Chelating diphosphines

In the skeleton of many chelating diphosphines, the phosphorus atoms bear two aryl substituents, not least because the traditional route to this class of compounds involves the nucleophilic substitution with alkali metal diarylphosphides of enantiopure ditosylates derived from optically active natural precursors, approach which is inapplicable to the preparation of P-alkylated analogs. The correct orientation of these aryl substituents in the coordination sphere has been identified as a stereo chemically important feature contributing to the recognition ability of the metal complex [11,18-20]. 

Interest has developed recently in cyclopentadienylcobalt carbonyl complexes. Oxidative addition of RI (R = Me or Et) to CpCo(CO)L (L = PPh3, PMe2Ph, or PMePh2) yields initially CpCo(CO)RL I, which then undergoes rapid CO insertion to CpCoL(COR) . The slow step has been studied kinetically 148). Compounds of the type CpCo(CO)(Rp)I afford ionic substitution products when treated with chelating diphosphines 106). 

The proposed reaction mechanism involves intermolecular nucleophilic addition of the amido ligand to the olefin to produce a zwitterionic intermediate, followed by proton transfer to form a new copper amido complex. Reaction with additional amine (presnmably via coordination to Cn) yields the hydroamination prodnct and regenerates the original copper catalyst (Scheme 2.15). In addition to the NHC complexes 94 and 95, copper amido complexes with the chelating diphosphine l,2-bis-(di-tert-bntylphosphino)-ethane also catalyse the reaction [81, 82]. 

Many procedures use Pb(OAc)2 or other Pd(II) salts as catalysts with the catalytic-ally active Pd(0) being generated in situ. The reactions are usually carried out in the presence of a phosphine ligand, with Lris-o-tolylphosphine being preferred in many cases. 7ra-(2-furyl)phosphine (tfp) is also used frequently. Several chelating diphosphines, shown below with their common abbreviations, are also effective. Phosphites are also good ligands.130... 

The strong backbonding from the chelated diphosphine Pd(0) metal center to the electron-poor exocylic C=C bond of the QM moieties results in remarkable stability of the complex, with the QM ligand remaining unaffected in water or alcohol, even upon heating. 

Use of the above conditions in conjunction with the enol tosylate 32, provided only low yields of 22, prompting an extensive screening of structurally diverse phosphine ligands/solvents and palladium sources to attempt to define suitable conditions. Quite quickly a number of conditions were found to be effective, with chelating diphosphines being superior to monodentate phosphines (Table 9.7). In... 

Diphosphinoamines (X2P-NR-PX2) are examples of a more unusual potentially chelating diphosphine. The dimer Co2(F2PN(CH3)PF2)3(CO)2 has three of these ligands bridging symmetrically between the two tetrahedral Co centers, which also carry a terminal CO group this molecule can undergo reduction with LiEt3BH in THF to form a stable mixed valence (d9—d10) radical anion.163... 

Dinuclear complexes with bridging phosphido, hydrido, and diphosphine ligands were formed via some interesting transformations, such as P—C bond formation, P—H bond activation, and conversion of a chelate diphosphine to one bridging two metal centers.259... 

Independently, triangulo-Hg3 compounds with stabilizing chelating diphosphine and diarsine ligands have been prepared, 82-387 crystal-structure analyses have been solved (the Hg—Hg bonds, 280 pm, are longer than before), and careful multinuclear NMR spectra measured and analysed. 

Phosphorus ligands are crucial for the stabilization of the systems and the complex cis-[PtCl2(PPh3)2] (16) is most often employed, but complexes with chelating diphosphines also have been studied extensively. The stability of the related alkyl- and acylplatinum(II) complexes has favored extensive mechanistic investigations based on studies of the reactivity of model complexes. 

The hydroformylation of styrene using rhodium systems containing the four structurally related diphosphines dppe, dppp, (86), and (87) has been studied. A systematic analysis of the effect of the pressure, temperature, and the ligand metal molar ratio shows that the five- and six-membered ring chelating diphosphines behave differently from one another.347 An analysis of the effect of pressure, temperature, and ligand metal molar ratio on the selectivity of styrene hydroformylation catalyzed... 

A water-soluble chelating diphosphine ligand (9) based on the xanthene backbone was also studied as supported aqueous phase catalysts. It was shown that this ligand performed well as SAPC since it is much more selective than other SAPC systems reported in literature [68]. Recycling experiments showed that these catalysts retained their activity and selectivity for at least ten consecutive runs, whereas under similar conditions the TPPTS based catalyst showed a reduced performance in the fourth run. 

The enantioselective hydrogenation of prochirai heteroaromatics is of major relevance for the synthesis of biologically active compounds, some of which are difficult to access via stereoselective organic synthesis [4], This is the case for substituted N-heterocycles such as piperazines, pyridines, indoles, and quinoxa-lines. The hydrogenation of these substrates by supported metal particles generally leads to diastereoselective products [4], while molecular catalysts turn out to be more efficient in enantioselective processes. Rhodium and chiral chelating diphosphines constitute the ingredients of the vast majority of the known molecular catalysts. 

Josiphos-rhodium systems have been also used to hydrogenate 2- or 3-substi-tuted pyridines and furans, yet both the activities (TOF = 1-2) and enantioselec-tivities were rather low (Scheme 16.22) [86, 87]. Comparable results were obtained with a number of chiral chelating diphosphines of various symmetries. 

Excellent ee-values (up to 94%) have been obtained for the hydrogenation of various 2-substituted N-acetyl indoles with an in-situ prepared rhodium catalyst modified with the trans-chelating diphosphine (S,S)-(R,R)-2,2"-bis[l-(diphenyl-phosphino)ethyl]-1,1"-biferrocene (Scheme 16.23) [88]. A strong base was required as co-reagent to observe both high conversion (TOFs of 50-100) and en-... 

Figure 6-3. Mechanism for the hydrogenation of a prochiral substrate methyl acet-amidocinnamate (MAC) with a catalyst containing a chiral chelating diphosphine ligand...

Figure 1. Diastereomeric catalyst-substrate adducts produced upon addition of olefin to solvated catalyst with chelating diphosphine ligand.

Since our first model system is achiral, we did not need to consider different diastereomeric manifolds. However, we did need to follow four different reaction pathways corresponding to the four possible cis -dihydride isomers (Figure 5). Intermediates with traits phosphorus orientations were not considered because the catalysts of interest have chelating diphosphine ligands. 

---