Tandem, dual-catalytic systems

Alkane Metathesis by Tandem, Dual-Catalytic Systems... 

Tandem, Dual Catalytic System Using Ir-Pincer Ligands and Mo-Alkylidene Complexes... 

The deactivation of this tandem, dual-catalytic system has been investigated. It was found that the low TONs obtained under these reaction conditions were mainly the result of the decomposition of the olefin metathesis catalyst. It was found that adding an additional amount of olefin metathesis catalyst at the end of a catalytic run reinitiated the process [18]. Therefore, to improve the catalytic efficiency, olefin metathesis catalysts as robust as the Ir-pincer dehydrogenation catalyst and more active at temperatures above 125 °C needed to be developed. A collaboration with Schrock and coworkers allowed for the synthesis of 40 different... 

Chapter 5. For this investigation, two approaches were studied (i) tandem alkane metathesis systems with either partial catalyst immobilization or both catalysts on the heterogeneous phase and (ii) a tandem, dual-catalytic system in which both catalysts independently operate at different reaction temperatures. 

In this section, the tandem system comprising of two catalysts for the alkane metathesis reaction will be discussed one catalyst for the dehydrogena-tion/hydrogenation steps and another one for the olefin metathesis step. In contrast to the multifunctional, single-catalyst systems, there are only a few reports that describe tandem, dual-catalyst systems for the alkane metathesis reaction [100]. Herein, the Chevron system will be discussed [8], followed by the recent development of another dual-catalytic system developed by Goldman and Brookhart [18]. 

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]. 

The discovery of the (de)hydrogenation steps using transition metals, in particular, the iridium pincer complexes, was essential to the success of this tandem, dualalkane metathesis reaction. As such, efforts made to improve these Ir catalytic systems will first be discussed. Subsequently, the application of these catalysts to the tandem, dual-alkane metathesis reaction will be elaborated. 

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