Synergistic catalytic cycles

Scheme 41.60 Proposed synergistic catalytic cycles for the KR of amines.

The synergistic effect described can also be used to increase the selectivity of a catalytic system. Hydroformylating norbomene instead of cyclohexene, catalytic cycles operate according to Scheme 2 [21]. In this case, besides the aldehyde, a lactone is formed in a side reaction. Adding the Ru species Ru3(CO)i2 to the system increases not only the norbomene conversion but also the selectivity of the system to aldehydes. For a fixed number of cobalt atoms, the norbomene conversion as well as the aldehyde selectivity of the system is plotted as a function of the quotient n Jnco in Figure 2. In this case, too, the synergistic effect is pronounced for the addition of small amounts of the bifunctional Ru species only. 

Since the last decades, chemists have described a huge variety of multi-catalytic systems and cooperative effects [6]. First of all, it has been shown that cooperative effects can appear by combining two catalytic functions within the same molecule (bifunctional catalysis) [7] or in two separate molecules (cooperative dual catalysis) [8,9], Both can participate to the same catalytic cycle by activating together the same substrate (double-activation catalysis) or its own substrate. The two catalytic centers can also activate simultaneously different substrates in two directly coupled catalytic reactions for giving a product (synergistic catalysis) [10]. Tandem reactions have been also described [11, 12]. In that case, the two catalytic centers operate consecutively in two independent catalytic cycles, the second catalytic cycle using the product of the first one as an intermediate and converts it as final product. The second catalytic function may also not interact with the substrates but contributes to the stability of the active metal center and acts as redox partner (restorative catalysis) [8]. 

MacMillan has reported examples of synergistic catalysis in which copper salts are used. Although these results were driven by ad hoc hypotheses, most of these transformations are related to a Cu(i)/Cu(m) catalytic cycle. In any case, the superior performances offered by copper(i) salts, compared to strong Lewis acids tested in the processes, is an indication that the Lewis acidity of the metal salt is not playing a decisive role in these transformations. The complexation of the enamine 7i-system with Cu(iii)-R is expected to lead to rjl-iminium organocopper species that, upon reductive elimination, will form a carbon-carbon bond and liberate the active Cu(i) catalyst. Hydrolysis of the resulting iminium will also release the imidazolidinone catalyst to complete the organocatalytic cycle as shown in Scheme 18.7. 

Cooperative (or Synergistic) [9-17] Catalysis. This is a process, in which two or more catalysts are present from the beginning of the reaction and share the same catalytic cycle. In this case, the desired reaction pathway is favored because of the narrowing of the HOMO-LUMO gap stemming from the individual activation of both intermediates. 

It is indeed often difficult to discern unambiguously between radical pathways and acid-induced trifluoromethylations. Great care has to be invested into the analysis of the analytical data to avoid misleading pitfalls. Indeed, as the example of the trifluoromethylation of thiols shows, both processes can, for instance, act in a synergistic fashion in a single catalytic cycle (Sala et al., unpublished results)... 

More interestingly, the same authors used their procedure for highly selective couplings of alkenes and aldehydes (Equation (10.32)). The catalytic system combined a strong a donor IPr and a strong tt acceptor P(OPh)3. This combination was proposed to produce a synergistic relationship by reducing the electron density at the coordinatively unsaturated Ni centre, which would accelerate the elimination step in the catalytic cycle. 

In the second example, a general method for the conversion of various 5-oxopentanals to substituted 8-lactones by the synergistic catalysis of samarium diiodide and 2-propanethiol has been demonstrated by Fang et al. [106] (Scheme 61). A series of 5-alkyl- and 5-phenyl-5-oxopentanals 283 were successfully converted to the corresponding 8-substituted-8-lactone 285 by the catalysis of SmV/PrSH (10-50 mol%) via the Tishchenko reaction. The reaction is believed to go through the transition state 284, and notably, no aldol or pinacol products were observed under the reaction conditions. In addition, the deliberate use of 2-propanethiol is beneficial to facilitate the catalytic cycle. 

The complex synergistic cycle of atomic oxygen transfer between the platinum group metal and ceria-zirconia promoter results in the catalytic nature of emissions control catalytic systems. 

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