fries transposition over phenylbenzoate. In the Fries reaction catalyzed by Lewis-type systems, aimed at the synthesis of hydroxyarylketones starting from aryl esters, the mechanism can be either (i) intermolecular, in which the benzoyl cation acylates phenylbenzoate with formation of benzoylphenylbenzoate, while the Ph-O-AfCL complex generates phenol (in this case, hydroxybenzophenone is a consecutive product of phenylbenzoate transformation), or (ii) intramolecular, in which phenylbenzoate directly transforms into hydroxybenzophenone, or (iii) again intermolecular, in which however the benzoyl cation acylates the Ph-O-AfCL complex, with formation of another complex which then decomposes to yield hydroxybenzophenone (mechanism of monomolecular deacylation-acylation). Mechanisms (i) and (iii) lead preferentially to the formation of p-hydroxybenzophenone (especially at low temperature), while mechanism (ii) to the ortho isomer. In the case of the Bronsted-type catalysis with zeolites, shape-selectivity effects may favor the formation of the para isomer with respect to the ortho one (11,12). 

Reaction (64) demonstrates the production of a metal formyl complex by intermolecular hydride transfer from a metal hydride which is expected to be regenerable from H2 under catalytic conditions. Further, it provides a plausible model for the interaction of [HRu(CO)4] with Ru(CO)4I2 during catalysis, and suggests a possible role for the second equivalent of [HRu(CO)4]- which the kinetics indicate to be involved in the process (see Fig. 23). Since the Ru(CO)4 fragment which would remain after hydride transfer (perhaps reversible) from [HRu(CO)4] is eventually converted to [HRu3(CO)),] [as in (64)] by reaction with further [HRu(CO)4], the second [HRu(CO)4]- ion may be involved in a kinetically significant trapping reaction. 

By a pH titration method, we have obtained log K = 2.0 and 2.5, respectively, for the formation constants of the 1 to 1 Cd and Ni complexes of glucosamine. These values are about 0.7 log unit lower than the corresponding metal complexes of imidazole (1), so that the metal ions would bind glucosamine less strongly than imidazole, and hence would exert a smaller effect. Moreover, in intramolecular catalysis, the catalytic amino group is already part of the glucose molecule, so that the catalytic influence would probably be relatively less affected by the presence of a metal ion than in the case of intermolecular catalysis. Our data also show that Ni(II) has a greater effect on intramolecular catalysis than Cd(II), and this is the same order as has been observed for intermolecular catalysis. 

Evidence for intramolecular hydrolysis of the methyl ester (62) by metal hydroxide has been provided.329 Molecular models of the metal complex (63) indicate that when complexation with the imidazole nitrogen and the phenolic hydroxyl group occurs, it is not possible for coordination of the ester carbonyl group to occur. This point, taken in conjunction with the observed pH rate profile which shows that ionization of the M—OH2 group is associated with catalysis, eliminates metal ion activation of the carbonyl bond to intermolecular attack by OH- as a contributing factor. For base hydrolysis of (62) kOH = 2.7 x 10-2 M-1 s-1 at 25 °C. The specific rate constants for intramolecular hydrolysis by the M—OH species are 0.245 s-1 and 2 x 10-2 s-1 for the Co11 and Ni11 complexes respectively. 

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