Nickel-Catalyzed Dimerization of Di- and Trisubstituted Olefins

The dimerization of a series of di- and trisubstituted C5-olefins catalyzed by hexafluoroacetylacetonate (acacCF3) nickel complexes is described. The dimerization of the trisubstituted olefin 2-methyl-2-butene by both a single-component nickel precatalyst and in situ activation of a bis(acacCF3) nickel precatalyst produced predominantly C10-dimers with nearly identical product distributions. The dimers produced by the nickel catalysts are less-branched than acid-catalyzed (e.g., BF3) dimers, due to the ability of the nickel intermediate to function as both an olefin isomerization and a dimerization catalyst, whereby C-C bond formation occurs preferentially between a primary carbon of one olefin and the less substituted carbon of the inserting olefin. The dimerization of the regioisomers of 2-methyl-2-butene, 2-methyl-1-butene, and 3-methyl-1-butene, utilizing the aforementioned nickel precatalysts, resulted in the same structural isomers of the dimers but different product distributions, indicating that dimerization occurred at similar rate or faster than isomerization. Stoichiometric experiments, product analysis, and observations from in situ nuclear magnetic resonance spectroscopy support a mechanism whereby a short-lived nickel-hydride intermediate promotes chain insertion, chain walking, and isomerization to give less-branched dimers from highly substituted olefins than would be achieved with acid-catalyzed dimerization.