Tethered NHCs

The properties and yield of the polymer product were correlated to the NHC identity, providing clear evidence that the NHC ligand was bound and influenced the reaction. Smaller R groups (Me, Et) on 39-R provided low molecular weights, yields, and detectable amounts of impurity. Sugiyama only examined the influence of sterics on the formation of PC, but the initial success inspired Tanaka and coworkers to extend this application by tethering NHC ligands to styrene beads [48]. 

The metallacycle ruthenium complex 39, bearing a tethered NHC-alkyli-dene unit, catalyzes the formation of cyclic polymers by ROMP of COD (Eq. 28). Polymer formation is believed to proceed through a transient macrocyclic complex in which both ends of the growing polymer chain remain attached to the ruthenium center. Subsequent intramolecular chain transfer releases cyclic polymer [90,91]. 

In this way, the indenyl tethered NHC complexes of yttrium, lutetiiim and scandium were synthesised [135] each still bearing two potentially reactive trimethylsilylmethyl substituents. The potential as single component or cationic (after activation with MAO) catalysts for the polymerisation of olefins was pointed out by the authors, but no studies have as yet been reported. The same can of course be said about the corresponding early transition metal complexes of titanium, zirconium, vanadium and chromium [134,136] where the synthesis of the proligands and the transition metal complexes were reported, but no catalytic studies performed. 

Scheme 15 Synthesis of the Ce(IV) tethered NHC complexes 24 and 25 from 23. Reproduced from [50] with permission of Royal Society of Chemistry...

Scheme 31.15 Proposed mechanism for polymerization of lactide by tethered NHC-catalysts 42-45. N" = N (SiMe3)2.

Early complexes developed by Lee and coworkers for use in Suzuki couplings were developed bearing various functionalities. Polydentate ligands involving a tethered NHC/phosphine or NHC/pyridine system were isolated, and their... 

An important advantage of the Cp tethered to NHC ligands compared to the nonlinked systems is that chelation does not consume an extra coordination site, leaving the metal complexes with an additional site for catalysis (Fig. 10.1). As we have already pointed out, the tethered NHC unit could not only increase the stability of the half-sandwich metal-NHC complexes by chelation, but also could enhance the catalytic activity of their metal complexes by the stabilization of intermediate active species. In addition, the hemilabile dynamic behavior of the Cp-NHC ligand may allow to efficiently control the reactivity and stability of the catalytically active center. 

Figure 10.9 (a-d) Iron(II) complexes with Cp-tethered NHC hgands. 

A quinoline-tethered NHC-rhodium complex was used as catalyst for the [3 + 2] cycloaddition of diphenylcyclopropenone and internal alkynes (Equation (8.11)). " In the presence of 2mol% of 23, the desired pentenone derivatives were isolated in up to 86% yield. Also, this catalyst tolerated functional substituents such as ketone, ester or hydroxyl groups. 

Figure 10.1 Half-sandwich and anion tethered [(NHC)Ni] complexes.

---