Coupled catalytic cycles

Allenylsilver compounds appear to enter the palladium coupling catalytic cycle via transmetallation. It was found that tert-butyl allenylsilver, and the corresponding argentate derivative, underwent palladium-catalyzed coupling with iodobenzene to... 

The major problem with using Wilkinson s catalyst is that it also constitutes an excellent hydrogenation catalyst [56]. Thus, alkynes and terminal alkenes are not tolerated under the conditions of the coupled catalytic cycles. This implies that radical cyclizations terminated by a CHAT cannot be carried out under these conditions. 

The use of hydrogen as terminal reductant has been accomplished by its activation with transition metal complexes. The resulting weak M-H bonds can be used in both radical generation and reduction through HAT. In this manner, conceptually novel radical chain reactions, such as hydrogen mediated cyclizations, or metal catalyzed processes with coupled catalytic cycles for radical generation and reduction, have been realized. The latter transformations are especially attractive for enantioselective synthesis. 

It is obvious that such equilibria would exist for all the other catalytic intermediates. The result of all this is coupled catalytic cycles and many simultaneous catalytic reactions. This is shown schematically in Fig. 5.5. The complicated rate expressions of hydroformylation reactions are due to the occurrence of many reactions at the same time. As indicated in Fig. 5.5, selectivity towards anti-Markovnikov product increases with more phosphinated intermediates, whereas more carbonylation shifts the selectivity towards Mar-kovnikov product. This is to be expected in view of the fact that a sterically crowded environment around the metal center favors anti-Markovnikov addition (see Section 5.2.2). 

Scheme of the coupled catalytic cycles for the asymmetric hydrogenation of MAC. 

The steady-state approximation indicates that the concentrations of reactive intermediates remain constant with time. In other words, the net rate of MAC binding to the catalyst to form an intermediate must be the same as the rate of hydrogenation (or disappearance) of the intermediate. The steady-state approximation for the coupled catalytic cycles is expressed mathematically as ... 

Schematic representation of two coupled catalytic cycles in the isomerization of n-pentane to i-pentane on a bifunctional catalyst consisting of platinum supported on acidic alumina...

Scheme 14.7 Pd-catalyzed allylic acetoxylation reaction featuring Pd-, quinone-, and metal macrocycle (LM)-coupled catalytic cycles.

Interactive mechanism for the Stille coupling catalytic cycle... 

Scheme 2 depicts a general cross-coupling catalytic cycle involving organic dihalides. Although the oxidative addition step is proposed to be reversible in some cross-coupling reactions [18], it is generally considered to be irreversible in many... 

Ruthenium compounds are widely used as catalysts for hydrogen transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. For example, BackvaU and coworkers [40] used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt-Schiff s base complex. The coupled catalytic cycle is shown in Figure 5.8. A low-valent ruthenium complex reacts with the alcohol to afford the aldehyde or ketone product and a ruthenium dihydride. The latter undergoes hydrogen transfer to the benzoquinone... 

Scheme 19 Suzuki cross-coupling catalytic cycle [96, 97]...

A similar compound with a bipyridyl central core was sought according to Scheme 3.42. In this manner, a greater degree of planarity could be achieved due to reduced interactions in the absence of 2- and 2 -steric interactions. To that end, 2-chloro-3-nitropyridine was homocoupled in the presence of copper/bronze and dimethylformamide. The bipyridine ring system was brominated at the 5-and 5 -position under harsh conditions (due to its electrophilicity) to afford intermediate 98 that was then coupled with two equivalents of TMSA. These coupling conditions unfortunately afforded the hydroxyamine and a very small amount of the dinitro-coupled product. The electron deficient 98 presumably underwent nitro loss and Pd-catalyzed reduction by the hydridopalladium species that are present in the coupling catalytic cycle to afford the undesired 99 (Scheme 3.42). ... 

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