Collins catalysts

R. M. Heck, J. M. Chen, and M. E. Collins "Oxidation Catalyst for Cogeneration AppHcations— Regeneration of Commercial Catalyst," paper... 

Collins and coworkers applied the bis(tetrahydroindenyl)zirconium triflate 32, which is used as a polymerization catalyst, to the asymmetric Diels-Alder reaction [50] (Scheme 1.61). A remarkable solvent effect was observed - although only a low optical yield was obtained in CH2CI2, high optical purity (91% ee) was realized in 2-nitropropane by use of only 1 mol% of the catalyst. The catalyst is also effective for crotonoyloxazolidinone, giving the cycloadduct in 90% ee. 

Dr. Collins Is the RMProcess basically an extension of existing refining technology with concurrent methanation, or the extension of methanation technology with concurrent reforming from the standpoint of catalyst performance requirements ... 

Collins et al. reported in 1995 that catalysts based on hafnium are desirable for the production of elastomeric polypropylene in that they polymerize propylene to a high molecular weight polymer and are indefinitely stable under typical polymerization conditions [8], Based on the theory that hafnium as a catalytic center leads to a significant increase of molecular weight in propene polymerization compared with the zirconium-based catalyst, Rieger et al. searched for hafnocene systems to obtain polymers with new properties. 

Based on Chien s research results, Collins et al. modified the basic structure of the catalysts and also achieved elastic material [8,18,19]. In both cases the elastic properties of the polymers are justified in a block structure with isotactic and atactic sequences. In 1999 Rieger et al. presented a couple of asymmetric, highly active metallocene catalysts, e.g., the dual-side catalyst rac-[l-(9-r 5-fluorenyl)-2-(5,6-cyclo-penta-2-methyl-l-q5-indenyl)ethane]zirconium dichloride (Fig. 3). These catalysts allowed building of isolated stereoerrors in the polymer chain to control the tacticity and therefore the material properties of the polymers [9],... 

Collins S, Gauthier WJ, Corrigan JF, Nicholas NJ (1995) Elastomeric poly(propylene) influence of catalyst structure and polymerization conditions on polymer structure and properties. Macromolecules 28 3771-3778... 

Collins S, Bailey L, Pigeon M (1998) Synthesis of elastomeric poly(propylene) using unsym-metrical zirconocene catalysts marked reactivity differences of rac - and meso -like dias-tereomers. Macromolecules 31 1000-1009... 

The first CL sensor for oxygen analysis was reported by Freeman and Seitz in 1978 [6], Collins and Ross-Pehrsson [12] investigated the effect of polymer type, pH, and metal catalyst incorporated within the film. Oxygen levels as low as 2.4 ppm in nitrogen have been detected using the oligomer fluoropolyol as the support matrix for immobilizing luminol, KOH, and the metal catalyst Fe2(S04)3. A sensor... 

Collins and co-workers have performed studies in the area of catalytic enantioselective Diels—Alder reactions, in which ansa-metallocenes (107, Eq. 6.17) were utilized as chiral catalysts [100], The cycloadditions were typically efficient (-90% yield), but proceeded with modest stereoselectivities (26—52% ee). The group IV metal catalyst used in the asymmetric Diels—Alder reaction was the cationic zirconocene complex (ebthi)Zr(OtBu)-THF (106, Eq. 6.17). Treatment of the dimethylzirconocene [101] 106 with one equivalent of t-butanol, followed by protonation with one equivalent of HEt3N -BPh4, resulted in the formation of the requisite chiral cationic complex (107),... 

Collins and co-workers have also reported on an enantioselective catalytic Diels—Alder cycloaddition, in which zirconocene and titanocene bis(triflate) complexes were used as catalysts [104], The influence of the solvent polarity on the observed levels of stereoselectivity is noteworthy. For example, as shown in Scheme 6.34, with 108 as the catalyst, whereas in CH2C12 (1 mol% catalyst) the endo product was formed with 30% ee (30 1 endoxxo, 88% yield), in CH3N02 solution (5 mol% catalyst) the enantioselectivity was increased to 89% (7 1 endoxxo, 85% yield). Extensive 1H and 19F NMR studies further indicated that a mixture of metallocene—dienophile complexes was present in both solutions (-6 1 in CH2C12 and -2 1 in CH3N02, as shown in Scheme 6.34), and that most probably it was the minor complex isomer that was more reactive and led to the observed major enantiomer. For example, whereas nOe experiments led to ca. 5 % enhancement of the CpH proton signals of the same ring when Hb in the minor complex was irradiated, no enhancements were observed upon irradiation of Ha in the major complex. 

With Cp2Zr(OtBu)(THF) as catalyst, selectivities are generally -20 1 endo exo. See S. Collins, B.E. Koene, R. Rama-chandran, N. J. Taylor, Organo-metallics 1991, 10, 2092—2094. 

The ketone catalyst was kindly provided by Professor Y. Shi (Colorado State University, Fort Collins, Colorado)... 

Catalysts and their effects on chemical reactions aid in efficiency, effectiveness and selectivity. A recent example of current research is redox and ligand exchange reactions of the oxygenation catalyst (N,N -bis(salicylidene)ethylenediaminato)co-balt(II), Co(SALEN)2 (below), and its one-electron oxidation product, Co(salen) 2-These were investigated in DMF, pyridine, and mixtures of these solvents. Solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition are important considerations (Eichhorn, 1997). The results in these solvents should be compared with other work with catalysts using more environmentally benign media (Collins et al., 1998). 

To estimate how much improvement may be possible, we can take as a first estimate the conclusions of Burch and Collins 68) and/or the modified data shown in Fig. 20—only one active site per MoS2 crystallite. The worst scenario would be that today s catalysts now have the maximum possible activity per site. In such a case, the only improvement possible is to increase the number of MoS2 crystallites on the catalyst surface or to increase the number of the special corner sites noted by Topspe. In the Burch and Collins model, 33 Mo atoms per crystallite were assumed. However, improved preparation procedures have reduced crystal sizes in experimental catalysts to less than 10 A or only about seven Mo atoms per crystallite. The maximum improvement would be to reduce the number of Mo atoms in an active crystallite to two (assuming the SBMS structures proposed by Startsev [2]). Thus, an improvement by a factor of only about 3 might be expected, relative to the most active reported catalysts. 

This fluorine-containing, oxidation-resistant alcohol is best oxidized by the Pfitzner-Moffatt reaction, using dichloroacetic acid as catalyst. Observe the use of toluene, instead of carcinogenic benzene, as solvent. A Swern oxidation was not reproducible, and caused substantial epimerization of the isobutyl side chain. Collins oxidation was successful, but ... 

Stringaro J-P, Collins P, Bailer O. In Cybulski A, Moulijn JA, eds. Structured Catalysts and Reactors. New York Marcel Dekker, 1998 393—416. 

K. R. and Collins, T.J. (1998) Designing ligands to achieve robust oxidation catalysts. Iron based systems. Coord. Chem. Rev., 174, 361. 

However, a serious issue for device integration with CNTs is posed by the inability to control whether the tubes or fibers are semiconducting, semimetallic, or metallic. This aspect will also play a role if carbon nanostructures are used as a catalyst support. Except for a selective destruction of metallic tubes (Collins et al., 2001) an interesting method to separate metallic from semiconducting CNTs is the use of AC DEP. This is done by bringing a suspension of the tubes in contact with a microelectrode array. Due to the different dielectric constant of the species with respect to the... 

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