Spectator ligands, carbenes

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. 

Fischer-type carbenes, 8, 218 iV-heterocyclic carbenes, 8, 219 non-heteroatom carbenes, 8, 217 overview, 8, 197—268 photochemistry, 1, 252 with Re(I), 5, 900 with rhodium, 7, 184 rotation, dynamic NMR studies, 1, 420 in Ru and Os half-sandwich preparations, 6, 575 silver-catalyzed insertions, 9, 568 with silver(I), 2, 203 as spectator ligands, 1, 66... 

The formation of transition-metal carbene complexes containing tin substituents on the metal is another example where the tin group is a spectator ligand (equations 169-171)395-401. 

For a long time, carbenes, neutral carbon species with a divalent carbon atom bearing six valence electrons, were considered to be too reactive to be isolated [1]. As a consequence, many chemists hesitated to make use of these compounds, especially as spectator ligands for transition metal chemistry. However, whereas the majority of carbenes are short-lived reactive intermediates, this picture does not hold for N-heterocyclic carbenes [2],... 

Other cyclizations of allenols have been realized by means of gold-catalysts [186, 187], gold and platinum catalysts [188], and with lanthanum amide catalysts [189]. Intermolecular additions of alcohols to allenes were also catalyzed by cationic gold(I) complexes with carbene [190] or phosphane spectator ligands... 

We now look in detail at compounds with multiple bonds between metal and ligand. We are chiefly concerned with multiple bonds to carbon, as in metal carbene complexes L M=CR2, which have a trigonal planar carbon and at least formally contain an M=C double bond, and ntetal carbyne complexes, L MsCR, which are linear and contain an M=C triple bond, but we also look at complexes with multiple bonds to O and N. These are more often actor rather than spectator ligands. 

Carbenes such as 11.6 are gaining increasing attention as spectator ligands in catalysis (Section 11.4) because they are strongly bound to the metal and successfully promote a variety of catalytic cycles much as do phosphines (Chapters 4 and 9). Like phosphines they can be turned both sterically and electronically. 

After the initial activity in the 1960s and 1970s, Arduengo drew attention back to the area in 1991 with the isolation of the first NHC in the free state, where bulky R groups stabilize the carbene center. From 1994, Herrmann developed the use of NHCs as spectator ligands in homogeneous catalysis. Perhaps the most dramatic success in area came from the modification of the Grubbs catalyst... 

N-heterocyclic carbenes (11.18) can be useful spectator ligands, tunable both electrrm-ically and sterically. 

Even though the different reaction conditions precluded comparison with the non-classical carbene complexes, these examples demonstrated the potential of non-classical carbenes as spectator ligands. Further improvements may be accomplished by adapting some of the principles developed for classical carbene complexes, such as introduction of sterically demanding substituents in the ortho position, perhaps paired with coordinatively labile ligands irons to the non-classical carbene. ... 

Carbenes in general are discussed in Chapter 11, but NHCs (4.15) are covered here because they have attained equal importance with phosphines as spectator ligands, particularly in catalysis Compared with phosphines, the range of accessible steric parameters is equally wide, but most NHCs are much stronger donors, best quantified in DFT calculations NHCs also seem to be modest tt acceptors ... 

A -heterocyclic carbenes (Section 11.4) are a rising class of spectator ligand. 

First, structure 7.34 is a generalized structure of many Mo and W complexes that are efficient metathesis precatalysts. In these complexes, X,Y, and NRj are the 5/ ecraror ligands. Similarly, in the Ru complexes 7.37-7.39, the NHC carbenes, chloride, and pyridine are spectator ligands. In all the complexes the alkylidene groups are the actor ligands. 

In recent times a number of palladium complexes with NHC carbenes (see Section 2.1.5) have been found to be effective precatalysts for a range of cross-coupling reactions. Complexes such as 7.58 and 7.59 are two examples. In both these complexes, the carbene and the other ligands are spectator ligands. Under the reaction conditions, the allyl ligand of 7.58 reacts with a base and is quickly lost. 

The push-spectator stabilization system enables one to employ various alkyl groups with different types of steric environment, which differentiate amino(alkyl) carbenes dramatically from the NHCs as ligands. Taking advantage of their steric and electronic properties, Bertrand et al. nicely demonstrated the utility of CAACs as ligands in the palladium catalyzed a-arylation of ketones. Depending on the nature of the aryl chloride used, dramatic differences were observed in the catalytic activity of Pd-complexes with CAACs featuring different types of steric environment [36]. 

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