Transition metal complexes, base hydrolysis

Base hydrolysis in transition-metal complexes. M. L. Tobe, Adv. Inorg. Bioinorg. Mech., 1983, 2,1 (269). 

The chemistry of these heterometallic compounds based on the M—O—motif covers main-group elements, transition metals, and lanthanides. The generation of the M—O—motif (21) requires the successful s)mtheses and stabilization of well-defined hydroxides. A considerable effort has been ongoing to stabilize terminal hydroxides of main-group and transition metals (22). Recently, a number of well-defined hydroxides of main-group and transition metals 1-11 (Chart 1) have been made (23-35) by careful hydrolysis of suitable precursors. Some of these hydroxides were used as building blocks to synthesize heterometallic complexes with M—O—backbones by reaction with catalyti-cally active transition metal complex precursors. 

Transition metal coordination of Cu(II) carboxylate groups and pyridine groups was employed as a means of coupling a telechelic butadiene-base polymer with a randomly functionalized styrenic polymer. Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) indicated partial miscibility of the two polymers and Fourier transform infrared (FTIR) spectroscopy demonstrated that interactions occurred on a molecular level. When compared with blends of PSVP and the free acid derivative of CTB, the compositions based on the transition metal complex had improved dimensional stability at elevated temperatures, though there remains some question as to the stability of the copper salt to hydrolysis. Electron spin resonance (ESR) spectroscopy showed that only the... 

The in situ racemization can be achieved by different means either spontaneously or catalytically. Due to their chemical properties certain substrates may racemize spontaneously under the reaction conditions. Useful catalysts could be ordinary chemicals such as bases, transition metal complexes and in theory another type of biocatalyst. Having identified a suitable enzyme promoting the enantiomer-differentiating process by hydrolysis or alcoholysis of a carboxylic ester or by acylation of an alcohol one has to find the appropriate racemizing catalyst. Lipase and catalyst must tolerate each other they must work under identical conditions. The product must be chemically and configurationally stable in the presence of the catalyst. 

M.L. Tobe, Base Hydrolysis of Transition Metal Complexes , Adv. Inorg. Bioinorg. Chem.", 2, 1 (1983). See also M.L. Tobe, Acc. Chem. Res., 3, 377 (1970) and M.L. Tobe, Comprehensive Coordination Chemistry, Vol. 1, page 281 Pergamon Press, Oxford 1987. 

Tetra-alkylammonium salts have a dramatically different effect from alkali-metal salts on rates of base hydrolysis of the dichromate ion the rate is nearly a hundred times faster in Ca than in equivalent Et4N+ media, at an added salt concentration of 1 mol dm. These effects are similar to those reported earlier for hydrolysis of species such as [VioOag] ", [MesNSOa], and [Fe(5-NO2phen)3]2+, 20 nd must reflect the very different eff ts of these cations on water structure and thus on reactant and transition-state hydration. An analysis of salt effects on the initial and transition states for the solvolysis of t-butyl chloride indicates an approach that should prove fruitful in discussing, for instance, the effects of alkali-metal and tetra-alkylammonium salts on reactivities of transition-metal complexes in aqueous solution. "... 

There have been a few reports of first generation coordination complex structural models for the phosphatase enzyme active sites (81,82), whereas there are some examples of ester hydrolysis reactions involving dinuclear metal complexes (83-85). Kim and Wycoff (74) as well as Beese and Steitz (80) have both published somewhat detailed discussions of two-metal ion mechanisms, in connection with enzymes involved in phosphate ester hydrolysis. Compared to fairly simple chemical model systems, the protein active site mechanistic situation is rather more complex, because side-chain residues near the active site are undoubtedly involved in the catalysis, i.e, via acid-base or hydrogenbonding interactions that either facilitate substrate binding, hydroxide nucleophilic attack, or stabilization of transition state(s). Nevertheless, a simple and very likely role of the Lewis-acidic metal ion center is to... 

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