Ligand congestion

In theory, one could utilise GC-A ligands to lower blood pressure and to reduce blood volume as they increase the excretion of water and salt. Nesiritide, human recombinant BNP, is the first member of this new class of drugs approved for the initial intravenous treatment of acutely decompensated congestive heart failure. Whether nesiritide can be a valuable addition to the standard therapy of decompensated heart failure remains to be demonstrated. 

In 2005, Kunieda et al. developed sterically congested 2-iminothioethers as novel and highly efficient chiral ligands for the test reaction, providing the product in excellent yield and enantioselectivity, as shown in Scheme 1.38. ... 

Scheme 1.38 Test reaction with congested 2-iminothioether ligand.

The coordination mode of bis(imidazolyl)(pyrazolyl)borates towards Ni11 depends on the steric congestion around the boron center caused by substituents in the pyrazol 5-position. Complexes (207) or (208) have been obtained upon reaction with Ni(N03)2.619 The tripodal ligand... 

The sterically encumbered R2 substituents give steric protection to the oxygen-donors that are attached to the metal centers from coordination with Lewis acids such as MAO, or from another molecule of the catalytically active cationic species, which are supposed to be highly electrophilic. The coordination increases steric congestion near the polymerization center, which at least hampers ethylene coordination to the metal. Even worse, it may cause catalyst decay by, for instance, loss of the ligand. 

Metal complexes of pinene-fused boratabenzene ligands, analogous to chiral metallocenes that have found application in catalysis and enantioselective synthesis, have been prepared.122-124 With late transition metals such as Mn and Fe, the complexes are obtained as mixtures of diastereomers (e.g., 97) with the sterically less congested exo form predominating, but the bis(ligand) Zr complex 98 was obtained as the pure exo,exo product.124 A lithium... 

To investigate the impact of steric congestion on the hydroformylation reaction, the authors prepared a new series of ligands differing in flexibility as a result of... 

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