Bidentate phosphane ligands

If a bidentate phosphane ligand is coordinated to a transition metal, an additional chemical shift attribntable to the size of the resulting metallacycle can be observed for rings containing three, four, five, and six members. 

Fig. 4. (a) Most widely used bidentate phosphane ligands possessing C stereogenic centers) and (b) most widely used bidentate phosphane and phosphinite ligands possessing C stereogenic centers). 

Figure 5.7 P-chiral bidentate phosphane ligands synthesized by Imamoto and coworkers [31].

If the Rh-catalyst is stabilized by a monodentate phosphane ligand, it can be destroyed because the lifetime of the oxidation of phosphine to phosphine oxide is less than 1 h. Yet, if the catalyst is stabilized by a bidentate ligand, and the catalyst has a lifetime of 2-3 h, then it can convert into different species that have no catalytic activity. However, such a synthetic approach would suffer as a result of a short lifetime and a low TON. 

When the labilities are reversed, that is when the NHC group is used as the anchor, ligand design again orientates itself on the successful hemilabile bidentate phosphanes. Now, we encounter pendant ether [34,35], tertiary amino [36,37], pyrido [11,38 0], imino [41-44] and carbonyl groups [30,45] in the sidechain. 

An enantioselective version of the cyclopentaannulation via [3 + 2] cycloaddition has been developed using cyclopent-2-enone and several different methylenecyclopropanes. Whereas chiral phosphane ligands, such as menthyldiphenylphosphane (21), or the camphor-derived sultam 22 only result in enantiomeric excesses of 31% at a maximum in nickel(0)-catalyzed reactions, the enantioselectivity dramatically increases when the bidentate azaphospholene ligand 23 is employed. The yields, however, are relatively low due to the competing formation of alkylation products. ... 

In order to increase interactions between the incoming nucleophile and the ligand, a chiral functional group capable of coordinating to the carbanion has been tethered to a bidentate phosphane. The resulting enhanced steric repulsion between the enolate and the chiral phos-phane now allows efficient differentiation between the enantiotopic faces of the prostereogenic enolate in favorable cases. Furthermore, directing the nucleophile accelerates the allylation process. 

The additional stability gained because of the formation of a bidentate chelate ligand has been used to achieve imidazole to NHC tautomerization for a N-phosphane-functionalized imidazole in the presence of iridium and ruthenium " metal fragments (structure 22 in Fig. 15). An analogous activation occurred when 2-pyridylbenzimidazole is refluxed in THF with... 

Stereoselectivity issues were also analyzed with the help of lMOMM(BP86 Sybil) calculations by Aguado-Ullate et al. [58] in the case of the reaction of styrene where the rhodium catalyst is carrying an unsymmetric bidentate phosphane phosphite ligand such as BINAPHOS. The behavior of the [Rh((/ ,5)-BlNAPHOS (C0)2H] catalyst was studied. The placement of the phosphane moiety in the apical site and the phosphite moiety on the equatorial site was shown to be critical for high enantioselectivity. The axial chirality of the phosphite discriminated one of the competitive equatorial-apical paths, whereas the chirality of the backbone discriminated one of the two enantiomers. QM/MM calculations were also used in the definition of QSAR descriptors for hydroformylation by the same group [59]. 

Keywords Group 13 metals (aluminum, gallium, indium, thalhum), Ambidentate ligands. Phosphorus-nitrogen bidentate ligands, Pyridyl phosphanes, Aminoiminophosphoranes, Lewis acid catalysis... 

Following the general trend of this account, monodentate phosphinous amide ligands and bidentate AT-phosphino phosphinous amides or bis(amino-phosphanes) are included in the following discussion, but not other bidentate ligands bearing additional, different phosphorus functionalities, as for instance phosphinous amide-phosphane bidentate ligands. 

The introduction of a sulfonate group by reaction with oleum is not limited to arylphosphanes. Tris(co-pbenylalkyl)phosphanes, P[(CH2) (C6H5)]3 (n = 1, 2, 3, and 6), can be sulfonated in the para position and to a lesser extent in the meta position (18). The technique of sulfonating water-insoluble ligand precursors can be applied to bidentate, polydentate, and chiral phosphanes (20-24) the compounds 1-3 are presented in Scheme 2 as examples. 

Metal complex chemistry, homogeneous catalysis and phosphane chemistry have always been strongly connected, since phosphanes constitute one of the most important families of ligands. The catalytic addition of P(III)-H or P(IV)-H to unsaturated compounds (alkene, alkyne) offers an access to new phosphines with a good control of the regio- and stereoselectivity [98]. Hydrophosphination of terminal nonfunctional alkynes has already been reported with lanthanides [99, 100], or palladium and nickel catalysts [101]. Ruthenium catalysts have made possible the hydrophosphination of functional alkynes, thereby opening the way to the direct synthesis of bidentate ligands (Scheme 8.35) [102]. 

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. 

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