Alumoxanes as Cocatalysts in the Palladium-Catalyzed Copolymerization of Carbon Monoxide and Ethylene:  Genesis of a Structure−Activity Relationship

Abstract

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-O2CCCl3)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) n PR2 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.

Novel Molecular Clusters Having Aluminum-Phosphorus, Aluminum-Arsenic, and Gallium-Arsenic Skeletons, and Synthesis of an Al4As6Li4 Rhombododecahedron

“…For instance, the alumoxanes [tBuAl(m 3 -O)] n (n 6, 7, 9) can act as cocatalysts in the Pd-catalyzed conversion of CO and ethene to furnish polyketone polymers [2] and are functional models of MAO (methylalumoxane) which is a cocatalyst in the Ziegler ± Natta polmerization process. [3] However, structure ± reactivity relationships concerning aluminum and gallium pnictide cage compounds have been investigated far less than the chalcogenides of type A [4] and the imino derivatives B.…”

Regioselective cyclotrimerization of phenylacetylenes to 1,2,4-triarylbenzenes catalyzed by iridium–diphosphine complexes

“…In the reactions catalyzed by HIr(cod)(dppm), selective formation of the one of the two regioisomers shown in Scheme 2 must be proposed, to account for the selective formation of 1,2,4-triarylbenzene. As in the series of catalysts under discussion the regioselectivity increases by decreasing the chelate ring size, i.e., by decreasing the ligand bite angle and therefore the steric crowding around the metal [15,16], preferred formation of the metallacycle with both aryl substituents adjacent to iridium can be suggested here.…”

Metallocene Catalysts

“…In the case of MAO, a cluster structure could not be isolated, while such a cluster with tertiary butyl groups instead of methyl groups were isolated and characterized by Barron (35). To increase the solubility of MAO, some methyl groups have to be exchanged with isobutyl or other alkyl groups (modified MAO).…”