Chromium catalysis

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

The formation of two rings by chromium catalysis (Scheme 25) is demonstrated <1995TL3027>. 

The concurrent formation of two rings by chromium catalysis is demonstrate in Scheme 6 <95TL3027>. Conversion of the hydrazone (48) into the complex (49) and subsequent reaction with hex-6-yn-l-ol affords the annulated complex (50) which is decomplexed by irradiation in benzene. The stereochemistry of the intermediate complex (50) was determined by recomplexation of the final product (51), which gave a separable mixture of the two possible diastereoisomers. 

Most of what known about chromium catalysis is rather empirical in naturebut seems to fit within this picture. For example, in sharp contrast to manganese, chromium catalyst appears to favor the formation of carbonyl compounds. This tendency is likely to be related to the ability of chromium ions to participate in two-electron change (heterolytic) redox reactions [77, 78] in addition to the one-electron change (homolytic) reactions noted above. These heterolytic reactions can involve both hydroperoxides and alcohols. 

Polymerisation by homogeneous chromium catalysis is overviewed while further developments of vanadium-siloxane ethene polymerisations are also available. Triflate or... 

Agapie T Selective ethylene oligomerization recent advances in chromium catalysis and mechanistic investigations. Coord Chem Rev 255(7—8) 861—880, 2011. 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

Mulzer J, Ohler E (2004) Olefin Metathesis in Natural Product Syntheses. 13 269-366 Muniz K (2004) Planar Chiral Arene Chromium (0) Complexes as Ligands for Asymetric Catalysis. 7 205-223 Murai S, see Kakiuchi F (1999) 3 47-79... 

The oxidation of tartaric and glycollic acid by chromic acid also induces the oxidation of manganous ions. In the presence of higher concentrations of manganese(II) the rate of oxidation of the acids is diminished to about one-third of that in the absence of manganous ions. The decrease of the rate has been attributed to manganese(II) catalysis of the disproportionation of the intermediate valence states of chromium probably chromium(IV). 

This complex and structurally related molecules served as a functional homogeneous model system for commercially used heterogeneous catalysts based on chromium (e.g. Cp2Cr on silica - Union Carbide catalyst). The kinetics of the polymerization have been studied to elucidate mechanistic features of the catalysis and in order to characterize the potential energy surface of the catalytic reaction. 

Our attempts to prepare chromium hydrides and to evaluate their role in polymerization catalysis eventually led to the isolation of a series of alkyls and hydrides lacking any ancillary ligands besides the cyclopentadienyl moiety (see below).[6] Reduced to the essence of alkyls, these complexes provided another piece of evidence in the growing case against polymerization activity of divalent chromium none of the alkyls even reacted with ethylene. The hydride underwent one insertion and stopped at the stage of an ethyl group. 

The alkyls Tp Cr-R are the best test case yet of the catalytic activity of CrU alkyls (see Section 1). However, they did not react with ethylene, even at elevated temperature. On the contrary, Tp - Cr-Et eventually decomposed by an apparent P-hydrogen elimination yielding Tp - Cr-H and ethylene. Thus our notion that divalent chromium alkyls are not the chain propagating species in polymerization catalysis receives further support... 

Shiny silvery metal that is relatively soft in its pure form. Forms a highly resistant oxide coat. Used mainly in alloys, for example, in construction steel. Tiny amounts, in combination with other elements such as chromium, makes steel rustproof and improves its mechanical properties. Highly suited for tools and all types of machine parts. Also applied in airplane turbines. Chemically speaking, the element is of interest for catalysis (for example, removal of nitric oxides from waste gases). Vanadium forms countless beautiful, colored compounds (see Name). Essential for some organisms. Thus, natural oil, which was formed from marine life forms, contains substantial unwanted traces of vanadium that need to be removed. 

One very efficient system which may work this way was reported by Chien and Kiang (32) who found that 1.5% chromium, introduced by the tard reaction, raised the oxygen index of polypropylene to 27 (Fig. 4) and char formation was promoted. The hypotheses as to mode of action included the idea that dehydrogenation catalysis might be involved. 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

The reaction of olefin epoxidation by peracids was discovered by Prilezhaev [235]. The first observation concerning catalytic olefin epoxidation was made in 1950 by Hawkins [236]. He discovered oxide formation from cyclohexene and 1-octane during the decomposition of cumyl hydroperoxide in the medium of these hydrocarbons in the presence of vanadium pentaoxide. From 1963 to 1965, the Halcon Co. developed and patented the process of preparation of propylene oxide and styrene from propylene and ethylbenzene in which the key stage is the catalytic epoxidation of propylene by ethylbenzene hydroperoxide [237,238]. In 1965, Indictor and Brill [239] published studies on the epoxidation of several olefins by 1,1-dimethylethyl hydroperoxide catalyzed by acetylacetonates of several metals. They observed the high yield of oxide (close to 100% with respect to hydroperoxide) for catalysis by molybdenum, vanadium, and chromium acetylacetonates. The low yield of oxide (15-28%) was observed in the case of catalysis by manganese, cobalt, iron, and copper acetylacetonates. The further studies showed that molybdenum, vanadium, and... 

The purpose here is not to consider the broad field of catalysis, but simply point out where ignition is important with regard to exothermic decomposition both with regard to sustained decomposition and consideration of safety in handling of such chemicals. Wolfe [22] found that copper, chromium, manganese,... 

Sola, M. Duran, M. Torrent, M. The Dotz reaction A chromium Eischer carbene-mediated benzannulation reaction. In Computational Modeling of Homogeneous Catalysis Maseras, E. Lledos, A. eds. Kluwer Academic Boston 2002, 269—287. (Review). 

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