176724-72-4Relevant 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 (1996)
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.
