Electron-rich diphosphines

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-... 

This is an expedient synthetic route to 1,2- and 1,3-amino alcohols 22, which are important building blocks for the preparation of many natural and pharmaceutical products. Good results have been obtained with the electron-rich diphosphines DuanPhos and BINAPINE (Table 7.6). 

A somewhat different approach to these systems has been taken by the group of Gridnev and Imamoto. They have used more electron-rich diphosphines, such as Mef BujPfCHjjjPf BujMe, which should favor oxidative addition of Hj and the hydride route. They have used low temperature NMR to characterize the species present under close to stoichiometric conditions of substrate and catalyst. This methodology can yield interesting chemical information, but one also needs to establish the relevance of the observations to the catalytic conditions where the substrate is in large excess. The following Scheme outlines the dominant species observed in a study with the same substrate as in Scheme 5.30. 

Regioselectivity of the copper(I)-catalysed hydroboration of unsymmetrical internal alkynes r1C=CR2 (R1 = Ar, COjR, amide, CHjOR, CH2NR2, CH2CH2OR R = alkyl, SiMe3) has been shown to be controlled by the choice of the catalytic species (copper hydride or boryl copper). Thus, the reaction with PinBH (Pin = pinacol), catalysed by CuCl chelated to an electron-rich diphosphine, affords R C(BPin)=CHR in the presence of t-BuONa switching to pinjBj and electron-poor diphosphine results in the formation of the opposite isomer, in both cases with >99 1 regioselectivity. ... 

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 closely related experiments it was shown that sp C—H activation takes place reversibly within the coordinahon sphere of the electron-rich Ir(I)-diphosphine complex 58 (Scheme 6.9) to form an alkyl-amino-hydrido derivative 57 reminiscent of the CCM intermediate 24 the solid-state structure of 57 is shown in Figure 6.13 [40]. It appears that C—H activation only takes place after coordination of the amine function to the Ir(I) center (complex 58, NMR characterized). Amine coordination allows to break the chloro bridge of 59 and to augment the electron density of the metal center, thus favoring oxidative addihon of the C—H bond. Most importantly, the microscopic reverse of this C—H activation process (i.e. C—H reductive elimination) models the final step of the CCM cycle (see Scheme 6.1) indeed, the reaction of Scheme 6.10 is cleanly reversible at 373 K. 

The bis-indole diphosphine delivers the best ee and as the heterocyclic units are also electron rich aromatics this also gives the added advantage of the highest activity of the catalyst system. This overcomes one drawback often encountered, that high hydrogen pressures are frequently needed for ruthenium-based catalysts. 

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. 

Scheme 1 shows the desired Heck reaction of alkoxy-DSB 1 with 2. The formation of 3 is accompanied by two destructive pathways the reductive debromination of 1 to 4 as a side reaction and the protodesilylation to 5 as a subsequent reaction. Particularly the latter limits the reaction conditions in terms of time and temperature. The phosphine is a decisive factor in this system consisting of three reactions a fine-tuning of the reaction conditions is possible via electronic and steric effects of the substituents in the phosphine electron-rich trialkylphosphines 6 and 7 strongly favor the reduction. Fast coupling reactions were observed with tris-o-tolylphosphine 8, the chelating diphosphine dppe 9 being even more efficient in terms of turnover, yield, and suppression of side reactions. Compared with Heck reactions of polycyclic or electron-deficient arenes with 2 [21, 22], the yield of 3 is only moderate. The reactivity of bromo-distyrylbenzenes 1 and 12 -14 in the coupling reaction is controlled by the substituents on the opposite side of the n-system (Fig. 1, Table 2) a compensation for the electron-donating alkoxy groups by a cyanide (13) or exchange of donors with electronically neutral alkyl side chains strongly improves the yields.

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. 

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