Palladium complexes ethylene

The conversion of ethylene to acetaldehyde using a soluble palladium complex, developed in the late 1950s, was one of the early applications of homogeneous catalysis and the first organo-palladium reaction practised on an industrial scale [40], Typically this reaction requires stoichiometric amounts of CuCl under aerobic conditions. The use of copper represents not only an environmental issue, but often limits the scope of ligands that can be used in conjunction with Pd. 

The first examples of highly active olefin polymerization catalysts based on late transition metals were nickel and palladium complexes containing bulky diimine ligands.310 312 For example, complex (120) was found to polymerize ethylene with an activity of ll,000gmmol h bar A range of PE materials with molecular weights up to 106 and... 

Several combinatorial approaches to the discovery of transition metal based catalysts for olefin polymerization have been described. In one study Brookhart-type polymer-bound Ni- and Pd-(l,2-diimine) complexes were prepared and used in ethylene polymerization (Scheme 3).60,61 A resin-bound diketone was condensed with 48 commercially available aminoarenes having different steric properties. The library was then split into 48 nickel and 48 palladium complexes by reaction with [NiBr2(dme)] and [PdClMe(COD)], respectively, all 96 pre-catalysts being spatially addressable. 

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

Chalk and Elarrod (11a) compared the above ethylene Pt(II) complex with chloroplatinic acid for hydrosilation, and found that each gave essentially the same results in terms of rate, yields, and products. Plati-num(II) complexes and rhodium(I) complexes were very much alike in their behavior. No system was found in which a palladium olefin complex brought about hydrosilation. In most systems the palladium complex was very rapidly reduced to the metal. 

Insertion of ethylene into the Ni-C bond in 3a leads to the alkyl complex 4a via the transition state TS[3a-4a] with a barrier [13a] of 17.5 kcal/mol relative to 3a It is worth to note that in TS[3a-4a both ethylene and the a-carbon of the growing (propyl) chain are situated in the N-Ni-N plane. For the corresponding palladium complex the insertion barrier [13c] is somewhat higher at 19.9 kcal/mol. 

In new studies heteropoly acids as cocatalysts were found to be very effective in combination with oxygen in the oxidation of ethylene.1311 Addition of phosphomo-lybdic acid to a chloride ion-free Pd(II)-Cu(II) catalyst system results in a great increase in catalytic activity and selectivity.1312 Aerobic oxidation of terminal alkenes to methy ketones can be performed with Pd(OAc)21313 or soluble palladium complexes. Modified cyclodextrins accelerates reaction rates and enhance selectivities in two-phase systems under mild conditions.1315 1316... 

One of the earliest uses of palladium(II) salts to activate alkenes towards additions with oxygen nucleophiles is the industrially important Wacker process, wherein ethylene is oxidized to acetaldehyde using a palladium(II) chloride catalyst system in aqueous solution under an oxygen atmosphere with cop-per(II) chloride as a co-oxidant.1,2 The key step in this process is nucleophilic addition of water to the palladium(II)-complexed ethylene. As expected from the regioselectivity of palladium(II)-assisted addition of nucleophiles to alkenes, simple terminal alkenes are efficiently converted to methyl ketones rather than aldehydes under Wacker conditions. 

During their work on the arylation of aromatic compounds by substitution, Fujiwara, et al. observed biaryl formation when aromatic compounds were placed in the presence of olefin-palladium complexes and silver nitrate.80 Developing this reaction as a method for biphenyl synthesis, these authors showed that the more stable the olefin-palladium complex was, the lower the yield. Ethylene dichloropalladium proved to be the best choice, when used with silver nitrate. However, the reaction required stoichiometric amounts of both catalysts (Scheme 10.47). Benzene derivatives substituted by electron-donating or -withdrawing groups reacted as well, but a mixture of regioisomers was produced, except for nitrobenzene, which only gave m,m -dinitrobiphenyl. 

Conversion of ethylene to acetaldehyde with a soluble palladium complex was one of the early applications of homogeneous catalysis. Traditionally, acetaldehyde was manufactured either by the hydration of acetylene or by the oxidation of ethanol. As most of the acetic acid manufacturing processes were based on acetaldehyde oxidation, the easy conversion of ethylene to acetaldehyde by the Wacker process was historically a significant discovery. With the... 

The catalytic cycle proposed for ethylene to acetaldehyde is shown in Fig. 8.2. The tetrachloro palladium anion 8.1 is used as the precatalyst. Conversion of 8.1 to 8.3 involves substitution of two chloride ligands by ethylene and water. Nucleophilic attack on coordinated ethylene leads to the formation of 8.4. The latter then undergoes substitution of another Cl- ligand. Conversion of 8.5 to 8.6 involves /3-hydride abstraction and coordination by vinyl alcohol. Intramolecular hydride attack to the coordinated vinyl group leads to the formation of 8.7. The latter eliminates acetaldehyde, proton, and CF and in the process is reduced to a palladium complex of zero oxidation state. 

Unlike early transition metal polymerization catalysts which do not tolerate functional groups, cationic palladium complexes are able to copolymerize ethylene with methyl acrylate.128... 

One of the more extraordinary recent developments in nickel and palladium polyalkene catalysis has been the development of a-diimines with bulky substituents as ligands in nickel and palladium complexes. When bulky aryl groups are used (R = isopropyl), these catalysts polymerize ethylene with high activities to high molecular weight highly branched... 

Palladium complexes figure prominently as well in the copolymerization of Q -olefins with carbon monoxide. Unlike the low molecular weight photodegradable random copolymers of ethylene and CO produced from a free-radical process, olefin/carbon monoxide copolymers produced from homogeneous palladium catalysts are perfectly alternating, the result of successive insertions of olefin and CO (Figure 19). Consecutive insertion of two similar monomers is either slow... 

Ethylene (tert-phosphine) complexes of zero-valent nickeP and platinum have been known for years. Analogous palladium complexes can be synthesized along the same lines as those reported for the nickel compounds, using ethoxy-diethylaluminum(III) as the reducing agent in the presence of ethylene. These palladium-ethylene complexes may serve as starting materials for oxidative addition reactions, since the ethylene ligand is loosely bonded. ... 

In 1938 Kharasch and co-workers described a method generally applicable for preparing mono-olefin palladium complexes (14 ). Palladium (II) chloride reacted with warm benzonitrile to form the complex bis(benzonitrile)-palladium chloride, and the latter reacted directly with olefins such as ethylene, styrene, cyclohexene, etc., as follows ... 

It has been shown that electron-rich cyclopropanes are able to displace ethylene from dichloro(ethylene)platinum to yield four-membered metallocyclic complexes (cf. equation 37). On the other hand 1,1,2,2-tetracyanocyclopropane (171) reacts under mild conditions with zerovalent platinum and palladium complexes of the type Pt(PPh3)2 (C2H4) or ML (n = 3, 4 M = Pd or Pt L = phosphines or triphenylarsines) to give metallocyclobutane derivatives (172) (equation 118) . 

Additions to nonactivated olefins and dienes are important reactions in organic synthesis [1]. Although cycloadditions may be used for additions to double bonds, the most common way to achieve such reactions is to activate the olefins with an electrophilic reagent. Electrophilic activation of the olefin or diene followed by a nucleophilic attack at one of the sp carbon atoms leads to a 1,2- or 1,4-addition. More recently, transition metals have been employed for the electrophilic activation of the double bond [2]. In particular, palladium(II) salts are known to activate carbon-carbon double bonds toward nucleophilic attack [3] and this is the basis for the Wacker process for industrial oxidation of ethylene to acetaldehyde [41. In this process, the key step is the nucleophilic attack by water on a (jt-ethylene)palladium complex. 

The reaction of HCN with bicyclo[2.2.1]heptene in the presence of Pd[P(OPh)3]4 gave only poor yields of 2-cyanobicyclo[2.2.1]heptane unless P(OPh)3 was also present, in which case a product that was almost exclusively the exo isomer could be obtained (J5). The suggested mechanism is outlined in Fig. 15. The palladium complex will also effectively catalyze the addition of HCN to ethylene. 

Another new synthesis of 77-allyl complexes is the reaction of divinyl carbinols with palladium(II) salts in methanol or ethylene glycols 270). In methanol, either 1,5- or 1,2-addition of methoxide occurred, whereas ethylene glycol gave a dioxanyl 77-allylic palladium complex. 

---