Heck pincer complex

Regarding bis-NHC chelating ligands, several structures that differ in the motifs used for the enlargement of the tether have been proposed as catalysts for the Mizoroki-Heck reaction. They range from non-functionalised aliphatic chains [23-25] to phenyl [26], biphenyl [27], binaphthyls [28] and to chains containing additional coordination positions like ethers [29], amines [30], and pyridines in an evolution towards pincer complexes [31-35], In most cases, the activity of aryl bromides in Mizoroki-Heck transformations was demonstrated to be from moderate to high, while the activation of chlorides was non-existent or poor (Scheme 6.7). 

Heterogenization of homogeneous metal complex catalysts represents one way to improve the total turnover number for expensive or toxic catalysts. Two case studies in catalyst immobilization are presented here. Immobilization of Pd(II) SCS and PCP pincer complexes for use in Heck coupling reactions does not lead to stable, recyclable catalysts, as all catalysis is shown to be associated with leached palladium species. In contrast, when immobilizing Co(II) salen complexes for kinetic resolutions of epoxides, immobilization can lead to enhanced catalytic properties, including improved reaction rates while still obtaining excellent enantioselectivity and catalyst recyclability. 

Figure 2 Immobilized PCP Pd(II) pincer complexes used in our Heck couplings. Other SCS and PCP pincers were also studied.

Immobilized Pd(II) pincer complexes were prepared as described previously (20-22). Heck couplings of iodoarenes and acrylates were carried out in DMF using tertiary amines as base and reaction kinetics were monitored as previously reported (20-22). 

Both complexes were used in the Heck reaction between activated aryl halides (Br, Cl) and styrene. The C,N,C pincer complex was by far more active than the C,C,C counterpart and produced a higher ratio of trani-stilbene with a significantly greater yield... 

A fluorous palladium pincer complex for use in Heck reactions was reported by Curran (Reaction Scheme 11). The tridentate ligand ensnred stability under the high temperatures (140 °C) needed for the reaction to go to completion. The complex was recovered and rensed three times without any sign of lowered catalytic activity. 

Research in our group has focused on two aspects of the catalytic behavior of palladium catalysts for Heck, Suzuki, and Sonogashira conpling reactions [38]. The first aspect is the study and development of active and recyclable immobilized palladium catalysts, specifically palladium pincer complexes (Fig. 20.8). The second aspect is the study of the true nature of the active catalytic species. 

Yu K, Sommer W, Week M, Jones CW (2004) Silica and polymer-tethered Pd-SCS-pincer complexes evidence for precatalyst decomposition to form soluble catalyhc species in Mizoroki-Heck chemistry. J Catal 226 101... 

Sommer W, Yu K, Sears JS, Ji Y, Zheng X, Davis RJ, Sherrill CD, Jones CW, Week M (2005) Inveshgahons into the stabihty of tethered paUadium(ll) pincer complexes during Heck catalysis. Organometalhcs 24 4351... 

Pincer complexes 2 are active catalysts for the Heck reaction. Interestingly, their catalytic activities are controlled by the central metal atom of the porphyrin nucleus. ... 

Also worth mentioning here are studies based around the preparation and use of silica-supported palladacyclic complexes. It was the use of these that gave valuable evidence for the decomposition of half-pincer and SCS pincer " complexes during Heck reactions, generating soluble Pd(0) species that are the true catalysts. 

Recently, the high activity of palladium/NHC complexes in the Heck reaction was combined with an efficient recyclability process [63]. Bis-carbene pincer complexes of palladium(II) were immobilized on montmorillonite K-10. The catalytic activity of the heterogeneous system is similar to that displayed by their homogeneous counterparts. The stability of the catalyst was tested in the reaction of phenyl iodide and styrene. The product yield decreases from 99 to 79% after ten cycles. 

Several studies have appeared that show beyond doubt that PCP and SCS pincer complexes also decompose during the Heck reaction and lead to the formation of colloidal palladium [121-124]. Evidence was based on immobilisation studies and on the application of the extensive Hg poisoning protocol developed by Finke, which proved the presence of palladium colloids [125]. 

Two hypothetical mechanisms have been proposed to explain the Heck reaction on the basis of Pd(II)/Pd(IV) cycles (Scheme 2.12). As discussed in Section 2.2.1, oxidative addition of aryl halides to Pd(II) precursors is both kinetically and thermodynamically difficult. The Pd(II)/Pd(IV) mechanism proposed by Shaw for the Heck reaction (Scheme 2.1) tried to elude this problem by postulating the intermediacy of anionic Pd(II) complexes with increased nucleophilicity, but it is not evident how this mechanism could be adapted to complexes containing PCP or related pincer ligands. With this problem in mind, Jensen [93] made an alternative proposal (Scheme 2.12a), which starts with the oxidative addition ofa C-H bond of the olefin to the Pd(II) pincer complex to afford a Pd(IV) vinyl-hydride intermediate. This idea was inspired by a similar reaction observed with an isostructural Ir(I) PCP complex, but such C-H bond activations are unusual in palladium chemistry. A theoretical analysis by Freeh [63] raled out such possibility, leading instead to the alternative Pd(II)/Pd(IV) cycle depicted in Scheme 2.12b. A key element... 

Scheme 2.12 Hypothetical Pd(ll)/Pd(IV) mechanisms for the Heck reaction catalyzed by pincer complexes, (a) Jensen s proposal and (b) Freeh s proposal.

Apart from intuitions based on experimental observations and support from computational work, the arguments in favor of Pd(II)/Pd(IV) mechanisms in the Heck reactions catalyzed by Pd pincer complexes are scarce. On the contrary, there is conclusive evidence indicating that in many cases the actual catalytic species results from the decomposition of pincer complexes [62, 76, 77, 97,100, 101, 103]. This conclusion can probably be extended to all systems that achieve exceptionally high TON numbers, such as 2 and 3, since the rate of the processes based on Pd(II)/Pd(IV) cycles would be always Hmited by the low reactivity of Pd(II) toward aryl halides. The observed influence of pincer ligands on the catalytic activity or the ability to catalyze difficult couplings (e.g., with aryl chlorides) can be rationalized on the basis of their ability to regulate the production of the actual catalytic species [11, 12, 96]. This, however, does not prevent the possibility that, in some specific cases, pincer complexes could act as true molecular catalysts for the Heck reaction or other closely related processes. In recent years, a couple of examples have been provided that demonstrate this possibility, as discussed below. 

Scheme 2.13 Mechanism of a Heck-type reaction involving Pd(ll) and Pd(IV) CNO pincer complexes.

The Role of Palladium Pincer Complexes in Heck Reactions 253... 

Even though various C-C cross-coupling reactions have been successfully catalyzed with palladium pincer complexes, often with excellent performances [22], their primarily application nowadays lies in the Heck reaction, of which the role of pincer-type catalysts in this reaction still is unclear and therefore under debate. 

Finally, it should be mentioned that two years later, an enantioselective oxidative (boron) Heck-type reaction was reported by Jung and coworkers [32] for a dinuclear NHC-derived CNO-based pincer complex for which for the first time a Pd /Pd" mechanism was suggested to be operative with palladium pincer-type crosscoupling catalysts. Reaction mechanisms without a change of the palladium s oxidation state have never been proposed to be operative for Heck cross-couphng reactions, but have been shown to be operative for xylene-derived selenium-based pincer complexes and related systems in the cross-couphng of vinyl epoxides (and aziridines) with organoboronic acids [24d, 33]. 

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