Phosphines BINAP

A fluorous version of the chiral bis-phosphine BINAP has been developed for asymmetric Heck reactions [5], Several fluorous-derivatized binaphthols and BINAP derivatives have been reported, (Scheme 10.4) [6], The silane spacer group present in one of the ligands was used to maximize the percentage fluorine on the molecule. Even so, the partition coefficient1 between FC-72 (see Chapter 3) and benzene was only 2.85, and not surprisingly, the reuse of the catalyst was poor. 

The initial step of the catalytic cycle is oxidative addition of aryl triflate to a BINAP-coordinated Pd(0) species. Since, in the actual catalytic system, Pd(OAc)2 and (/ )-BINAP are used as the precursors of the Pd(0) species, reduction of Pd(OAc)2 into the BINAP-coordinated Pd(0) species should be operative prior to the catalytic reaction. Although Pd(OAc)2 is the most commonly used precursor of a Pd(0) species in many palladium-catalyzed organic reactions, no direct information has been reported so far on its reduction process. In this study, we confirmed for the first time that the reduction proceeds according to the process involving a combination of tertiary phosphine (BINAP) and water as the reducing reagent (Scheme 8) (Ozawa, F. Kubo, A. Hayashi, T., submitted for publication). 

An amine can be coupled with an aryl bromide, iodide or triflate in the presence of a palladium catalyst, a base, typically KOBu or CSCO3, and a ligand such as the bidentate phosphine BINAP. These reactions are known as Buchwald or Buchwald-Hartwig reactions (Scheme 10.24). A catalytic cycle is again involved, with the amine displacing X from Ar-Pd -X to form Ar-Pd -NHR2. Abstraction of a proton by the base produces Ar-Pd -NRj, which undergoes a reductive elimination. 

The normal neutral pathway (22 24 25 27) was ruled out by conducting the reaction with monodentate phosphine BINAP ligand mimics (Scheme 12.5). The products obtained were of low enantiomeric excess relative to reactions employing BINAP. The direct cationic pathway (24-> 26) was also eliminated due to the fact that the opposite stereochemistry was obtained under cationic conditions with the addition of silver salts. The switch in stereoselectivity in the presence of silver salts, moreover, indicates that oxidative insertion is not the enantioselective step. j8-Hydride elimination was also discounted as the enantioselective step due to the influence of the double-bond geometry of the starting material on the enantioselectivity of the cyclization. The proposed enantioselective step is the formation of the cationic intermediate 26 by an associative displacement (24-> 28-> 26). In the case of square planar pafladium(n) complexes, substitution chemistry can occur through associative processes. Axial coordination of the alkene would form the pentacoordinate pafladium(II) complex 28. Reports of isolated and characterized pentacoordinate palladium(II) species provide support for this proposed intermediate. 

More recently, the Heck reaction has been used in cascade cyclisations, and under enantioselective conditions. For example, the intramolecular synthesis of decalin derivative 9 has been achieved in good enantiomeric excess, using an optically pure phosphine (Binap) as part of the palladium catalyst. ... 

Catalytic Asymmetric Hydroboration. The hydroboration of olefins with catecholborane (an achiral hydroborating agent) is cataly2ed by cationic rhodium complexes with enantiomericaHy pure phosphines, eg, [Rh(cod)2]BE4BINAP, where cod is 1,5-cyclooctadiene and BINAP is... 

The numerous chiral phosphine ligands which are available to date [21] can be subclassified into three major categories depending on the location of the chiral center ligands presenting axial chirality (e.g., BINAP 1 and MOP 2), those bearing a chiral carbon-backbone (e.g., DIOP 3, DuPHOS 4), and those bearing the chiral center at the phosphorus atom (e. g., DIPAMP 5, BisP 6), as depicted in Fig. 1. 

Fig. 1. shows the P MAS NMR chemical shifts for the immobilized and homogeneous catalyst. The chemical shifts at the -15.2 and -13.7 ppm correspond to PTA while the chemical shifts in the range from 20 and 40 ppm correspond to phosphine oxide. The chemical shifts at the 66 and 118 ppm seems to be those of BINAP ligand, which is confirmed by the spectrum of Ru-BINAP catalyst. This spectrum shows that PTA exist in large amount on the surface of immobilized catalyst and that BINAP ligand is intact after immobilization. 

Table 3.12 surveys current industrial applications of enantioselective homogeneous catalysis in fine chemicals production. Most chiral catalyst in Table 3.12 have chiral phosphine ligands (see Fig. 3.54). The DIP AMP ligand, which is used in the production of L-Dopa, one of the first chiral syntheses, possesses phosphorus chirality, (see also Section 4.5.8.1) A number of commercial processes use the BINAP ligand, which has axial chirality. The PNNP ligand, on the other hand, has its chirality centred on the a-phenethyl groups two atoms removed from the phosphorus atoms, which bind to the rhodium ion. Nevertheless, good enantio.selectivity is obtained with this catalyst in the synthesis of L-phenylalanine. 

The fundamental concepts of enantioselective hydrogenation were introduced in Section 2.5.1 of Part A, and examples of reactions of acrylic acids and the important case of a-acetamido acrylate esters were discussed. The chirality of enantioselective hydrogenation catalysts is usually derived from phosphine ligands. A number of chiral phosphines have been explored in the development of enantioselective hydrogenation catalysts,21 and it has been found that some of the most successful catalysts are derived from chiral 1, l -binaphthyldiphosphines, such as BINAP.22... 

Related catalysts include both a chiral BINAP-type phosphine and a chiral diamine ligand. A wide range of aryl ketones gave more than 95% enantioselectivity when substituted-l,l -binaphthyl and ethylene diamines were used.52... 

Early work in the field of asymmetric hydroboration employed norbornene as a simple unsaturated substrate. A range of chiral-chelating phosphine ligands were probed (DIOP (5), 2,2 -bis(diphenyl-phosphino)-l,l -binaphthyl (BINAP) (6), 2,3-bis(diphenylphosphino)butane (CHIRAPHOS) (7), 2,4-bis(diphenylphosphino)pentane (BDPP) (8), and l,2-(bis(o-methoxyphenyl)(phenyl)phos-phino)ethane) (DIPAMP) (9)) in combination with [Rh(COD)Cl]2 and catecholborane at room temperature (Scheme 8).45 General observations were that enantioselectivities increased as the temperature was lowered below ambient, but that variations of solvent (THF, benzene, or toluene) had little impact. 

Intramolecular hydrosilylation of the fe-alkenyl silane yields the chiral spirosilane with high diastereoselectivity (Scheme 30). With 0.3-0.5 mol.% of catalyst consisting of [Rh(hexadiene)Cl]2 and a range of chelating phosphines P-P (P-P = (R)-BINAP (6), (R,R)-DIOP (5)), a maximum chemical yield of spirosilane of 82% was found with 83% enantiomeric excess. These values were improved considerably by the use of the new ligand... 

The use of chiral bidentate phosphines (such as BINAP) in enantioselective cross-coupling reactions has been described, though only modest stereoselectivity has been achieved.431... 

Palladium-catalyzed aromatic C—O bond formation is less developed than palladium-catalyzed aryl amination. Except when the aryl halide is strongly electron deficient,107-110 catalysts ligated by the conventional aryl phosphines such as DPPF and BINAP are ineffective for coupling of... 

Catalytic arsination has also been reported.226 The arsine analog of BINAP (BINAs) has been prepared from the reaction of diphenylarsine with the optically pure ditriflate that is generated from binaphthol. Triarylarsines themselves have been used like triphenyl phosphine, in this case as a reagent for the conversion of aryl triflates to mixed aryl diphenylarsines. 

In asymmetric reactions, chiral phosphine ligands such as BINAP derivatives are used as effective chiral ligands in silver complexes. In particular, an Agr-BINAP complex activates aldehydes and imines effectively and asymmetric allylations,220-222 aldol reactions 223 and Mannich-type reactions224 proceed in high yield with high selectivity (Scheme 51). 

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