Catalytic cycle of olefins

In this work, we have compared the potential energy profiles of the model catalytic cycle of olefin hydrogenation by the Wilkinson catalyst between the Halpern and the Brown mechanisms. The former is a well-accepted mechanism in which all the intermediates have trans phosphines, while in the latter, proposed very recently, phosphines are located cis to each other to reduce the steric repulsion between bulky olefin and phosphines. Our ab initio calculations on a sterically unhindered model catalytic cycle have shown that the profile for the Halpern mechanism is smooth without too stable intermediates and too high activation barrier. On the other hand, the key cis dihydride intermediate in the cis mechanism is electronically unstable and normally the sequence of elementary reactions would be broken. Possible sequences of reactions can be proposed from our calculation, if one assumes that steric effects of bulky olefin substituents prohibits some intermediates or reactions to be realized. 

Fig. 14. Proposed catalytic cycle of olefin hydrogenation and isomerization catalyzed on [HRu3(CO), (OSi=)] and [HOs3(CO)i,(OSi=)] species.

The equilibrium and transition state structures involved in the catalytic cycles of olefin hydrogenation by Wilkinson s catalyst and the mechanism of the reaction was examined by the ONIOM method. The energy profiles for both trans- and cis-forms were optimized and determined." ... 

Furthermore, in homogeneous catalytic reactions involving transition metal complexes, solvent molecules may coordinate to unsaturated intermediates and transition states, playing an important role in determining the reaction pathway. For example, it is believed that a solvent molecule coordinates to three- and five-coordinated unsaturated intermediates in the catalytic cycle of olefin hydro-genafion by RhCl(PPh3)3 (92). Such effects have recently been quantified by fhe Ab Inifo Molecular Orbifal Sfudy of the Full Cycle of Olefin Hydroformylafion Cafalyzed by RhH(CO)2(PH3)2 (93). Coordination of an olefin represenfafive of a solvent molecule into various intermediates was found to have a dramatic... 

Tandem procedures under hydroformylation conditions cannot only make use of the intrinsic reactivity of the aldehyde carbonyl group and its acidic a-position but they also include conversions of the metal alkyl and metal acyl systems which are intermediates in the catalytic cycle of hydroformylation. Metal alkyls can undergo -elimination leading to olefin isomerization, or couplings, respectively, insertion of unsaturated units enlarging the carbon skeleton. Similarly, metal acyls can be trapped by addition of nucleophiles or undergo insertion of unsaturated units to form synthetically useful ketones (Scheme 1). 

Figure 3.31 Catalytic cycle of the ruthenium-catalysed cleavage of olefins in the presence of percarboxylic acids.

In this double catalytic cycle the olefin is first transformed into aldehyde and then into alcohol. Owing to the increased sieric crowding of the bulky tertiary phosphine Hgand, all the stcric factors discussed in the previous section, which favour the production of normal aldehydes, are much favoured here. 

These recent findings indicate overall that the ligand that remains on the metal affects the energetics of the catalytic cycle, specifically olefin coordination, and the accessibility of the metallocyclobntane stmcture, the properties of the phosphane ligand control initiation rates, and thus how much of the catalyst can enter the catalytic cycle. The results of these carefiil analyses (Table 2) are sure to germinate the next generation of efficacious olefin metathesis catalysts. 

It has been our goal to design a catalytic system theoretically. To the end of this goal, we have so far analyzed the organometallic reactions by using the ab initio MO calculations. Recently, we have completed the theoretical study of the catalytic cycle of hydrogenation by the Wilkinson catalyst (2), of which mechanism has been proposed by Halpern (3). This catalytic cycle shown in Scheme 1 consists of oxidative addition of H, coordination of olefin, olefin insertion, isomerization, and reductive elimina-... 

This is a new entry for alkylation of benzene, though the applicability of this reaction is narrow. These authors proposed that a catalytic cycle involves olefin/acetonitrile ligand exchange followed by olefin insertion into the Ru-Ar bond. The C-H bond activation in another arene allows elimination of alkylbenzenes. 

Another example, to which we will return later, is square-planar [Rh(PH3)3 Cl] (Fig. 2). This molecule is used as a model for Wilkinson s catalyst, [Rh(PPh3)3Cl], in an extensive investigation of the catalytic cycle for olefin hydrogenation (Sect. 4.4) [85]. However, Bertran has demonstrated [86]... 

Scheme 1 Tentative catalytic cycle of the [Ni ]-catalyzed co-oligomerization of 1,3-butadiene and ethylene affording linear and cyclic Cio-olefins. Based on experimental studies of Wilke and co-workers [5]. Please note that only the favorable forms of la and lb are displayed... 

Scheme 3 Condensed Gibbs free-energy profile (kcal mol ) of the complete catalytic cycle of the co-oligomerization of 1,3-butadiene and ethylene catalyzed by zerovalent bare nickel complexes affording linear and cyclic Cio-olefins, focused on viable routes for individual elementary steps. The favorable [Ni (ri -frans-butadiene)2(ethylene)] isomer of the active catalyst species lb was chosen as reference. Activation barriers for individual steps are given relative to the favorable stereoisomer of the respective precursor (given in italics)...

The thermodynamic analysis of a complex chemical process has both the advantage and the limitation of being concerned only with the reactants and products of the reaction. This blindness to the mechanistic details ensures that the analysis is correct even if the model designed for the path is not, but misses all the important chemical events that account for the transformation of the molecules. This point can be illustrated through Scheme 6, which represents the catalytic cycle of alkene hydrogenation. While the net thermodynamic efficiency of the process is only dictated by the enthalpies of formation of the olefin, hydrogen, and the alkane, its feasibility depends on the thermodynamics of each elementary step. 

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