Ligands diphosphanes

Chiral Bidentate and Multidentate Phosphorus Ligands (Diphosphanes and Polyphosphanes, Diphosphites, Diphosphinites, and Diphosphonites). After testing a number of monodentate phosphanes, a real breakthrough came in the area of enantioselective hydrogenation when various types of chiral diphosphanes were applied. The first two diphosphanes employed were DIPAMP and DIOP (see later) introduced by Knowles and Kagan, respectively. Their achievements stimulated research on a variety of bidentate chiral diphosphanes. 

Ruthenium hydride complexes, e.g., the dimer 34, have been used by Hofmann et al. for the preparation of ruthenium carbene complexes [19]. Reaction of 34 with two equivalents of propargyl chloride 35 gives carbene complex 36 with a chelating diphosphane ligand (Eq. 3). Complex 36 is a remarkable example because its phosphine ligands are, in contrast to the other ruthenium carbene complexes described so far, arranged in a fixed cis stereochemistry. Although 36 was found to be less active than conventional metathesis catalysts, it catalyzes the ROMP of norbornene or cyclopentene. 

Asymmetric synthesis of tricyclic nitro ergoline synthon (up to 70% ee) is accomplished by intramolecular cyclization of nitro compound Pd(0)-catalyzed complexes with classical C2 symmetry diphosphanes.94 Palladium complexes of 4,5-dihydrooxazoles are better chiral ligands to promote asymmetric allylic alkylation than classical catalysts. For example, allylic substitution with nitromethane gives enantioselectivity exceeding 99% ee (Eq. 5.62).95 Phosphi-noxazolines can induce very high enatioselectivity in other transition metal-catalyzed reactions.96 Diastereo- and enantioselective allylation of substituted nitroalkanes has also been reported.9513... 

Very large organogold rings (16), as well as the related dimeric [2]catenane, have been generated by the self-assembly of complex digold(I) diacetylide with a diphosphane ligand <00AG(E)3819>. 

A series of non-f, -symmetrical ferrocene-based 1,5-diphosphane ligands (TaniaPhos) has been developed by Knochel.88,88a,88b The ligands have been effectively used in Rh- or Ru-catalyzed asymmetric hydrogenations. The ligand 39, which has an MeO group at the chiral carbon center, has shown excellent applications in the hydrogenation of several olefin and ketone substrates.89 Weissensteiner and Spindler have reported a series of structurally different... 

Scheme 5.4 Three processes in which nickel diphosphane complexes can be involved under catalytic hydrogenation conditions. (A) ligand exchange (B) oxidation and (C) hydrogenation of 1-octene. For simplicity, the reac-...

Silica-supported iron materials catalyze oxidation reactions [152]. Fe(II) silsesqui-oxanes have been reported [153, 154]. On the complex based on the bisilanol 2a and diphenyl phosphinomethane, dppm [154], possible structural similarities with iron-zeolites where identified after loss of the diphosphane ligand by addition of water. The Fe-POSS were tested in the catalytic oxidation of benzene with N2O with little success [154]. 

Ruthenium(II) complexes bearing atropisomeric diphosphine ligands have proved to be efficient systems for the hydrogenation of a wide range of prochiral substrates. A new catalytic system has been developed based on ruthenium complexes having SYNPHOS and DIFLUORPHOS as chiral diphosphanes (Figure 3.6). 

A highly informative review over the initial problems associated with the use of diphosphane ligands in homogeneous catalyst P. W. N. M. van Leeuwen, P. C. 

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