Nucleophilic reactivity transition metal bases

Table I. Nucleophilic Reactivity and Basicity of a Series of Transitional Metal Bases...

For all these reasons, some chemical or genetical modifications have been applied into the binding sites of antibodies in order to improve their reactivity [22]. Antibodies can be modified by the incorporation of natural or synthetic catalysts into the antibody recognition site, as for instance transition metal complexes, cofactors, and bases or nucleophiles, to carry other catalytic functions, which open the way to... 

Carbenes, generated by several methods, are reactive intermediates and used for further reactions without isolation. Carbenes can also be stabilized by coordination to some transition metals and can be isolated as carbene complexes which have formal metal-to-carbon double bonds. They are classified, based on the reactivity of the carbene, as electrophilic heteroatom-stabilized carbenes (Fischer type), and nucleophilic methylene or alkylidene carbenes (Schrock type). 

In retrospect, it is ironic to it that when I met Ernst Schumacher in 1969 (he was then Professor at the University of Bern in Switzerland) we did not talk about the experiments he did at Zurich in the same building where I was at that time. Instead, his interest focussed on our work on borazine transition metal compounds and we discussed in some detail whether it would be possible to incorporate metal atoms like chromium or molybdenum between the layers of hexagonal boron nitride (BN) in a similar way as it can be done with graphite. In the course of these discussions I did not mention that, after I had moved to Zurich, we had begun to investigate the reactivity of nickelocene towards both nucleophilic and electrophilic substrates. The reason was that we were still at the beginning, and while we had been able to prepare a series of monocyclopentadienyl nickel complexes from Ni(C5H5)2 and Lewis bases, our attempts to obtain alkyl- or acyl-substituted nickelocenes by the Friedel-Crafts reaction failed. 

The key feature of the adequate transition element seems to be in this ability to stabilize both electrophiles and nucleophiles within the same complex. Extra ligands may or may not increase the reactivity of one of these two species and therefore makethe design of a transition metal zeolite based catalyst within the same ease or difficulty as their homogeneous analogues. 

Recently, the reactivity (base, nucleophile, catalyst) and the role in modem organic synthesis of N-heterocyclic carbenes (cychc carbenes bearing at least one amino substituent) has been set-up by Ender et al. [60,61]. The possible utilization of NHCs as ligands for transition-metal catalyst [62,63] and as organocatalyst has been emphasized [61]. The inversion of the normal reactivity (umpolung) induced via NHCs has been extensively discussed. Classical carbon-carbon-bond-forming reactions (benzoin condensation, Stetter reaction, etc.) have been re-examined using ionic liquids as pre-catalysts in the presence of bases. 

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