Phosphine-based systems

The different ligand properties compared to phosphine ligands can be seen when comparing the NHC complexes with known analogous phosphine chelators. The differences become obvious from the coordination polyhe-dra and the electronic properties. The TIMEN hgands lead to high spin carbonyl complexes while the phosphine-based systems form diamagnetic complexes. 

Although most of the structurally characterized gold clusters are phosphine-based systems, in recent years some studies have focused on the synthesis of gold clusters with other stabilizers coexisting with phosphines such as thiols, arsines and boranes. Certain heteroleptic gold clusters stabilized with these ligands and arylphosphines in the same molecule have been structurally characterized. 

It is important to note that no reaction was observed in the absence of the carbene salt and that as for the phosphine-based systems, the reaction proceeded with overall retention of stereochemistry. The use of NHC-imine ligands in asymmetric allylic alkylation reaction has also been reported [20]. Excellent yields but low ee were obtained. The best example is shown in Scheme 3. 

This mixture of diastereomers of 27 was tested in the catalytic hydrogenation of dimethylitaconate and methyl Z-acetamidoacrylate (Scheme 18). The catalytic activity of these complexes is relatively high although lower than that of some of the previously studied purely phosphine based systems, while the enantioselectivity proved to be excellent (ee s between 98 and 99% under optimized conditions). 

In 2008, Bhanage and co-workers reported two examples of a phosphine-free version of the direct C2-arylation of N-methylindoles using Pd (TMHD)2 as the catalyst (Scheme 12,08TL1045). The authors noted that Pd (TMHD)2 is very stable and not air-sensitive, in this respect, offering advantages over the phosphine-based systems. 

Even though aminophosphine- and phosphite-based pincer complexes, as well as other systems such as SCS-based pincer-type Heck catalysts, were shown to serve as stable and clean sources of palladium nanoparticles in Heck reactions [24a, 25], and also xylene-derived phosphine-based systems were found to decompose under Heck reaction conditions in the presence of organic bases and hence palladium nanoparticles generally are considered to be the catalytically active form of palladium pincer Heck catalysts [27], catalytic cycles with the involvement of Pd intermediates could not have been excluded to be operative in pincer-catalyzed versions of the Heck reaction. In contrast, experimental observations, such as halide exchange reactions... 

In a subsequent report, the CAAC-based metathesis catalysts were examined for selectivity in the formation of Z E olefins, as well as their activity for ethenolysis [12]. Both of these processes require kinetic selectivity to produce the thermodynamically less-favored Z and terminal olefins, respectively. It was discovered that the CAAC catalysts displayed improved conversion to the Z olefin EIZ= 1.5—2.5 after 70% conversion) for the cross metathesis of cis-l,4-diacetoxy-2-butene with allylbenzene, relative to that observed using the classical NHC- and phosphine-based systems ElZ = 3-4) at comparable conversion (Scheme 4.3). 

In 2001, Mori and coworkers reported a nickel-NHC-catalyzed system for the reductive coupling of aldehydes and 1,3-dienes that had distinct properties from a nickel-phosphine based system. Both systems generated the homoallylic silyl ether however, use of phosphines resulted in selectivity, while using an NHC reversed selectivity and resulted in the Z isomer (Figure 13.42) [94]. 

Another family of diphosphines displaying a // 7/7i--coordination mode, SPANphos 63a-d, has also been tested in this reaction.Surprisingly, even if the corresponding frans-[Rhl(CO)63] render systems as active as the Siiss-Fink analogs, when tested in an elementary step, they show no reaction in the oxidative addition of Mel (vide supra). The observed catalytic activity has been attributed to dinuclear species formed under catalytic conditions. The dinuclear compounds [Rh2(/x-Cl)2(CO)263a], 87, react with Mel at 25 °G with a kx value of ca. 0.025 s M and currently they represent the fastest phosphine-based systems reported for methanol carbonylation. According to spectroscopic and GC-MS analysis, the products of the Mel oxidative addition are dinuclear monoacetyl derivatives 88 (Scheme 11). 

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