Ligand steric

Tetrakisligand nickel(0) complexes have tetrahedral stmctures. Electronic stmctures have been studied and conformational analysis performed. Quantitative equiUbria measurements of the ligands in these complexes imply a dominant role for ligand steric effects when the complexes are employed as catalysts (94). 

Davies [30] studied the PyBOx-induced conformational effects by testing several ligands sterically hindered on the oxazoUne moieties (Scheme 11, structures 18 and 19). However, these new ligands gave poorer results in terms of yields and enantioselectivities than ligand 16 for the Ru-catalyzed cyclopropanation reaction, indicating unfavorable steric interactions between styrene and the carbene complex. 

Professor Cotton s studies have considerably clarified our understanding of di- and tri-nuclear metal-metal bonded compounds. For higher nuclearity clusters, rationalisation of structures, bonding, reactivity etc., must be much more tenuous because of the increased number of variables (metal-metal, metal-ligand, steric effects) now present. However, I would briefly like to present a few trends which seem to be emerging in this area. 

Electronic effects. To study the nature of the electronic effect in the rhodium diphosphine catalysed hydroformylation, a series of thixantphos 18 ligands with varying basicity was synthesized 25-30 (Figure 8.11). In this series of ligands, steric differences are minimal so purely electronic effects could be investigated. 

Jacobsen and co-workers have reported that chiral diimine 33a serves as an effective chiral auxiliary for the copper-catalyzed aziridination of aryl-substituted Z-olefins (Scheme 6B.35) [80], For example, the aziridination of 6-cyanochromene proceeds with high enantioselectivity (>98% ee). Comparison of ligands 33a-33c has revealed that the o-substituents in the ligands sterically and electronically influence the enantioselectivity of the reaction, that is, the introduction of chlorines at o-positions not only prolongs catalyst lifetime but also enhances enantioselectivity. The reactions of other Z-substrates and cinnamate esters catalyzed by 33a show moderate-to-high enantioselectivity, whereas that of -stilbene gave low enantioselectivity (Table 6B.3). 

It is not easy to explain Irving and Williams rule that complex formation constants show a maximum for Cu(II) between Ni(II) and Zn(II) as a consequence of ligand field stabilization. It is true that multidentate ligands sterically predisposed to low ratios of tetragonality (such as 1.35 for ethylenediaminetetra-acetate) also show this effect to a less pronounced extent. Thus, log/f j can be compared6 22,23 with bidentate en and tridentate den ... 

Guezi, I.A. and Wendt, M. (2006) An improved method for the computation of ligand steric effects based on solid angles. Dalton Trans., 3991. 

The screening software SPECITOPE, developed by Schnecke et al.156, uses distance matrix comparisons as a first filter step. Elowever, flexibility of molecules is not modeled by distance intervals. Instead, a weighting scheme is defined to scale down the contributions of more flexible atom pairs in the overall score. In addition, a special optimizer is added that allows the removal of protein-ligand steric clashes after the placement calculation. 

Two studies have been conducted that outline the effects of ligand steric and electronic properties on the relative rates for reductive elimination of amine and P-hydrogen elimination from amides. One study focused on the amination chemistry catalyzed by P(o-C6H4Me)3 palladium complexes [111], while the second focused on the chemistry catalyzed by complexes containing chelating ligands [88]. 

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