PCP-pincer

Palladium complexes of inexpensive, easily synthesized bis(phosphinite) PCP -pincer ligands show good activity in the Suzuki coupling of deactivated and sterically hindered aryl bromides.256... 

Scheme 3 Syntheses of metal complexes of fluorous PCP pincer ligands 10-Rf ...

Fig. 6 Structure of the fluorous palladium PCP pincer complex lO-Rfs top, ORTEP representation middle, view with atoms at van der Waals radii bottom, packing diagram...

The iridium(l) PCP pincer complexes 1 exhibit remarkable activity in the catalytic dehydrogenation of unfunctionalized alkanes (Scheme 12.1). The H2, which is formally produced during this process, may be transferred to either tert-butyleth-ylene (TBE) or norbomene (NBE) as a sacrificial hydrogen acceptor. For example, complex la converts cyclooctane (COA) to cyclooctene (COE) in the presence of TBE, which in turn is reduced to tert-butylethane (TBA ueo-hexane) [6]. 

The high activity of iridium PCP pincer complexes in transfer dehydrogenation has been applied in a very elegant approach to devise the first homogeneous alkane metathesis process (Equation 12.5) [3]. 

Recently, Kaska and coworkers ]37] reported a rigid PCP pincer system based on an anthracene backbone (29) (Scheme 13.17). The iridium derivative of this... 

In spite of the success that Ir—PCP pincer compounds have enjoyed in the development of more efficient and promising alkane dehydrogenation systems, one caveat of using these complexes has been the sometimes difficult or tedious synthesis of the pincer ligands and complexes. An answer to this problem was... 

Since then, these ligands and their complexes have become increasingly important due to the fact that they exhibit the same characterishcs of robustness and thermal stability and, in most of cases, an enhanced reactivity compared to their phosphine counterparts. Thus, phosphinite PCP pincer complexes have been used intensely during the most recent era of catalytic processes and reactions involving the activation of aliphatic C—H bonds [40]. 

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.

Johansson et al. [48] were interested in improving this bimetallic system, and proposed the existence of PCP-pincer ligands (2,6-bis[(di-t-butylphosphino)methyl]... 

Subsequent to the first PCP-pincer study, Johansson used NMR spectroscopy to investigate the reactivity of the PCP-Pd complexes in transmetallation reactions with organotin derivatives. Somewhat surprisingly, only those palladium complexes with electron-withdrawing groups and less sterically demanding substituents could react efficiently with tributylallyltin [50]. 

Schwartsburd, E. Poverenov, L. J. W. Shimon, and D. Milstein, Organometallics, 2007, 26, 2931. The steric bulk of the PCP pincer ligand helps to stabilize the Pt-CO complex, and the electronic properties of the naphthyl group enhance the susceptibility of the CO group... 

Figure 13 Structure of an ir-PCP pincer complex and of a Cu-PyBox complex...

PCP Pincer Complexes as Catalysts for Novel Organic Transformations. University of Washington, 2/24/02. 

Recent Progress in the Development of PCP Pincer Complexes as Catalysts for Organic Transformations. 2000 International Chemical Congress of the Pacific Basin Societies, Honolulu, Hawaii, 12/17/01. 

Pincer (mer, tridentate) phosphines have proved resistant to degradation and useful in alkane dehydrogenation catalysis. Recently, Xu et al. [118] found that a dihydrido Ir complex containing a tridentate monoaifionic aryl bis(phosphino) (PCP) pincer, 31 (R = terf-butyl), is highly active catalyst for dehydrogenation... 

Figure 2.3 Computational energy profile for the oxidative addition of bromobenzene to a Pd PCP pincer complex. Energy is in kilocalories per mole.

Figure 3.17 Acceptorless dehydrogenation of alcohols by Gelman s Ir-PCP-pincer complex.

Figure 5.1 Diphosphine-derived nickel PCP-pincer complexes.

The anionic, tridentate piticer stracture suppresses the formation of an electrophilic it-aUylpalladium intermediate as established by Prof Szabo [42] in the PCP-pincer system. It is assumed that the nucleophilicity of the o-allylpalladium is further enhanced by strong trans influence and electron-donating nature of the Si atom, reahzing a facile nucleophilic addition to less reactive COj. 

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