Bidentate bisphosphine ligands

The monodentate ligand (R)-CAMP was soon after replaced by the bidentate bisphosphine ligand (Rp,Rp)-DIPAMP that increased enantioselectivity to 95% ee (Scheme 29.2). " " Application of asymmetric hydrogenation for the industrial synthesis of l-DOPA represents one of the rare examples of very fast transfer of basic knowledge into commercial application. 

Even though the outlined approach allowed the successful rationalisation of many experimentally observed shift/structure and shift/reactivity correlations, Leitner et al. have pointed out that such relations cannot be expected to be universally valid and require that structural variations are modest and avoid large simultaneous changes in parameters that may have opposite effects on metal chemical shifts.61 To overcome these drawbacks and establish a more rational interpretation of chemical shift trends, they used a combination of experimental and computational efforts to assess the importance of different electronic and structural factors on the metal chemical shifts of a series of rhodium complexes with bidentate chelating bisphosphine ligands. The basis of their approach is first the validation of experimentally observed metal shifts by... 

Bidentate N-heterocyclic carbene complexes of group 13 hydrides have been reported recently. Treatment of carbene (31) with two equivalents of [MH3(NMc3)] (M = Al, Ga, In) or LilnTLi (equation 4) resulted in the formation of the 1 2 adducts (32-34). The Al and Ga analogues are moderately thermally stable (dec. 122-124 and 168-171 °C) but less stable than compounds (21-24) and (26). In contrast, the indane adduct (34) decomposes at 10 12°C. The stmctures of (32) and (34) were determined and found to be isostructural. Both (32) and (34) are monomeric and the bidentate carbene ligand (31) bridges two MH3 fragments in a similar manner to bisphosphine complexes. 

The regioselectivity of hydroesterification of alkyl acrylates or aromatic olefins catalyzed by [PdCl2L2] complexes (L = phosphine ligand) could be largely controlled by variation of the ligands. Triphenylphosphine promotes preferential carboxylation to the branched isomer, whereas with bidentate bisphosphines the linear product is produced overwhelmingly [14]. 

Figure 11 Bite angle and natural bite angle of bidentate ligands, with representative examples of small (dppm) and large (Xantphos) bite angle bisphosphine ligands...

A novel bidentate P-ligand was prepared from the 3-diphenylphosphinoxido-l-phenyl-tetrahydrophosphinine oxide (54, Z = Ph) by double deoxygenation. Reaction of the bisphosphine (64) so obtained with dichlorodibenzonitrileplatinum (II) afforded cis chelate complex 65 (Scheme 29) [39],... 

Bisphosphines are oxidized to the monooxides, thus bidentate phosphorus ligands of mixed oxidation states are readily accessible. 

The increase in yield cannot be simply attributed to the bidentate nature of the phosphine ligand. A number of bisphosphine ligands were employed in the coupling of -hexylamine with 3,5-dimethylbromobenzene, and the results are summarized in Table 13.2 (38a). 

The large bite angle of this bisphosphine (131.F) relative to related bisphosphine ligands (e.g., Xantphos, 111.7 ) structurally inhibits its chelation in a bidentate fashion toward transition metals. Thus, limited synthetic uses have been found for DBFphos (10). 3... 

Palladium complexes with monocoordinated phosphine ligands were synthesized by mixing the corresponding phosphine dendrimer with [Pd(cod)MeCl], which afforded exclusively trans dendrimer(MePdCl) complexes (P/Pd = 2). Complexes in which the ligand is coordinated in a bidentate fashion were synthesized by reaction of the respective bisphosphine den-... 

One of the drawbacks of the use of bisphosphines is the elaborate syntheses necessary for their preparation. Many efforts have been directed towards the development of bis-phosphonites and bis-phosphites. However, surprisingly mono-dentate phosphinates, phosphates and phosphoramidates recently emerged as effective alternatives for bidentate phosphines (Fig. 3.29). This constitutes an important breakthrough in this area as these can be synthesized in one or two steps, and the cost of these ligands is an order of magnitude lower Monophos can be made in a single step from BINOL and HMPT. 

Simple chiral phosphines have already been mentioned (Section 3.1.3) and the macrocycle enantiomers are discussed below (Section 4.6). Current research in this area is concentrated on bidentate chiral phosphines, such as the ligands (24)-(27). Although their transition metal complexes are normally used for stereospecific synthesis, Whitmire and coworkers used the molybdenum complexes to resolve their racemic bisphosphines via flash chromatography. The phosphines were decomplexed by reductive cleavage at low temperatures (-78 °C) using sodium naphthalenide (Scheme 1). 

Most of the chiral phosphine ligands prepared so far and used for catalytic asymmetric reactions are the bisphosphines,5 which are expected to construct an effective chiral environment by bidentate coordination to metal they have been demonstrated to be effective for several types of asymmetric reactions. On the other hand, there exist transition metal-catalyzed reactions where the bisphosphine-metal complexes cannot be used because of their low catalytic activity and/or low selectivity... 

Studies suggest that the mechanism of AHRs is quite similar to the achiral version, which includes the same fundamental steps in the catalytic cycles that we have seen already. The major difference is the presence of chiral bidentate ligands, such as bisphosphines [R2P-Y-PR2], phosphine-phosphites [R2P-Y-P(OR)2], bisphospites [(RO)2P-Y-P(OR)2], aminophosphines [R2N-Y-PR2], or diamines (such as (-)-sparteine), which we encountered earlier in Chapter 12. The bidentate variation of the cationic cycle, shown in Scheme 12.15b, seems to explain most aspects of the mechanism of AHR when X = OTf or I (in the presence of halide scavengers). Throughout the cycle, the ligand remains bidentate, and this factor seems to enhance enantioselectivity. The neutral cycle variant, which ought to be... 

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