Diphosphines as ligands

Sanger [34] studied the effect that the addition of common diphosphines exerts on catalyst 1 (2 mM in benzene) at room temperature and 1 bar. While the amount of ligand added strongly influenced the rate, the l b ratio of the aldehyde product ranged fi om 3.2 to 3.9 for aU diphosphines (dppe, dppp, dppb, DIOP). With the exception of dppb, which forms also dimeric species, it is expected that these diphosphines will form chelated intermediates of the structures 2ae and 3c, due to the small bite angle of the phosphines. The behavior in such mixtures of ligands (PPha and diphosphine) remains hard to disentangle. 

Pittman [37b] also studied the effect of added diphosphines (dppe, dppp, and dppb) to catalyst 1 (10 mM) using higher temperatures and pressures (60-120 °C, 7 bar) on the hydroformylation of 1-pentene. He found low I b ratios of only 1. He concluded that apparently a cis coordination as in 3c gives low I b ratios. Thus, under these standard conditions the I b ratio is much lower than that found by Sanger at ambient conditions. The presence ofPPhs in both sets of experiments would make an explanation speculative. 

Later studies have shown that dppp and other ligands containing a propane bridge smoothly form complexes 11a [18, 35]. It was noted by Brown that dppe formed less clean solutions. The small bite angle of 84° is 

DIOP was first used as a bidentate hgand added to 1 as early as 1973 by ConsigUo while aiming at asymmetric catalysis. For 1 -aUcenes they observed high I b ratios of 13 at 25 °C and 1 bar [37a]. 

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More successful attempts at asymmetric hydroformylation have involved rhodium and platinum complexes. As in asymmetric hydrogenation, best results have been obtained with optically active chelating diphosphines as ligands, but some studies of monophosphines have been made. Using... 

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. 

The general synthetic scheme outlined in Fig. 2.1 allows the facile preparation of a series of chiral catalysts using commercial chiral diphosphines as ligands. The use of complexes of the type [(P-P)Pt(C6F5)(H2O)]OTf represents the most versatile, active. 

As a conclusion, the use of diphosphines as ligands instead of the COD ligand does not produces stable rhodium catalysts under hydroformylation conditions. 

Pignolet and coworkers [8] found that, compared to RhCl(CO)(PPh3)2, chelating diphosphines as ligands for rhodium accelerated the decarbonylation by two orders of magnitude and allowed even a clean reaction at such temperatures, which are typical also for hydroformylation reactions (Scheme 8.4). Maximum TOF (turnover frequency) of 225 h at 50% conversion was calculated with [Rh(dppp)2]BF4 at 115 C. The catalyst remained stable for several days. 

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-... 

Finally, Sinou el al. have employed a chiral tetrasulfonated diphosphine as a ligand in the Rh-catalysed reduction of the Schilf base depicted in Scheme 8.34. In the presence of an aqueous-organic two-phase solvent system, a quantitative yield combined with a moderate enantioselectivity of 34% ee were obtained for this reaction. 

Mono and binuclear platinum(II) complexes with diphosphines have been reported as catalysts in the hydroformylation reaction. Dppp and related diphosphines are used as ligands in platinum/Sn systems for the hydroformylation of different substrates.99-107... 

CHIRAPHOS (86), bdpp (87), DIOP (85), deguphos (117), and related chiral diphosphines have been used as ligands in asymmetric hydroformylation of styrene and related substrates.255 347-349... 

A broad screening of ligands and ionic liquids was carried out by Feng et al. [104]. For rhodium-catalyzed hydrogenation of enamides the best catalysts were found to be the rhodium-ferrocenyl-diphosphine complexes with taniaphos, josiphos, walphos and mandyphos as ligands (Fig. 41.9). 

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