Copper-based systems

The use of metal-catalyzed aziridination methods with chiral ligands has also been reported. The copper-based system paired with ligand 56 provides the expected cinnamyl aziridine in good yield and excellent ee <06MI4568>. It is interesting to note that the /-butyl ester is obtained with 99% ee while the smaller methyl ester is obtained in only 88% ee. The binaphthyl ruthenium catalyst 57 has been found to aziridinate a number of olefins with moderate enantioselectivity <06TL1571>. Both p-nitrophenyl (Ns) and trimethylsilyloxy (SES) sulfonamides work well with this catalytic system. As is usually seen, the aziridination of aliphatic olefins proceeds in only 32% yield and 56% ee. 

Important advances in propargylic etherification have come from the use of copper-based systems that achieve efficient, catalytic O-progargylation of phenols (Scheme 8).245,246 While the mechanism of this transformation remains unclear, the products of these reactions have been readily converted into chromenes through subsequent Claisen rearrangement,... 

The copper and palladium transition metal catalysts noted in Table 18 proved to be superior to nickel, ruthenium and rhodium catalysts. The nature of the reacting species has not been unequivocally defined, but the following experimental observations may provide some insight (i) tetrahydrofuran solvent is essential for the palladium-mediated reactions, since complex reaction mixtures (presumably containing carbinols) were observed when the reactions were performed in either benzene or methylene chloride (ii) the reaction is truly catalytic with respect to palladium (2 mmol alkylaluminum, 0.05 mmol of Pd(PPh3)4), whereas the copper catdyst is stoichiometric and (iii) in the case where a direct comparison may be made (entries 1-8, Table 18), the copper-based system is superior to palladium catalysis with regard to overall yield. 

A large number of styrenic monomers have been investigated in metal-catalyzed radical polymerizations. Polymerization of styrene (M-19) can be controlled with copper,28,84,85 152 176 ruthenium,57 60 62 66 86,205 iron,71 75 rhodium,86 140 rhenium,141 and molybdenum catalysts.144 The polymerizations have actively been studied with the copper-based systems, among which precisely controlled molecular weights and very narrow MWDs (MJMn =1.1) were obtained in a homogeneous system consisting of 1-13 (X = Br), CuBr, and L-3 in the bulk at 130 °C.85 Similar well-controlled polymerizations are feasible with several ruthenium (Ru-5)60 and iron (Fe-2,72 Fe-3,73 and Fe-471) complexes in conjunction with a bromide or iodide initiator. Even a chloride initiator (1-25, X = Cl) can afford narrow MWDs (MJMn =1.1) when coupled... 

Metal-catalyzed living radical polymerizations of vinylpyridines were investigated with the copper-based systems. One of the difficulties in the polymerization is a decrease of catalytic activity imposed by the coordination of the monomers by the metal complex. Controlled radical polymerization of 4-vi-nylpyridine (M-33) was achieved by an initiating system consisting of a strong binding ligand such as L-32 and a chloride-based system [1-13 (X = Cl)/ CuCl] in 2-propanol at 40 °C.214 The Mn increased in direct proportion to monomer conversion, and the MWDs were narrow (MJMn = 1.1 —1.2). In contrast, 2-vinylpyridine (M-34) can be polymerized in a controlled way with chlorine-capped polystyrene as an initiator and the CuCl/L-1 pair in / -xylene at 140 °C.215 Block copolymers with narrow MWDs (Mw/Mn = 1.1 —1.2) were obtained therein. 

Methanol is a good solvent for HEMA and its polymer, and thus can be employed for its homogeneous living radical polymerization with Ru-3, which is also highly soluble in methanol.59 Copper-based systems also give homogeneous living radical polymerizations of HEMA in a mixture of methyl ethyl ketone and 1-propanol,243 of acrylamides in methanol,117172 and of 4-vinylpyridine (M-33) in 2-pro-panol.214... 

Water is the solvent of choice for ionic monomers such as sodium methacrylate, where a direct radical polymerization (i.e., with the nonprotected form of the monomer) is carried out with the copper-based systems in aqueous media (pH 8—9) at 90 °C to afford controlled molecular weights and narrow MWDs.200 Another ionic monomer, sodium 4-vinylbenzoate, is polymerized very fast in aqueous media (pH 11) at 20 °C.247 An ammonium salt monomer, [2-(methacryl-oxy)ethyl]trimethylammonium chloride (FM-6 Figure 12), was polymerized in water with CuBr/L-1 in conjunction with a surface-confined initiator, while... 

Amino- and amido-functionalized monomers can also be polymerized directly with metal catalysts. Living radical polymerization of 2-(dimethylamino)-ethyl methacrylate (FM-5) was achieved with 1-31 (X = Br)/CuBr/L-29 in dichlorobenzene at 50 °C.314 Its ammonium salt (FM-6) was polymerized from the surface of a cross-linked polystyrene latex with CuBr/ L-l in water at 80 °C to generate hydrophilic shells, although there were no data for polymer molecular weight.248 As described above (section II.C.6), (methacrylamides with at least their amido protons unprotected (FM-7, FM-8, and FM-9) can be polymerized with copper-based systems,117168,217,218 but a further optimization seems to be necessary. 

Most of the block copolymers consisting of methacrylates and acrylates (B-8 to B-12) have been prepared via macroinitiator methods. AB- and BA-type block copolymers of MMA and MA (B-876 135,359 and B-9359) were prepared with nickel, copper, and iron catalysts. Due to the higher activity of the carbon—halogen terminals in poly(methacrylate) s than in poly(acrylate)s, block copolymerization from PMMA is successfully performed via both sequential and macroinitiator methods, where the controllability seems better in the copper-based system. Similar... 

A wider range of acrylate/styrene block copolymers have been prepared by copper catalysts, partially because the homopolymerizations of both monomers can be controlled with common initiating systems. Both AB- (B-15 to B-17)202,230,254,366,367 and BA-type (B-18 to B-21)28,112,169,230,366,368,369 block copolymers were obtained from macroinitiators prepared by the copper-based systems. The block copolymerizations can also be conducted under air230 and under emulsion conditions with water.254 Combination of the Re-and Ru-mediated living radical polymerizations in... 

Acrylamide-based block copolymers B-2 7370 and B-28117 were prepared by the ruthenium- and copper-based systems, respectively. The vinylpyridine (B-29)214,371 segment can be introduced into the block copolymers with MMA. The polystyrene-based block copolymers B-30 with short segments of a bicyclic monomer had a higher decomposition temperature than the homopolystyrene with C—Br terminals.223... 

In short, we expect that the synthetic utility of both biocatalytic and biomimetic copper-based systems will be further improved in the future. This will provide economically viable catalytic methodologies for the green, atom efficient oxidations of alcohols employing molecular oxygen as the primary oxidant, and affording water as the sole byproduct. 

Dri] Drits, M.E., Bochvar, N.R., Guzei, L.S., Lysova, E.V., Padezhnova, E.M., Rokhlin, L.L., Turkina, N.L, Cu-Fe-Mn (in Russian) in Binary and Multicomponent Copper-Base Systems , Nauka Moscow, 112-113 (1979) (Phase Diagram, Phase Relations, Review, 3)... 

Diels-Alder reaction between N-acryloyloxazoIidinone and cyclopentadiene for the case of the magnesium-based bis(oxazoline) complexes [65]. Poor enantiomeric excesses and conversion were observed for the analogous copper-based systems. 

Copper based systems are generally distinguished by their superior reactivity and their excellent compatibility with functional groups, whereas ruthenium complexes turned out to be less active despite the beneficial effect of Lewis acid cocatalysts. In view of the preliminary observations indicating that styrene polymerised under conditions used for olefin metathesis and/or cyclopropanation, we were prompted to probe the performance of some ruthenium complexes under ATRP conditions. 

Catalyst coatings under development for methanol steam reforming may be divided into copper-based systems and precious metal systems, namely Pd/ZnO. 

The knots based on neutral, purely organic molecules are obviously not prone to classical diastereomer resolution, and, while chromatographic methods were not suitable for the separation of the two enantiomers of the metal-templated trefoil knot, they have been proved successful in the amide-containing knots. As far as these knotted molecules are concerned, it must be noted that they incorporate classical stereogenic centers (carbon atoms), which makes them very different from the copper-based systems in terms of chirality. In the first instance, the separation of the two enantiomers of six different knots was achieved with a colunm that was not conunercially available (chiral-AD type). Trichloromethane was needed to obtain an optimal separation. The silica gel and the chiral stationary phase were covalently bound so that the material did not bleed out when the lipophilic eluent was used. Moreover, comparison of the experimental CD of the pure enantiomers of a knot with a theoretically calculated CD (based on X-ray structure and a fiiUy optimized AMI geometry) permitted assignment of the absolute configuration of this knot. The latter preparation of soluble knots based on substitution of the 5-position of the pyridine moieties in 13 afforded molecules that were soluble in solvents which could be used in commercially available chiral columns." On the other hand, the substitution of a racemic mixture of knots with chiral auxiliaries allows the separation of the diastereomeric product." ... 

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