Hydrolysis coordinated water catalyst

The coordination of the alkyne to the rhodium catalyst allows the carborhodation of the triple bond to afford the vinylrhodium intermediate 47 (Scheme 14). The rearrangement of this organometallic compound into the 2-(alkenyl)phenylrhodium intermediate 48 is evidenced by one deuterium incorporation resulting from the deuter-iolysis of the Rh-C bond. The addition of the phenylrhodium intermediate 45 must occur before its hydrolysis with water. The 2-(alkenyl)phenylrhodium intermediate 45, generated by the phenylrhodation of an alkyne followed by... 

A series of diaquatetraaza cobalt(III) complexes accelerated the hydrolysis of adenylyl(3 -50adenosine (ApA) (304), an enhancement of 10 -fold being observed with the triethylenetetramine complex (303) at pH 7. The pentacoordinated intermediate (305), which is formed with the complex initially acting as an electrophilic catalyst, then suffers general acid catalysis by the coordination water on the Co(III) ion to yield the complexed 1,2-cyclic phosphate (306), the hydrolysis of which occurs via intracomplex nucleophilic attack by the metal-bound hydroxide ion on the phosphorus atom. Neomycin B (307) has also been shown to accelerate the phosphodiester hydrolysis of ApA (304) more effectively than a simple unstructured diamine. 

As discussed in this chapter, there are three direct modes of activation that metal ions can provide for hydrolysis reactions, i.e., by Lewis acid, nucleophile and leaving group. In addition, metal coordinated water molecules can act as general acid catalysts and metal coordinated hydroxides can act as general base catalysts. We are mainly concerned with the three direct modes of activation since there is no particular advantage to using metal-based general acids and bases. 

Lewis-Acid Catalyzed. Recently, various Lewis acids have been examined as catalyst for the aldol reaction. In the presence of complexes of zinc with aminoesters or aminoalcohols, the dehydration can be avoided and the aldol addition becomes essentially quantitative (Eq. 8.97).245 A microporous coordination polymer obtained by treating anthracene- is (resorcinol) with La(0/Pr)3 possesses catalytic activity for ketone enolization and aldol reactions in pure water at neutral pH.246 The La network is stable against hydrolysis and maintains microporosity and reversible substrate binding that mimicked an enzyme. Zn complexes of proline, lysine, and arginine were found to be efficient catalysts for the aldol addition of p-nitrobenzaldehyde and acetone in an aqueous medium to give quantitative yields and the enantiomeric excesses were up to 56% with 5 mol% of the catalysts at room temperature.247... 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

Hydrolysis of letraa I koxy silane (TMOS or TEOS) is generally performed in the presence of a catalyst which can be an acid, a base or a nucleophile. This is also the case for the hydrolysis of R/Si(OMe)350. In the case of TMOS and TEOS, the acid catalysis is due to the reversible protonation of the alkoxy group which converts it to a better leaving group. However, the nucleophilic attack of the oxygen atom of water is still a key step (equation 17). In the case of basic catalysis, nucleophilic attack of the OH- anion at the silicon centre leads to a penta-coordinated intermediate, followed by the elimination of the RO group (equation 18). For nucleophilic catalysis (promoted by F, HMPA, imidazole, 7V,7V-dimethylaminopyridine as well as OH ) the formation of a penta-coordinated species (equation 19) increases the reactivity of the silicon atom towards the nucleophilic attack of water that leads to an hexa-coordinated intermediate, which finally leads to the product of hydrolysis or condensation. 

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