Heck reaction Cationic cycle

In an oxidative addition, Pd(0) complex 22 with BINAP as a ligand accepts alkenyl triflate It. The resulting Pd complex 23 is cationic, since the triflate anion is bound only loosely to the palladium and dissociates from the complex.1 Syn insertion of one of the two enantiotopic double bonds of the cyclopentadienc into the alkenyl-Pd bond of complex 23 leads firs to q -allyl-Pd complex 24. This is in rapid equilibrium with t 3-allyl-Pd complex 25. Neither 24 nor 25 contains a p-H atom in a yn relationship to palladium. Moreover, internal rotation is impossible in the con form a-tionaily fixed ring system. For this reason there is no possibility of a subsequent p-hydride elimination that would once again release the palladium catalyst. In a normal Heck reaction (see discussion) the catalytic cycle would be broken at this point. 

Eberlin also studied the Heck reaction of aryltellurides [45] and Svennebring [19] reported the identification of three types of cationic, catalytic intermediates (Fig. 4A-C) in the microwave-assisted, phosphine-containing Heck arylation of electron rich olefins. In the latter case the authors support a Pd(0)/Pd(II) cycle as opposed to a Pd(II)/Pd(IV) cycle based on ESI-MS evidence for reduction of the palladium(II) precatalyst to Pd(0) by the ligand, and oxidative addition of the aryl substrate to a Pd(0) species. None of the expected Pd-bound olefin intermediates were observed however, this is often the case either because the olefin-bound species is neutral or because OA (oxidative addition) is the turnover-limiting step and the subsequent steps occur too quickly to be observed by the sampling method (in this case sampling included quenching and dilution of samples from a reaction vessel). 

The mechanism of the Heck reaction is not fully understood and the exact mechanistic pathway appears to vary subtly with changing reaction conditions. The scheme shows a simplified sequence of events beginning with the generation of the active Pd catalyst. The rate-determining step is the oxidative addition of Pd into the C-X bond. To account for various experimental observations, refined and more detailed catalytic cycles passing through anionic, cationic or neutral active species have been proposed. ... 

In this context, a functionalized ionic liquid, 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate [hemim][BF4], is reported as an efficient and recyclable reaction medium for the palladium catalyzed Heck reaction. The olefination of iodoarenes and bromoarenes with olefins generates the corresponding products in good to excellent yields under phosphine-ffee reaction conditions. After separation of the product, fresh starting materials are charged into the recovered ionic liquid which entraps the palladium catalyst. The reactions still proceed quantitatively for six cycles, without significant loss of catalytic activity. " The effect of both the cation and the anion on the chemical yield is shown in Figure 28. 

The key step in the catalytic cycle with regard to enantioselectivity is clearly B), association of the alkene 2 and insertion of it into the Pd-R bond. As with the Heck reaction itself, the mechanism for this process remains a matter for conjecture, with the overall rationale currently in favor having been proposed in 1991 by Ozawa and Hayashi [18] and independently by Cabri [19] (although the cationic pathway via 8 and 9 had been proposed as early as 1990 [20]). Its development and subsequent evolution has recently been reviewed by the latter author [ 12]. 

Scheme 10.10 Generally proposed catalytic cycle of the Heck reaction catalyzed by palladium pincer complexes involving Pd intermediates and cationic styrene complexes as resting state.

A detailed discussion of the current understanding of the mechanism of the Mizoroki-Heck reaction can be found in earUer chapters of this book and in several excellent reviews [7]. Two mechanistic pathways, typically termed neutral and cationic, have been proposed to account for the differences in reactivity and enantioselectivity observed in asymmetric Mizoroki-Heck cycUzations of unsaturated trillates and halides. These pathways differ in the degree of positive charge and the number of available coordination sites assignable to the palladium(II) intermediates of the catalytic cycle. Because catalytic asymmetric Mizoroki-Heck cyclizations are typically carried out with bidentate Ugands, these pathways will be illustrated with a chelating diphosphine Ugand. 

For the first time, therefore, several cationic intermediates of the oxidative addition step of the Heck reaction involving arene diazonium salts were detected and characterized by ESI-MS(/MS). A dynamic, time-dependent process with ligand equilibria between several ionic intermediates was observed for the oxidative addition step. The most reactive intermediate for olefin addition (the ion of m/z 488 in Figure 3.5) was also detected and characterized, and this species predominates after mixing the arene diazonium salt and [Pd2(dba)3] dba after about 90 min. Therefore, a novel protocol for the Heck reaction with a delay of 90 min for olefin addition was established for maximum yield. A detailed catalytic cycle for the Heck... 

A rhodium(l)-catalyzed system in THF is also effective in the Mizoroki-Heck-type reaction of arylsilanediols with acrylates (Scheme 4).53 Interestingly, the use of aqueous THF switches the reaction to 1,4-addition forming /3-arylated esters. The proposed catalytic cycles for these reactions involve 1,4-addition of an arylrhodium species to an acrylate. The change of the reaction pathway is probably because, in aqueous THF, the resultant Rh enolate 6 undergoes protonolysis rather than /3-elimination. Similar Rh-catalyzed 1,4-additions to a,/3-unsaturated carbonyl compounds have been achieved with arylsilicones,54 arylchlorosilanes,55 and aryltrialkoxysilanes.56,57 The use of a cationic Rh-binap complex leads to highly enantioselective 1,4-additions of alkenyl- and arylsilanes.58 583... 

This represents the first ESI-MS complete study of Matsuda-Heck arylation with aryldiazonium salts and allowed the authors to propose a detailed catalytic cycle for this reaction, outlined in Scheme 7.11. The main cationic intermediates of this catalytic cycle were detected and structurally characterized directly from the reaction mixture. 

The general catalytic cycle for the Heck-Matsuda reaction using acyclic olefins starts with the oxidative addition of the Pd(0) catalyst A to the aryldiazonium salt and sequential elimination of nitrogen to produce the cationic palladium species C. This intermediate is very electrophilic and it promptly... 

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