Ethylene complexes with palladium

An important feature of this process is that it uses the ability of palladium to form complexes with the reactant ethylene, with the important chemistry of ethylene occurring while it is attached to the metal. In other words, the palladium modifies the chemical behavior of ethylene to enable reactions to occur that would not be possible for free ethylene. Incidentally, the first ethylene complex with palladium in Figure 14-17 is isoelectronic with Zeise s complex, [PtCl3(iri -H2C=CH2)]. ... 

The obvious limitation of this reaction so far is in the electrophile as we have suggested naturally electrophilic enones as ideal partners for aryl or vinyl palladium o-complexes. In fact, complexation with palladium makes all alkenes electrophilic and nucleophilic addition can in principle occur to a simple alkene while it and the nucleophile are both bound to the palladium atom. Such reactions are known for aryl halides. Even ethylene itself does satisfactory Heck reactions and its reaction with the bromopyridine 132 is the basis for a large scale process leading to a drug.21... 

Fluoroalkylated heteroaromatic polymers can be prepared from 2,5-dibromo-3-perfluoropropylthiophene with NiCl2 [567]. The cross coupling reaction of dihaloheteroaromatic compounds and l,2-bis(tributylstannyl)ethylene catalyzed with palladium complexes leads in a single step to poly(thienylene vinylene) [353,568, 569]. The synthesis of structurally homogeneous PATs is described in Sect. 5.1.8. 

Nickel and palladium react with a number of olefins other than ethylene, to afford a wide range of binary complexes. With styrene (11), Ni atoms react at 77 K to form tris(styrene)Ni(0), a red-brown solid that decomposes at -20 °C. The ability of nickel atoms to coordinate three olefins with a bulky phenyl substituent illustrates that the steric and electronic effects (54,141) responsible for the stability of a tris (planar) coordination are not sufficiently great to preclude formation of a tris complex rather than a bis (olefin) species as the highest-stoichiometry complex. In contrast to the nickel-atom reaction, chromium atoms react (11) with styrene, to form both polystyrene and an intractable material in which chromium is bonded to polystyrene. It would be interesting to ascertain whether such a polymeric material might have any catal3dic activity, in view of the current interest in polymer-sup-ported catalysts (51). 

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. 

The last stage is supposed to be limiting. However, the limiting stage can be the transformation of the TT-complex into the o-complex of ethylene with palladium (preliminary stage). The rate constant k (water, T 298 K, ion force = 3g ion L-1) has the following values [246]. 

The details of the organic chemistry of the reaction of ethylene with PdCl2 (equation (1) above) are also known and are shown in Fig. 9.2. The palladium ion complexes with ethylene and water molecules and the water adds across the bond while still complexed to palladium. The palladium then serves as a hydrogen acceptor while the double bond reforms. Keto-enol tautomerism takes place, followed by release of an acetaldehyde molecule from the palladium. 

Figure 7.35. Dinuclear palladium(II) and platinum(ll) complexes with phenylene- and ethylene-bridged bis(oxazolone) ligands.

My last comment concerns the reaction of palladium olefin complexes with carbon monoxide discovered by Tsuji. I agree that this is most likely to proceed by an insertion rather than an ionic mechanism. Chloride attack on coordinated olefin is rare however. Chloride ion is an inhibitor, for example in the palladous chloride catalyzed hydration of ethylene (0). I, therefore, wondered whether carbon monoxide was affecting the ease with which chloride attacks olefin. One can postulate that carbon monoxide participates in this insertion either as a gas phase reactant or by first forming a carbonyl olefin complex. Such complexes of the noble metals were unknown, but examining the reaction between carbon monoxide and the halogen bridged olefin complexes of platinum revealed that they are formed very readily... 

Another important piece of mechanistic evidence comes from the fact that acetaldehyde obtained from ethylene and deuterium-labeled water does not have any deuterium incorporation. In other words, all four hydrogen atoms of the ethylene molecule are retained. This means vinyl alcohol, which would certainly exchange the hydroxyl proton with deuterium, cannot be a free shortlived intermediate that tautomerizes to acetaldehyde. However, in spite of many elegant synthetic accomplishments in organometallic chemistry, realistic model complexes of palladium for 8.4 to 8.7 remain unknown. 

When a sulfonated diphosphine is used as the ligand, the complex formed with palladium(0) catalyzes the alternating copolymerization of ethylene and... 

Similarly, the ethylene diamine complex of palladium ([Pd(en)2]2+) can be prepared. These positively charged complexes can be used for the study of interfacial reaction with montmorillonite. 

Similar cts-bis-carbene chelate complexes of palladium(Il) [327,330,331], but without the hydroxy functional groups on the wingtips, were used by the same research group for the copolymerisation of ethylene and CO. Once again, chelating bisphosphane complexes inspired the synthesis and application of their NHC counterparts [332,333]. The actual, defined catalyst precursors were the cationic complexes formed after haUde abstraction with silver salts in acetonitrile as donor solvent. 

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. 

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 ... 

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. 

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