Phosphite complexes of palladium

Preparation of phosphine and phosphite complexes of palladium ) have been reported to result from reduction of palla-dium(II) complexes in the presence of the desired ligand.1" The products are generally formulated as PdL4 (where n = 0, 1), depending upon the nature and amount of the ligand used. A related complex, [Pd P(C6H5)3 2]re, has also been reported.6... 

It has been shown 4 that Ph3CBp4 can function as a one-electron oxidant and is able to oxidize metallic copper with formation of the homoleptic solvento-complex, [Cu(MeCN)4][BF4]. Yamazaki S has reported that the phosphine and phosphite complexes of palladium(n), [Pd(PPh3)4]X2 (X = BF4, PF, and Pd P(OPh)3 4][PF6]2, have been synthesized in 60% and... 

The chemistry of phosphite, phosphonite and phosphinite complexes of palladium(II) generally resembles that of their phosphine analogues. Its description here will therefore be brief, attention being paid to those areas where differences in behaviour are apparent. 

The d phosphine and phosphite complexes of zerovalent nickel, palladium and platinum possess long-lived emissive excited states in both fluid solution and in the solid state.The lifetimes of the Ni and Pd complexes are in the 1.39-5.38 ps range, with the platinum complex Pt(PPh3)4 having the shorter excited state lifetime of 0.07 //s because of spin-orbit coupling. These long lifetimes allow for bimolecular reactions to occur, and under photochemical conditions chlorobenzene will add to the complex Pd(PPh3)3 (Ref. 75) ... 

Since Pd complexes are well-known catalysts for enantioselective allylic substitution reactions, here the catalytic behaviour of palladium NPs for this reaction is examined (Scheme 1). One example involving a chiral phosphite with a carbohydrate backbone, able to coordinate firmly at the surface of NPs together with oxygen atoms capable to interact weakly with this surface, is presented. In particular. 

The susceptibility of phosphites to hydrolysis limits their application as ligands for homogeneous catalysis. The fused tricyclic monophosphites derived from ca 1 i x[4]arenes260-262 form coordination complexes with palladium.263... 

Electronic ligand effects are highly predictable in oxidative addition reactions a-donors strongly promote the formation of high-valence states and thus oxidative additions, e.g. alkylphosphines. Likewise, complexation of halides to palladium(O) increases the electron density and facilitates oxidative addition [11], Phosphites and carbon monoxide, on the other hand, reduce the electron density on the metal and thus the oxidative addition is slower or may not occur at all, because the equilibrium shifts from the high to the low oxidation state. In section 2.5 more details will be disclosed. 

The planar phosphite complexes [PdX2 P(OR)3 2] have been prepared by a variety of methods, such as the reaction of P(OR)3 with [PdX p in ethanol in the presence of LiX.88 The palladium(II) iodide and P(0-o-tolyl)3 complexes have trans structures or are cisftrans mixtures, but the others are predominantly cis isomers. The phosphonites, P(OR)2Ph, and... 

The Suzuki reaction of aryl bromides and chlorides is efficiently catalyzed by palladium/ phosphite complexes generated in situ. The influence of the ligand, base, and various additives was examined. The process tolerates various functional groups, and catalyst turnovers of up to 820,000 were obtained, even with deactivated aryl bromides [94]. 

Ligand size determines coordination numbers as well as reactivity. Thus phosphine complexes of Pd°, frequently used as precursors in palladium catalyzed reactions, may be 4-, 3-, or 2-coordinate, as in Pd(PMe3)4, Pd(PPr )3, and Pd(PPhBu 2)2, respectively. For nickel phosphine and phosphite complexes, the dissociation constant Kd for the equilibrium... 

Similarly, palladium phosphite complexes catalyze the addition of B—Si bonds of silylboranes to alkynes the reaction is a typical cis addition with high regioselectivity.44... 

The purity can be checked conveniently by complexometric titration of palladium - after destruction of the complex with sulfuric and nitric acids and by volumetric determination of the ethylene upon displacement with triphenyl phosphite. Anal. Calcd. for C38Hs4P2Pd Pd, 16.1 C2H4, 4.26 C, 69.3 H, 5.2. Found Pd, 16.2 C2H4, 4.06 C, 68.9 H, 4.9. 

Water-soluble palladium(O) complexes have also been used as homogeneous catalysts in aqueous-solution alkylation reactions. The particular complex that has been used is Pd(TPPMS>3. Aryl or heteroaromatic halides can be coupled with aryl or vinyl boronic acids, alkynes, alkenes, or dialkyl phosphites with this palladium(0) complex. This complex in aqueous solution can also be used for the coupling of alkynes with unprotected iodonucleotides, iodonucleosides, and iodoamino acids (133). 

Trialkyl phosphines and phosphites generally form with palladium (II) chloride well-defined square planar complexes of the type [PdCl2L2], which exist as either cis or trans isomers. 

The trialkyl phosphine complexes are usually prepared by reaction of the phosphine with sodium tetrachloropalladate(II) in alcohol, with hydrated palladium(II) chloride in ethanol, or with a suspension of anhydrous palladium(II) chloride in benzene. However, these methods cannot be e.xtended to include ligands, or their derived complexes, which undergo solvolysis or which do not react with anhydrous palladium(II) chloride suspensions. Although trialkyl phosphite complexes have been... 

The scheme illustrates that E-olefins lead to virtually pure trans-adducts. However, Z-olefins produce mixtures of as- and tra .9-methylenecyclopentanes, the loss of stereospecificity being partially due to Z/E isomerization of the olefin under the reaction conditions. The stepwise nature of the addition also contributes to the loss in stereochemical integrity. Although in these examples the palladium catalyst used was the prereduced tetrakis(triphenylphosphane)palladi-um(0), studies have shown that there is no significant difference in yield between this approach and the use of palladium acetate with triisopropyl phosphite. Indeed, this latter system is often much easier to apply since the nature of the phosphane can be altered and the handling of air-sensitive complexes is avoided. The procedure with teti akis(triphenylphosphane)palladium(0) as catalyst is exemplified by the addition to coumarin. 

These cycloadditions are more sensitive to the quality of the catalyst, the major side reaction being protodesilylation of the allylsilane subunit. Since this can not be measured readily either in the case of the tetrakis(triphenylphosphane)palladium(0) or the palladium acetate/triisopropyl phosphite methods, an improved method for generating the palladium(O) species has been developed22. This involves in situ preparation of tetrakis(triisopropyl phos-phite)palladium(O) by direct reduction of palladium acetate with butyl lithium. This method is illustrated by the addition of the methyl-substituted TMM-Pd complex to eyelopentenone. 

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