Ligand electronic asymmetry

The range in Mo-C(Cp) distances reflects the electronic asymmetry about the molybdenum caused by the nature and spatial arrangement of the other ligands bonded to the metal. In other words, these variations are a manifestation of the trans-influence. 

There has been an increased interest in mixed-donor and hemilabile ligands owing to their successful use in catalysis owing to their electronic asymmetry or ability to provide access to an unsaturated site via dissociation of one of the termini. Typical ligands used for catalysis that make use of their hemilability involve a phosphine and an oxygen donor, such as an ether. A weak donor, such as an ether (if it binds at all), is relatively easily displaced, and rapture of the M-0 bond and retention of the M-P bond is expected to provide an ideal hemilabile system. With P,N- ligands, either... 

Another impressive exanqtle for the importance of electronic asymmetry in the design of chelating chiral ligands was reported by RajanBabu and Casalnuovo for the asymmetric hydrocyanation reaction . As chiral ligands 3,4-phosphinites from D-fructofiiranoside derivatives were synthesized. The unsymmetrical phosphinite with the more electron-deficient phosphorous at the C4-position of fiuctose gave superior enantioselectivities for the hydrocyanation of 6-methoxy-2-vinylnaphthalene. 

Obviously, basic research is needed to provide a more comprehensive insight into the dependence of enantioselectivities on ligand electronics which is only poorly understood compared to steric effects. Nevertheless the concept of electronic asymmetry to enhance enantio selectivity in other ligand systems is very appealing and should be considered to a greater extent for future ligand optimization studies. 

Keywords Asymmetric hydrocyanation. Hydrogen cyanide, Vinylarenes, 2-Arylpropanoic acids, Naproxen, Arylphosphinites, Carbohydrate ligands. Electronic effects. Electronic asymmetry... 

Effect of Electronic Asymmetry Fine Tuning of a Ligand. 9... 

As we have argued before, if the primary effect of electron-withdrawing phosphorus aryl substituents is to accelerate the final reductive elimination, then the enhancement of enantioselectivity may result because one of the diastereomers of the benzylic complex (Fig. 4) may be disproportionately affected. The precise factors favoring a particular diastereomer (its formation or its decomposition) are presently unclear, but the results with the a-methyl fructofuranoside ligand frame strongly suggest the importance of a stereoelectronic component. For example, the effect of electronic asymmetry may reflect the importance of a trans... 

The electronic asymmetry key to the regioselective nucleophilic attack arises from different jr-acidity of two ligands, e.g. NO and PPhs in 30, or NO and CO in... 

In all compounds the extent of electron delocalisation is rather large. For the osmium containing compounds electron delocalisation is smaller, most likely because of the reduced overlap for the Os-bpt orbitals. The data in Table 2 and Figure 4 clearly show that the intervalence bands of two mixed metal compounds possess quite different characteristics. This is a clear indication of the asymmetry in the bridging ligand. This asymmetry is further reflected in the different oxidation potentials observed for the two compounds (See Table 1) and in the values obtained for k (See Table 2). 

Even bidentate ligands that rely on donor atoms of the same element can still impart an electronic asymmetry onto the allylpalladium moiety. This can be achieved by changing the environment of the donor atoms relative to one another. A clear-cut example of this approach is seen in the case of ligand 71, where the two phosphorus donor atoms are electronically quite distinct from each other.t There have also been many examples of non-C2-symmetric dinitrogen ligands reported.f " Their success may be attributed either to steric effects or, in some cases, an electronic difference between the two nitrogen donor atoms (Scheme 14). 

An interesting and specific characteristic of poly-NHC ligands is the possibility of combining different carbene donors to form the so-called hetero-poly-NHC hgands. As already reported by Huynh and coworkers, the combination of two different donors did not lead always to Hgands with intermediate properties with respect to the corresponding homoleptic compounds. In fact, enhanced properties (e.g., catalytic properties) may be recorded in the corresponding complexes, which have been interpreted in terms of an electronic asymmetry . The most recent examples on this topic are reported in Fig. 8. 

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