Palladium diphosphine ligands

Palladium acetate in conjunction with a diphosphine ligand, xantphos, is active for arylation of amides, ureas, oxazolidinones and sulfonamides.174... 

The studies on palladium-phosphine complexes, especially those with diphosphine ligands, have mainly been focused on the complexes of palladium in lower oxidation state of 0 and 1 84,580,581,583,766,787,788,811-816 suc]1 as [Pd2(dppm)2]2+, [Pd2(dppm)3],815 [Pd3(dppm)3(/i3-... 

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],... 

Insertion of alkenes. Alkene insertions have also been widely studied and many insertion products have been isolated [31], Alkene insertions follow a migratory mechanism in the palladium and platinum square planar complexes with diphosphine ligands [18],... 

The study of alkene insertions in complexes containing diphosphine ligands turned out to be more complicated than the study of the CO insertion reactions [13], When one attempts to carry out insertion reactions on acetylpalladium complexes decarbonylation takes place. When the reaction is carried out under a pressure of CO the observed rate of alkene insertion depends on the CO pressure due to the competition between CO and ethene coordination. Also, after insertion of the alkene into the acetyl species (3-elimination occurs, except for norbomene or norbomadiene as the alkene. In this instance, as was already reported by Sen [8,27] a syn addition takes place and in this strained skeleton no (3-elimination can take place. Therefore most studies on the alkene insertion and isolation of the intermediates concern the insertion of norbomenes [21,32], The main product observed for norbomene insertion into an acetyl palladium bond is the exo species (see Figure 12.8). 

While apparently no metallasilsesquioxane complexes of nickel and palladium have yet been prepared, several literature reports have appeared on platinum compounds containing silsesquioxane ligands. Abbenhuis reported the synthesis and characterization of three platinum(II) complexes stabilized by the chelating diphosphine ligand dppe (= 1,2-bis(diphenylphosphino)ethane) as outlined in Scheme 63. ... 

Figure 8.7 Encapsulation of palladium in a heterocapsule formed by ionic interactions between a concave Xantphos-based diphosphine ligand and a tetraanionic calix[4]arene.

The desymmetrization of dicarbonate 206 was initiated by the addition of one equivalent of N-(3-butenyl) nosylamide 207 under palladium catalysis in the presence of Trost s chiral diphosphine ligand 205. When the first allylic substitution was completed, the reaction was warmed and the resulting intermediate 208 was treated in situ with one equivalent of a second nosylamide 209. Product 210 resulting from this double substitution reaction was submitted to a tandem intramolecular ROM/RCM to furnish key precursor 211, which was engaged in the final cyc-lization step by the reduction of the double bonds, followed by the HCl-promoted domino deprotection of the acetal and aminal formation. 

Figure 5-52. The reaction of the palladium diphosphine complex with diacetyl gives a palladium complex of a macrocyclic P4 donor ligand.

Benvenuti F, Carlini C, Lami M, Marchionna M, Patrini R, Galletti AMR, Sbrana G (1999) Telomerization of 1,3-hutadiene with alcohols catalyzed by homogeneous palladium(O) complexes in the presence of mono- and diphosphine ligands. J Mol Catal A Chem 144 27-40... 

Trost has reported enhanced enantioselectivity in the desymmetrization of mero-biscarbamates in the presence of triethylamine. Under these conditions, high yields (>80%) and enantiomeric excesses (93-99% ee) are obtained. This methodology has been applied to the synthesis of (—)-swainsonine. a-Amino esters have been used as nucleophiles in the reaction with acyclic allylic esters and isoprene monoepoxide, providing access to diastereoselective N-alkylated a-amino esters. By employing the feature ligand, asymmetric palladium(0)-catalyzed cychzation of 2-(tosylamino)phenol with ( -l,4-bis[(methoxycarbonyl)oxy]but-2-ene provides 2-vinylbenzomorpholine in 79% ee. A number of alternative diphosphine ligands were studied and found to be inferior. 

Palladium-catalyzed asymmetric allylic alkylation [529] of suitable amines with two equivalents of racemic butadiene monoxide ((-r)-411) allows for the expedient synthesis of trans-and cA-2,5-dihydropyrroles derivatives 416 and 417 that are versatile chrrons towards the s)m-thesis of a wide variety of iminosugars [530]. In the presence of 0.4% [(allyl)PdCl)2], 1.2% of enantiomerically pure diphosphine (/ ,/ )-ligand, and Na2C03 a 1 1 mixture of ( )-411 and phthalimide reacted in CH2CI2 at room temperature giving (5)-412 in 99.6% yield and... 

Even in the early days of homogeneous hydrocyanation the reaction of norbor-nene with hydrogen cyanide in the presence of tetrakisftriphenyl phosphite)palla-dium(O) 12 indicated the influence of steric factors, since exo-5-cyanobicy-clo[2.2.1]heptane (Structure 10) is obtained stereospecifically. This result was confirmed in similar reactions showing that the entering cyano group is directed into the exo-position of the norbomene system [28]. This is due to the complexa-tion of the palladium(O) center to the exo-face of norbomene. Recent experiments have also utilized the bicyclic system to demonstrate asymmetric hydrocyanation induced by chiral palladium diphosphine complexes. Depending on the applied ligand system 11-17, an enantiomeric excess (ee) up to 40% is obtained [25]. 

The most effective palladium catalysts are those in which a PdFe bimetallic compound carries a bridging diphosphine ligand, when the reaction is thought to occur at the palladium centre with the iron playing an important role.25 Optically active methyl-phenylneophyltin hydride, [a] +28.9°, reacts with palladium on charcoal to give the optically active distannane, [a] +13.2°, which is optically stable for several weeks.28... 

Phenols are used as the nucleophile in the asymmetric aUylation of 7r-aUylpalladium complexes. Trost and Toste attained asymmetric phenyl ether formation in high enantiomeric excess (ee) using diphosphine ligand derived from chiral 1,2-cyclohexanediamine (equation 10). Dynamic kinetic resolution of the racemic secondary aUylic carbonate is conducted in the presence of tetrabutylammonium chloride, which increases the rate of ft—a—ft isomerization of the jr-allyl palladium intermediate (equation 11). Lautens and coworkers cleaved meio-oxabicyclic alkenes with phenol in the presence of a catalytic amount of a chiral ferrocenyldiphosphine and a rhodium complex (equation 12). ... 

Okada. Y. Minami. T., Yamamoto, T., and Ichikawa, J., A novel type of chiral diphosphine ligand, rra 75-2.3-/ zT(diphenylphosphino)-l-methyl-l-cyclopropanecai boxylic acid and asymmetric allylic alkylation by the use of its palladium complex, Chem. Lett., 547, 1992. 

Intermolecular hydroamination of alkynes, which is a process with a relatively low activation barrier, has not been used for the synthesis of chiral amines, since the achiral Schiff base is a major reaction product. However, protected aminoalkynes may undergo an interesting intramolecular allylic cyclization using a palladium catalyst with a chiral norbomene based diphosphine ligand (Eq. 11.9) [115]. Unfor tunately, significantly higher catalyst loadings were required to achieve better enantioselectivities of up to 91% ee. 

Since the AAA in aqueous solution has been most studied with Pd(diphosphine)-bearing catalysts, we screened the library of biotinylated diphosphine ligands that were initially used in the hydrogenation study (Sect. 2.1). Our initial experiments focused on symmetric 1,3-diphenylallyl acetate and dimethytmalonate as soft nucleophiles (Scheme 3) [50]. It is widely accepted that the enantiodiscrimmation event occurs through the external attack of a soft nucleophile on a palladium allyl intermediate. The AAA thus bears resemblance to enzymatic reactions, in which a reactant need not necessarily bind to the active site of the enzyme for the reaction to proceed with high stereoselectivity [51]. The second coordination sphere may thus play an important role in ensuring selectivity. 

From experimental results of acceleration of CO insertion into cationic methyl-palladium complexes [Pd(CH3)(CH3CN)(P-P)]+ in the order of DPPE < DPPF < DPPP DPPB (DPPF = l,2-bis(diphenylphosphino)ethane DPPF = 1,F-bis(diphenylphosphino)ferrocene DPPP = l,3-bis(diphenylphosphino)propane DPPB = l,4-bis(diphenylphosphino)butane) the kinetic barriers to migratory insertion in [Pd(CH3)(CO)(P-P)]+ were found to decrease with increase of the P-Pd-P angle as well as the steric bulk of the diphosphine ligand [50]. 

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