By Hélène Pellissier
The objective of this ebook is to teach the excessive strength of chiral sulfur-containing ligands to advertise various uneven catalytic variations. the real variety of reviews seemed within the literature during the last 35 years usually highlighted fantastic ends up in phrases of potency and enantioselectivity, permitting entry to many biologically vital molecules, which essentially demonstrates that those ligands can now be acknowledged as genuine opponents to the extra ordinary phosphorus- or nitrogen-containing ligands. A key aspect of reference for post-graduate scholars, researchers and lecturers.
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Extra info for Chiral Sulfur Ligands: Asymmetric Catalysis (RSC Catalysis Series)
Example text
On the other hand, a novel chiral xylofuranose-based phosphinooxathiane ligand was found by Nakano et al. 50 This ligand was easily prepared from commercially available 1,2-O-isopropylidene-D-xylofuranose in five steps. In order to explain the absolute configuration of the product, these authors have proposed a nucleophilic attack occurring at the allyl terminus trans to the phosphorus atom, which was the better p-acceptor. 17 Furanoside thioether-phosphinite ligand for Pd-catalysed allylic substitutions of acyclic and cyclic allylic substrates.
They proposed a possible mechanism for the chirality transfer, controlled by the steric environment of the chiral imine-sulfur chelate ligand with the reaction occurring trans to the imine donor. 74 In 2005, Kunieda et al. 75 The first successful use of simple 1,2-aminothioethers as hybrid ligands in the test reaction was reported by Bulman Page et al. 39), whereas poor enantioselectivities were obtained when the sulfur atom was substituted with an aromatic group. These authors explained these results by steric and not electronic considerations.
Due to the fact that the steric effect near the sulfur atom was less important than near the nitrogen atom, the enantioselection was much weaker. In 2001, a new chiral bis(benzothiazines) ligand was reported by Harmata et al. 101 The authors studied, in particular, different reaction conditions to perform this reaction and proved that good yields and enantioselectivities were obtained in relatively nonpolar solvents, whereas the reaction remained racemic in dichloromethane S O R R = CH2i-Pr: 87% ee = 16% (S) R = i-Pr: 89% ee = 56% (S) R = Bn: 84% ee = 14% (R) R = Ph: 86% ee = 19% (S) O R R = CH2i-Pr: 88% ee = 74% (R) R = i-Pr: 86% ee = 75% (R) R = Bn: 83% ee = 73% (R) R = Ph: 90% ee = 86% (R) Test reaction with bis(thiazolines) ligands bridged by a dibenzo[a,c]cycloheptadiene backbone.