Kinetics and mechanism of the hydrolysis of functional groups in the side-chain of macrocyclic Cu2+ -com piexes
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چکیده
A series o f functionalized tetraazamacrocycles 2-8 carrying in their pendant side-chain a group which can be hydrolyzed, have been synthesised and their Cu(I1) complexes prepared. The structure analysis of the Cu(I1) complex with 5 shows a pentacoordinate Cu(I1) surrounded by the four nitrogens of the macrocycle and the carbonyl oxygen o f the ester group in a distorted square pyramidal geometry. Whereas the Cu(I1) complexes with the amide derivatives 7 and 8 do not react, the nitrile, the esters of the carbonic acids and of the phosphonate are hydrolyzed in alkaline solution. The kinetics of these reactions were measured and the rate constants determined. The pH-profiles log k/pH are linear with pH for the esters 3 5 , whereas those of the nitrile 2 and the phosphonate ester 6 exhibit a plateau at high pH. This allows to state that the hydrolysis o f these last two compounds proceeds through an internal OHattack. For the mechanism o f the carbonic acid ester hydrolysis several possibilities are discussed, but no definitive choice is yet possible. INTRODUCTION In several esterases and peptidases metal ions play an important role at t h e active site (ref.1). In general they are essential for activity and it is thought that the metal ion acts as a Lewis-acid and polarizes the bonds o f the reactive group so that a nucleophilic attack is facilitated and thus hydrolysis takes place. However, it has also been shown that metal ions can act as organizers, around which the reactands are oriented sterically in such a way that the desired reaction can proceed. In model reactions both types of mechanisms have also been observed (ref.2). So the hydrolysis o f amino acid esters in the presence of Cu(I1) is one o f the early examples and has extensively been studied (ref.3). For a detailed discussion o f the mechanistic pathways, however, the investigation of hydrolytic reactions in the coordination sphere of Co(II1) has given t h e deepest insight (ref.41, since with a kinetically inert metal ion an easier and more direct correlation between structure and reactivity can be made (ref.5). In these examples it has been clearly shown that there are two main possibilities: either a free OHattacks the coordinated substrate, which through coordination to the metal ion has been activated, or a coordinated OHreacts with the substrate, which is kept in the vicinity of the metal centre. The distinction between external and internal OHattack is much more difficult for labile metal ions, since a rapid equilibrium could produce a small amount of the reactive species, the structure of which generally is not easy to determine. To avoid this we have started to use mono-functionalized macrocycles to keep the metal ion in a fixed geometry and at the same time to bring the reactive group close to the metal ion. We have previously shown that a functional group at t h e end of the side chain of a tetraazamacrocycle, especially when it has donor properties, can bind t o the metal ion when the lenght o f the chain is designed in such a way that a five o r six-membered chelate ring can be formed (ref.6) and there is no steric interaction which would interfere (ref.7). Such systems seem therefore ideal for the study o f metal ion promoted reactions such as hydrolyses. A first example for this type o f reactions was the rapid hydrolysis o f the
منابع مشابه
Cu2+ PROMOTED HYDROLYSIS REACTIONS OF SIDE CHAIN FUNCTIONAL GROUPS. MODELS FOR HYDROLYTIC METALLO ENZYMES
Using three examples it is shown how the metal ion incorporated i n the macrocyclic ring modifies the reactivity of functional groups i n the side chain. The first series includes Cu2+ promoted hydrolysis reactions of a nitrile 2 , of several esters 3-5, of a phosphonate ester 6 and of an amide 7. The mechanism of the hydrolysis of the nitrile and of the phosphonate ester are consistent with an...
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تاریخ انتشار 2005