Single catalytic cycles

In transition metal-catalyzed domino reactions, more than one catalyst is often employed. In Tietze s definition and the classification of domino reactions, no distinction has been made between transformations where only one or more transition metal catalyst is used for the different steps, provided that they take place in a chronologically distinct order. Poli and coworkers [13] differentiated between these processes by calling them pure-domino reactions (which consisted of a single catalytic cycle driven by a single catalytic system) or pseudo-domino reactions . The latter type was subdivided into ... 

Sauna, Z. E., Ambudkar, S. V., Characterization of the catalytic cycle of ATP hydrolysis by human p-glycoprotein. The two ATP hydrolysis events in a single catalytic cycle are kinetically similar but affect different functional outcomes, J. Biol. Chem. 2001, 276, 11653-11661. 

As an extension of the Heck reaction, Pd-catalyzed hydroarylation of alkynes and alkenes continnes to attract high level of research interest in simple couphng processes and in cyclization reactions. The use of this type of transformation as part of a domino reaction will be of increasing interest. The research in the field of domino reactions is attracting considerable attention in synthetic organic chemistry since it enables the rapid assembly of complex molecirles in one-pot processes. Very elegant examples of palladium-catalyzed cascade processes where a single catalytic cycle entails several sequential bond transformations have been recently reported [la, b, 2a, b, c]. 

Recent mechanistic studies using HP infrared equipment, as well as HP-NMR measurements involving the use of CO and CH3I, have allowed the iridium intermediates which are present in solution as methyl acetate and water, and are consumed to produce acetic acid [.12, 34, 41-43], to be followed. All of these observations can be rationalized by a single catalytic cycle (see Figure 8.5), in which equilibria exist between the neutral and anionic complexes for all species. The main species involved in the carbonylation, which are detected in batch mode under carbonylation conditions [34], and correspond to the slower steps of catalysis, are the methyl—iridium and acetyl-iridium complexes [Ir(CH3)l3(CO)2] and [Ir(COCH3)l3(CO)2] respectively. 

General formula for single catalytic cycles Christiansen mathematics... 

As at the start of this chapter, we consider the single catalytic cycle... 

A general formula for single catalytic cycles with arbitrary number of members and arbitrary distribution of catalyst material has been derived by Christiansen. Unfortunately, the denominator of his rate equation for a cycle with k members contains k2 additive terms. Such a profusion makes it imperative to reduce complexity. If warranted, this can be done with the concept of relative abundance of catalyst-containing species or the approximations of a rate-controlling step, quasi-equilibrium steps, or irreversible steps, or combinations of these (the Bodenstein approximation of quasi-stationary states is already implicit in Christiansen s mathematics). In some fortunate instances, the rate equation reduces to a simple power law. 

Fagnou et al. were able to exploit these two complimentary reactivity modes within a single catalytic cycle to achieve a highly selective Ar-H/Ar-H crosscoupling (Scheme 32). 

In an alternative mechanism, the substrate molecule is again coordinated to tetrahedral Zn, with the coordinated water molecule now serving as a site for transient proton transfer, thereby generating a penta-coordin ated zinc intermediate. Formation of this intermediate during substrate turnover was supported by time-resolved freeze-quenched X-ray absorption fine spectroscopic analysis of the thermophilic bacterium Thermoanaerobaaer brockii alcohol dehydrogenase (TbADH). These results thus provided further evidence for the dynamic alteration of the Zn from a tetrahedral to a penta-coordinated form with detection of two new penta-coordinated intermediate states these included the water molecule in the zinc coordination sphere during a single catalytic cycle. 

Catalysis by transition metals can often be represented by reaction mechanisms which correspond to the case of single catalytic cycles. Only one route exits but it could contain several steps. If the reaction mechanism can be simplified to only one intermediate besides the free catalyst form (the most abundunt surface intermediate or catalytic species) while other intermediates even if present are in inferior quantities, then the mechanism corresponds to the two-step sequence described in detail in section 4.3. As an example heterolytic oxidation can be considered (Figure 5.9). The catalyst can exist in two forms with different oxidation states and the catalytic cycle consists effectively of oxidation-reduction steps. The reaction rate in such a case is described by the kinetic equation (4.88). 

We need to study a system in which the metathesis products do not themselves metathesize, so that we can be sure that we are seeing the initial products. Perhaps the best example is shown in Eq. 11.54, in which labeled 11.19 is converted into ethylene and phenanthrene, neither of which metathesize further with the particular Mo catalyst chosen. The initial isotope distributions in the products will then truly reflect a single catalytic cycle. The results of this reverse double cross showed a 1 2 1 mixture of the d , d, and... 

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)... 

The ground spin state of chromium (acetylene) adducts is known to be of quintet, and the most plausible reaction pathway on quintet surface needs to overcome two activation barriers to finish a single catalytic cycle, as depicted in Scheme 3.4. However, the reaction on quintet surface is prohibited by presenting a ffee-energy barrier of 31.1 kcal/mol that transforms two coordinated acetylene into the key intermediate 4D. Thus, the turnover of frequency (TOP) for the catalytic cycle on the quintet surface is 1.36 X 10 h, which rules out the quintet reaction mechanism for acetylene cyclotrimerization by Cr(II)/Si02 model catalyst. 

---