Ligand aryl effect

Although chelating aryl diphosphine ligands are effective for hydrogenating dehydroamino acids with high stereoselectivity, minor changes in the composition of the substrate can drastically lower the enantiomeric excess. The best results are obtained for the following structures ... 

The ferrocenylbisphosphines 8f—h bearing the amino pendant side chain are unique ligands that effect the rhodium-catalyzed asymmetric hydrogenation of tetrasubstituted olefms 57 (Scheme 2-43) [61]. Thus, the hydrogenation of a-aryl-acrylic acid 57a in the presence of a cationic rhodium catalyst coordinated with 8g gives a quantitative yield of carboxylic acid 58a with 98.4% ee. Other tetra-... 

Copper(I) complexes catalyse a variety of organic reactions which are of synthetic and industrial importance.305 In such processes that involve halide abstraction from aryl or alkyl halides, the abstraction step by a Cu(I) catalyst is believed to be the rate-determining step. In order to circumvent the property of facile disproportionation of Cu, various methods of stabilising Cu(I) and influencing reaction rates were considered.306 A kinetics study of ligand (L) effects on the reactivity of Cu(I)L complexes towards C13CC02 was undertaken. The results indicated that the rate of the chlorine abstraction reaction was affected by several factors. These were the redox potential of the Cu(II/I)L couple, the hybridisation on Cu(I) in the Cu(I)L complex, steric hindrance, and electron density on the central Cu(I) cation at the binding site of the chlorine atom to be abstracted. The volume of activation,... 

Chan et al. have utilized hybrid binaphthyl amino alcohol for this transformation. The various ligauds prepared by them are listed in Scheme 21.14. AU these ligands were effective only in the case of aryl aldehydes in the presence of dimethyl zinc. The e.e. values varyied from 60 to 90%, and the chemical yields were found to be in the 90% range. 

As part of the studies of ligand effects on the reductive elimination of diaryl ethers, arylpalladium phenoxide complexes containing a DPPF analog that had P-CF3 substituents on each of the ligand aryl groups was prepared. This CF3-DPPF complex underwent reductive elimination of diaryl ether only twice as fast as the simple DPPF complex (Eq. 7). ... 

Historically, this distinction was first made by Heck on the basis of two different systems, one based on a simple palladium salt for aryl iodides [2] and another based on the PhsP or (2-tolyl)3P complex of palladium for aryl bromides [6-8]. However, as the performance of these initial systems was far from the levels achieved later, we cannot conclude for sure that a ligand-accelerating effect is observed in these protocols. We know today that aryl iodides and reactive aryl bromides are highly reactive practically with any form of labile palladium complex, so that the ligand-accelerating effect cannot be reliably established for these substrates in the many published protocols (type 1 and type 2 systems). 

Despite electron-rich bulky side-arms as in phosphine pincers 190,191 [245] or 192 [246] (Figure 2.24), these complexes behave strikingly different from their respective dialkyl or trialkylphosphine palladium complexes the latter complexes show t)q)e3 activtity (cf. Hartwig-Fu protocol see above). PCP-pincer complexes 190-192, however, are typical SRPCs exclusively suitable for type 1 reactions of aryl iodides and activated aryl bromides (Table 2.9, entries 1-6). Ligand-acceleration effects are not observed, which unequivocally underlines that the cleavage of these pincer complexes under the reacation conditions occurs to release nonphosphine palladium complexes with indeterminate coordination shell. 

Zou, G., Huang, W., Xiao, Y. and Tang, J. (2006) Heckreaction catalysed by palladium supported with an electron-rich benzimidazoly lidene generated in situ remarkable ligand electronic effects and controllable mono- and di-arylation. New J. Chem., 30, 803-9. 

A solvent-free variant used octahedral CU2O nanoparticles with added PPhj ligand, which effectively catalyzed the cross-coupling reactions of various aryl and heteroaryl halides with terminal alkynes [148]. The catalyst was reusable without any observable loss of activity, but worked only at temperatures above 100 °C. Further improvements to this approach involved the elimination of the Hgand and carrying out the reaction in a mixture of DMSO, K2CO3, and catalytic amounts of CU2O nanoparticles [149]. 

A palladium-catalyzed intramolecular cyclization of enolate O-aiylation and thio-enolate S-arylation was reported by Willis and co-workers in 2006. A catalyst generated from Pd2(dba)3 and the ligand DPEphos effects the key bond formation to deliver a variety of substituted products from both cyclic and acyclic precursors. The analogous thio-ketones undergo C-S bond formation using identical reaction conditions and were converted to ben-zothiophene products. A cascade sequence that produces the required a-atyl ketones in situ has also been developed (Scheme 2.6). 

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