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The polymerization of methyl acrylate by pentafluorophenyl complexes [Pd2(μ-X)2(C6F5)2L2] (L = tetrahydrothiophene (tht), X = Cl, 2; L = tht, X = Br, 3; L = AsPh3, X = Br, 4) gives atactic polymers in good yields. Mechanistic studies reveal that the polymerization of methyl acrylate starts by insertion of methyl acrylate in the Pd−aryl bond of the precatalyst to give the alkyl complexes trans-[Pd2(μ-Cl)2{CH(CO2Me)-CH2C6F5}2(tht)2] (5) and trans-[Pd2(μ-Cl)2{CH(C6F5)CH2CO2Me}2(tht)2] (6). These complexes can be isolated, and the X-ray crystal structure of 5 has been determined. Complexes 5 and 6 decompose mainly by β-H elimination but also by homolytic cleavage of the Pd−C bond in the light. In the presence of methyl acrylate, insertion of MA in hydrido−Pd species produces the alkyl complex trans-[Pd2(μ-Cl)2{CH(CO2Me)CH3}2(tht)2] (9). Then a radical polymerization is initiated by small amounts of the radicals generated from these complexes (5, 6, or 9). Formation of 9 is the regeneration pathway of radicals after a termination reaction has occurred by recombination of the growing radical with palladium and β-H elimination. The success of the polymerization requires a slow but steady supply of radicals by slow decomposition of alkyl complexes (5 and 6) or by slow generation of Pd−H species that provide new alkyl complexes (9), as well as an efficient recycling of the Pd−H generated in the termination step to 9.

The neutral palladium complex [Pd(C6F5)Br(NCMe)2] (1) was found to effect the polymerization of acrylates upon addition of 1 equiv of a monodentate phosphine or pyridine or an excess of halide. Methyl methacrylate was not polymerized, and furthermore, its addition stopped the progress of independently initiated methyl acrylate polymerization in the phosphine-based system. Addition of ethene also inhibited the polymerization of methyl acrylate. However, over 10 mol % incorporation of 1-hexene in the polymer was achieved when the latter was added together with methyl acrylate. The polymerizaion mechanism is discussed.

The behavior of palladium C-bound enolates [Pd(CH2C(O)CR3)Cl(PPh3)2] (R = H, 1; R = Me, 2) and [Pd(CH2C(O)CR3)(PPh3)2(NCMe)](BF4) (R = H, 5; R = Me, 6) has been studied. Dimeric species with bridging enolate moieties are formed in solution when a coordination site on the metal is made available, either with Pd2{μ-κ²-C,O-CH2C(O)CR3}2 or with mixed Pd2{μ-κ²-C,O-CH2C(O)CR3}(μ-X) (X = Cl, OH) bridges. It is proposed that π back-donation is important to stabilize oxygen bonding. Complexes 1 and 2 undergo exchange between free and coordinated phosphine in solution. Kinetic experiments support an intramolecular associative mechanism which could involve an oxoallyl-like transition state. The reactivity of the complexes has been explored. Some reactions typical of Pd-alkyls have been observed such as insertion of CO to give CR3C(O)CH2COOH. Electrophilic attack on oxygen is very important:  the hydrolysis of the enolate complexes has been studied and also the reaction with ClSiMe3 to give silyl enol ethers.

The synthesis of enantiomerically pure palladium cyclic allyl complexes is accomplished by reaction of “PdHBr” with several chiral cyclic terpenes. The strategy used involves a 100% stereoselective Pd-migration from an attached chain to the ring through a chiral carbon (“gated migration”). “PdHBr” species are produced by β-H elimination in the benzylic moiety of [Pd2(μ-Br)2(η³-PfCH2CHPh)2] (1, Pf = C6F5). The allylic complexes prepared are transformed by carbonylation reactions into chiral unsaturated methyl esters.

The reaction of [Pd(C6F5)Br(NCMe)2] with the allylic sulfides R¹SCH2CH&dbd;CH2 (R¹ = −CH2CH&dbd;CH2, Me), n-BuSCHMeCH&dbd;CH2, and n-BuSCH2CH&dbd;CHMe results in insertion of the double bond into the Pd−C6F5 bond and β-SR elimination (C−S cleavage) to give the corresponding (pentafluorophenyl)alkenes and a palladium thiolate. With alkenyl sulfides of longer carbon chains RS(CH2)nCH&dbd;CH2 (R = n-Bu, Ph; n = 2, 3) five-membered (n = 2, 13, 14) or six-membered (n = 3, 16, 17) σ-yl-κS-palladacycles were obtained. The six-membered derivatives isomerize to the corresponding five-membered palladacycles (18, 19) by one-step Pd-migration (Pd−H elimination−readdition). Inversion of the coordinated sulfur is observed in these derivatives, and the dynamic process has been studied for the monomeric complexes [(σ-κS-(Rthio)alkyl)Pd(acac)] (13b, 14b, 18b, 19b). ΔG^&thermod;Tc values show that the S-inversion is easier for the Ph-substituted than for the n-Bu-substituted complexes. The decomposition of the σ-yl-κS palladacycles occurs through Pd-migration and, when Pd and S are placed three bonds apart in the chain, β-SR elimination; this is the same process observed in the reaction of [Pd(C6F5)Br(NCMe)2] with allylic sulfides. Double bond isomerization of the (pentaflurophenyl)alkene products formed in the decomposition reactions is observed, catalyzed by “Pd−H” intermediates. Double pentafluorophenyl arylation is also observed in some cases.

A three-membered dimeric (phenylthio)alkylpalladacycle (mixture of diastereoisomers) has been synthesized by Pd−C6F5 addition to the double bond of phenyl vinyl sulfide. The coordination mode of the organic moiety changes slowly in solution from a chelating σ-κS-yl to a bridging μ(σ-κS-yl), and the palladacyclopropane isomerizes to an unusual tetrameric palladium complex; the X-ray crystal structure of this latter derivative has been determined, and it shows that each pair of palladium atoms are linked by either two bridging chlorine atoms or two bridging (phenylthio)alkyl moieties. The tetramer has four chiral carbon atoms and has S4 symmetry (meso form), showing that the crystals are formed stereospecifically from the diastereomeric mixture of dimers. The decomposition pathways of the palladacyclopropane complex have been analyzed. Two main routes have been observed, i.e., (a) 1,2-hydrogen shift and Pd−SR β-elimination, which occurs in refluxing toluene, and (b) hydrolysis of the C−S bond to give an aldehyde and a palladium thiolate, predominant at room temperature in the presence of ligands such as tetrahydrothiophene.

η¹−η²-Enylpalladium complexes have been detected and/or isolated in the reaction of [PdPfBr(NCMe)2] (Pf = C6F5) with the dienes R−CH&dbd;CH−(CH2)n−Y−(CH2)m−CH&dbd;CH2 (R = H, Me; n, m = 0, 1; Y = CCl2, C(COOMe)2, CO2, O, SiMe2, SO2). Insertion of one double bond into the Pd−aryl bond and coordination of the remaining double bond gives the above-mentioned organometallic derivatives. The dienes tested have a non-hydrogen-containing link (Y), so Pd migration to give η³-allyl derivatives is blocked. The evolution and decomposition processes observed for the η¹-η²-enyls reveal that β-X elimination is the main operating pathway for Y = CCl2 (X = Cl) or Y = OCO, O, SiMe2 (X = YR‘). When Y = C(COOMe)2 or SO2, C−X cleavage is not observed and intramolecular insertion to give cyclic products or Pd−H elimination to generate a substituted diene predominates, respectively. From these results, a trend in C−X cleavage easiness in the presence of palladium can be estimated:  C−C ≪ C−SO2 < C−Cl < C−O (ether) < C−O (ester) ≈ C−Si.

Palladium(II) complexes with the asymmetric aryl groups o-C6BrF4 and m-C6BrF4 have been synthesized, namely [Pd2(C6BrF4)2(μ-Cl)2(tht)2], trans-[Pd(C6BrF4)Cl(tht)2] (tht = tetrahydrothiophene), trans-[Pd(C6BrF4)2L2], and cis-[Pd(C6BrF4)2L2] (L2 = 2tht, COD, Me2bipy (4,4‘-dimethyl-2,2‘-bipyridine), OPPy2Ph (Py = 2-pyridyl), 2PMe3, 2CNMe). A mixture of atropisomers (syn-Br,Br and anti-Br,Br) is found for most bis(aryl) complexes, and the ratio of syn:anti atropisomers depends upon the R group, the cis or trans geometry of the complexes, and the ancillary ligand:  A ratio close to 1:1 is observed for the cis- and trans-m-C6BrF4 derivatives and for the trans-o-C6BrF4 complexes, but the anti atropisomer is preferred in the cis-o-C6BrF4 derivatives, particularly if the ancillary ligands extend out of the coordination plane. Strong through-space coupling between syn Fortho atoms of mutually cis R groups is observed in the ¹⁹F NMR spectra for cis-[Pd(C6BrF4)2L2] complexes. J-values were determined which prove useful to unequivocally identify the atropisomers formed and to detect distortions in the structure of the complexes. Atropisomerization by aryl rotation is observed in several cases at different rates:  Faster for m-C6BrF4 than for o-C6BrF4 derivatives, and faster for small-in-plane than for bulk-in-plane ancillary ligands. The fastest rotation occurs in the complexes with CNMe, which do not show atropisomers by ¹⁹F NMR at room temperature.

The reactions of [PdPfBr(NCMe)2] (Pf = C6F5) with stoichiometric amounts of 1,5-hexadiene, 1,6-heptadiene, or 1,7-octadiene at low temperature result in the formation of several (η¹-η²-enyl)palladium complexes that isomerize sequentially at different temperatures depending on the ring size of the palladacycles (Tisom:  7.5- < 6.5- < 5.5-membered). These (η¹-η²-enyl)palladium derivatives are intermediates in the Pd-migration process, arrested by coordination of the unattacked double bond. The final products of their isomerization are several isomeric Pf−(η³-allyl)palladium complexes (Pf = C6F5). The major allylic derivative in each case arises from Pd migration to the terminal double bond. Minor amounts of (η³-allyl)palladium complexes formed by double bond switches in the process of Pd migration are also seen, but this occurs only on putative 1,5- or 1,6-diene−hydrido−palladium intermediates. A small amount of cyclic organic derivatives coming from the cyclization of (η¹-η²-enyl)palladium intermediates is detected in each case. The use of excess diolefin gives rise to additional (η³-allyl)palladium complexes without the Pf group and to the corresponding Pf-substituted linear dienes. These arise via displacement of the Pf dienes by the starting diolefin in a hydrido−palladium intermediate during the Pd-migration process.

Intramolecular hydride transfer in a dimer between Pd atoms and reductive elimination of H and a benzyl moiety to give PhCH2CH2C6F5 (3) is the main decomposition pathway for the hydrido species generated from the η³-benzylpalladium derivative [Pd2(μ-Br)2(η³-CHPhCH2C6F5)2] (1). The commonly accepted decomposition of the palladium hydride to give Pd(0) and HX, followed by acid attack on 1 to produce the alkane, is ruled out in this case.