ALUMOXANES AS COCATALYSTS IN THE PALLADIUM-CATALYZED COPOLYMERIZATIONOF CARBON-MONOXIDE AND ETHYLENE - GENESIS OF A STRUCTURE-ACTIVITY RELATIONSHIP

The palladium-catalyzed copolymerization of carbon monoxide and ethyle ne to give polyketone polymers, [CH2CH2C(O)](n), has been accomplished by the use of either (dppp)Pd(OAc)(2) or (dppp)Pd[C(O)(t)Bu]Cl in the presence of a tert-butyl alumoxane, [((t)Bu)Al(mu(3)-O)](n) (n = 6, 7 , 9) or [((t)Bu)(7)Al-5(mu(3)-O)(3)(mu-OH)(2)] cocatalyst. The effects on the catalytic activity of the alumoxane and palladium concentratio ns, 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)(t)Bu]Cl the alumoxane abstracts chloride to give a catalytically active catio nic palladium complex directly. In contrast, the alumoxane initially a lkylates the palladium in (dppp)Pd(OAc)(2) and subsequently abstracts the remaining acetate anion, yielding the active cationic palladium co mplex. The catalytic activity is highly dependent on the structure of the alumoxane. A comparative study indicates the cocatalytic activity to be [((t)Bu)Al(mu(3)-O)](7) > [((t)Bu)Al(mu(3)-O)](6) > [((t)Bu)Al(m u(3)-O)](9) >> [((t)Bu)(7)Al-5(mu(3)-O)(3)(mu-OH)(2)]. This observed c ocatalytic 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 )Bu)(6)Al-6(mu(3)-O)(4)( mu-OH)(2)(mu-O2CCCl3)(2)], prepared by the reaction of [((t)Bu)Al(mu(3 )-O)](6) with HO2CCCl3. The steric effects of the catalyst active site , as determined by the alkyl bridge length (n) in R(2)P(CH2)(n)PR(2) a nd the alkyl substituents R, were probed for the catalyst precursor co mpounds [R(2)P(CH2)(n)PR(2)]Pd[C(O)(t)Bu]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 oli gomers is indicative of catalyst turnover via a beta-hydride-eliminati on chain termination. A proposed catalyst mechanism and a pathway to c atalyst activation are presented. The molecular structures of (dppp)Pd [C(O)(t)Bu]Cl, (dppe)Pd[C(O)(t)Bu]Cl, (dmpe)Pd[C(O)(t)Bu]Cl, (dcpe)Pd[ C(O)(t)Bu]Cl, and t)Bu)(6)Al-6(mu(3)-O)(4)(mu-OH)(2)(mu-O2CCCl3)(2)] h ave been determined by X-ray crystallography.