Spectator catalysis

Venkatasubban and Schowen34 provided a key refinement in which they specify that the role of a pre-associated catalyst is to quench a reactive intermediate. Since the catalyst does not participate in stabilizing the transition state it has another role and a special name The [catalyst] is a spectator during the bond fission, and the catalysis is of the type we have called spectator catalysis... It is conceivable that the diffusional approach of BH to the tetrahedral adduct is the rate-limiting step. ... 

Promising applications for metal-NHC compounds in materials science and medicinal chemistry are based on the strong metal-carbon bond and the high donor capability of the NHC. The most extensive investigations have been carried out in the field of homogeneous catalysis. Here, NHCs have to be considered as typical directing or innocent spectator ligands, best comparable to trialkylphosphines. The recent successful applications of NHCs in ruthenium-catalyzed olefin metathesis... 

From the temperature variation of the equilibrium constant, thermodynamic parameters for the reaction were also obtained. The extent of formation of [Mo(CO)5l]" was found to be cation-dependent, and while equilibrium constants of 39 and 21 atm L moF were obtained for Bu4P and pyH+, none of the anionic iodide complex was observed for Na. Despite this variation, there seemed to be no correlation between the concentration of [Mo(CO)5l]" and the rate of the catalytic carbonylation reaction. It was proposed that [Mo(CO)5] and [Mo(CO)5l] are spectator species, with the catalysis being initiated by [Mo(CO)5]. Based on the in situ spectroscopic results and kinetic data, a catalytic mechanism was suggested, involving radicals formed by inner sphere electron transfer between EtI and [Mo(CO)5]. 

Sensitivity and complexity represent challenges for ATR spectroscopy of catalytic solid liquid interfaces. The spectra of the solid liquid interface recorded by ATR can comprise signals from dissolved species, adsorbed species, reactants, reaction intermediates, products, and spectators. It is difficult to discriminate between the various species, and it is therefore often necessary to apply additional specialized techniques. If the system under investigation responds reversibly to a periodic stimulation such as a concentration modulation, then a PSD can be applied, which markedly enhances sensitivity. Furthermore, the method discriminates between species that are affected by the stimulation and those that are not, and it therefore introduces some selectivity. This capability is useful for discrimination between spectator species and those relevant to the catalysis. As with any vibrational spectroscopy, the task of identification of a species on the basis of its vibrational spectrum can be difficult, possibly requiring an assist from quantum chemical calculations. 

Alkoxyl species form very readily from the reaction of alkyl halides on alkali, alkaline earth, transition metal, and lanthanide exchanged zeolites (128, 129). The more basic the zeolite, the more readily the reaction proceeds. Alkyl halides have been used to generate methoxyl, ethoxyl, isopro-poxyl, and ferf-butoxyl species on metal-exchanged zeolites. The mechanistic significance of alkoxyl species in zeolite acid catalysis is not in general clear in some reactions they may be true intermediates, and in others mere spectators. 

The efficiency of /-elements in catalysis originates from unconventional electrophilic pathways. In contrast to rf-elements oxidative addition/reductive elimination sequences are not accessible. Instead, substrate adduct formation, ligand exchange and insertion reactions rule the mechanistic scenarios. Therefore, the main emphasis is put on the fine-tuning of the spectator ligand of the precatalyst. 

Although there is a lot of information available about surfaces and surface species there is unfortunately no direct translation between such information and a more detailed understanding of the actual heterogeneous catalysis processes. There are several reasons for this surface studies normally have to be carried out in ultra-high vacuum and at low temperatures, a far cry from the conditions of a catalytic reaction which may typically only occur at temperatures of greater than 100°C and pressures of several bar. The residence times of the species that participate in catalyses are normally very short and their concentration is very low, thus it has only quite recently become possible to derive any meaningful information from the surface studies about the catalyses. Furthermore, although many species are detected on surfaces, most are spectator species, which do not participate in, and are unaffected by, the desired catalyses. 

The configurationally robust [CpRe(NO)(PPh3)X] complexes, easily obtained in an enantiomerically pure form (except for X = OR, NR2), were used as building blocks for new chiral hgands with spectator chiral-at-metal fragments. Both chiral diphosphine (117) and diamine ligands (118) have been prepared and successfully tested in catalysis. ... 

Compelling evidence that the radial allyl species which forms is important in the catalysis and not just a spectator species comes from measurement of the... 

From their genesis, polypyrazolylborates have found applications in many areas of chemistry such as the modeling of active sites of met-alloenzymes, perspective materials, organic synthesis, homogenous catalysis, or as spectator ligands in the development of reactive metal centers. A number of miscellaneous items will also be discussed in this chapter. 

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