Phosphine as ligand

Piet W. N. M. van Leeuwent, Charles P. Casey, Gregory T. Whiteker  

van Leeuwen and C. Claver (eds.), Rhodian CatalyzedHydroformylttiion, 63-105. 2000 Kluwer Academic Publishers. Printed in the Netherlands. 

Like CO, coordinated NO can give the migratory insertion reaction  

Tertiary phosphines, PR3, are important because they constitute one of the few series of ligands in which electronic and steric properties can be altered in a systematic and predictable way over a very wide range by varying R. They also stabilize an exceptionally wide variety of ligands of interest to the organometallic chemist as their phosphine complexes (R3P) M—L. 

Like NH3, phosphines have a lone pair on the central atom that can be donated to a metal. Unlike NH3, they are also ir acids, to an extent that depends on the nature of the R groups present on the PR3 ligand. For alkyl phosphines, the it acidity is weak aryl, dialkylamino, and alkoxy groups are successively more effective in promoting it acidity. In the extreme case of PF3, the IT acidity becomes as great as that found for CO. 

CARBONYLS, PHOSPHINE COMPLEXES. AND LIGAND SUBSTITUTION REACTIONS 

FIGURE 4.3 The a orbitals of the P—R bond play the role of acceptor in metal complexes of phosphorus ligands. As the atom attached to phosphorus becomes more electronegative, the empty a orbital of the P—X bond moves to lower energy and becomes a better acceptor from the metal. 

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The preparation of a series of transition metal complexes (Co. Ni. Pd. Pt, Ir. Au. Cu. Ag) with ambident anion (70) and phosphines as ligands has been reported recently (885). According to the infrared and NMR spectra the thiazoline-2-thione anion is bounded through the exocyclic sulfur atom to the metal. The copper and silver complexes have been found to be dimeric. 

Miscellaneous Reactions of Phosphines.- The role of chiral phosphines as ligands in the catalysis of reactions leading to the formation of chiral products has been reviewed.1111 A procedure for the determination of the enantiomeric excess in chiral phosphines has been developed, based on 13C n.m.r. studies of the diastereoisomeric complexes formed by phosphines with the chiral pinenyl nickel bromide complex. 111 Studies of the sulphonation of triphenylphosphine and of chiral arylphosphines have been reported in attempts to prepare water soluble ligands which aid... 

In 1998, Miyaura reported a Rh(acac)(CO)2/dppp-catalyzed addition of aryl or alkenylboronic acids to aldehydes in aqueous organic mixtures under an inert atmosphere (Eq. 8.85).216 The use of electron-rich tri(tm-butyl)phosphine as ligand was found to be beneficial for obtaining good yields of the corresponding aldehyde addition products.217... 

Under microwave heating, the Heck olefinations were achieved in 30-60 min, as opposed to 10-40 h by conventional heating. The recyclable heterogeneous LDH-Pd(0) catalytic system circumvents the need to use expensive and air-sensitive basic phosphines as ligands in the palladium-catalyzed coupling of chloroarenes. This novel Mg-Al layered double-hydroxide (LDH) support in the catalytic system stabilizes the nanopalladium particles and also supplies adequate electron density to the anchored palladium(O) species and facilitates the oxidative addition of the deactivated electron-rich chloroarenes. 

One mole of isoprene reacted with one mole of acetoacetate by using a bidentate phosphine as ligand (56). Reaction of 2,3-dimethylbutadiene with acetoacetate was carried out by using PdCl2 in the presence of sodium phenoxide. When PPh3 was used, a 1 2 adduct was obtained. On the other hand, use of P-phenyl-l-phospha-3-methyl-3-cyclopentene (105) at 100°C caused the 1 1 addition to give 3-carbomethoxy-5,6-dimethyl-5-hepten-2-one (106), from which 5,6-dimethyl-5-hepten-2-one (107) was formed. This compound is the useful intermediate for a-irone synthesis (96). 

Following Wilkinson s discovery of [RhCl(PPh3)3] as an homogeneous hydrogenation catalyst for unhindered alkenes [14b, 35], and the development of methods to prepare chiral phosphines by Mislow [36] and Horner [37], Knowles [38] and Horner [15, 39] each showed that, with the use of optically active tertiary phosphines as ligands in complexes of rhodium, the enantioselective asymmetric hydrogenation of prochiral C=C double bonds is possible (Scheme 1.8). 

Water-soluble transition-metal complexes have been used recently for transfer hydrogenolysis of halocarbons. Paetzold and Oehme [110] have realized the reductive dehaiogenation of allyl or benzyl halides in the presence of [(phosphine) 2PdCl2] complexes with sulfonated phosphines as ligands (e.g., Ph2P(CH2)3S03K) by... 

Non-activated aryl bromides (but not fluorides) can be used as substrates for palla-dium(0)-catalyzed aromatic nucleophilic substitutions with aliphatic or aromatic amines. These reactions require sodium alcoholates or cesium carbonate as a base, and sterically demanding phosphines as ligands. Moreover, high reaction temperatures are often necessary to achieve complete conversion (Entries 7 and 8, Table 10.4 Experimental Procedure 10.1). Unfortunately, the choice of substituents on the amine... 

Compared with the analogous hydrogenation of aldehydes, the reaction requires somewhat more drastic conditions (about 200°C and 6 hrs), but the temperature is still within the stability range of the cobalt carbonyl phosphine complexes containing tertiary alkyl phosphines as ligands. If aryl phosphines are used, a more or less pronounced decomposition of the carbonyl complexes can be observed (as indicated by the IR... 

Using chiral phosphines as catalyst components, asymmetric induction may be expected if R R. This assumption was tested with acetophenone as substrate (R = Ph, R = Me) and two different optically active tertiary phosphines as ligands (S)-(+ )-PMePrPh and (S)-( — )-PMePh(CH2Ph). Only insignificant optical activity of the product was achieved in both cases (optical yields below 1%). High reaction temperature and/or the low coordination number of the intermediate complexes (lack of steric factors) may be the cause of this negative result. 

It is clear from the preceding section that the field of tethered arene-metal complexes is dominated by ruthenium and by arene-phosphines as ligands. In part, this situation has arisen because of the current surge of interest in the catalytic properties of ruthenium complexes in organic synthesis.85,86 Moreover, the tethered arene complexes are usually air-stable, crystalline solids with a well-defined, half-sandwich molecular geometry that, in principle, can lock the configuration at the metal centre. These compounds should, therefore, be ideal both for the study of the stereospecificity of reactions at the metal centre and for stereospecific catalysis. 

Asymmetric hydroboration of prochiral alkenes has been achieved using transition metal catalysts and chiral phosphines as ligands to obtain enantiomerically pure alkyl boronates <1997CC173>. Catalysts such as Rh(COD)2+BF4 , Rh(COD)2+Cl, Rh+BF4 , etc., in combination with chiral phosphines like DIOP 71, BINAP 72, CHIRAPHOS 73, DIPAMP 74, BDPP 75, ferrocene-based diphosphines 76<1999TL4977>, etc., have been employed for the asymmetric hydroboration of prochiral alkenes with moderate to high ee (DIOP = 2,3-0-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane BINAP = 2,2-bis(diphenyl-phos-phanyl)-l,1-binaphthyl CHIRAPHOS = 2,3-bis(diphenylphosphino)butane DIPAMP = l,2-bis[(2-methoxyphe-nyl)phenylphosphino]ethane BDPP = 2,4-bis(diphenylphosphino)pentane). 

The use of secondary phosphines as ligands for transition metals has led to some interesting complexes (2,3). Our interest in unusual macrocyclic ligand systems led us to the synthesis of the first macrocycles (VI, VII, Scheme 1) which contain secondary-phosphino ligating sites. 

Asymmetric C-C bond formation catalysed by metallic complexes with chiral amino-phosphines as ligands. 

Scheme 8. Phase-tagged phosphines as ligands in Pd-catalyzed cross-coupling reactions.

The arylation of cyclopentadiene (3) with 2-chlorobromobenzene using tris(7ert-butyl)phosphine as ligand leads to a mixture of the tetra-arylated product 5e and its pentaarylated congener, both isolated in 17-18 % yield. With the less reactive ortho-dichlorobenzene the process stops at the stage of the triarylated species 4e, which can then be further arylated with 2-chlorobromobenzene. 

Much attention has been focused on the nature of P Sn coordination bonds in phosphines as ligands. The shift of the Sn resonance to higher field upon adduct formation, the multiplicity of the resonance, the variation of P- Sn coupling with the nature of the substituent and the change in the shape of the signals with concentration and temperature were used to determine the stoichiometry and the structure of the phosphine adducts of Ph3SnCl, R2SnCl2 (R = Et, Pr, Bu, f-Bu and Ph) and RSnC (R = Bu and Ph)" >. 

Enantioselective catalytic hydrogenation. Hoffmann-LaRoche chemists have examined the efficiency of DIOP (1) and a number of related phosphines as ligands for asymmetric hydrogenation of acrylic acids for synthesis of (R)-6-methyl-tryptophan (6), of potential value as a non-nutritive sweetening agent. Of these, 2 and the less available 3 showed the most promise. Thus catalytic hydrogenation of 4... 

More recently there has been a strong growth of interest in the use of transition metals as homogeneous catalysts in aqueous solution. These processes involve the intermediacy of organometallic compounds for cases where the catalysts are simple transition-metal halide salts or where the metal complexes contain water-soluble tertiary phosphines as ligands 3,4). 

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