Chromium catalysis, addition

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. 

Biaryls merit special interest due to their axial element of chirality and are among the most widely used ligands in enantioselective synthesis and catalysis. Their coordination by a tricarbonyl chromium fragment following benzannulation provides an additional stereogenic element in terms of a chiral plane to the molecule [68]. Biaryl quinones are similarly relevant to natural product synthesis and enantioselective catalysis. 

The gas phase homogeneous catalysis of the CO + O2 reaction in shock waves by addition of chromium, iron and nickel carbonyls has been described by Izod et al. [507], and Matsuda [508, 509]. It will not be discussed further here. 

One of the first applications of electron spin resonance (ESR) spectroscopy to catalysis was in a study of the chromia-alumina system, and during the last five years or so a number of publications have appeared dealing with this subject. The ESR spectra of supported chromia catalysts have been interpreted in terms of various chromium ion configurations or phases, each of which wiU be discussed below. It will be seen that these data substantiate many of the conclusions drawn from the magnetic susceptibility data described above, and, in addition, they provide a deeper insight into the molecular structure of chromia-alumina catalysts than can be obtained from static susceptibility measurements alone. This body of research serves as a very good illustration of the potential usefulness of ESR spectroscopy to the catalytic chemist, particularly when one considers that all of the data to be discussed below were obtained on poorly crystallized, high surface area powders, typical of practical catalysts. 

The use of chiral esters as 2tx-substrates permit the production of diastereomer-ically enriched cycloadducts. For example, the menthyl ester 48 was obtained as a 4 ldiastereomeric mixture. The most intriguing feature of polystyrene-bound catalyst 46 is the fact that it can be removed from the reaction mixture by simple filtration and repeatedly reused. This means that the two basic problems of homogeneous catalysis, separation and recycling of the catalyst, can be solved by using the solid-supported complex 46. Additionally, the environmental problems associated with chromium can also be effectively eliminated. 

The dimerization of chromium-coordinated carbene ligands typically requires temperatures above 120°C. Whereas the addition of catalytic amounts of Rh2(OAc)4 allows to only a minor decrease in temperature to 100°C, the dimerization occurs already at room temperature under palladium catalysis. [95a,96] When the reaction of chromium arylcarbene 16 was promoted by Pd(OAc)2 (10 mol-%) in the presence of NEt3 using THF as a solvent, a mixture of isomeric carbene dimers 95 was obtained with an E Z ratio of 2 1 (Scheme 42). The effects of the eatalyst load, phosphine additives, the reaction temperature, the solvent and a series of other palladium catalysts have been investigated systematically, but did not reveal significant changes. [96]... 

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