Linear alpha-olefins, such as 1-hexene and 1-octene, are important comonomers in the production of linear low-density polyethylene (LLDPE). The conventional method of producing 1-hexene and 1-octene is by oligomerization of ethylene, which yields a wide spectrum of linear alpha-olefins (LAOs). While there exists several processes for producing 1-hexene via ethylene trimerization, a similar route for the selective production of 1-octene has so far been elusive. We now, for the first time, report an unprecedented ethylene tetramerization reaction that produces 1-octene in selectivities exceeding 70%, using an aluminoxane-activated chromium/((R2)2P)2NR1 catalyst system.

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Oxidative addition of SeCN(-) to tertiary phosphine ligands has been investigated in methanol at 298 K by use of UV-Vis stopped-flow and conventional spectrophotometry. In most cases k(obs) vs. [SeCN(-)] plots were linear with zero intercepts corresponding to a rate expression of k(obs) = k(1)[SeCN(-)]. Reactions rates are dependent on the electron density of the phosphorus centre with k(1) varying by five orders of magnitude from 1.34 +/- 0.02 x 10(-3) to 51 +/- 3 mol(-1) dm(3) s(-1) for P(2-OMe-C(6)H(4))(3) to PCy(3) respectively. Activation parameters range from 27 +/- 1 to 49.0 +/- 1.3 kJ mol(-1) for DeltaH(double dagger) and -112 +/- 9 to -140 +/- 3 J K(-1) mol(-1) for DeltaS(double dagger) supporting a S(N)2 mechanism in which the initial nucleophilic attack of P on Se is rate determining. Reaction rates are promoted by more polar solvents supporting the mechanistic assignment. Reasonable linear correlations were observed between log k(1)vs. pK(a), (1)J(P-Se) and chi(d) values of the phosphines. The reaction rates are remarkably sensitive to the steric bulk of the substituents, and substitution of phenyl rings in the 2 position resulted in a decrease in the reaction rate. The crystal structures of SePPh(2)Cy and SePPhCy(2) have been determined displaying Se-P bond distances of 2.111(2) and 2.1260(8) A respectively.

Complex equilibria in methanol/chloroform/dichloromethane solutions containing Zeise's anion, [PtCl(3)(C(2)H(4))](-) (1), the solvento species, trans-[PtCl(2)(C(2)H(4))(MeOH)] (2), and the dinuclear complex, trans-[PtCl(2)(C(2)H(4))](2) (3), have been studied by UV-vis, (1)H, and (195)Pt NMR spectroscopy, giving average values of K(Cl) = (1.6 +/- 0.2)10(3) M(-)(1) and K(S) = (0.16 +/- 0.02) M(-)(1) for the equilibrium constants between 2 and 1 and 3 and 2, respectively. The bridged complex 3 is completely split into monomeric solvento complexes 2 in methanol and in chloroform or dichloromethane solutions with [MeOH] > 0.5 M. Ethene exchange at the mononuclear complexes 1 and 2 was studied by (1)H NMR line-broadening experiments in methanol-d(4). Observed overall exchange rate constants decrease with an increase in free chloride concentration due to the displacement of the rapid equilibrium between 1 and 2 toward the more slowly exchanging parent chloro complex 1. Ethene exchange rate constants at 298 K for complexes 1 and 2 are k(ex1) = (2.1 +/- 0.1)10(3) M(-)(1) s(-)(1)and k(ex2) = (5.0 +/- 0.2)10(5) M(-)(1) s(-)(1), respectively, with corresponding activation parameters DeltaH(1)() = 19.1 +/- 0.3 kJ mol(-)(1), DeltaS(1)() = -117 +/- 1 J K(-)(1) mol(-)(1), DeltaH(2)() = 10.2 +/- 0.4 kJ mol(-)(1), and DeltaS(2)() = -102 +/- 2 J K(-)(1) mol(-)(1). The activation process is largely entropy controlled; the enthalpy contributions only amounting to approximately 30% of the free energy of activation. Ethene exchange takes place via associative attack by the entering olefin at the labile site trans to the coordinated ethene, which is either occupied by a chloride or a methanol molecule in the ground state. The intimate mechanism might involve a two-step process via trans-[PtCl(2)(C(2)H(4))(2)] in steady state or a concerted process via a pentacoordinated transition state with two ethene molecules bound to the platinum(II).