Mechanism catalyst system

Suitable catalysts are /-butylphenylmethyl peracetate and phenylacetjdperoxide or redox catalyst systems consisting of an organic hydroperoxide and an oxidizable sulfoxy compound. One such redox initiator is cumene—hydroperoxide, sulfur dioxide, and a nucleophilic compound, such as water. Sulfoxy compounds are preferred because they incorporate dyeable end groups in the polymer by a chain-transfer mechanism. Common thermally activated initiators, such as BPO and AIBN, are too slow for use in this process. 

Fluidized-bed reactor systems put other unique stresses on the VPO catalyst system. The mixing action inside the reactor creates an environment that is too harsh for the mechanical strength of a vanadium phosphoms oxide catalyst, and thus requires that the catalyst be attrition resistant (121,140,141). To achieve this goal, vanadium phosphoms oxide is usually spray dried with coUoidal siUca [7631-86-9] or polysiUcic acid [1343-98-2]. Vanadium phosphoms oxide catalysts made with coUoidal sUica are reported to have a loss of selectivity, while no loss in selectivity is reported for catalysts spray dried with polysUicic acid (140). 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

An improved design undertaken by Sacony used high-velocity gases to replace the mechanical elevator systems as catalyst carriers. These so-called air-lift units improved upon the Thermofor process both in terms of economies and octane numbers. It was, however, only with the fluid cracking process that catalytic technology realized fully continuous production. 

RhCl(PPhi)i as a homogenous hydrogenation catalyst [44, 45, 52]. The mechanism of this reaction has been the source of controversy for many years. One interpretation of the catalytic cycle is shown in Figure 2.15 this concentrates on a route where hydride coordination occurs first, rather than alkene coordination, and in which dimeric species are unimportant. (Recent NMR study indicates the presence of binuclear dihydrides in low amount in the catalyst system [47].)... 

Dall Asta and Motroni (44, 57) provided direct experimental evidence for the transalkylidenation mechanism in the case of cycloalkenes. With a catalyst system consisting of WOCI4, C2H6A1C12, and benzoyl peroxide they prepared a random copolymer of cyclooctene and cyclopentene, the cyclo-pentene double bond being labeled with 14C. The distribution of the radioactivity in the copolymer formed will depend on the site of ring opening. 

Because in metathesis reactions with most catalyst systems a selectivity of nearly 100% is found, a carbene mechanism seems less likely. Banks and Bailey ( ) reported the formation of small quantities of C3-C6-alkenes, cyclopropane, and methylcyclopropane when ethene was passed over Mo(CO)6-A1203, which suggests reactions involving carbene complexes. However, similar results have not been reported elsewhere most probably the products found by Banks and Bailey were formed by side reactions, typical for their particular catalyst system. 

Because of its industrial importance and the relative simplicity of its reaction mechanism and the catalyst system, much fundamental work has been done on this reaction. For an overview we refer the reader to R.A. van Santen and H.P.C.E. Kui-pers, Adv. Catal. 35 (1987) 265. 

Poly(2,6-dimethyl-l,4-oxyphenylene) (poly(phenylene oxide), PPG) is a material widely used as high-performance engineering plastics, thanks to its excellent chemical and physical properties, e.g., a high 7 (ca. 210°C) and mechanically tough property. PPO was first prepared from 2,6-dimethylphenol monomer using a copper/amine catalyst system. 2,6-Dimethylphenol was also polymerized via HRP catalysis to give a polymer exclusively consisting of 1,4-oxyphenylene unit, while small amounts of Mannich-base and 3,5,3, 5 -tetramethyl-4,4 -diphenoquinone units are always contained in the chemically prepared PPO. 

A comparison of several of the biphenylphosphine ligands has provided some insight into the mechanism of catalyst activation.168 The results of this study suggest that dissociation of the diphosphino to a monophosphino complex is an essential step in catalyst activation, which would explain why some of the most hindered phosphines are among the best catalyst ligands. This study also indicated that deprotonation of the amine ligand is an essential step. Finally, in catalyst systems that are based on Pd(II) salts, there must be a mechanism for reduction to the active Pd(0) species. In the case of amines, this may occur by reduction by the amine ligand. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

Platinum complexes with chiral phosphorus ligands have been extensively used in asymmetric hydroformylation. In most cases, styrene has been used as the substrate to evaluate the efficiency of the catalyst systems. In addition, styrere was of interest as a model intermediate in the synthesis of arylpropionic acids, a family of anti-inflammatory drugs.308,309 Until 1993 the best enantio-selectivities in asymmetric hydroformylation were provided by platinum complexes, although the activities and regioselectivities were, in many cases, far from the obtained for rhodium catalysts. A report on asymmetric carbonylation was published in 1993.310 Two reviews dedicated to asymmetric hydroformylation, which appeared in 1995, include the most important studies and results on platinum-catalogued asymmetric hydroformylation.80,81 A report appeared in 1999 about hydrocarbonylation of carbon-carbon double bonds catalyzed by Ptn complexes, including a proposal for a mechanism for this process.311... 

The polymerization of olefins and di-olefins is one of the most important targets in polymer science. This review article describes recent progress in this field and deals with organo-transition metal complexes as polymerization catalysts. Recent developments in organometallic chemistry have prompted us to find a precise description of the mechanism of propagation, chain transfer, and termination steps in the homogeneously metal-assisted polymerization of olefins and diolefins. Thus, this development provides an idea for designing any catalyst systems that are of interest in industry. 

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