4-methyl palladium complex

Modification of the behavior of a palladium methyl complex by coordination of a second transition metal complex was reported to enhance the CO insertion rate. The CO insertion into palladium-methyl bond in a Pd-Co heterodinuclear complex, (dppe)(CH3)Pd-Co(CO)4, proceeded with a higher reaction rate than into the mononuclear palladium complex, Pd(CH3)Cl(dppe) (Eq. 7.5) [66]. The outcome of the reaction using CO was consistent with the mechanism of the preferential insertion of the carbonyl on Co center into the Pd-C bond and the coordination of incoming CO to Co. Theoretical studies suggested that the... 

Brookhart on the insertion of a single ethylene into a branched alkyl complex of nickel and an example of the 2,1-insertion of vinylacetate into the palladium-methyl complex to form a stable chelated product. Equation 9.65 shows the insertion of a vinylarene into a palladium-aryl complex that is an intermediate in the vinylation of aryl halides known as the Heck reactioa... 

Few direct comparisons between the rates for insertions of alkenes and alkynes have been made. However, one comparison indicates that the insertions of alkenes are slower than die insertions of alkynes. The insertion of acetylene into the cationic palladium-alkyl complex in Equation 9.7H° is directly analogous to the insertions of ethylene into cationic palladium alkyl complexes. This insertion of acetylene is faster than the insertion of ettiylene into ttie same palladium-methyl complex. The insertion of 1-hexyne ismuch slower than the insertion of acetylene and gives a mixture of the two vinyl complexes that result from 1,2- and 2,1-insertion. 

The cationic palladium methyl complex [(phen)Pd(CH3)-(OEt2)](BAr4) (where Ar = 3,5-(CF3)2CgH3) has shown high efficiency in carbon-silicon bond forming processes such as hydrosilylation of unfunctionalized olefins and tandem cycliza-tion/hydrosilylation of functionalized 1,6-dienes (eq 2). ... 

Chen YC, Chen CL, Chen JT, Liu ST (2001) Well-controlled block polymerization/ copolymerization of alkenes and/or carbon monoxide by cationic palladium methyl complexes. Organometallics 20 1285... 

Reddy KR, Surekha K, Lee GH, Peng SM, Liu ST (2001) Study of insertion of olefins and/ or carbon monoxide into phosphine-imine palladium methyl complexes. Organometallics 20 1292... 

Other examples that involve intermediate allyl cations are illustrated in Scheme 1.4. The cationic palladium(II) complex [Pd(dppp)(PhCN)2](BF4)2 coordinates the carbonyl oxygen of benzaldehyde and the activated carbonyl carbon attacks the isoprene, forming the allyl cation 10 which then cyclizes to give the 4-methyl-6-phenyl-5,6-dihydro-2H-pyran [22]. 2-Oxopropyl acrylate 11, in the presence of trimethylsilyltrifluoromethane sulfonate (TMSOTf) and methoxytrimethylsilane (MeOSMT), generates the cation 11a which is an efficient dienophile that reacts easily with the cyclohexadiene to give the Diels-Alder adduct in good yield [23]. 

Early studies into the decomposition of ethyl diazoacetate by a ic-allyl palladium chloride complex in the presence of acetonitrile led to the isolation of 2-methyl-5-ethoxyoxazole in... 

The addition of cyclohexyl isocyanide to several palladium(II) methyl complexes has proven particularly interesting 169, 170). From... 

Following the synthesis of the first methyl-palladium NHC complexes it was subsequently found that the complexes undergo a facile thermal decomposition process in which the NHC is lost as 2-methylimidazolium salt and the Pd is rednced to Pd(0) (Scheme 13.1) [15-17]. In ensuing studies investigating the reaction behavionr of a range of hydrocarbyl Pd and Ni carbene complexes, it was found that the decomposition reaction is ubiquitous. It occurs with varying ease, for mono-NHC, bis-NHC and donor functionalised-NHC complexes [16-23]. 

From the beginning of the 1970s unhl the mid 1980s, several examples of the telomerization of dienes with water [76, 77] or methanol [78, 79] to isomeric mixtures of dienols or dienol ethers catalyzed by palladium-phosphine complexes in the presence of carbon dioxide have been reported. Neither the yield nor the selectivity were very high. However, when allene was employed as a diene , 3-methyl-2-meth-ylene-3-buten-l-ol was obtained with fairly good selectivity (up to 98%) (Eq. 6.43) [78]. 

Phosphine ligands based on the ferrocene backbone are very efficient in many palladium-catalyzed reactions, e.g., cross-coupling reactions,248 Heck reaction,249 amination reaction,250 and enantioselective synthesis.251 A particularly interesting example of an unusual coordination mode of the l,l -bis(diphenylphosphino)ferrocene (dppf) ligand has been reported. Dicationic palladium(II) complexes, such as [(dppf)Pd(PPh3)]2+[BF4 ]2, were shown to contain a palladium-iron bond.252,253 Palladium-iron bonds occur also in monocationic methyl and acylpalladium(II) complexes.254 A palladium-iron interaction is favored by bulky alkyl substituents on phosphorus and a lower electron density at palladium. 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

A number of examples have been reported documenting the use of palladium phosphine complexes as catalysts. The dialkyl species [PtL2R2] (L2 = dmpe, dppe, (PMe3)2 R = Me, CH2SiMe3) catalyze the reaction of [PhNH3]+ with activated alkenes (acrylonitrile, methyl acrylate, acrolein).176 Unfunctionalized alkenes prove unreactive. The reaction mechanism is believed to proceed via protonation of Pt-R by the ammonium salt (generating PhNH2 in turn) and the subsequent release of alkane to afford a vacant coordination site on the metal. Coordination of alkene then allows access into route A of the mechanism shown in Scheme 34. Protonation is also... 

Van Koten and Frey used a hyperbranched poly(triallylsilane) as the support for palladium- pincer complexes.[63] The supported palladium-pincer complexes were applied in the catalytic aldol condensation of benzaldehyde and methyl isocyanate. Their activity was similar to that of single site Pd catalysts. According to the authors, the complex is suitable for continuous membrane applications, as demonstrated by their purification by means of dialysis. 

Highly branched ethene-methyl acrylate polymers. The cationic palladium diimine complexes are remarkably tolerant towards functional groups, although the rates decrease somewhat when polar molecules are added. In ETM catalysis addition of polar molecules or monomers kills the catalyst and therefore it was very interesting to see what the new palladium catalysts would do in the presence of polar monomers. Indeed, using methyl acrylate a copolymerisation... 

The data for the insertion rate of CO into a palladium-methyl bond for dppp as the ligand has been studied with considerably more precision by Brookhart and co-workers [24], The kinetics were studied in the temperature range between 191 and 210 K for a reaction similar to that of Figure 12.6, i.e. the starting material was the CO adduct of the methylpalladium(dppp)+BArF complex. A AG of 62 kJ.mol"1 was observed and since AS was close to zero, a half-life time of 10 s is calculated for the CO-adduct at 235 K, much shorter than the value for dppe given above (150 s). 

In spite of the difficulties mentioned above, Brookhart and co-workers succeeded in measuring the barrier for ethene insertion into (dppp)PdC(0)CH3+ at 160 K, starting from the ethene adduct, generated at still lower temperatures, in the absence of CO. The barrier measured (AG ) amounted to only 51.4 kJ/mol, i.e. the reaction is faster than the insertion of CO in an ionic alkylpalladium complex. The barrier of insertion of ethene into a palladium methyl species or palladium ethyl species was higher, at 67 kJ/mol at 233 K. As for the CO insertion described above, these values concern the barriers in preformed ethene adducts at higher temperatures the overall barrier will be higher, because alkene coordination will be disfavoured by entropy and competition with CO and solvent. Formation of CO adducts will also be less favourable at higher temperatures. 

Methyl iodide is likely generated from methyl acetate and HI (equation 7) a portion of the iodide species in the reaction mixture certainly exists in the form of hydrogen iodide (equations 11, 12). Iodide ion tends to attack the -carbon of the alkyl portion of unhindered esters ( 7). Thus, methyl acetate, which would be in general inert to the metal complexes used, is converted to a reactive species which can readily oxidatively add to a palladium(0) complex. An... 

The oxidation to methyl ketones without cleavage of the double bond was reported recently for a palladium NHC complex [108]. When the authors used the previously described catalyst 13 in THF with dioxygen for the oxidation of styrene they found that together with the phenylmethylketone a significant amount of y-butyrolactone was formed. Analysis of the mechanism led to the conclusion that THF is oxidized to a hydroperoxide species which is the real oxidant. They therefore tried tert-butylhydroperoxide (TBHP) and found immediate conversion without any induction period. Optimized conditions include 0.75 mol % of the previously described dimeric complex... 

Similarly,palladium, nickel complex [49] or methyl viologen (MV2+) [50] have also been used to catalyze the reaction of perfluoroalkyl iodides with aldehydes in the presence of zinc to give the corresponding a-perfluoroalkyl carbinols (Scheme 17). 

Backwall and coworkers have extensively studied the stereochemistry of nucleophilic additions on 7r-alkenic and ir-allylic palladium(II) complexes. They concluded that nucleophiles which preferentially undergo a trans external attack are hard bases such as amines, water, alcohols, acetate and stabilized carbanions such as /3-diketonates. In contrast, soft bases are nonstabilized carbanions such as methyl or phenyl groups and undergo a cis internal nucleophilic attack at the coordinated substrate.398,399 The pseudocyclic alkylperoxypalladation procedure occurring in the ketonization of terminal alkenes by [RCC PdOOBu1], complexes (see Section 61.3.2.2.2)42 belongs to internal cis addition processes, as well as the oxidation of complexed alkenes by coordinated nitro ligands (vide in/ra).396,397... 

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