Ligands spectator

The steric environment of COP-X 46 and 47a around the catalytic palladium site mainly differs in a Ph (47a) and an i-Pr group (46) next to the coordinating N-site and the type and distance of the spectator ligand. While the distance of the two sandwich ligands differs only slightly between COP and 47a (3.4 A vs. 3.3 A), oxidation of the ferrocene to a ferrocenium species is expected to shorten this distance further. Overall, the steric hindrance to access the Pd-center is more distinct for 47a. These steric effects are capable to explain the higher ee obtained with 47a. 

Several studies have been undertaken in an attempt to understand factors that affect the susceptibility of a complex to reductive elimination. These include investigations on Me-Pd-CNC pincer complexes [39] studies on the influence of the geometry of the complexes [40] the impact of A-substituents [41] and a study on the influence of chelating spectator ligands in complexes of the type [PdMe(NHC) (P-P)]BF [42]. 

Experimental studies show that chelating spectator ligands impart a degree of stability to complexes of type 23 (Scheme 13.10) [42]. If monodentate phosphine ligands are used decomposition is rapid at 20°C, however, using dppp no decomposition is detected after 24 h [19]. It was found that the rate of decomposition could be linked to the chelate ring size at 65°C, with dppp decomposition was complete after 6 h, with dppe only a small amount of decomposition occurred after this time [42]. 

More than 80 % of all organotransition metal compounds are cyclopentadienyl complexes with Cp (C5H5) and Cp (CsMes) being the most prominent cyclopentadienyl systems used [1]. However, during the past few years functionalized cyclopentadienyl systems which do not just act as innocent spectator ligands have become very attractive. 

Recently considerable attention has been given to chiral 72-bonded olefin spectator ligands for the asymmetric conjugate addition.122 Hayashi and Carreira independently reported that new chiral diene ligands 83,123 84,124 and 85125 showed high enantioselectivities in Rh(i)-catalyzed conjugate addition of arylboronic acids to enones (Scheme 41). 

Addition of disilanes to isocyanides is catalyzed by palladium complexes, giving A-substituted bis(silyl)imino-methanes (Equation (53)).132 A wide range of isocyanides including aryl isocyanides and alkyl isocyanides can take part in the reaction. However, it is important to note that tert-alkyl isocyanides hardly undergo the bis-silylation reaction. This low reactivity of / r/-alkyl isocyanides allows their use as spectator ligands in the catalytic bis-silylations. 

Promising applications for metal-NHC compounds in materials science and medicinal chemistry are based on the strong metal-carbon bond and the high donor capability of the NHC. The most extensive investigations have been carried out in the field of homogeneous catalysis. Here, NHCs have to be considered as typical directing or innocent spectator ligands, best comparable to trialkylphosphines. The recent successful applications of NHCs in ruthenium-catalyzed olefin metathesis... 

Abstract A-Heterocyclic carbenes (NHCs) have developed into an important class of ligands in transition metal coordination chemistry. They have been employed successfully as spectator ligands in various catalytically active metal complexes and as organocatalysts. In this chapter we present some important synthetic methods for the preparation of various NHCs and their metal complexes. 

Scheme 7 Schlenk equilibrium of reactive and spectator ligands of the homo- and heteroleptic forms complexes of calcium (a) and barium (b) [53-55]...

Recently, rare-earth metal complexes have attracted considerable attention as initiators for the preparation of PLA via ROP of lactides, and promising results were reported in most cases [94—100]. Group 3 members (e.g. scandium, yttrium) and lanthanides such as lutetium, ytterbium, and samarium have been frequently used to develop catalysts for the ROP of lactide. The principal objectives of applying rare-earth complexes as initiators for the preparation of PLAs were to investigate (1) how the spectator ligands would affect the polymerization dynamics (i.e., reaction kinetics, polymer composition, etc.), and (2) the relative catalytic efficiency of lanthanide(II) and (III) towards ROPs. 

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