Modeling of Catalysts and Catalytic Cycles

For an ea HRh(CO)(alkene)(diphosphine), in which the hydride is assumed, as in Figure 3, to be in axial position, alkene have two coordination sites available, four conformations for each site, two rotation sides, N ligand conformations, and therefore 16xN TS s. Computation of the full catalytic cycle, all intermediates and TS s, from the entry of the substrate to the departure and regeneration of the catalyst, complemented with IRC calculations to confirm the connection between TS s and intermediates is out of reach for current computational resources. However, suitable modeling strategies can reduce of the problem, and still provide useful insight. 

Figure 6.5 Mechanism and catalytic cycle for propylene polymerization with a model metallocene catalyst. Conversion of 6.16 to 6.17 and 6.19 to 6.20 involve insertion of (n + 1) propylene molecules.

From Iron(III) Tetraarylporphyrins and Alkenes. N-alkyl porphyrins are formed via side reactions of the normal catalytic cycle of cytochromes P-450 with terminal alkenes or alkynes. N-alkylpor-phyrins formed from terminal alkenes (with model iron porphyrin catalysts under epoxidation conditions) usually have a covalent bond between the terminal carbon atom of the alkene and a pyrrole nitrogen. The double bond is oxidized selectively to an alcohol at the internal carbon. Mansuy (23) showed that, in isolated examples, terminal alkenes can form N-alkylated products in which the internal carbon is bound to the nitrogen and the terminal carbon is oxidized to the alcohol. Internal alkenes may also form N-alkyl porphyrins (24, 25). 

This chapter is organized as follows. In the following section the tentative catalytic cycle proposed by Wilke and co-workers is outlined, followed, in the next section, by a short description of the computational approach employed and the catalyst model chosen. The structural and energetic aspects of all critical elementary steps of the complete catalytic cycle are presented after that. Then we propose a theoretically verified, refined catalytic reaction cycle, and follow that with the elucidation of the product distribution between linear and cyclic Cjo-olefins. Finally, the catalytic reaction courses of the [Ni°]-catalyzed co-oligomerization of 1,3-butadiene and ethylene and of the cyclooligomerization of 1,3-butadiene are compared. 

Sequential Iminium-Enamine Catalysis. Directed Electrostatic Activation. A comparison of the standard catalytic cycles for enamine activation (Scheme 2.1) and for iminium ion activation (Scheme 2.12) show that iminium catalysis proceeds, after the addition of the nucleophile, via an ( )-enamine. In the presence of a suitable electrophile, this enamine gives rise to an iminium ion that after hydrolysis can give rise to an a,p-diftmctionalyzed carbonyl (Scheme 2.13) [85]. Scheme 2.13 also shows that when using a chiral 2-substituted pyrrohdine or an imidazolidinone as the catalyst, the sequential apphcation of the steric model for Michael addition to iminium ions (Figure 2.15) and of the steric model for electrophilic attack to enamines (Figure 2.IB) predicts the absolute stereochemistry of the major isomer obtained in the reaction. 

The design and catalytic activity of dibenzobarrelene-based bifunctional PC(5p )P pincer catalysts for acceptor-less dehydrogenation of primary and secondary alcohols to give carbonylic and carboxylic compounds has been described. The mechanism of the H2 formation involves intra-molecular cooperation between the structurally remote functionality and the metal centre. The feasibility of the complete catalytic cycle was studied using a stoichiometric model. ... 

Today, iridium compounds find so many varied applications in contemporary homogeneous catalysis it is difficult to recall that, until the late 1970s, rhodium was one of only two metals considered likely to serve as useful catalysts, at that time typically for hydrogenation or hydroformylation. Indeed, catalyst/solvent combinations such as [IrCl(PPh3)3]/MeOH, which were modeled directly on what was previously successful for rhodium, failed for iridium. Although iridium was still considered potentially to be useful, this was only for the demonstration of stoichiometric reactions related to proposed catalytic cycles. Iridium tends to form stronger metal-ligand bonds (e.g., Cp(CO)Rh-CO, 46 kcal mol-1 Cp(CO)Ir-CO, 57 kcal mol ), and consequently compounds which act as reactive intermediates for rhodium can sometimes be isolated in the case of iridium. 

The previous section has described how one can compute accurately a system of about 30 atoms including one transition metal. The problem is, as mentioned above, that these are usually not the real catalysts, but model systems where the bulky substituents have been replaced by hydrogen atoms. Calculations on model systems are usually at least indicative of the nature and the energy barriers of the steps involved in a catalytic cycle, but they are often unable to provide information on some of the most interesting features, namely enantioselectivity and regioselectivity. The reason for this failure is simply that selectivity is often associated to the presence of the bulky substituents which are deleted when defining the model system. 

The proper treatment of the electronic subtleties at the metal center is not the only challenge for computational modeling of homogeneous catalysis. So far in this chapter we have focused exclusively in the energy variation of the catalyst/substrate complex throughout the catalytic cycle. This would be an exact model of reality if reactions were carried out in gas phase and at 0 K. Since this is conspicously not the common case, there is a whole area of improvement consisting in introducing environment and temperature effects. 

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