Macroligands

A second approach that should allow for catalyst recycling is based on amphiphilic block copolymers, where the catalyst is covalently bound to the hydrophobic block. The groups of G. Oehme in Rostock and O. Nuyken in Munich are working on such systems that are sometimes described as metallosurfactants. The appending polymers without the catalyst are called macroligands or amphiphihzed ligands [4, 50]. 

Scheme 6.3 Amphiphilic poly(2-oxazoline) block copolymers bearing triphenylphosphine and bipyridine moieties respectively as polymeric macroligands for micellar catalysis.

In the first set of experiments O. Nuyken et al. studied the rhodium-catalyzed hydroformylation of 1-octene to its corresponding -aldehyde (Scheme 6.5) [51-53]. The active catalyst species was formed in situ by mixing the appropriate amount of polymeric macroligand, Rh(CO)2acac and 1-octene in water. The results are summarized in Tab. 6.3. 

Scheme 6.5 Hydroformylation of 1 -octene as a model reaction for micellar catalysis using tri-phenylphosphine-functionalized poly(2-oxazoline)s as macroligands.

Tab. 6.4 Recycling experiments Table 6.3, reaction time = 3 h). and subsequent reuse of the polymeric catalyst (macroligand 3,...

Macroligand trans-stilbene I, I -diphenylethy-lene (%)"> cis-stilbene i%r>... 

After an induction period of approximately 1 h the reaction started. Gas chromatographic analysis indicated 80% yield of trans-stilbene after 14 h reaction time based on iodobenzene. Considering the mild reaction conditions, this is a particularly remarkable result. As side products 1,1-diphenylethylene and cis-stilbene were identified in very low amounts of 2.8% and 0.7% respectively (after 14 h reaction time). Furthermore, catalytic activity depends strongly on the stracture of the macroligand used (Tab. 6.5) [57]. 

ATRP of Methyl Methacrylate in the Presence of an Amphiphilic, Polymeric Macroligand... 

Much research has already been devoted in the past couple of years to (i) the immobilization of ATRP active metal catalysts on various supports to allow for catalyst separation and reycycling and (ii) ATRP experiments in pure water as the solvent of choice [62]. A strategy to combine these two demands with an amphiphilic block polymer has recently been presented. Two types of polymeric macroligands where the ligand was covalently linked to the amphiphilic poly(2-oxazo-line)s were prepared. In the case of ruthenium, the triphenylphosphine-functiona-lized poly(2-oxazoline)s described in section 6.2.3.2 were used, whereas in the case of copper as metal, 2,2 -bipyridine functionalized block copolymers were prepared via living cationic polymerization [63] of 2-methyl-2-oxazoline and a bipyridine-functionalized monomer as shown in Scheme 6.8. 

Complex formation takes place in an organic solvent or in a water/monomer mixture by reaction of the macroligand with a metal compound (e.g. a Cu(I)-ha-lide). It is supposed that the conditions in the reaction mixture are comparable to those in conventional emulsion polymerization, where monomer droplets stabilized by surfactant molecules coexist with monomer swollen micelles [64]. Reaction sites are presumably the hydrophobic core of the micelles and the monomer droplets as well. Initial results of the micellar-catalyzed ATRP of methyl methacry-... 

Fig. 6.7 First-order kinetic plot for ATRP experiments with MMA in the presence of macroligand Me3o(4Bpy7)e.

It seems worth pointing out, that 137 and human semm albumin contain no pendant phosphines and the donor atoms in the complexes formed with rhodium can be only O (137) or O, N and perhaps S (HSA), which are not the most suitable for stabilizing low oxidation state metal ions. Still these macroligands gave active and stable catalysts with rhodium, which shows that perhaps in the high local concentration provided by the polymer even these hard donor atoms are able to save the metal ion against hydrolysis or other deterioration. 

P-Cyclodextrin was modified by attaching 2-(diphenylphosphinoethyl)-thio- (127) and 2-bis(diphenylphosphinoethyl)amino- (126) moieties at the C-6 position [8-11]. The resulting macroligands were reacted with [ RhCl(NBD) 2] to provide the corresponding cationic rhodium-bisphosphine complexes. These catalysts showed pronounced selectivity due to complexation of the substrate by the CD unit adjacent to the catalyticaUy active metal center. For example, in competitive hydrogenation of similarly substituted terminal olefins (Scheme 10.4), 4-phenyl-but-l-ene was... 

Macromolecular metal complexes can be classified into three main categories, taking into consideration the manner of binding of a metal compound to suitable macroligands [33] (Fig. 1). Type 1 metal complexes are those with the metal ion or metal chelate at a macromolecular chain, network, or surface. One possible approach to synthesize such polymers is using the polymerization of vinyl-substituted metal complexes. 

Johnson RM, Fraser CL. Iron tris(bipyridine)-centered star block copolymers chelation of triblock macroligands generated by ROP and ATRP. Macromolecules 2004 37 2718-2727. 

SCHEME 84. Epoxidation of cyclohexene with tungsten catalysts immobilized on organophosphoryl macroligands... 

The binding of functionalized chiral ligands to water-soluble polymers has also been shown a (diphenylphosphino)pyrrolidine derivative reacts with poly(acrylic acid) to form a macroligand that is useful in biphasic reduction (35). 

Recently, Kaneda and coworkers developed a new strategy for the design of high-performance heterogeneous catalysts utilizing hydroxyapatite as a macroligand for catalytically active centers [8] an efficient hydroxyapatite-bound Ru catalyst (RuHAP) was developed for selective oxidation of various alcohols using 02 [9]. 

Main Approaches to Calculating the Equilibrium Constants in Metal Ion — Chelating Macroligand Systems... 

The assessment of the chelating ability of macroligands and applicability of such systems requires the data on the quantitative evaluation of the chelation processes such a stability constants and formation function. Contemporary methods are largely based on the composition of the products formed. The major methods of analyzing the quantitative parameters of MX chelation use predominantly the same techniques that are applied to the description of complexing reactions with participation of monofunctional macroligands [5, 7b, 13a]. 

MX — macroligand systems (linear homo- and copolymers) and, (2) when the polymer ligand is suspended in the reaction medium. 

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