Palladium tertiary phosphine complexes

Tertiary phosphine complexes of platinum and palladium M(PR3)2X2 are important [95]. The cis- and trans- somers are readily obtained for platinum,... 

The palladium-catalyzed coupling of aiyl and vinyl halides to organotin compounds, known as Stille coupling, is one of the most important catalytic methods of carbon-carbon bond formation. The reaction is generally conducted in polar organic solvents, such as dimethylformamide, with tertiary phosphine complexes of palladium, although phosphine-free complexes or simple Pd-salts are also frequently used as catalysts [8]. 

Dimerization(46,47) with a palladium acetate/tertiary phosphine complex (di-ace tate-bis--[ tri-phenyl phosphine] palladium) in the absence of air gives primarily the desired linear dimer with smaller amount of branched, cyclic, and heavy product (Equation 13.). The reaction product is a complex mixture of cis and trans geometric isomers which makes analysis difficult. In order to simplify this problem, the product analysis is made after hydrogenation over 5% palladium on carbon. 

The complexes [Ni(acrylonitrile)2] and [Ni(COD)2] catalyze [3 + 2] cycloadditions of (26) with electron deficient l,2 isubstituted alkenes to afford 2,3- or 3,4-disubstituted methylenecyclopentanes such as (32) and (33). Similar reactions have been reported by use of tertiary phosphine complexes of nickel(0) and palladium(0) (equation 13 and Table 1). The reaction proceeds regioselectively to give (32) or (33) depending on both the alkene stmcture and catalytic system. Reactions catalyzed by phos-phine-palladium(0) complexes afford only products of the type (32), via selective cleavage of the C(2)— C(3)bondof(26). 

The most widely used methods of preparation of palladium(O) phosphine complexes are the reduction of palladium(II) salts in the presence of tertiary phosphine ligands (eq (48)-(50)) [65-67]. Palladium(II) dichloride and bis(2,4-pentanedionalo)palladi-um(II) (Pd(acac)2) are the typical starting materials. A variety of reducing agents including hydrazine [65], sodium borohydride [68], sodium alkoxide [69], and triethyl-alminum [66] were employed. 

Another convenient method to synthesize palladium(O) phosphine complexes involves the reduction of CpPd( 7r-all i) [70], The starting 7c-allyl complex can be readily prepared and stored as a relatively stable crystalline solid. In the presence of tertiary phosphine ligands in solution, this complex undergoes reductive elimination to give palladium(O) phosphine complexes. The reaction proceeds cleanly under mild conditions without formation of inorganic by-products, leading to a facile synthesis of kinetically unstable species. This method was used to prepare coordinatively unsaturated bis(tertiary phosphine)pa)ladium(O) complexes (eq (51)) [71]. 

Grushin, V.V. and Alper, H. (1993) Alkali-induced disproportionation of paUadium(II) tertiary phosphine complexes [L2PdCl2], to LO and palladium(O). Key intermediates in the biphasic carbonylation of ArX catalyzed by [L2PdCl2]. Organometallics, 12, 1890-901. 

A proposed mechanism [9] for the hydrosilylation of olefins catalyzed by platinum(II) complexes (chloroplatinic acid is thought to be reduced to a plati-num(II) species in the early stages of the catalytic reaction) is similar to that for the rhodium(I) complex-catalyzed hydrogenation of olefins, which was advanced mostly by Wilkinson and his co-workers [10]. Besides the Speier s catalyst, it has been shown that tertiary phosphine complexes of nickel [11], palladium [12], platinum [13], and rhodium [14] are also effective as catalysts, and homogeneous catalysis by these Group VIII transition metal complexes is our present concern. In addition, as we will see later, hydrosilanes with chlorine, alkyl or aryl substituents on silicon show their characteristic reactivities in the metal complex-catalyzed hydrosilylation. Therefore, it seems appropriate to summarize here briefly recent advances in elucidation of the catalysis by metal complexes, including activation of silicon-hydrogen bonds. 

In 1974, Heck reported the synthesis of ester using Pd-catalyzed carbonylation by a very simple procedure.f In this procedure, a solution of aryl iodides and an alcohol was heated under carbon monoxide (1 atm) at 100 °C in the presence of a tertiary amine and a catalytic amount of palladium-triphenyl phosphine complex to form ester. Pd(OAc)2,... 

Ph3P)4Pd and certain Pd(II) complexes in the presence of an excess of a tertiary phosphine also function as active catalysts (128). This indicates that palladium species may have potential provided they are protected from destructive reduction by the choice of suitable ligands. A complex species [(Ph3P)2Pd]jf gradually forms in the PhjP—Pd metal mixture ( 28). 

The palladium complex of the dibenzofuran-based water-soluble tertiary phosphine 49 was found catalytically active for the internal Heck reaction of N-aUyl-o-iodoaniline in CH3CN/H2O l/l(Scheme 6.6) [21],... 

The reaction is catalyzed by a group VIII metal species, particularly that of rhodium or palladium. The initial metal species may be any variety of complexes (e.g., PdCl2 Pd acetate, etc.). A source of halide is necessary iodide is especially effective. The most convenient source is methyl iodide, since it is likely a reaction intermediate. In addition, an organic promoter must be included for catalytic activity. These promoters are generally tertiary phosphines or amines. Also, chromium complexes were found to have an important promotional effect. 

The inclusion of the nitrogen donor atom in an aromatic heterocycle allows the possibility of -bonding with the metal centre, giving these ligands some similarities to tertiary phosphines. Thus pyridyl and bipyridyl ligands stabilize organometallic complexes of palladium (see companion volume).2 ... 

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