Bidentate diphosphine

Coordination of an additional metallic fragment to a free donor atom of a bidentate diphosphine, dppm [172, 175], (PPh2)2C=CH2 (vinylidenebis(diphenylpho-... 

Even bidentate diphosphines do not act as catalyst poisons. For example, Pt(0) catalyzed smooth addition of ethyl acrylate to 1,2-diphosphinobenzene (Scheme 5-12, Eq. 1) no reaction took place in the absence of Pt(0), and radical initiators gave a mixture of products. 

For ruthenium, special precursors are required to synthesize defined bidentate diphosphine complexes. With Taniaphos for instance, it is possible to synthesize such complexes starting from unusual rathenium(ll) species. The complexes were characterized by NMR and single crystal analysis. 

Three generations of dendritic phosphines have been prepared from 3,5-diaminobenzoylglycine and 9-fluorenylmethoxycarbonyl-L-phenylalanine. The dendrimers were then attached to MBHA resin, treated with CH20 and Ph2PH, and converted to their Rh complexes. The polymer-supported complexes are excellent catalysts for the hydroformylation of alkenes, which could be recycled.283 The bidentate diphosphine A,A-bis-(P-(phosphabicyclo[3.3.1] nonan) methyl)aniline was prepared by phosphanomethylation of aniline. It forms a Rh-complex which is a highly regioselective catalyst in the hydroformylation of citronellene.284... 

Nonetheless, among bidentate diphosphines and with the notable exception of BINAP 23, there have been only sporadic examples of ligands whose rhodium complexes give enantioselectivities above 85% in hydroboration Knochel s dicyclohexylphosphine 34,80 Togni s Josiphos 41,85 and TADDOL derivatives 48, 50-52.90 Even... 

In order to keep the mild conditions, hydroxycarbonylation has been performed in biphasic media, maintaining the catalyst in the aqueous phase thanks to water-soluble mono- or diphosphine ligands. In the presence of the sodium salt of trisulfonated triphenylphosphine (TPPTS), palladium was shown to carbonylate efficiently acrylic ester [19], propene and light alkenes [20,21] in acidic media. For heavy alkenes the reduced activity due to the mass transfer problems between the aqueous and organic phases can be overcome by introducing an inverse phase transfer agent, and particularly dimeihyl-/-i-cyclodextrin [22,23]. Moreover, a dicationic palladium center coordinated by the bidentate diphosphine ligand 2,7-bis(sulfonato)xantphos (Fig. 2) catalyzes, in the presence of tolylsulfonic acid for stability reasons, the hydroxycarbonylation of ethylene, propene and styrene and provides a ca. 0.34 0.66 molar ratio for the two linear and branched acids [24],... 

The ( -C6H5PMe2)V complex is coordinated to the CpMn(CO) fragment as a bidentate diphosphine ligand and its reduction becomes irreversible with E- c = —2.68 V. On the other hand, the V-based oxidation is hardly affected, shifting 30 mV to values more negative. 

Bidentate diphosphines, such as bis(diphenylphosphino)methane (dppm) [242— 244], l,2-bis(diphenylphosphino)ethane (dppe) [245], l,2-bis(hydroxymethylpho-sphinojethane (hmpe) [246], l,2-bis(dicydohexyphosphine)ethane (dcype) [247] or bis(diphenylphosphino)isopropane (dppip) [248] build structures of stoichiometry [Au2(P P)2]2+ such as those shown in Figure 2.45. The crystal structure of [Au2(dmpe)2]X2-nH20 (dmpe = l,2-bis(dimethylphosphino)ethane X = Cl, n = 2 Br, = 1.5) consist of polymeric chains anion/water molecule/cation/anion/cat-ion/anion/water molecule [249]. 

Figure 2.46 Different geometries in gold complexes with bidentate diphosphines and halide ligands.

In the 1980s Drent and colleagues at Shell found a remarkable change in catalytic behaviour when monodentate phosphines were replaced with certain bidentate diphosphines. Cationic palladium(II) catalysts generated in methanol from Pd(OAc)2, PPhs and the acid of a weakly coordinating anion (e.g. tosylate, triflate) catalyzed the methoxycarbonylation of ethylene to... 

The three-coordinate intermediate in Equation 19 would then allow for greater mobility of the groups from which reductive elimination could proceed.(19) Rearrangement followed by a very rapid reductive elimination the cis isomer is also a possibility. However, our unsuccessful attempts to synthesize cis-aryl-methyl-nickel(ll) complexes still leave open the question of reductive elimination from such stereoisomers. The results we obtained in studies with the bidentate diphosphine ligand (dppe), though qualitatively in this direction, unfortunately lack definitiveness as yet. 

Figure 2 Structures of some silver(I) complexes with bridging bidentate diphosphines...

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