Electron-rich diphosphine ligands

In 2000, these authors also developed a very efficient diphosphine-bithiophene ligand, tetraMe-BITIOP, which is depicted in Scheme 8.29. The ruthenium complex of this electron-rich diphosphine was used as the catalyst in asymmetric hydrogenation reactions of prostereogenic carbonyl functions of a-... 

Significant advance in the field of asymmetric catalysis was also achieved with the preparation of l,2-bis(phospholano)benzene (DuPHOS 4) and its confor-mationally flexible derivative (l,2-bis(phospholano)ethane, known as BPE) by Burk et al. [59]. Two main distinctive features embodied by these Hgands, as compared to other known chiral diphosphine ligands, are the electron-rich character of the phosphorus atoms on the one hand and the pseudo-chirality at phosphorus atoms, on the other. These properties are responsible for both the high activity of the corresponding metal complex and an enantioselection indepen-... 

The diphosphines 29-31 induced low asymmetries (8-15% ee) irrespective of the reaction conditions. The more electron-rich ligand (R, R)-32 gave (R)- 1-phenylethanol with a much-improved 65% ee (entry 1). Conversely, the application of the more electron-poor diphosphine (R, R)-33 resulted in the (A)-enantiomer, but with the same level of enantioselection (entry 2), highlighting the importance of the electron density of the phosphorus center on the... 

The success of Burk s alkyl diphosphines spurred development of a number of other ligands with electron rich phosphines, such as Zhang s PennPHOS (7) [36-38], Marinetti s /Pr-CnrPHOS (8) [39], and Imamoto s BisP ligands (9) [40],... 

Figure 15.8 a simple example is presented of a subsequent insertion of CO and methanolysis of the palladium acyl intermediate [14], This is not a very common reaction, because both the ligand requirements and the redox conditions for Wacker and carbonylation chemistry are not compatible. For insertion reactions one would use cis coordinating diphosphines or diimines, which makes the palladium centre more electron-rich and thus the nucleophilic attack in the Wacker part of the scheme will be slowed down. In addition, the oxidants present may lead to catalytic oxidation of carbon monoxide. 

In 1998, Hamann and Hartwig reported that electron-rich, ferrocene-based diphosphines such as 13 allowed for the coupling of cyclic amines with aryl chlorides [72,73]. The known ligand 13 proved to be most generally useful for this transformation, Eq. (42). The 13/Pd-catalyzed arylation reaction was performed with cyclic amines as well primary amines, however, no reactions with acyclic secondary amines were reported. 

P. Knochel and co-workers used diphosphines as ligands in the rhodium-catalyzed asymmetric hydroboration of styrene derivatives." The best results were obtained with the very electron rich diphosphane, and (S)-1-phenylethanol was obtained in 92% ee at -35 °C, with a regioselectivity greater than 99 1 (Markovnikoff product). A lower reaction temperature resulted in no reaction, while a higher temperature resulted in lower enantioselectivity and regioselectivity. The regioselectivity was excellent in all cases. Irrespective of the electronic nature of the substituents, their position and size had a profound effect on the enantioselectivity. 

Since the diphosphine is appreciably more electron-rich than is BINAP, the major ruthenium complex is a more active hydrogenation catalyst than the parent. Increased electron-rich ligation may be the reason for the success of heterocyclic analogues of BINAP in which the binaphthalene is replaced by a bi(ben-zothiophene) or biindolyl the resulting Ru complexes are effective both in terms of enantioselectivity and reactivity [139]. Readers of the related Chapter 6.1 on the asymmetric hydrogenation of carbonyl compounds will encounter the Ru complexes of ligands in the DUPHOS family, where the ease of modification of the alkyl substituents of the phospholane enhances the power of the system, since it permits the easy optimization of ee for any substrate [140]. 

Obligatory ancillary ligands Not needed Not needed Electron rich, bulky monophos- phine Chelating diphosphine... 

This chapter describes the preparation of the compounds depicted in Schemes 5.2 and 5.3 by the enantioselective deprotonation protocol. It has become an extremely important procedure to prepare some interesting families of ligands including many electron-rich bulky diphosphines, thus complementing the Juge Stephan method. 

The electron-rich t-Bu-MiniPHOS ligand provided good yield but moderate enantioselectivity (entry 1). Much better results have been obtained with the related ligand t-Bu-BisP (entry 2) and with the QuinoxP ligands (entries 3-5). The diphosphine of entries 6 and 7, containing an unsubstituted ethynyl group, leads to excellent yields and almost perfect enantioselectivities. 

In 1979, Kumada/Hayashi extended the scope of the palladium-catalyzed reaction using the diphosphine dppf to react alkyl Grignard reagents without any isomerization (Scheme 19.19) [24a, b]. Aryl bromides react at rt, which means that the dppf ligand accelerates the oxidative addition (electron-rich ligand) when compared to PPh. Remarkably, dppf also accelerates the reductive elimination cis and bulky ligand) and suppresses the p-hydride elimination. 

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