Catalysts and Mechanistic Studies

In the hydrocyanation process, zero-valent nickel phosphine or phosphite complexes are used as precatalysts. These are used in combination with Lewis acid promoters such as zinc chloride, trialkyl boron compounds, or trialkyl borate ester. The Lewis acid is very important for the facile and selective formation of 3PN and ADN in the first and the second stage, respectively. 

As shown by structure 535, the essential role of the Lewis acid is to act as an electron acceptor of the nitrogen lone pair of the coordinated cyano group. Such an interaction weakens the Ni-CN bond. It also increases the steric crowding in the cataljdic intermediates wherever Ni-CN bonds are present. The net effects are increased reaction rates because of the weakened Ni-CN bond and higher selectivity toward the linear isomer because of steric crowding. 

The following points deserve attention. First, interactions of Lewis acid with the coordinated cyano groups, like in structure 5.55, are present in intermediates such as 5.56 and 5.57. However, for clarity, they are not shown. Second, coordination of butadiene followed by its insertion into the nickel hydrogen bond produces 5.57. 

This Tj -allyl intermediate undergoes conversion to 5.58 or 5.59, the anti-Markovnikov and the Markovnikov products, by insertion of allyl into the nickel-cyano bond. The formal electron-pushing mechanism for these transformations is shown by reaction 5.6.1.2. 

Most of the reactions of the catalytic cycle are significantly reversible, and this makes isomerization of 2M3BN to 3PN possible. Also, 3PN is the thermodynamically more stable isomer a mixture of 3PN and 2M3BN if allowed to reach a thermodynamic equilibrium over the catalyst would have the concentration ratio of approximately 9 1. Reaction of 2M3BN with NiLj follows the reversible pathway and produces 5.58 through the intermediacy of 5.59, 5.57, and 5.56. In other words, free butadiene is formed during the isomerization. 

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As in COMC (1982) and COMC (1995), catalysis results are only mentioned in those cases where rt-bonded organometallic complexes have been isolated or characterized. However, since the discovery of the a-diimine nickel catalysts in 1996, the interest in the field has been strongly polarized toward the study of the new olefin polymerization and oligomerization catalysts, many of them cr-organonickel compounds themselves. In order to account for this important aspect of the chemistry of (j-bonded organonickel compounds, the different kinds of catalysts and mechanistic studies are discussed in Section 8.02.3.4.4. 

Hydrogenation of epoxides lends itself well to both synthetic applications and mechanistic studies. The reaction is complex, for either carbon-oxygen bond may break with or without inversion of configuration, and the product may contain deoxygenated products (92,93) as well as ketones derived by isomerization (26). The reaction is especially sensitive to both catalyst and environment (74). 

Catalyst characterization and Mechanistic Studies 3.2.1 Steady-State isotopic tracing experiments... 

Jusys Z, Behm RJ. 2001. Methanol oxidation on a carbon-supported Pt fuel cell catalyst—A kinetic and mechanistic study by differential electrochemical mass spectrometry. J Phys ChemB 105 10874-10883. 

Mahzoul, H., Brilhac, J.F. and Gilot, P. (1999) Experimental and Mechanistic Study of NOx Adsorption over NOx Trap Catalysts, Appl. Catal. B Environ., 20, 47. 

Rhodium compounds have also been used as catalysts since the late 1960s and mechanistic studies date from the 1970s.534,578-582 The binuclear rhodium complex [(Ph3P)4Rh2(//-OH)2] was found to be an effective catalyst for the reductive carbonylation of nitrobenzenes to carbamate esters. Electron-withdrawing groups at the para-position enhance the reactivity of the substrate.583... 

A possible mechanistic pathway for the M3(CO)i2 (M = Ru or Os) Fischer-Tropsch catalysts is presented in Scheme 3. It should be emphasized that most of the ideas outlined above are extremely speculative. However, it is to be hoped that with the advent of homogeneous catalyst systems, detailed kinetic and mechanistic studies will lead to a clarification of the situation in the not-too-distant future. 

Metal ions play an important role as catalysts in many autoxidation reactions and have been considered instrumental in regulating natural as well as industrial processes. In these reactive systems, in particular when the reactions occur under environmental or in vivo biochemical conditions, the metal ions are involved in complicated interactions with the substrate(s) and dioxygen, and the properties of the actual matrix as well as the transport processes also have a pronounced impact on the overall reactions. In most cases, handling and analyzing such a complexity is beyond the capacity of currently available experimental, computational and theoretical methods, and researchers in this field are obliged to use simplified sub-systems to mimic the complex phenomena. When the simplified conditions are properly chosen, these studies provide surprisingly accurate predictions for the real systems. In this paper we review the results obtained in kinetic and mechanistic studies on the model systems, but we do not discuss their broad biological or environmental implications. 

Kinetic and mechanistic studies by Casey et al. provided further insight into the mechanistic details of the hydrogenation of ketones and aldehydes, using a more soluble analogue of Shvd s catalyst (with p-tolyl groups instead of two of the Ph groups) [72]. The kinetics of hydrogenation of benzaldehyde by the Ru complex shown in Eq. (43) were first order in aldehyde and first order in the Ru complex the... 

Catalytic asymmetric methylation of 6,7-dichloro-5-methoxy-2-phenyl-l-indanone with methyl chloride in 50% sodium hydroxide/toluene using M-(p-trifluoro-methylbenzyDcinchoninium bromide as chiral phase transfer catalyst produces (S)-(+)-6,7-dichloro-5-methoxy-2-methyl-2--phenyl-l-indanone in 94% ee and 95% yield. Under similar conditions, via an asymmetric modification of the Robinson annulation enqploying 1,3-dichloro-2-butene (Wichterle reagent) as a methyl vinyl ketone surrogate, 6,7 dichloro-5-methoxy 2-propyl-l-indanone is alkylated to (S)-(+)-6,7-dichloro-2-(3-chloro-2-butenyl)-2,3 dihydroxy-5-methoxy-2-propyl-l-inden-l-one in 92% ee and 99% yield. Kinetic and mechanistic studies provide evidence for an intermediate dimeric catalyst species and subsequent formation of a tight ion pair between catalyst and substrate. 

Darensbourg and coworkers reported a systematic investigation of ROP catalyst performance along with kinetic and mechanistic studies for the polymerization of L- and rac-lactide using calcium complexes derived from tridentate Schiff base ligands (Fig. 13) [90, 91]. With calcium catalysts 77a-d, used in melt and solution polymerization of L-lactide, it was found that calcium salen catalyst 77d with bis (phosphoranylidene)ammonium azide as a co-catalyst is much less active than the calcium complexes with tridentate Schiff base ligands, as reflected in the monomer L-lactide conversions of 59-80% for 77a-c but only 35% for 77d as initiator. 

These complexes, either as [Ru(H30)(EDTA)] , Ru(H30)(EDTA.H) or even mixtures of RuClj and EDTA.H, are ubiquitous oxidation catalysts, and a number of kinetic and mechanistic studies on a variety of oxidations with them as starting... 

Other effective catalysts to synthesize formic acid are Rh complexes with formate114 or hexafluoroacetylacetonate115 ligands. New results of theoretical and mechanistic studies for these and other systems have been disclosed98 103 116-121... 

The catalytic hydration of olefins can also be performed in a three-phase system solid catalyst, liquid water (with the alcohol formed dissolved in it) and gaseous olefin [258,279,280]. The olefin conversion is raised, in comparison with the vapour phase processes, by the increase in solubility of the product alcohol in the excess of water [258]. For these systems with liquid and vapour phases simultaneously present, the equilibrium composition of both phases can be estimated together with vapour-liquid equilibrium data [281]. For the three-phase systems, ion exchangers, especially, have proved to be very efficient catalysts [260,280]. With higher olefins (2-methylpropene), the reaction was also performed in a two-phase liquid system with an ion exchanger as catalyst [282]. It is evident that the kinetic characteristics differ according to the arrangement (phase conditions), i.e. whether the vapour system, liquid vapour system or two-phase liquid system is used. However, most kinetic and mechanistic studies of olefin hydration were carried out in vapour phase systems. 

This reaction was one of the first examples of catalysis by a supported organometallic compound. In 1964 it was observed that Mo (CO) 6/ A1203, after activation by heating in vacuo at 120°C, catalyzed the conversion of propylene into ethylene and 2-butene (82). The nature of the active site in this catalyst system is still not fully defined (83). Since the initial discovery many heterogeneous and homogeneous catalyst systems have been reported (84, 85), the latter being more amenable to kinetic and mechanistic studies. 

The role of various elements in the supported catalyst requires additional work to fully understand their function. There is also a dearth of information dealing with the nature of the surface of the newer catalysts subjected to various pretreatment and exposed to model compounds such as alkyl-substituted dibenzothiophene or other feedstocks. There is a need to correlate that data from such characterization with kinetic and mechanistic studies. 

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