81174-30-3Relevant academic research and scientific papers
Alumoxanes as cocatalysts in the palladium-catalyzed copolymerization of carbon monoxide and ethylene: Genesis of a structure-activity relationship
Koide, Yoshihiro,Bott, Simon G.,Barron, Andrew R.
, p. 2213 - 2226 (2008/10/08)
The palladium-catalyzed copolymerization of carbon monoxide and ethylene to give polyketone polymers, [CH2CH2C(O)]n, has been accomplished by the use of either (dppp)-Pd(OAc)2 or (dppp)Pd[C(O)tBu]Cl in the presence of a tert-butyl alumoxane, [(tBu)Al(μ3-O)]n (n = 6, 7, 9) or [(tBu)7Al5(μ3-O) 3(μ-OH)2] cocatalyst. The effects on the catalytic activity of the alumoxane and palladium concentrations, the alumoxane structure, and the identity of the phosphine ligands were determined. The function of the alumoxane is shown to depend on the choice of palladium catalyst precursor. With (dppp)Pd[C(O)tBu]Cl the alumoxane abstracts chloride to give a catalytically active cationic palladium complex directly. In contrast, the alumoxane initially alkylates the palladium in (dppp)Pd(OAc)2 and subsequently abstracts the remaining acetate anion, yielding the active cationic palladium complex. The catalytic activity is highly dependent on the structure of the alumoxane. A comparative study indicates the cocatalytic activity to be [(tBu)Al(μ3-O)]7 > [(tBu)Al(μ3-O)]6 > [(tBu)Al(μ3-O)]9 ? [(tBu)7Al5(μ3-O) 3(μ-OH)2]. This observed cocatalytic activity correlates with the predicted latent Lewis acidity of the alumoxanes. A discussion of the palladium-alumoxane complex is presented with respect to the model compound [(tBu)6Al6(μ3-O) 4(μ-OH)2(μ-O2-CCCl3) 2], prepared by the reaction of [(tBu)Al(μ3-O)]6 with HO2CCCl3. The steric effects of the catalyst active site, as determined by the alkyl bridge length (n) in R2P(CH2)nPR2 and the alkyl substituents R, were probed for the catalyst precursor compounds [R2P(CH2)n-PR2]Pd[C(O) tBu]Cl (R = Ph, n = 2 (dppe), 3 (dppp), 4 (dppb); R-Me (dmpe), C6H11 (dcpe), n = 2). The concept of "pocket angle" has been developed to account for the observed steric effects. The detection of vinyl end groups on low-molecular-weight oligomers is indicative of catalyst turnover via a β-hydride-elimination chain termination. A proposed catalyst mechanism and a pathway to catalyst activation are presented. The molecular structures of (dppp)Pd[C(O)tBu]Cl, (dppe)Pd[C(O)tBu]Cl, (dmpe)Pd[C(O)tBu]Cl, (dcpe)Pd[C(O)tBu]Cl, and [(tBu)6Al6(μ3-O) 4(μ-OH)2(μ-O2CCCl3) 2] have been determined by X-ray crystallography.
On the mechanism of the hydrocarbalkoxylation of olefins catalyzed by palladium complexes
Cavinato, G.,Toniolo, L.
, p. 187 - 195 (2007/10/02)
The acyl complex PdCl(COR)(PPh3)2 (R = Et, n-Hex), isolated during the course of hydrocarbalkoxylation reactions catalyzed by the precursor system PdCl2(PPh3)2-PPh3 (95 deg C, P(CO) 100-120 atm; Pd:P = 1 : 3-4), in ethanol or higher alkanols as solvents, reacts with an alkanol R'OH in the presence of added PPh3 (Pd:P = 1 : 3) to yield the ester RCOOR' and a mixture of PdCl2(PPh2)2 and Pd(PPh3)3 or 4.Moreover, when it is employed as catalyst precursor (R = n-Hex; Pd:P = 1:4) in the hydrocarbalkoxylation of ethylene, it is recovered as its ethyl analog and it yields almost stoichiometric amounts of n-HexCOOR'.When the dicarbomethoxy complex PdCl(COOMe)(PPh3)2, isolated in mixture with PdCl(COR)(PPh3)2 in hydrocarbomethoxylation experiments, is treated with 1-hexene in methanol (Pd:P:1-hexene:MeOH = 1:3:40:125), under nitrogen, in the absence of carbon monoxide, at 95 deg C, methyl heptanoate ester is not formed, and the starting complex is recovered almost quantitatively (92percent).When PdCl2(PPh3)2 or PdCl(COOMe)(PPh3)2 are used as catalyst precursors for the carbonylation of 1-hexene in MeOH, in the absence of added PPh3 and in the presence of NEt3 or of carboxylic acid anions (both of them are known to favor the formation of the carbomethoxy complex), no catalytic activity is observed and the precursors are recovered as palladium(0)carbonylphosphine complexes, ultimately mixed with the carbomethoxy complex. The results support the view that, of the two commonly accepted mechanisms for the catalytic hydrocarbalkoxylation of olefins, involving M-H or M-COOR' addition to the olefinic double bond, only the first one, in which a key intermediate is a Pd-acyl species, is probably involved. When n-BuOH is used as solvent the catalytic activity remains high even after 5-6 reuses of the catalyst, whereas in MeOH the activity falls significantly below its initial value because of decomposition of the catalyst into inactive palladium(0) complexes and palladium metal, probably via a carbomethoxypalladium complex.
