Anchoring ligands

Rhodium(I) complexes anchored by phosphine ligands usually have high selectivity but low stability.270-273 One of the possible way to tackle this problem is to bond the metal complexes by oxygen-containing anchoring ligands (14 in Fig. 7.12).274 275... 

Herein we have reported the synthesis, characterization, and reactivity of both transition metal and actinide elements supported by tripodal ligand scaffolds. The carbon-anchored ligands TIME normally form polynuclear species while the nitrogen-anchored ligands TIMEN can coordinate to transition metals in a 1 1, fashion, rendering the coordinated metal centers in well-protected pockets. 

A novel photostabilization process of anchoring ligands onto a polystyrene support which would eliminate the possibility of leaching the metal moieties by the coordinating solvent is due to Cais et al. [24]. A typical system is the... 

Figure 7.1. Chiral diphosphine, diphosphite, phosphine-phosphite, phosphinc-phosphinite, aminc-phosphinite, and polymer-anchored ligands used for asymmetric hydroformylation.

H. Schreier, P. Moran, and I. W. Caras. Specific targeting of liposomes to cells using a GPI-anchored ligand influence of liposome composition on intracellular trafficking, J. Biol. Chem. 269 9090-9098 (1994). 

Although the recovery of the polytartrate ligand by filtration amounts to about 90%, the Ti recovery levels are significantly lower (34). The fact that part of the Ti remains in the solution means that the binding of Ti to the anchored ligand is less efficient than it is to the ligand in solution. It seems unlikely that the polymeric nature of the ligand allows the formation of the same 2 2 Ti tartrate complexes that are formed in solution. Moreover, no recycle experiments have provided evidence that the catalyst is reusable. 

When the labilities are reversed, that is when the NHC group is used as the anchor, ligand design again orientates itself on the successful hemilabile bidentate phosphanes. Now, we encounter pendant ether [34,35], tertiary amino [36,37], pyrido [11,38 0], imino [41-44] and carbonyl groups [30,45] in the sidechain. 

If the molecular catalyst contains a labile ligand, a ligand exchange may occur involving the anchored ligand and the ligands on the complex. This type of attachment is exemplified by the following ... 

F. Montanari, M. Penso, S. Quici, P. Vigano, Highly efficient sodium hypochlorite olefin epoxidations catalyzed by imidazole or pyridine "tailed" manganese porphyrins under two-phase conditions. Influence of pH and of the anchored ligand, /. Org. Chem. 50 (1985) 4888. 

Polycondensation (sol-gel process) of suitable alkyl sulfonic acid functionalized organosilane monomers leads to solids with a siliceous matrix and a high concentration of anchored ligand groups, thus leading to the development of specially designed polysiloxane based solid acid catalysts Deloxan ASP (like in formula 1,2a, 2b, and 3). 

Wang, Y., Liu, H. and Jiang, Y. (1989) A new method for immobilization of polymer-protective colloidal platinum metal via co-ordination capture with anchored ligands. /. Chem. Soc., Chem. Commun., 1878-1925. 

Additional modifications include the immobilization of the catalyst on an insoluble surface. Silica-anchored ligands have been reported based on the DPP 33 and PYR 34 cores.24 Their use in AD reactions were comparable to the untethered versions of the chiral ligands. Alkenes substituted with alkyl or aryl groups and with internal and terminal double bonds gave diol products with yields ranging 51-93% and optical purities of 61-99% ee. The differentiation between 33 or 34 was the ability to readily recover and recycle the chiral ligands. 

Subsequent to this, the 6- and 6 -anchored PS-PEG MOP ligands 140 and 141 have been prepared and the catalytic activity in asymmetric tr-allylic reduction of their corresponding allylpalladium complexes compared and contrasted. The 6-anchored ligand was found to be the best and the 6 -analog the worst. This has been attributed to the fact that the 6 -position on a MOP ligand is in close proximity with the substrate moiety and thus the effect of attaching a sterically demanding tether at this position may well be to reduce catalytic activity. 

This oxidative reaction with the oxide surface has been observed even when the triosmium clusters are first bound to anchored ligands, such as PPh2 (0112)2 SIL or 118(0112)3 SIL. Analysis of the osmium L(III) edge EXAFS of the product of Os (00) j 2 with the latter functionalised oxide was entirely in accord with the structure Os H (00) j q(M-S(CH2) 3SIL) with Os-Os bond lengths of 2.848. On thermal decomposition at 48 h, a new component was... 

First, heterogeneous catalytic systems involved iV-anchoring ligands on inorganic supports such as zeolithes [76], clays [77]. ... 

Recently, Islam et al. (2012) reported that a polymer-supported Cu(II) complex could promote the C-N and C-0 coupling of arylboronic acids with Af-/0-nucleophiles. Three polymer-supported Cu complexes were prepared from the polymer-anchored ligand and the corresponding Cu salts Cu(II) complex [CuL(OAc)2l from Cu(OAc)2 was proven to be the best catalyst for N-arylation. A variety of Af-aryl imidazoles/ benzimidazoles were synthesized in good to excellent yields in methanol (MeOH) at 40°C (Scheme 4.35). The system is air and moisture stable and the catalyst can be reused for several cycles with consistent catalytic activity. 

Ffe. 6 Molecular orbital comprising the back-donation towards the 2ti molecular orbital of O2, denoting the participation from the anchoring ligand... 

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