Pyridine(diimine) Chromium η,¹η³-Metallacycles as Precatalysts for Alkene-Diene [2 + 2] Cycloaddition

The synthesis, characterization, and catalytic cycloaddition reactivity of aryl-substituted pyridine(diimine) (PDI) chromium ether complexes and alkene-diene derived metallacycles are described. The reduction of either (PDI)CrCl₂ or (PDI)CrCl₃ with sodium and catalytic amounts of naphthalene furnished the corresponding chromium THF or diethyl ether complexes. Exposure of the reduced chromium compounds to a mixture of butadiene and ethylene induced rapid oxidative cyclization and yielded isolable, thermally stable chromium η,¹η³ trans-metallacycles. In the presence of pure butadiene, oxidative cyclization was also observed, and a η,¹η³ diene-diene chromacycloheptane with vinyl substitution was isolated and crystallographically characterized. Reductive C(sp³)-C(sp³) elimination of the resulting vinylcyclobutane products was promoted by either one-electron oxidation or irradiation with visible light in the presence of diene or coordinating solvents such as THF. Under thermal catalytic conditions, the chromacycles converted butadiene and ethylene to a mixture of vinylcyclobutane arising from [2 + 2] cycloaddition and cis-1,4-hexadiene resulting from competitive hydrovinylation. Visible light irradiation at ambient temperature enabled selective catalytic [2 + 2] cycloaddition. Isotopic labeling studies with ¹³C-labeled ethylene established that the alkene-diene chromacycle is thermodynamically preferred over the diene-diene alternative and that both oxidative cyclization and reductive coupling steps are reversible. Visible light accelerated the reductive coupling step, while subsequent product displacement from the metal center occurs in the presence of excess diene.