Dimerization of alkenes catalyzed by Ni complexes

The Institut Fran ais du Petrole has developed and commerciaHzed a process, named Dimersol X, based on a homogeneous catalyst, which selectively produces dimers from butenes. The low-branching octenes produced are good starting materials for isononanol production. This process is catalyzed by a system based on a nickel(II) salt, soluble in a paraffinic hydrocarbon, activated with an alkylalumini-um chloride derivahve directly inside the dimerization reactor. The reaction is sec- 

The reaction takes place at low temperature (40-60 °C), without any solvent, in two (or more, up to four) well-mixed reactors in series. The pressure is sufficient to maintain the reactants in the liquid phase (no gas phase). Mixing and heat removal are ensured by an external circulation loop. The two components of the catalytic system are injected separately into this reaction loop with precise flow control. The residence time could be between 5 and 10 hours. At the output of the reaction section, the effluent containing the catalyst is chemically neutralized and the catalyst residue is separated from the products by aqueous washing. The catalyst components are not recycled. Unconverted olefin and inert hydrocarbons are separated from the octenes by distillation columns. The catalytic system is sensitive to impurities that can coordinate strongly to the nickel metal center or can react with the alkylaluminium derivative (polyunsaturated hydrocarbons and polar compounds such as water). 

Despite aU the advantages of this process, one main limitation is the continuous catalyst carry-over by the products, with the need to deactivate it and to dispose of wastes. One way to optimize catalyst consumption and waste disposal was to operate the reaction in a biphasic system. The first difficulty was to choose a good solvent N,N -DiaIkyHmidazoHum chloroaluminate ionic Hquids proved to be the best candidates. These can easily be prepared on an industrial scale, are liquid at the reaction temperature, and are very poorly miscible with the products. They play the roles both of the catalyst solvent and of the co-catalyst, and their Lewis acidities can be adjusted to obtain the best performances. The solubihty of butene in these solvents is high enough to stabilize the active nickel species (Table 5.3-3), the nickel 

Despite the utmost importance of physical limitations such as solubility and mixing efficiency of the two phases, an apparent first-order reaction rate relative to the olefin monomer was determined experimentally, ft has also been observed that an increase of the nickel concentration in the ionic phase results in an increase in the olefin conversion. 

In the homogeneous Dimersol process, the olefin conversion is highly dependent on the initial concentraHon of monomers in the feedstock, which limits the applicability of the process. The biphasic system is able to overcome this limitation and promotes the dimerization of feedstock poorly concentrated in olefinic monomer. 

---

Another photocatalytic reaction of some interest is the cyclic dimerization of alkenes, which has been reported for a number of different metal carbonyls including Fe(CO)s, Cr(CO)6, Ni(CO)4 and several substituted analogs [83-85]. Although such dimerizations are catalyzed thermally by some of the same species, the stereochemistries of the products differ depending both upon which complex is utilized and whether the reaction is carried out thermally or photochemically. The dimerization of butadiene to 4-vinylcyclohexene photocatalyzed by the iron dicarbonyl Fe(CO>2(NO)2 has been studied by examining the intermediates formed when the photolysis was carried out in liq. Xe [85]. The key species was found to be the disubstituted species Fe(NO)2(Ti -butadiene)2 which was proposed to undergo isomerization to Fe(NO)2( n -butadiene)(ri2-butadiene) which can undergo an internal Diels-Alder cyclization to the vinyl cyclohexene. 

Formation of n-alkylnickels by addiction of Ni hydride to an alkene is important in many nickel-catalyzed reactions such as alkene dimerization and isomerization. However, stoichiometric formation of alkylnickels from alkenes and Ni hydrides is not synthetically useful because of the reactivity of alkylnickels. The regiochemistry of the addition of HNiX(PR3)2 to propene is affected by the nature of the trialkylphosphine group used in the Ni hydride complex. The observed ratios of n-propyl- to isopropyl-nickel found after the insertion step vary from 82 18 to 82 20 to 19 81 as the phosphine is changed from P(OC6Hs)3 to P(c-C6Hu) to P(t-C4H9)2(i-C3H7). ... 

Metal-mediated cyclizations that rely on the initial complexation of an alkene or alkyne around a low oxidation state metal center are often sensitive to the presence of additional substituents (particularly electron-donating substituents), and relatively more stringent reaction conditions are often required for successful cychzation. This effect was noted in the Ni-catalyzed formal [4 -I- 4] cycloaddition reactions developed by Wender and Tebbe and is apparent when one compares the reported facility of Pd-catalyzed linear dimerization of 1,3-butadiene versus that of substituted 1,3-dienes. Similarly, the initial attempts at Pd-catalyzed cyclization of bisdiene 70a (Scheme 22) were rather disappointing. Using 0.05 equiv of [Pd(OAc)2/3 PhjP] (THF, 65 °C, 12 h), only a small... 

---