Other Pyridine Ligands

After successful application of the silver catalyst shown in olefin aziridination (Section 6.1.1), He and coworkers showed that intramolecular amidation was possible with both hydrocarbon-tethered carbamates and sulfamate esters.24 They found that only the Bu3tpy silver complex could catalyze efficient intramolecular amidation, while other pyridine ligands gave either dramatically lower yields or complicated product mixtures. In an interesting control study, both copper and gold were also tested in this reaction. Both the copper and gold Bu tpy complexes can mediate olefin aziridination, but only silver can catalyze intramolecular C-H amidation, indicating that the silver catalyst forms a more reactive metal nitrene intermediate. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Pyridine-based N-containing ligands have been tested in order to extend the scope of the copper-catalyzed cyclopropanation reaction of olefins. Chelucci et al. [33] have carefully examined and reviewed [34] the efficiency of a number of chiral pyridine derivatives as bidentate Hgands (mainly 2,2 -bipyridines, 2,2 6, 2 -terpyridines, phenanthrolines and aminopyridine) in the copper-catalyzed cyclopropanation of styrene by ethyl diazoacetate. The corresponding copper complexes proved to be only moderately active and enantios-elective (ee up to 32% for a C2-symmetric bipyridine). The same authors prepared other chiral ligands with nitrogen donors such as 2,2 -bipyridines 21, 5,6-dihydro-1,10-phenanthrolines 22, and 1,10-phenanthrolines 23 (see Scheme 14) [35]. 

On the other hand, sulfur-containing pyridine ligands have been prepared by Chelucci et al These ligands gave only moderate enantioselectivities (<51% ee) in the enantioselective addition of ZnEt2 to benzaldehyde, as shown in Scheme 3.17. 

A) between each unit, producing a tiara framework (Fig. 16). One Mo atom of the dinuclear core is in a square pyramidal environment and the other one, to which the pyridine ligand is bound, is in a distorted octahedral environment. The polyhedra share edges within the ring. In the solid state, the rings stack on top of each other, forming channels. 

Scheme 3. Stoichiometric mechanism of demetalation of robust FeIII-TAMLs such as lm by picolinic acid (L) in the presence of other pyridine bases (P). Axial aqua ligands are omitted for clarity.

Some other common ligands are pyridine (py), ethylenediamine (en), cyanide ion (CN ), thiocyanate ion (SCN, acetylacetonate (acac), sahcy-laldehyde (sal), triphenylphosphine (PPhs) and various trihalophosphines. Ethylenediame and acetylacetonate are examples of two common bidentate hgands. 

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