Preferred donor

A number of mixed crown ligands have been synthesized combining N, O, or S ligands. The larger, more flexible, ligand systems can show interesting coordination patterns as the zinc binds to its preferred donor set. 

Palladium is the most active and most frequently used catalyst in transfer hydrogenations.77 78 Cyclohexene, a cheap, readily available, highly reactive molecule, is the preferred donor compound. Alternatively, tetralin and monoterpenes and, in general, any hydroaromatic compound, may be used. Mainly alcohols are employed as the donor with Raney Ni. 

The donor element is an important factor in chelate stability. Generally, certain metals form more stable complexes when bonded to a preferred donor... 

The criterion used is that class (a) acids form more stable complexes with the donor atoms to the right, and class (b) acids prefer donor atoms to the left. This is essentially the same criterion used by Schwarzenbach and Chatt. Often the existence or non-existence of related compounds, or complexes, could be used as an indicator. Rates of reaction could also be used by considering the activated complex to be an acid-base complex, A B. 

A common metal may be defined simply by its geo-availability, but from a coordination chemistry perspective it is more appropriate to define common in terms of aspects such as preferred oxidation state, number of coordinated donors or even preferred donor types. 

At the present time there are obviously two different sets of structural requirements which are to be imposed on optimally effective chelating agents for toxic metals. The first of these arises from the chemical requirements of the toxic metal ion. The second arises from the nature of the biological systems which must be traversed by the chelating agent if it is to reach the site at which the toxic metal ion is present, react with the toxic metal and transform it into a new and readily excreted metal complex. The most effective way to combine these two sets of structural requirements in a general manner must be developed for each toxic metal ion and is dependent upon its preferred biological sites for deposition and its preferred donor atoms and their attendant requirements. Some further examples may be found in recent reviews (47,48). 

The ThDP-dependent enzyme from Serratia marcescens (PigD), is an interesting carboligase since it has the ability to catalyze umpolung, Stetter type 1,4-additions to a,p-unsaturated ketones using pyruvate as donor substrate (Table 10.7) [51]. Aliphatic, aromatic as wdl as heterocydic a,p-unsaturated ketones selectively furnish the 1,4-adducts in the presence of pyruvate with PigD catalysis. Pyruvate is the preferred donor substrate, but 2-oxobutanoate can also be employed depending on the acceptor [51]. 

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